, the scientific study of cells, was born in 1663 when Robert Hooke observed the empty cell walls of cork and coined the word cellulae (“little cells”) to describe them

Soon he studied thin slices of fresh wood and saw living cells "filled with juices”—a fluid later named cytoplasm.

By the mid- 1800s, with increasingly sophisticated instruments and methods of observation, scientists arrived at certain generalizations about cells that we now call the cell theory.

This is credited especially to German physician-physiologist Theodor Schwann (1810-82) and German botanist Matthias Schleiden (1804-81)

is therefore the foundation for all biological understanding of life.

We will shortly examine the structure of a generic cell, but the generalizations we draw shouldn't blind you to the diversity of cellular form and function in humans.

There are about 200 kinds of cells in the human body, with a variety of shapes, sizes, and functions.

thin, flat, scaly shape, often with a bulge where the nucleus is, much like the shape of a fried egg “sunny side up.” Squamous cells line the esophagus and air sacs (alveoli) of the lungs, and form the surface layer (epidermis) of the skin.

—squarish-looking in frontal sections and about equal in height and width; liver cells are a good example.

-distinctly taller than wide, such as the inner lining cells of the stomach and intestines.

-having irregularly angular shapes with four, five, or more sides, like the wax cells of a honeycomb. The densely packed cells of many glands are polygonal.

having multiple pointed processes projecting from the body of a cell, giving it a somewhat starlike shape. The cell bodies of many nerve cells are stellate.

to ovoid—round to oval, as in egg cells and white blood cells.

-disc-shaped, as in red blood cells.

—spindle-shaped; elongated, with a thick middle and tapered ends, as in smooth muscle cells. Fibrous-long, slender, and threadlike, as in skeletal muscle cells and the axons (nerve fibers) of nerve cells.

long, slender, and threadlike, as in skeletal muscle cells and the axons (nerve fibers) of nerve cells.

Some of these shapes refer to the way a cell looks in typical tissue sections, not to the complete three-dimensional shape of the cell.

A cell that looks squamous, cuboidal, or columnar in a tissue section, for example, usually looks polygonal if viewed from its upper surface.

The most useful unit of measure for designating cell sizes is the micrometer (μm), formerly called the micron-one-millionth (10-6) of a meter, one-thousandth (103) of a millimeter.

The smallest objects most people can see with the naked eye are about 100 μm, which is about one-quarter the size of the period at the end of a typical sentence of print

A few human cells fall within this range, such as the egg cell and some fat cells, but most human cells are about 10 to 15 μm wide.

The longest human cells are nerve cells (sometimes over a meter long) and muscle cells (up to 30 cm long), but both are usually too slender to be seen with the naked eye.

There are several factors that limit the size of cells. If a cell swelled to excessive size, it could rupture like an overfilled water balloon.

In addition, cell size is limited by the relationship between its volume and surface area.

The surface area of a cell is proportional to the square of its diameter, while volume is proportional to the cube of its diameter.

Thus, for a given increase in diameter, volume increases much more than surface area.

Picture a cuboidal cell 10 μm on each side (fig. 3.2). It would have a surface area of 600 μm2 (10 μm × 10 μm × 6 sides) and a volume of 1,000 μm3 (10 × 10 × 10 μm).

Now, suppose it grew by another 10 μm on each side. Its new surface area would be 2,400 μm2 (20 μm × 20 μm × 6) and its volume would be 8,000 μm3 (20 × 20 × 20 μm).

Large Cell Surface Area =

20 μm x 20 μm cell x 6 = 2,400 μm^2

20 μm x 20 μm cell x 20 μm = 8,000 μm^3

10 μm x 10 μm x 10 μm = 1000 μm^3

Effect of Cell Growth

Diameter (D) increased by a factor of 2 Surface Area increased by a factor of 4 (D^2) Volume increased by a factor 8 (=D^3)

The Relationship Between Cell Surface Area and Volume.

As a cell doubles in diameter, its volume increases eightfold, but its surface area increases only fourfold. A cell that is too large may have too little plasma membrane to serve the metabolic needs of the increased volume of cytoplasm.

The time required for diffusion is proportional to the square of distance, so if a cell diameter doubled, the travel time for molecules within the cell would increase fourfold.

If it took 10 seconds for a molecule to diffuse from the surface to the center of a cell with a 10 μm radius, then we increased the cell radius to 1 mm, it would take 278 hours to reach the center-far too slow to support the cell's life activities.

Having organs composed of many small cells instead of fewer large ones has another advantage.

The death of one or a few cells has less effect on the structure and function of the whole organ.

In the nineteenth century, little was known about cells except that they were enclosed in a membrane and contained a nucleus. .

The fluid between the nucleus and surface membrane, its cytoplasm, was thought to be little more than a gelatinous mixture of chemicals and vaguely defined particles

The transmission electron microscope (TEM), invented in the mid-twentieth century, radically changed this view.

Using a beam of electrons in place of light, the TEM enabled biologists to see a cell's ultrastructure, a fine degree of detail extending even to the molecular level.

The most important thing about a good microscope isn't magnification but resolution-the ability to reveal detail, to distinguish small, close-together objects from each other.

Any image can be photographed and enlarged as much as we wish, but if enlargement fails to reveal any more useful detail, it is empty magnification.

A big blurry image isn't nearly as informative as one that's small and sharp. The TEM reveals far more detail than the light microscope (LM)

A later invention, the scanning electron microscope (SEM), produces dramatic three-dimensional images at high magnification and resolution, but can view only surface features.

Magnification Versus Resolution. Cross sections of a skeletal muscle cell viewed by

(a) light microscopy (LM) and (b) transmission electron microscopy (TEM). The TEM, with its better resolution, shows significantly finer detail, down to the protein filaments that produce muscle contraction.

A stunning application of SEM, often seen in this book, is the vascular corrosion cast technique for visualizing the blood vessels of an organ. The vessels are drained and flushed with saline, then carefully filled with a resin.

After the resin solidifies,, the actual tissue is dissolved with a corrosive agent such as potassium hydroxide. This leaves only a resin cast of the vessels, which is then photographed with the SEM.

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Chapter 3

Question	Answer
Cytology	, the scientific study of cells, was born in 1663 when Robert Hooke observed the empty cell walls of cork and coined the word cellulae (“little cells”) to describe them
Robert Hooke	Soon he studied thin slices of fresh wood and saw living cells "filled with juices”—a fluid later named cytoplasm.
By the mid- 1800s, with increasingly sophisticated instruments and methods of observation, scientists arrived at certain generalizations about cells that we now call the cell theory.	This is credited especially to German physician-physiologist Theodor Schwann (1810-82) and German botanist Matthias Schleiden (1804-81)
All activities of an ----- (including the human body) stem from the activities of its constituent cells.	organism
Cytology	is therefore the foundation for all biological understanding of life.
All cells arise from ----- cells, not from nonliving matter, and they pass hereditary information from generation to generation of cells.	preexisting
We will shortly examine the structure of a generic cell, but the generalizations we draw shouldn't blind you to the diversity of cellular form and function in humans.	There are about 200 kinds of cells in the human body, with a variety of shapes, sizes, and functions.
Squamous	thin, flat, scaly shape, often with a bulge where the nucleus is, much like the shape of a fried egg “sunny side up.” Squamous cells line the esophagus and air sacs (alveoli) of the lungs, and form the surface layer (epidermis) of the skin.
Cuboidal	—squarish-looking in frontal sections and about equal in height and width; liver cells are a good example.
Columnar	-distinctly taller than wide, such as the inner lining cells of the stomach and intestines.
Polygonal	-having irregularly angular shapes with four, five, or more sides, like the wax cells of a honeycomb. The densely packed cells of many glands are polygonal.
Stellate	having multiple pointed processes projecting from the body of a cell, giving it a somewhat starlike shape. The cell bodies of many nerve cells are stellate.
Spheroidal	to ovoid—round to oval, as in egg cells and white blood cells.
Discoidal	-disc-shaped, as in red blood cells.
Fusiform	—spindle-shaped; elongated, with a thick middle and tapered ends, as in smooth muscle cells. Fibrous-long, slender, and threadlike, as in skeletal muscle cells and the axons (nerve fibers) of nerve cells.
Fibrous	long, slender, and threadlike, as in skeletal muscle cells and the axons (nerve fibers) of nerve cells.
Some of these shapes refer to the way a cell looks in typical tissue sections, not to the complete three-dimensional shape of the cell.	A cell that looks squamous, cuboidal, or columnar in a tissue section, for example, usually looks polygonal if viewed from its upper surface.
The most useful unit of measure for designating cell sizes is the micrometer (μm), formerly called the micron-one-millionth (10-6) of a meter, one-thousandth (103) of a millimeter.	The smallest objects most people can see with the naked eye are about 100 μm, which is about one-quarter the size of the period at the end of a typical sentence of print
A few human cells fall within this range, such as the egg cell and some fat cells, but most human cells are about 10 to 15 μm wide.	The longest human cells are nerve cells (sometimes over a meter long) and muscle cells (up to 30 cm long), but both are usually too slender to be seen with the naked eye.
There are several factors that limit the size of cells. If a cell swelled to excessive size, it could rupture like an overfilled water balloon.	In addition, cell size is limited by the relationship between its volume and surface area.
The surface area of a cell is proportional to the square of its diameter, while volume is proportional to the cube of its diameter.	Thus, for a given increase in diameter, volume increases much more than surface area.
Picture a cuboidal cell 10 μm on each side (fig. 3.2). It would have a surface area of 600 μm2 (10 μm × 10 μm × 6 sides) and a volume of 1,000 μm3 (10 × 10 × 10 μm).	Now, suppose it grew by another 10 μm on each side. Its new surface area would be 2,400 μm2 (20 μm × 20 μm × 6) and its volume would be 8,000 μm3 (20 × 20 × 20 μm).
The ---- has eight times as much cytoplasm needing nourishment and waste removal, but only four times as much membrane surface through which wastes and nutrients can be exchanged. A cell that is too big cannot support itself.	20 μm cell
Large Cell Diameter =	20 μm cell
Large Cell Surface Area =	20 μm x 20 μm cell x 6 = 2,400 μm^2
Large Cell Volume =	20 μm x 20 μm cell x 20 μm = 8,000 μm^3
Small Cell Diameter =	10 μm
Small Cell Surface Area =	10 μm x 10 μm x 600 μm ^2
Small Cell =	10 μm x 10 μm x 10 μm = 1000 μm^3
Effect of Cell Growth	Diameter (D) increased by a factor of 2 Surface Area increased by a factor of 4 (D^2) Volume increased by a factor 8 (=D^3)
The Relationship Between Cell Surface Area and Volume.	As a cell doubles in diameter, its volume increases eightfold, but its surface area increases only fourfold. A cell that is too large may have too little plasma membrane to serve the metabolic needs of the increased volume of cytoplasm.
Further, if a cell were too large, molecules couldn't diffuse from place to place fast enough to support its -----	metabolism
The time required for diffusion is proportional to the square of distance, so if a cell diameter doubled, the travel time for molecules within the cell would increase fourfold.	If it took 10 seconds for a molecule to diffuse from the surface to the center of a cell with a 10 μm radius, then we increased the cell radius to 1 mm, it would take 278 hours to reach the center-far too slow to support the cell's life activities.
Having organs composed of many small cells instead of fewer large ones has another advantage.	The death of one or a few cells has less effect on the structure and function of the whole organ.
In the nineteenth century, little was known about cells except that they were enclosed in a membrane and contained a nucleus. .	The fluid between the nucleus and surface membrane, its cytoplasm, was thought to be little more than a gelatinous mixture of chemicals and vaguely defined particles
The transmission electron microscope (TEM), invented in the mid-twentieth century, radically changed this view.	Using a beam of electrons in place of light, the TEM enabled biologists to see a cell's ultrastructure, a fine degree of detail extending even to the molecular level.
The most important thing about a good microscope isn't magnification but resolution-the ability to reveal detail, to distinguish small, close-together objects from each other.	Any image can be photographed and enlarged as much as we wish, but if enlargement fails to reveal any more useful detail, it is empty magnification.
A big blurry image isn't nearly as informative as one that's small and sharp. The TEM reveals far more detail than the light microscope (LM)	A later invention, the scanning electron microscope (SEM), produces dramatic three-dimensional images at high magnification and resolution, but can view only surface features.
Magnification Versus Resolution. Cross sections of a skeletal muscle cell viewed by	(a) light microscopy (LM) and (b) transmission electron microscopy (TEM). The TEM, with its better resolution, shows significantly finer detail, down to the protein filaments that produce muscle contraction.
A stunning application of SEM, often seen in this book, is the vascular corrosion cast technique for visualizing the blood vessels of an organ. The vessels are drained and flushed with saline, then carefully filled with a resin.	After the resin solidifies,, the actual tissue is dissolved with a corrosive agent such as potassium hydroxide. This leaves only a resin cast of the vessels, which is then photographed with the SEM.
Visible to the Naked Eye (Resolution 70–100 µm)	Human egg, diameter: 100 µm
Visible with the Light Microscope (Resolution 200 nm): Most human cells, diameter	10–15 µm
Visible with the Light Microscope (Resolution 200 nm): Cilia, length	7–10 µm
Visible with the Light Microscope (Resolution 200 nm): Mitochondria, width × length	0.2 × 4 µm
Visible with the Light Microscope (Resolution 200 nm): Bacteria (Escherichia coli), length	1–3 µm
Visible with the Light Microscope (Resolution 200 nm): Microvilli, length	1–2 µm
Visible with the Light Microscope (Resolution 200 nm): Lysosomes, diameter	0.5 µm = 500 nm
Visible with the Transmission Electron Microscope (Resolution 0.5 nm): Nuclear pores, diameter	30–100 nm
Visible with the Transmission Electron Microscope (Resolution 0.5 nm): Centriole, diameter × length	20 × 50 nm
Visible with the Transmission Electron Microscope (Resolution 0.5 nm): Poliovirus, diameter	30 nm
Visible with the Transmission Electron Microscope (Resolution 0.5 nm): Ribosomes, diameter	15 nm
Visible with the Transmission Electron Microscope (Resolution 0.5 nm): Globular proteins, diameter	5–10 nm
Visible with the Transmission Electron Microscope (Resolution 0.5 nm): Plasma membrane, thickness	7.5 nm
Visible with the Transmission Electron Microscope (Resolution 0.5 nm): Plasma membrane channels, diameter	2.0 nm
Visible with the Transmission Electron Microscope (Resolution 0.5 nm): DNA molecule, diameter	0.8 nm
The cell is surrounded by a plasma (cell) membrane made of proteins and lipids.	The composition and functions of this membrane can differ significantly from one region of a cell to another, especially between the basal, lateral, and apical (upper) surfaces of cells like the one pictured.
The cytoplasm is crowded with fibers, tubules, passages, and compartments. It contains the cytoskeleton, a supportive framework of protein filaments and tubules;	an abundance of organelles, diverse structures that perform various metabolic tasks for the cell; and inclusions, which are foreign matter or stored cell products.
cell may have 10 billion protein molecules, including potent enzymes with the potential to destroy the cell if they're not contained and isolated from other cellular components.	You can imagine the enormous problem of keeping track of all this material, directing molecules to the correct destinations, and maintaining order against nature's incessant trend toward disorder.
Cells maintain order partly by compartmentalizing their contents in the organelles.	The cytoskeleton, organelles, and inclusions are embedded in a clear fluid called the cytosol3 or intracellular fluid (ICF). All body fluids not contained in the cells are collectively called the extracellular fluid (ECF).
The ECF located amid the cells is also called tissue (interstitial) fluid.	Some other extracellular fluids include blood plasma, lymph, and cerebrospinal fluid.
In summary, we regard cells as having the following major components:	Plasma membrane Cytoplasm Cytoskeleton Organelles (including nucleus) Inclusions Cytosol
In summary, we regard cells as having the following major components: Plasma membrane	----- Cytoplasm Cytoskeleton Organelles (including nucleus) Inclusions Cytosol
In summary, we regard cells as having the following major components: Cytoplasm	Plasma membrane ----- Cytoskeleton Organelles (including nucleus) Inclusions Cytosol
n summary, we regard cells as having the following major components: Cytoskeleton	Plasma membrane Cytoplasm ----- Organelles (including nucleus) Inclusions Cytosol
In summary, we regard cells as having the following major components: Organelles (including nucleus)	Plasma membrane Cytoplasm Cytoskeleton ------ Inclusions Cytosol
In summary, we regard cells as having the following major components: Inclusions	Plasma membrane Cytoplasm Cytoskeleton Organelles (including nucleus) ---------- Cytosol
In summary, we regard cells as having the following major components: Cytosol	Plasma membrane Cytoplasm Cytoskeleton Organelles (including nucleus) Inclusions -----------
What are the basic principals of cell theory?	Cell theory is a fundamental principle in biology, akin to the foundation of a building. It posits that all living organisms are composed of cells, the basic unit of life.
Squamous	Squamous (type I) alveolar cells are like a thin, expansive pizza crust, covering about 95% of the alveolar surface.
Columnar	These cells, found in simple columnar and pseudostratified columnar epithelia, are tall and narrow, with each cell reaching the basement membrane, much like each building is anchored to the ground.
Fusiform	This shape allows for a strong contraction, as seen in the biceps brachii and gastrocnemius.
Intracellular fluid (ICF) and extracellular fluid (ECF)	ICF is the fluid inside cells, making up 65% of body water. ECF is outside cells, holding 35%, and includes fluids like blood plasma and tissue fluid.
Many physiologically important processes occur at the surface of a cell—immune responses, the binding of egg and sperm, ------signaling by hormones, and the detection of tastes and smells, for example.	cell-to-cell
The plasma membrane defines the boundaries of the cell, governs its interactions with other cells, and controls the passage of materials into and out of the cell.	It appears to the electron microscope as a pair of dark parallel lines with a total thickness of about 7.5 nm
The side that faces the cytoplasm is the intracellular face of the membrane, and the side that faces outward is the extracellular face.	Similar membranes enclose most of a cell's organelles and control their uptake and release of chemicals.
plasma membrane	—an oily film of lipids with proteins embedded in it.
Typically about 98% of the membrane molecules are lipids, and about 75% of those are phospholipids.	These amphipathic molecules arrange themselves into a sandwichlike bilayer, with their hydrophilic phosphate-containing heads facing the water on each side and their hydrophobic tails directed toward the center, avoiding the water.
The phospholipids drift	laterally from place to place, spin on their axes, and flex their tails. These movements keep the membrane fluid.
What would happen if the plasma membrane were made primarily of a hydrophilic substance such as carbohydrate?	If the plasma membrane were primarily hydrophilic, like carbohydrates, it would disrupt its essential barrier function, allowing water and solutes to pass freely.
Cholesterol molecules	, found near the membrane surfaces amid the phospholipids, constitute about 20% of the membrane lipids
By interacting with the ---- and holding them still, cholesterol can stiffen the membrane (make it less fluid) in spots.	phospholipids
Higher concentrations of -----, however, can increase membrane fluidity by preventing phospholipids from packing closely together.	cholesterol
The remaining 5% of the membrane lipids are ----- with short carbohydrate chains on the extracellular face of the membrane.	glycolipids-phospholipids
They contribute to the -----, a carbohydrate coating on the cell surface with multiple functions, described shortly.	glycocalyx
Although proteins are only about 2% of the molecules of the ----, they're larger than lipids and average about 50% of the membrane by weight.	plasma membrane
There are two broad classes of membrane proteins:	transmembrane and peripheral.
Transmembrane proteins	pass completely through the phospholipid bilayer, protruding from it on both sides
Transmembrane proteins	They have hydrophilic regions in contact with the water on both sides, and hydrophobic regions that pass back and forth through the lipid
Most transmembrane proteins are glycoproteins, bound to oligosaccharides on the extracellular side.	Many of these proteins drift about freely in the phospholipid film, like ice cubes floating in a bowl of water. Others are anchored to the cytoskeleton—an intracellular system of tubules and filaments
Peripheral proteins	don't protrude into the phospholipid layer but adhere to either the inner or outer face of the membrane. Those on the inner face are typically anchored to a transmembrane protein as well as to the cytoskeleton.
Transmembrane Proteins.	A transmembrane protein has hydrophobic regions embedded in the phospholipid bilayer and hydrophilic regions projecting into the intracellular and extracellular fluids.
Transmembrane Proteins.	The protein may cross the membrane once (left) or multiple times (right). The intracellular regions are often anchored to the cytoskeleton by peripheral proteins.
The functions of membrane include	Receptors, Enzymes, Channel proteins, Carriers, Cell-identity markers, Cell-adhesion molecules
The functions of membrane include: Receptors	The functions of membrane include: -----, Enzymes, Channel proteins, Carriers, Cell-identity markers, Cell-adhesion molecules
The functions of membrane include: Enzymes	The functions of membrane include: Receptors, ----, Channel proteins, Carriers, Cell-identity markers, Cell-adhesion molecules
The functions of membrane include: Channel proteins	The functions of membrane include: Receptors, Enzymes, -----, Carriers, Cell-identity markers, Cell-adhesion molecules
The functions of membrane include: Carriers	The functions of membrane include: Receptors, Enzymes, Channel proteins, ------, Cell-identity markers, Cell-adhesion molecules
The functions of membrane include:, Cell-identity markers	The functions of membrane include: Receptors, Enzymes, Channel proteins, Carriers, ------, Cell-adhesion molecules
The functions of membrane include: Cell-adhesion molecules	Receptors, Enzymes, Channel proteins, Carriers, Cell-identity markers, ----
Receptors	Many of the chemical signals by which cells communicate (epinephrine, for example) cannot enter the target cell but bind to surface proteins called receptors.
Receptors	are usually specific for one particular messenger, much like an enzyme that is specific for one substrate.
Plasma membranes	also have receptor proteins that bind chemicals and transport them into the cell
When a messenger binds to a surface receptor, it may trigger changes within the cell that produce a second messenger in the cytoplasm.	This process involves both transmembrane proteins (the receptors) and peripheral proteins.
Enzymes: in the plasma membrane carry out the final stages of starch and protein digestion in the small intestine,	help produce second messengers, and break down hormones and other signaling molecules whose job is done, thus stopping them from excessively stimulating cell.
Channel proteins	Channels are passages that allow water and hydrophilic solutes to move through the membrane
Channel proteins	A channel is a tunnel that passes through a complex of multiple proteins or between subunits of an individual protein.
Channel proteins	Some of them, called leak channels, are always open and allow materials to pass through continually.
Channel proteins	Others, called gates (gated channels), open and close under different circumstances and allow solutes through at some times, but not others
Channel proteins These gates respond to three types of stimuli:	ligand-gated channels respond to chemical messengers, voltage-gated channels to changes in electrical potential (voltage) across the plasma membrane, and mechanically gated channels to physical stress on a cell, such as stretch and pressure.
Channel proteins	By controlling the movement of electrolytes through the plasma membrane, gated channels play an important role in the timing of nerve signals and muscle contraction
Channel proteins	Some receptors double in function as gated channels.
Channel proteins	When a nerve stimulates a muscle, for example, a chemical from the nerve fiber binds to a receptor on the muscle fiber and the receptor opens to allow sodium and potassium ions to flow through and excite the muscle.
Channel proteins	Defects in channel proteins are responsible for a family of diseases called channelopathies.
Carriers	Carriers are transmembrane proteins that bind to glucose, electrolytes, and other solutes and transfer them to the other side of the membrane. Some carriers, called pumps, consume ATP in the process.
Cell-identity markers Glycoproteins contribute to the , which acts like an “identification tag” that enables the to tell which cells belong to one's body and which are foreign invaders.	immune system, glycocalyx
Cell-adhesion molecules	Cells adhere to one another and to extracellular material through membrane proteins called cell-adhesion molecules (CAMs).
Cell-adhesion molecules	Special events such as sperm-egg binding and the binding of an immune cell to a cancer cell also require CAMs
Receptor	A receptor that binds to chemical messengers such as hormones sent by other cells.
Enzyme	An enzyme that breaks down a chemical messenger and terminates its effect.
Channel	A channel protein that is constantly open and allows solutes to pass into and out of the cell.
Gated channel	A gate that opens and closes to allow solutes through only at certain times.
Cell-identity marker	A glycoprotein acting as a cell-identity marker distinguishing the body's own cells from foreign cells.
Cell-adhesion molecule (CAM)	A cell-adhesion molecule (CAM) that binds one cell to another.
Calcium channel blockers are a class of drugs that show the therapeutic relevance of understanding gated membrane channels.	The walls of the arteries contain smooth muscle that contracts or relaxes to change their diameter. These changes modify the blood flow and strongly influence blood pressure.
Blood pressure rises when the arteries constrict and falls when they relax and dilate.	Excessive, widespread vasoconstriction can cause hypertension (high blood pressure), and vasoconstriction in the coronary blood vessels of the heart can cause pain (angina) due to inadequate blood flow to the cardiac muscle.
In order to contract, a smooth muscle cell must open calcium channels in its plasma membrane and allow calcium to enter from the extracellular fluid.	Calcium channel blockers prevent these channels from opening and thereby relax the arteries, increase blood flow, relieve angina, and lower the blood pressure.
Second messengers are of such importance that they require a closer look.	You will find this information essential for your later understanding of hormone and neurotransmitter action.
Let's consider how the hormone epinephrine stimulates a cell.	Epinephrine, the “first messenger," can't pass through the plasma membrane, so it binds to a surface receptor.
The receptor is linked on the intracellular side to a peripheral G protein (fig. 3.8).	G proteins are named for the ATP-like chemical, guanosine triphosphate (GTP), from which they get their energy.
When activated by the receptor, a G protein relays the signal to another membrane protein, adenylate cyclase (ah-DEN-ih-late SY-clase).	Adenylate cyclase removes two phosphate groups from ATP and converts it to cyclic AMP (CAMP), the second messenger
Cyclic AMP then activates cytoplasmic enzymes called kinases.	which add phosphate groups to other cellular enzymes. This activates some enzymes and deactivates others, but either way, it triggers a great variety of physiological changes within the cell. Up to 60% of drugs work by altering the activity of G proteins.
1	A messenger such as epinephrine (red triangle) binds to a receptor in the plasma membrane.
2	The receptor releases a G protein, which then travels freely in the cytoplasm and can go on to step 3 or have various other effects on the cell
3	The G protein binds to an enzyme, adenylate cyclase, in the plasma membrane. Adenylate cyclase converts ATP to cyclic AMP (cAMP), the second messenger.
4	cAMP activates a cytoplasmic enzyme called a kinase.
5	Kinases add phosphate groups Pi to other cytoplasmic enzymes. This activates some enzymes and deactivates others, leading to varied metabolic effects in the cell.
Is adenylate cyclase a transmembrane protein or a peripheral protein? What about the G protein?	adenylate cyclase is a transmembrane protein. The G protein is peripheral
A receptor protein spans across a cell membrane and binds with a signal molecule on the outside of the cell. In the cytoplasm	in the cell's interior, guanosine triphosphate (GTP) binds with the G protein before it migrates to and activates a single adenylyl cyclase molecule. This releases cyclic adenosine monophosphate (cAMP), which is acting as the second messenger.
The ------ stimulate a molecule, protein kinase, in the cell, which activates an enzyme and produces several enzymatic product molecules.	cAMP molecules
"Receptor proteins interact with signal molecules at the surface of the cell. In most cases, the signals are relayed to the cytoplasm or the nucleus by second messengers, which influence the activity of one or more enzymes or genes inside the cell.	However, most signaling molecules are found in such low concentrations that their effects in the cytoplasm would be minimal unless the signal were amplified."
A diagram labeled signal amplification appears. The signal and receptor molecules are the same as before, and there are four molecules of adenylyl cyclase not yet activated. In the cell, there are five each of GTP and G protein molecules.	A single signal molecule binds with the receptor protein and activates the entire series of four G protein molecules, one after the other.
Therefore, most enzyme-linked and G protein–linked receptors use a chain of other protein messengers to amplify the signal as it is being relayed	. In the case of a protein kinase cascade, one cell surface receptor activates many G protein molecules.
Each G protein activates many adenylyl cyclases. Each cyclic AMP in turn will activate protein kinases, which then activates several molecules of a specific enzyme.	The end result is rapid production of high levels of the final product."
External to the plasma membrane, all animal cells have a fuzzy coat called	the glycocalyx, composed of the carbohydrate moieties of membrane glycolipids and glycoproteins.
It is chemically unique in everyone but identical twins, and acts like an identification tag that enables the body to distinguish its own healthy cells from transplanted tissues, invading organisms, and diseased cells.	Human blood types and transfusion compatibility are determined by glycolipids.
Protection	Cushions the plasma membrane and protects it from physical and chemical injury
Immunity to infection	Enables the immune system to recognize and selectively attack foreign organisms
Defense against cancer	Changes in the glycocalyx of cancerous cells enable the immune system to recognize and destroy them
Transplant compatibility	Forms the basis for compatibility of blood transfusions, tissue grafts, and organ transplants
Cell adhesion	Binds cells together so tissues don't fall apart
Fertilization	Enables sperm to recognize and bind to eggs
Embryonic development	Guides embryonic cells to their destinations in the body
Many cells have surface extensions called microvilli, cilia, flagella, and pseudopods.	These aid in absorption, movement, and sensory processes.
Microvilli are extensions of the plasma membrane that serve primarily to increase a cell's surface area	They are best developed in cells specialized for absorption, such as the epithelial cells of the intestines and kidneys. They give such cells 15 to 40 times as much absorptive surface area as they would have if their apical surfaces were flat.
Microvilli and the Glycocalyx (TEM). The microvilli are anchored by microfilaments of actin, which occupy the core of each microvillus and project into the cytoplasm. (a) Longitudinal section, perpendicular to the cell surface.	(b) Cross section.
(a) Epithelium of the uterine (fallopian) tube (SEM). The short, mucus-secreting cells between the ciliated cells show bumpy microvilli on their surfaces.	(b) Three-dimensional structure of a cilium.
(c) Cross section of a few cilia and microvilli (TEM).	(d) Cross-sectional structure of a cilium. Note the relative sizes of cilia and microvilli in parts (a) and (c).
Individual microvilli can't be distinguished very well with the light microscope because they're only 1 to 2 μm long.	On some cells, they're very dense and appear as a fringe called the brush border at the apical cell surface.
With the scanning electron microscope, they resemble a deep-pile carpet. With the transmission electron microscope, microvilli typically look like finger-shaped projections of the cell surface.	They show little internal structure, but some have a bundle of stiff filaments of a protein called actin.
Actin filaments attach to the inside of the plasma membrane at the tip of the microvillus, and at its base they extend a little way into the cell and anchor the microvillus to a protein mesh called the terminal web.	When tugged by another protein in the cytoplasm, actin can shorten a microvillus to milk its absorbed contents downward into the cell.
In contrast to the long, shaggy microvilli of absorptive cells, those on many other cells are little more than tiny bumps on the surface.	On cells of the taste buds and inner ear, they are well developed but serve sensory rather than absorptive functions.
Cilia are hairlike processes about 7 to 10 μm long. Nearly every human cell has a single, nonmotile primary cilium a few micrometers long	Its function in some cases is still a mystery, but many of them are sensory, serving as the cell's "antenna" for monitoring nearby conditions.
Cilia: In the inner ear, they play a role in the sense of balance	in the retina of the eye, they're highly elaborate and form the light-absorbing part of the receptor cells; and in the kidney, they're thought to monitor the flow of fluid as it is processed into urine
In some cases, they open calcium gates in the plasma membrane, activating an informative signal in the cell.	Sensory cells in the nose have multiple nonmotile cilia that bind odor molecules.
Defects in the development, structure, or function of cilia—especially these nonmotile primary cilia—	are sometimes responsible for birth defects and hereditary diseases called ciliopathies.
Motile cilia are less widespread, but more numerous on the cells that do have them.	They occur in the respiratory tract, uterine (fallopian) tubes, internal cavities (ventricles) of the brain, and short ducts (efferent ductules) associated with the testes.
There may be 50 to 200 cilia on the surface of one cell.	They beat in waves that sweep across the surface of an epithelium, always in the same direction, propelling such materials as mucus, an egg cell, or cerebrospinal fluid.
Each cilium bends stiffly forward and produces a power stroke that pushes along the mucus or other matter.	Shortly after a cilium begins its power stroke, the one just ahead of it begins, and the next and the next-collectively producing a wavelike motion.
After a ----completes its power stroke, it pulls limply back in a recovery stroke that restores it to the upright position, ready to flex again.	cilium
Cilia of an epithelium moving mucus along a surface layer of saline.	Power and recovery strokes of an individual cilium. The cilium goes limp on the recovery stroke to return to its original position without touching the mucus above.
Cilia couldn't beat freely if they were embedded in sticky mucus Instead,	they beat within a saline layer at the cell surface.
Chloride pumps in the apical plasma membrane produce this layer by pumping Cl- into the extracellular fluid.	Sodium ions follow by electrostatic attraction and water follows by osmosis. Mucus essentially floats on the surface of this layer and is pushed along by the tips of the cilia.
The significance of chloride pumps is especially evident in cystic fibrosis (CF), a hereditary disease affecting primarily White children of European descent.	CF is usually caused by a defect in which cells make chloride pumps but fail to install them in the plasma membrane.
Consequently, there is an inadequate saline layer on the cell surface and the mucus is dehydrated and overly sticky.	This thick mucus plugs the ducts of the pancreas and prevents it from secreting digestive enzymes into the small intestine, so digestion and nutrition are compromised.
In the respiratory tract, the mucus clogs the cilia and prevents them from beating freely. The respiratory tract becomes congested with thick mucus, often leading to chronic infection and pulmonary collapse.	The mean life expectancy of people with CF is about 44 years.
The structural basis for ciliary movement is a core called the axoneme, which consists of an array of thin protein cylinders called ----.	microtubules
There are two central microtubules surrounded by a ring of nine microtubule pairs—an arrangement called the 9+ 2 structure.	In cross section, it is reminiscent of a Ferris wheel
The central microtubules stop at the cell surface, but the peripheral microtubules continue a short distance into the cell as part of a basal body that anchors the cilium.	In each pair of peripheral microtubules, one tubule has two little dynein arms
Dynein, a motor protein, uses energy from ATP to crawl up the adjacent pair of microtubules. On one side of the cilium, activated dynein arms reach out and attach to a microtubule of an adjacent pair, then flex and pull that microtubule toward them.	Dynein arms on the other side of the cilium are inhibited at that time so they don't resist the bending. Then the first dyneins are inhibited and the others are activated, causing a reverse bend in that direction.
Activation and inhibition	repeatedly switch sides, causing the back and forth bending of the cilium's power and recovery strokes.
The microtubules of a cilium also act like monorail tracks along which motor proteins carry materials up and down the cilium for use in its growth and maintenance.	The primary cilia, which cannot move, lack the two central microtubules and dynein arms, but still have the nine peripheral pairs; they are said to have a 90 structure.
The only functional flagellum in humans is the whiplike tail of a sperm.	It is much longer than a cilium and has an axoneme surrounded by a sheath of coarse fibers that stiffen the tail and give it more propulsive power.
A flagellum beats by the same dynein-powered mechanism as cilia, but not in power and recovery strokes. Instead, it beats in a more undulating, snakelike or corkscrew fashion. It is described in further detail as part of	sperm structure in
Pseudopods are cytoplasm-filled extensions of the cell varying in shape from fine, filamentous processes to blunt fingerlike ones. Unlike the other three kinds of surface extensions, they change continually.	Some form anew as the cell surface bubbles outward and cytoplasm flows into a lengthening pseudopod, while others are retracted into the cell by disassembling protein filaments that supported them like a scaffold.
Pseudopods.	(a) Amoeba, a freshwater organism that crawls and captures
A neutrophil	(white blood cell) that similarly uses pseudopods for locomotion and capturing bacteria.
(c) A ----- extending filamentous pseudopods to snare and "reel in" bacteria (the red rods).	macrophage
The freshwater organism Amoeba furnishes a familiar example of pseudopods, which it uses for locomotion and food capture.	White blood cells called neutrophils crawl about like amebae by means of fingerlike pseudopods, and when they encounter a bacterium or other foreign particle, they reach out with their pseudopods to surround and engulf it.
Macrophages-tissue cells derived from certain white blood cells-reach out with thin filamentous pseudopods to snare bacteria and cell debris and "reel them in" to be digested by the cell.	Like little janitors, macrophages thereby keep our tissues cleaned up. Blood platelets reach out with thin pseudopods to adhere to each other and to the walls of damaged blood vessels, forming plugs that temporarily halt bleeding.
How does the structure of a plasma membrane depend on the amphipathic nature of phospholipids?	The amphipathic nature of phospholipids is crucial to plasma membrane structure. This arrangement creates a bilayer, allowing lateral movement, spinning, and tail flexing. Such dynamics maintain membrane fluidity, essential for cell function.
Define peripheral versus transmembrane proteins.	Transmembrane proteins span the entire phospholipid bilayer, with hydrophilic regions interacting with water inside and outside the cell, and hydrophobic regions within the lipid layer.
Cell-adhesion molecules	----- function like Velcro, helping cells stick together and form tissues.
Pumps	----- actively transport ions or molecules across membranes, akin to water pumps moving fluids against gravity.
Receptors	---- are like cellular antennas, detecting signals from the environment and initiating responses.
What three factors open and close membrane gates?	Three factors that open and close these gates include voltage changes (like a light switch), ligand binding (like a key), and mechanical stress (like a pressure sensor).
What related roles do cAMP, adenylate cyclase, and kinases play in cellular function?	cAMP, adenylate cyclase, and kinases form a crucial signaling pathway. Think of it like a relay race: adenylate cyclase, a transmembrane protein, converts ATP to cAMP, the baton.
Identify several reasons why the glycocalyx is important to human survival.	The glycocalyx is crucial for human survival due to its role as a cellular identification tag, helping the body recognize its own healthy cells while distinguishing them from foreign invaders or diseased cells.
How do microvilli and cilia differ in structure and function?	Microvilli and cilia are both cellular surface extensions but differ in structure and function.
shows our current concept of the molecular structure of the plasma membrane-an oily film of lipids with proteins embedded in it.	Typically about 98% of the membrane molecules are lipids, and about 75% of those are phospholipids.
phospholipids	These amphipathic molecules arrange themselves into a sandwichlike bilayer, with their hydrophilic phosphate-containing heads facing the water on each side and their hydrophobic tails directed toward the center, avoiding the water.
One of the most important functions of cellular membranes is to control the passage of materials into and out of the organelles and the cell as a whole. The plasma membrane is both a barrier and gateway between the cytoplasm and ECF.	It is selectively permeable-it allows some things through, such as nutrients and wastes, but usually prevents other things, such as proteins and phosphates, from entering or leaving the cell.
The methods of moving substances through the membrane can be classified in two overlapping ways: as passive or active mechanisms and as carrier-mediated or not.	Passive mechanisms require no energy (ATP) expenditure by the cell
In most cases, the random molecular motion of the particles themselves provides the necessary energy.	Passive mechanisms include filtration, diffusion, and osmosis.
Active mechanisms, however, consume ATP. These include active transport and vesicular transport.	Carrier-mediated mechanisms use a membrane protein to transport substances from one side of the membrane to the other, but some transport processes, such as osmosis, do not involve carriers.
Filtration	is a process in which a physical pressure forces fluid through a selectively permeable membrane.
The weight of the water drives water and dissolved matter through the filter, while the filter holds back larger particles (the coffee grounds). In physiology, the most important case of filtration is seen in the blood capillaries,	where blood pressure forces fluid through gaps in the capillary wall
This is how water, salts, nutrients, and other solutes are transferred from the bloodstream to the tissue fluid and how the kidneys filter wastes from the blood.	In most cases, water and solutes filter through narrow gaps between the capillary cells.
Capillaries hold back larger particles such as blood cells and proteins.	In some capillaries, however, the cells have large filtration pores through them, like the holes in a slice of Swiss cheese, allowing for more rapid filtration of large solutes such as protein hormones.
Blood pressure in capillary forces water and small solutes such as salts through narrow clefts between capillary cells.	Clefts hold back larger particles such as red blood cells.
Filtration through a wall of a blood capillary	Water and small solutes pass through gaps between cells, while
Simple diffusion is the net movement of particles from a place of high concentration to a place of lower concentration as a result of their constant, spontaneous motion.	In other words, substances diffuse down their concentration gradients
Molecules move at astonishing speeds. At body temperature, the average water molecule moves about 2,500 km/h (1,500 mi./h)!	However, molecules are so crowded they can travel only a short distance before colliding with each other and careening off in a new direction, like colliding billiard balls.
Molecules move at astonishing speeds. At body temperature, the average water molecule moves about 2,500 km/h (1,500 mi./h)!	The rate of diffusion, therefore, is much slower than the speed of molecular motion.
Diffusion occurs readily in air or water and doesn't necessarily need a membrane-for example, when an odor spreads from its source to your nose.	However, if there is a membrane in the path of the diffusing molecules, and if it is permeable to that substance, the molecules will pass from one side of the membrane to the other.
This is how oxygen passes from the air we inhale into the bloodstream.	Dialysis treatment for kidney patients is based on diffusion of solutes through artificial dialysis membranes.
Diffusion rates are important to cell survival because they determine how quickly a cell can acquire nutrients or rid itself of wastes. Some factors that affect the rate of diffusion through a membrane are as follows:	Temperature
Diffusion rates are important to cell survival because they determine how quickly a cell can acquire nutrients or rid itself of wastes. Some factors that affect the rate of diffusion through a membrane are as follows:	Molecular weight
Diffusion is driven by the kinetic energy of the particles, and temperature is a measure of that kinetic energy.	The warmer a substance is, the more rapidly its particles diffuse. This is why sugar diffuses more quickly through hot tea than through iced tea
Molecular weight	Heavy molecules such as proteins move more sluggishly and diffuse more slowly than light particles such as electrolytes and gases. Small molecules also pass through membrane pores more easily than large ones.
"Steepness" of the concentration gradient.	The steepness of a gradient refers to the concentration difference between two points. Particles diffuse more rapidly if there is a greater concentration difference, like a ball rolling faster down a steeper slope.
• Membrane surface area.	As noted earlier, the apical surface of cells specialized for absorption (for example, in the small intestine) is often extensively folded into microvilli. This makes more membrane available for particles to diffuse through.
Membrane permeability.	Diffusion through a membrane depends on how permeable it is to the particles.
Membrane permeability.	For example, potassium ions diffuse more rapidly than sodium ions through a plasma membrane.
Membrane permeability.	Nonpolar, hydrophobic, lipid-soluble substances such as oxygen, nitric oxide, alcohol, and steroids diffuse through the phospholipid regions of a plasma membrane.
Membrane permeability.	Water and small charged, hydrophilic solutes such as electrolytes don't mix with lipids but diffuse primarily through channel proteins in the membrane.
Membrane permeability.	• Cells can adjust their permeability to such a substance by adding channel proteins to the membrane, by taking them away, or by opening and closing membrane gates.
Osmosis is the net flow of water from one side of a selectively permeable membrane to the other.	It is crucial to the body's water distribution (fluid balance). Imbalances in osmosis underlie such problems as diarrhea, constipation, hypertension, and edema (tissue swelling); osmosis also is a vital consideration in intravenous (IV.) fluid therapy.
Osmosis is the net flow of water from one side of a selectively permeable membrane to the other.	It is crucial to the body's water distribution (fluid balance).
Imbalances	in osmosis underlie such problems as diarrhea, constipation, hypertension, and edema (tissue swelling); osmosis also is a vital consideration in intravenous (I.V.) fluid therapy
Osmosis occurs through nonliving membranes, such as cellophane and dialysis membranes, and through the plasma membranes of cells.	The usual direction of net movement is from the more watery side, with a lower concentration of dissolved matter, to the less watery side, with a greater concentration of solute.
The reason for the accumulation of water on the high-solute side is that when water molecules encounter a solute particle, they tend to associate with it to form a hydration sphere (see fig. 2.9).	Even though this is a loose, reversible attraction, it does make those water molecules less available to diffuse back across the membrane to the side they came from.
In essence, solute particles on one side of the membrane draw water away from the other side. Thus, water accumulates on the side with the most solute.	All of this assumes that the solute molecules in question can't pass through the membrane, but stay on one side. The rate and direction of osmosis depend on the relative concentration of these nonpermeating solutes on the two sides of the membrane.
Significant amounts of water pass even through the hydrophobic, phospholipid regions of a plasma membrane, but water passes more easily through channel proteins called aquaporins, specialized for water.	Cells can increase the rate of osmosis by installing more aquaporins in the membrane or decrease the rate by removing them.
Certain cells of the kidney, for example, regulate the rate of urinary water loss by adding or removing -----.	aquaporins
A cell can exchange a tremendous amount of water by osmosis.	In red blood cells, for example, the amount of water passing through the plasma membrane every second is 100 times the volume of the cell.
is a conceptual model of osmosis. Imagine a chamber divided by a selectively permeable membrane.	Side A contains distilled water and side B contains large particles of a nonpermeating solute-that is, a solute such as protein that cannot pass through the membrane pores because of its size or other properties.
Water molecules pass from side A to B (fig. 3.14a) and cling to the solute molecules on side B, hindering their	movement back to side A.
Under such conditions, the water level in side A would fall and the level in side B would rise.	It may seem as if this would continue indefinitely until side A dried up. This would not happen, however, because as water accumulated in side B, it would become heavier and exert more force, called hydrostatic pressure, on that side of the membrane.
At some point, the rate of filtration would equal the rate of “forward” osmosis, water would pass through the membrane equally in both directions, and net osmosis would slow down and stop.	At this point, an equilibrium (balance between opposing forces) would exist. The hydrostatic pressure required on side B to halt osmosis is called osmotic pressure. The more nonpermeating solute there is in B, the greater the osmotic pressure.
If the solute concentration on side B was half what it was in the original experiment, would the fluid on that side reach a higher or lower level than before?	if the solute concentration on side B is halved, the osmotic pressure decreases.
Reverse osmosis is a process in which a mechanical pressure applied to one side of the system can override osmotic pressure and drive water through a membrane against its concentration gradient	. This principle is used to create highly purified water for laboratory use and to desalinate seawater, converting it to drinkable freshwater-important for arid countries and ships at sea.
The body's principal pump, the heart, drives water out of the smallest blood vessels (the capillaries) by ----- —a process called capillary filtration.	reverse osmosis
The equilibrium between osmosis and filtration will be an important consideration when we study fluid exchange by the ----	capillaries
Blood plasma also contains albumin.	In the preceding discussion, side B is analogous to the high-protein bloodstream and side A to the low-protein tissue fluid surrounding the capillaries.
Water	leaves the capillaries by filtration, but this is approximately balanced by water reentering the capillaries by osmosis.
The ------, or osmotic concentration, of body fluids has such a great effect on cellular function that it is important to understand the units in which it is measured.	osmolarity
Physiologists and clinicians usually express this in terms of ------, a unit of measure that expresses the quantity of nonpermeating particles per liter of solution	milliosmoles per liter (mOsm/L)
The basis of this unit of concentration is explained in appendix B. Blood plasma, tissue fluid, and intracellular fluids measure about	300 mOsm/L
Tonicity	is the ability of a solution to affect the fluid volume and pressure in a cell. If a solute cannot pass through a plasma membrane but remains more concentrated on one side than on the other, it causes osmosis
A hypotonic solution has a lower concentration of nonpermeating solutes than the	intracellular fluid (ICF)
Cells in a hypotonic solution absorb water, swell, and may burst (lyse)	Distilled water is the extreme example; a sufficient quantity given to a person intravenously would lyse the blood cells, with dire consequences.
A hypertonic 19 solution is one with a higher concentration of nonpermeating solutes than the ICF.	It causes cells to lose water and shrivel (crenate)
Such cells may die of torn membranes and cytoplasmic loss. In isotonic solutions, the total concentration of nonpermeating solutes is the same as in the	ICF—hence, isotonic solutions cause no change in cell volume or shape
Effects of Tonicity on Red Blood Cells (RBCs).	(a) RBC swelling in a hypotonic medium such as distilled water. (b) Normal RBC size and shape in an isotonic medium such as 0.9% NaCl. (c) RBC shriveling in a hypertonic medium such as 2% NaCl.
It is essential for cells to be in a state of ----- with the fluid around them, and this requires that the ECF have the same concentration of nonpermeating solutes as the ICF	osmotic equilibrium
Intravenous fluids given to patients are usually isotonic solutions, but hypertonic or hypotonic fluids are given for special purposes.	A 0.9% solution of NaCl, called normal saline, is isotonic to human blood cells.
It is important to note that osmolarity and tonicity are not the same.	Urea, for example, is a small organic molecule that easily penetrates plasma membranes.
If cells are placed in 300 mOsm/L urea, urea diffuses into them (down its concentration gradient), water follows by osmosis, and the cells swell and burst. Thus, 300 mOsm/L urea is not isotonic to the cells.	Sodium chloride, by contrast, penetrates plasma membranes poorly. In 300 mOsm/L NaCl, there is little change in cell volume; this solution is isotonic to cells.
The processes of membrane transport described up to this point don't necessarily require a cell membrane; they can occur as well through artificial membranes.	Now, however, we come to processes for which a cell membrane is necessary, because they employ transport proteins, or carriers.
Thus, the next three processes are classified as carrier-mediated transport. In these cases, a solute binds to a carrier in the -----, which then changes shape and releases the solute to the other side.	plasma membrane
Carriers can move substances into or out of a cell, and into or out of organelles within the cell.	The process is very rapid; for example, one carrier can transport 1,000 glucose molecules per second across the membrane.
Carriers act like enzymes in some ways: The solute is a ligand that binds to a specific receptor site on the carrier, like a substrate binding to the active site of an enzyme.	The carrier exhibits specificity for its ligand, just as an enzyme does for its substrate.
A glucose carrier, for example, can't transport fructose.	Carriers also exhibit saturation; as the solute concentration rises, its rate of transport increases, but only up to a point.
When every carrier molecule is occupied, adding more solute can't make the process go any faster. The carriers are saturated—no more are available to handle the increased demand, and transport levels off at a rate called the	transport maximum (Tm)
You could think of carriers as ------ to buses. If all the buses on a given line are full (“saturated”), they can't carry any more passengers, regardless of how many people are waiting at the bus stop.	analogous
An important difference between a carrier and an enzyme is that carriers don't chemically change their -----; they simply pick them up on one side of the membrane and release them, unchanged, on the other.	ligands
Carrier Saturation and Transport Maximum. Up to a point, increasing the solute concentration increases the rate of transport through a membrane.	At the transport maximum (Tm), however, all carrier proteins are busy and cannot transport the solute any faster, even if more solute is added
There are three kinds of carriers: uniports, symports, and antiports.	A uniport carries only one type of solute.
For example, most cells pump out calcium by means of a uniport, maintaining a low intracellular concentration so calcium salts don't crystallize in the cytoplasm.	Some carriers move two or more solutes through a membrane simultaneously in the same direction; this process is called cotransport and the carrier protein that performs it is called a symport.
For example, absorptive cells of the small intestine and kidneys have a symport that takes up sodium and glucose simultaneously. Other carriers move two or more solutes in opposite directions;	this process is called countertransport and the carrier protein is called an antiport.
For example, nearly all cells have an antiport called the sodium-potassium pump that continually removes Na+ from the cell and brings in ----.	K+
There are three mechanisms of carrier-mediated transport:	facilitated diffusion, primary active transport, and secondary active transport.
Facilitated diffusion is the carrier-mediated transport of a solute through a membrane down its concentration gradient. It requires no expenditure of metabolic energy (ATP) by the cell.	It transports solutes such as glucose that cannot pass through the membrane unaided. The solute attaches to a binding site on the carrier, then the carrier changes conformation and releases the solute on the other side of the membrane.
1	The solute particle enters the channel of a membrane protein (carrier).
2	The solute binds to a receptor site on the carrier and the carrier changes conformation.
3.	The carrier releases the solute on the other side of the membrane.
Primary active transport is a process in which a carrier moves a substance through a cell membrane up its concentration gradient using energy provided by ATP.	Just as rolling a ball up a ramp would require you to push it (an energy input), this mechanism requires energy to move material up its concentration gradient.
ATP supplies this energy by transferring a phosphate group to the transport protein. The calcium pump mentioned previously uses this mechanism.	Even though Ca2+ is already more concentrated in the ECF than within the cell, this carrier pumps still more of it out. Active transport also enables cells to absorb amino acids that are already more concentrated in the cytoplasm than in the ECF.
Secondary active transport also requires an energy input, but depends only indirectly on ATP.	For example, certain kidney tubules have proteins called sodium- glucose transporters (SGLTs) that simultaneously bind sodium (Na) and glucose and transport them into the tubule cells. saving glucose from being lost in the urine
An SGLT itself doesn't use ATP.	However, it depends on the fact that the cell actively maintains a low internal Na+ concentration, so Na+ diffuses down its gradient into the cell.
Glucose "hitches a ride" with the incoming Na. But what keeps the intracellular Na+ concentration low is that the basal membrane of the cell has an ATP-driven sodium-potassium pump that constantly removes Na from the cell.	If not for this, the Na* and glucose inflow via the SGLT would soon cease.
Therefore, the SGLT doesn't use ATP directly, but depends on ATP to drive the Na-K pump; it is therefore a secondary active- transport protein.	(Secondary active transport is an unfortunate name for this, as the SGLT is actually carrying out facilitated diffusion, but its dependence on a primary active-transport pump has led to this name.)
Secondary Active Transport.	In this example, the sodium-glucose transporter (SGLT) at the apical cell surface carries out facilitated diffusion, but depends on active transport by the Na+-K* pump at the base of the cell to keep it running.
The sodium-potassium (Na-K) pump itself (fig. 3.19) is a good example of primary active transport. It is also known as Na1-K+ ATPase because it is an enzyme that hydrolyzes ATP.	The Na+-K+ pump binds three Na1 simultaneously on the cytoplasmic side of the membrane, releases these to the ECF, binds two K simultaneously from the ECF, and releases these into the cell.
Each cycle of the pump consumes one ATP and exchanges three Na1 for two K‡. This keeps the K+ concentration higher and the Na+ concentration lower within the cell than they are in the ECF.	These ions continually leak through the membrane, and the Na+-K pump compensates like bailing out a leaky boat.
Why would the Na+-K+ pump, but not osmosis, cease to function after a cell dies?	The Na+-K+ pump requires ATP, whereas osmosis does not. ATP is quickly depleted after a cell dies.
Lest you question the importance of the Na-K pump, consider that half of the calories you use each day go to this purpose alone.	The pump typically operates at about 10 cycles/s, but under certain conditions it can achieve 100 cycles/s.
Various types of cells have from just a few hundred Na*-K+ pumps (red blood cells) to millions of them (nerve cells), so an average cell may exchange -----	30 million Na+ ions and 20 million K+ ions and consume 10 million ATPs per second.
Secondary active transport.	It maintains a steep Na* concentration gradient across the membrane. Like water behind a dam, this gradient is a source of potential energy that can be tapped to do other work. The SGLT described previously is an example of this.
Regulation of cell volume.	Certain anions are confined to the cell and can't penetrate the plasma membrane. These "fixed anions,” such as proteins and phosphates, attract and retain cations.
Regulation of cell volume.	If there were nothing to correct for it, the retention of these ions would cause osmotic swelling and possibly lysis of the cell.
Regulation of cell volume.	Cellular swelling, however, elevates activity of the Na-K pumps.
Regulation of cell volume.	Since each cycle of the pump removes one ion more than it brings in, the pumps are part of a negative feedback loop that reduces intracellular ion concentration, controls osmolarity, and prevents cellular swelling.
Maintenance of a membrane potential.	All living cells have an electrical charge difference called the resting membrane potential across the plasma membrane.
Like the two poles of a battery, the inside of the membrane is negatively charged and the outside is positively charged.	This difference stems from the unequal distribution of ions on the two sides of the membrane, maintained by the Na-K pump.
Heat production.	When the weather turns chilly, we turn up not only the furnace in our home but also the “furnace" in our body.
Heat production.	Thyroid hormone stimulates cells to produce more Na-K pumps. As these pumps consume ATP, they release heat from it, compensating for the body heat we lose to the cold air around us.
An important characteristic of proteins is their ability to change shape in response to the binding or dissociation of a ligand Explain how this is essential to carrier-mediated transport.	Proteins in carrier-mediated transport are like adaptable keys that change shape to unlock specific doors, or in this case, transport molecules across cell membranes.
An important characteristic of proteins is their ability to change shape in response to the binding or dissociation of a ligand Explain how this is essential to carrier-mediated transport.	When a ligand binds to a protein, it alters the protein's shape, enabling it to transport substances like ions or nutrients through the membrane efficiently.
In summary, carrier-mediated transport is any process in which solute particles move through a membrane by means of a transport protein.	The protein is a uniport if it transports only one solute, a symport if it carries two types of solutes at once in the same direction, and an antiport if it carries two or more solutes in opposite directions.
If the carrier doesn't depend on ATP at all and it moves solutes down their concentration gradient, the process is called facilitated diffusion.	If the carrier itself consumes ATP and moves solutes up their concentration gradient, the process is called primary active transport.
If the carrier doesn't directly use ATP,	but depends on a concentration gradient produced by ATP-consuming Na*-K+ pumps elsewhere in the plasma membrane, the process is called secondary active transport.
So far, we have considered processes that move one or a few ions or molecules at a time through the plasma membrane.	Vesicular transport processes, by contrast, move large particles, droplets of fluid, or numerous molecules at once through the membrane, contained in bubblelike vesicles of membrane.
Vesicular processes that bring matter into a cell are called endocytosis and those that release material from a cell are called exocytosis.	These processes employ motor proteins whose movements are energized by ATP.
There are three forms of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis.	Phagocytosis, or “cell eating,” is the process of engulfing particles such as bacteria, dust, and cellular debris—particles large enough to be seen with a microscope.
For example, ----- (a class of white blood cells) protect the body from infection by phagocytizing and killing bacteria.	neutrophils
A neutrophil spends most of its life crawling about in the connective tissues by means of its pseudopods.	When a neutrophil encounters a bacterium, it surrounds it with pseudopods and traps it in a vesicle called a phagosome—a vesicle in the cytoplasm surrounded by a membrane
A lysosome merges with the phagosome, converting it to a phagolysosome, and contributes enzymes that destroy the invader.	Some cells called macrophages (literally, “big eaters”) phagocytize the equivalent of 25% of their own volume per hour.
Step 1: Encounter	A phagocytic cell encounters a particle of foreign matter.
Step 2: Engulfment	The cell surrounds the particle using its pseudopods.
Step 3: Phagosome Formation	The particle is phagocytized and contained within a phagosome.
Step 4: Fusion	The phagosome fuses with a lysosome, forming a phagolysosome.
Step 5: Digestion Enzymes	from the lysosome digest the foreign matter.
Step 6: Membrane Fusion	The phagolysosome fuses with the plasma membrane.
Step 7: Exocytosis	The indigestible residue is voided from the cell by exocytosis.
"In phagocytosis, phagocytes are attracted to the area of invasion by chemical products of the microorganism, phospholipids released by injured mammalian cells, or by components of the complement system.	The phagocyte moves into the area of invasion and then attaches to the microorganism.
For example, the C3B component of complement codes bacteria or other particles and then binds to C3B receptors on the phagocyte. This process of coding to enhance phagocytosis is called opsonization."	(The phagocyte moves into the area of invasion and then attaches to the microorganism)The attachment is mediated by a variety of surface receptors including antibody, lipopolysaccharide, and complement receptors.
A small, rod-shaped microorganism is shown. A large, irregularly shaped phagocyte, containing tiny spherical lysosomes, approaches and then attaches to the microorganism. microorganism..	The surface of the microorganism is covered in small, circular bumps, which are called out as C3B complements.
The surface of the phagocyte has C3B receptors with indented shapes that perfectly fit the C3B complements on the microorganism.	The phagocyte’s C3B receptors are shown attaching to C3B complements on the microorganism
The microorganism is then engulfed by the phagocyte into a vacuole known as a phagosome. Vesicles in the cytoplasm called lysosomes fuse with the phagosome, releasing digestive enzymes such as lysozyme and proteases into the phagosome.	The structure resulting from this fusion is called a phagolysosome. Inside the phagolysosome, microorganisms are killed then digested. Finally, the digested contents of the phagolysosome are eliminated from the phagocyte by exocytosis.
The phagocyte envelopes the microorganism, which is now surrounded in a spherical vacuole, or phagosome, inside the phagocyte.	Several small, spherical lysosomes are shown approaching and then entering the phagosome vacuole, where they each split into multiple smaller spheres, which are called out as digestive enzymes.
The phagosome with the digestive enzymes and the microorganism together is called out as a phagolysosome. The digestive enzymes in the phagolysosome are shown breaking down the microorganism into many pieces.	The phagolysosome then moves to the edge of the phagocyte where the vacuole opens to the outside and releases the digested microorganism.
Pinocytosis	, or “cell drinking," is the process of taking in droplets of ECF containing molecules of some use to the cell.
While phagocytosis occurs in only a few specialized cells, pinocytosis occurs in all human cells. The process begins as the plasma membrane dimples, or caves in, at points.	These pits soon separate from the surface membrane and form small membrane-bounded pinocytotic vesicles in the cytoplasm.
The vesicles contain droplets of the ----- with whatever molecules happen to be there.	ECF
Receptor-mediated endocytosis	is a more selective form of either phagocytosis or pinocytosis. It enables a cell to take in specific molecules from the ECF with a minimum of unnecessary matter. Particles in the ECF bind to specific receptors on the plasma membrane.
The receptors then cluster and the membrane sinks in at this point, creating a pit coated with a peripheral membrane protein called clathrin.	The pit soon pinches off to form a clathrin-coated vesicle in the cytoplasm.
Clathrin	may serve as an “address label" on the coated vesicle that directs it to an appropriate destination in the cell, or it may inform other structures in the cell what to do with the vesicle.
①	Extracellular molecules bind to receptors on plasma membrane; receptors cluster together.
②	Plasma membrane sinks inward, forms clathrin-coated pit.
③	Pit separates from plasma membrane, forms clathrin-coated vesicle containing concentrated molecules from ECF.
One example of receptor-mediated endocytosis is the uptake of low-density lipoproteins (LDLs)–protein-coated droplets of cholesterol and other lipids in the blood.	The thin endothelial cells that line our blood vessels have LDL receptors on their surfaces and absorb LDLs in clathrin-coated vesicles.
Inside the cell, the ----- is freed from the vesicle and metabolized, and the membrane with its receptors is recycled to the cell surface.	LDL
Endothelial cells also imbibe insulin by receptor-mediated endocytosis.	Insulin is too large to pass through channels in the plasma membrane, yet it must somehow get out of the blood and reach the surrounding cells if it is to have any effect.
Endothelial cells take up insulin by receptor-mediated endocytosis, transport the vesicles across the cell, and release the insulin on the other side, where tissue cells await it.	Such transport of material across a cell (capture on one side and release on the other) is called transcytosis
transcytosis	This process is especially active in muscle capillaries and transfers a significant amount of
Transcytosis. An endothelial cell of a capillary imbibes droplets of blood plasma at sites indicated by arrows along the left.	This forms pinocytotic vesicles, which the cell transports to the other side. Here, it releases the contents by exocytosis at sites indicated by arrows along the right side of the cell.
Why isn't transcytosis listed as a separate means of membrane transport, in addition to pinocytosis and the others?	Transcytosis is a combination of endocytosis and exocytosis
Receptor-mediated endocytosis	isn't always to our benefit; hepatitis, polio, and AIDS viruses trick our cells into engulfing them by receptor-mediated endocytosis, thus exploiting this mechanism to establish infection.
Exocytosis is a process of discharging material from a cell. It occurs, for example, when endothelial cells release insulin to the tissue fluid,	sperm cells release enzymes for penetrating an egg, mammary gland cells secrete milk sugar, and other gland cells release hormones
Exocytosis	It bears a superficial resemblance to endocytosis in reverse.
A secretory vesicle in the cell migrates to the surface and “docks” on peripheral proteins of the plasma membrane.	These proteins pull the membrane inward and create a dimple that eventually fuses with the vesicle and allows it to release its contents.
Stage of exocytosis	1. A secretory vesicle approaches the plasma membrane and docks on it by means of linking proteins. The plasma membrane caves in at that point to meet the vesicle.
Stage of exocytosis	2. The plasma membrane and vesicle unite to form a fusion pore through which the vesicle contents are released
The question might occur to you, If endocytosis continually takes away bits of plasma membrane to form intracellular vesicles, why doesn't the membrane grow smaller and smaller?	Another purpose of exocytosis, however, is to replace plasma membrane that has been removed by endocytosis or that has become damaged or worn out
. Empty vesicles are transported to the inner surface of the plasma membrane and fuse with it. Exocytosis must keep pace with the rate of membrane removal by endocytosis, or else a cell would shrivel and die.	Plasma membrane is continually recycled from the cell surface into the cytoplasm and back to the surface.
Transport Without Carriers	Filtration, Simple diffusion, Osmosis
Transport Without Carriers: Filtration	------, Simple diffusion, Osmosis
Transport Without Carriers: Simple diffusion	Filtration, ------, Osmosis
Transport Without Carriers: Osmosis	Filtration, Simple diffusion, -----
Filtration (Movement of Material Without the Aid of Carrier Proteins:)	The movement of water and solutes through a selectively permeable membrane due to hydrostatic pressure.
Simple diffusion Movement of Material Without the Aid of Carrier Proteins):	The diffusion of particles through a membrane, down their concentration gradient, without the assistance of membrane carriers.
Osmosis (Movement of Material Without the Aid of Carrier Proteins):	The net flow of water across a selectively permeable membrane, driven by either a difference in solute concentration or a mechanical force.
Carrier-Mediated Transport	Movement of Material Through a Cell Membrane by Carrier Proteins
Carrier-Mediated Transport	Facilitated diffusion, Primary active transport, Secondary active transport, Countertransport (Antiport)
Fasciliated diffusion (Movement of Material Through a Cell Membrane by Carrier Proteins)	The transport of particles across a membrane, down their concentration gradient, via a carrier that does not directly consume ATP.
Primary active transport (Movement of Material Through a Cell Membrane by Carrier Proteins)	The transport of solute particles across a membrane, up their concentration gradient, by a carrier that consumes ATP.
Secondary active transport (Movement of Material Through a Cell Membrane by Carrier Proteins)	Transport of solute particles through a selectively permeable membrane, up their concentration gradient, by a carrier that doesn't use ATP itself but depends on concentration gradients produced by primary active transport elsewhere in the membrane.
Cotransport (Movement of Material Through a Cell Membrane by Carrier Proteins)	Simultaneous transport of two or more solutes in the same direction through a membrane by a carrier protein called a symport, using either facilitated diffusion or active transport.
Countertransport (Movement of Material Through a Cell Membrane by Carrier Proteins)	Transport of two or more solutes in opposite directions through a membrane by a carrier protein called an antiport, using either facilitated diffusion or active transport.
Vesicular (Bulk) Transport	Movement of Fluid and Particles Through a Plasma Membrane by Way of Membrane Vesicles; Consumes ATP
Endocytosis (Movement of Fluid and Particles Through a Plasma Membrane by Way of Membrane Vesicles; Consumes ATP)	Vesicular transport of particles into a cell.
Phagocytosis (Movement of Fluid and Particles Through a Plasma Membrane by Way of Membrane Vesicles; Consumes ATP)	Process of engulfing large particles by means of pseudopods; "cell eating".
Pinocytosis (Movement of Fluid and Particles Through a Plasma Membrane by Way of Membrane Vesicles; Consumes ATP)	Process of imbibing extracellular fluid in which the plasma membrane sinks in and pinches off small vesicles containing droplets of fluid; "cell drinking"
Receptor-mediated endocytosis (Movement of Fluid and Particles Through a Plasma Membrane by Way of Membrane Vesicles; Consumes ATP)	Phagocytosis or pinocytosis in which specific solute particles bind to receptors on the plasma membrane, and are then taken into the cell in clathrin-coated vesicles with a minimal amount of extraneous matter.
Exocytosis (Movement of Fluid and Particles Through a Plasma Membrane by Way of Membrane Vesicles; Consumes ATP)	Process of eliminating material from a cell by means of a vesicle approaching the cell surface, fusing with the plasma membrane, and expelling its contents
Exocytosis (Movement of Fluid and Particles Through a Plasma Membrane by Way of Membrane Vesicles; Consumes ATP)	used to release cell secretions, replace worn-out plasma membrane, and replace membrane that has been internalized by endocytosis.
Movement across the Membrane	Substances move either passively or actively into or out of the cell by crossing the semipermeable plasma membrane. Passive transport processes do not require the use of ATP while active transport processes do
Simple Diffusion: Substances that can directly cross the plasma membrane unassisted, such as small, lipid-soluble molecules, do so by simple diffusion.	The directional movement of these molecules is determined by the concentration gradient, with substances moving from a higher to a lower concentration.
Facilitated diffusion	allows substances to cross the cell membrane by utilizing specific protein channels or protein carriers. Channel proteins can be non-gated, which are always open, or gated, which are triggered to open in response to a specific stimulus.
Ligand-gated ion	channels involve the molecular binding of a substance to the protein channel.
Voltage-gated	channels involve changes in the membrane potential.
Mechanically-gated	channels respond to a mechanical stimulus.
Diffusion of Water across the Membrane: In biological systems, cellular homeostasis is managed by either the movement of the solvent (water) or solutes [ions and/or molecules) across cell membranes.	The diffusion of water across the semi-permeable cell membrane is called osmosis. Water can cross the plasma membrane directly, or move through specialized membrane channels called aquaporins.
Physiological saline is defined as the relationship of physiologically relevant salts in solution. For example, in mammalian systems this normally refers to the concentration of sodium chloride [NaCl) in water within the cells	which is essentially isotonic with tissue fluids and blood. This balance is a dynamic process by which the water and ions can move selectively as required by homeostasis.
Osmolarity	is the measure of solute in a solution. The directional movement of water occurs from a solution of lesser osmolarity to a solution of greater osmolarity
Passive Transport: Osmosis Hypertonic vs. Hypotonic Solutions A net influx or efflux of water into or out of the cell changes its shape, or tonicity.	A cell in an isotonic solution will have no net gain or loss of water and will retain its shape. Cells surrounded by either a hypertonic or hypotonic solution will shrink or swell accordingly in response to the resulting water loss or gain by the cell.
ATP Powers Active Transport Processes Active transport processes drive the movement of molecules across the plasma membrane against their concentration gradients.	ATP produced in the mitochondria provides the energy for active transport to occur due to the breakdown of ATP into ADP and inorganic phosphate.
Primary Active Transport: In primary active transport, carrier proteins found within the membrane become phosphorylated as the energy is released from ATP molecules.	he resulting phosphorylation induces a change in the shape of the protein, which drives movement of the solute across the membrane. Carrier proteins may transfer one (uniport) or more molecules at a time
Secondary Active Transport: Secondary active transport ties the movement of one substance following its concentration gradient to the movement of another substance against its concentration gradient.	ATP is used indirectly in this process. This coupled movement of substances can occur in the same direction (symport), or in the opposite direction (antiport).
Vesicular Transport Mechanisms: Materials can enter or exit the cell through vesicular transport mechanisms. Because these processes require the use of ATP, they are classified as active transport.	Materials produced within the cell are packaged into vesicles and released into the extracellular environment through exocytosis, while materials found in the extracellular environment enter the cell through the process of endocytosis.
Exocytosis	Intracellular materials produced in the organelles are packaged into membrane-bound secretory vesicles. These vesicles travel through the cell and fuse with the cell membrane, releasing their contents into the extracellular environment.
Endocytosis: Pinocytosis There are three general categories of endocytosis: pinocytosis, phagocytosis, and receptor-mediated endocytosis.	Through the process of pinocytosis, cells engulf small samples of the interstitial fluid, thus ingesting a representative sample of the ions and molecules present in the fluid
Endocytosis: Phagocytosis - Specific cell types are capable of engulfing large extracellular particles through the process of phagocytosis. During this process, which literally means "cell eating," cytoplasmic extensions extensions called pseudopodia	extend from the cell, surrounding the particle and forming a vesicle around it. Once engulfed, the vesicle is internalized and fuses with a lysosome containing digestive enzymes. This results in the chemical breakdown of the engulfed particle.
Receptor-Mediated Endocytosis: Receptor-mediated endocytosis is triggered by the binding of a molecule to specific receptors on the plasma membrane. Once binding has occurred, a vesicle forms in which the material is transferred into the cell.	These vesicles are often surrounded with clatharin-coated pits (proteins) that appear to act as an identification marker for the substance inside the vesicle.
The combination of passive and active transport processes allows cells to use a multitude of membrane transport mechanisms to maintain homeostasis and to adapt to different	cellular, tissue, organ, and organism needs.
What is the importance of filtration to human physiology?	This process occurs in the kidneys, where blood pressure forces water and solutes through a membrane, retaining essential proteins and cells while eliminating waste. Filtration helps maintain homeostasis by regulating fluid balance and blood pressure.
What does it mean to say a solute moves down its concentration gradient?	When a solute moves down its concentration gradient, it travels from an area of higher concentration to one of lower concentration.
How does osmosis help to maintain blood volume?	Osmosis maintains blood volume by balancing fluid exchange between blood and tissues.
Define osmolarity and tonicity, and explain the difference between them.	While osmolarity measures solute quantity, tonicity considers the impact on cells, influencing water movement across membranes.
Hypotonic	refers to a condition where the extracellular fluid (ECF) has a lower concentration of solutes compared to the inside of cells.
Isotonic	Isotonic solutions maintain a balance of solute concentration between the inside and outside of a cell, like a well-tuned see-saw.
Hypertonic	A hypertonic solution has a higher concentration of nonpermeating solutes compared to the intracellular fluid (ICF)
Why are these concepts are important in clinical practice: Hypotonic, Isotonic, Hypertonic	In clinical practice, knowing about hypotonic, isotonic, and hypertonic solutions helps doctors give the right fluids to patients, ensuring cells stay healthy and balanced.
What do facilitated diffusion and active transport have in common? How are they different?	Facilitated diffusion and active transport both use carrier proteins to move substances. Facilitated diffusion needs no energy, while active transport uses energy to move substances.
How does the Na+-K+ pump exchange sodium ions for potassium ions across the plasma membrane? What are some purposes served by this pump?	The Na+–K+ pump uses energy to move three sodium ions out of the cell and two potassium ions into the cell. This helps cells work properly.
How does phagocytosis differ from pinocytosis?	Phagocytosis engulfs large particles like bacteria, while pinocytosis takes in fluids and small molecules.
Describe the process of exocytosis. What are some of its purposes?	Exocytosis is when cells push materials out. It helps release hormones, enzymes, and other substances like insulin and milk sugar into the body.

Created by: Russells3709

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