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3.2a
| Question | Answer |
|---|---|
| Many physiologically important processes occur at the surface of a cell—------, the binding of egg and sperm, cell-to-cell signaling by hormones, and the detection of tastes and smells, for example | immune responses |
| 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 (fig. 3.5a). |
| The side that faces the cytoplasm is the intercellular 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. |
| 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. | 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? Which of the major themes at the end of chapter 1 does this point best exemplify? | If the plasma membrane were primarily hydrophilic, like carbohydrates, it would compromise its integrity. |
| Cholesterol molecules, found near the membrane surfaces amid the phospholipids, constitute about 20% of the membrane lipids. | By interacting with the phospholipids and holding them still, cholesterol can stiffen the membrane (make it less fluid) in spots. |
| Higher concentrations of cholesterol, however, can increase ------ by preventing phospholipids from packing closely together. | membrane fluidity |
| The remaining 5% of the membrane lipids are glycolipids—phospholipids with short carbohydrate chains on the extracellular face of the membrane. | They contribute to the glycocalyx, a carbohydrate coating on the cell surface with multiple functions, described shortly. |
| Although proteins are only about 2% of the molecules of the plasma membrane, they’re larger than lipids and average about 50% of the membrane by weight. | 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. | They have hydrophilic regions in contact with the water on both sides, and hydrophobic regions that pass back and forth through the lipid (fig. 3.6). |
| 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 discussed later. |
| 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. |
| A transmembrane protein has hydrophobic regions embedded in the phospholipid bilayer and hydrophilic regions projecting into the intracellular and extracellular fluids. | The protein may cross the membrane once (left) or multiple times (right). The intracellular regions are often anchored to the cytoskeleton by peripheral proteins. |
| 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, as discussed later in this chapter. |
| 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. Second-messenger systems are discussed shortly in more detail. |
| 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 a cell. |
| Channels are passages that allow water and hydrophilic solutes to move through the membrane. A channel is a tunnel that passes through a complex of multiple proteins or between subunits of an individual protein. | Some of them, called leak channels, are always open and allow materials to pass through continually. Others, called gates (gated channels), open and close under different circumstances and allow solutes through at some times, but not others (fig. 3.7d). |
| 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. |
| By controlling the movement of electrolytes through the plasma membrane, ------ play an important role in the timing of nerve signals and muscle contraction (see Deepen Insight 3.1). Some receptors double in function as gated channels. | gated channels |
| 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. | 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 glycocalyx, which acts like an “identification tag” that enables the immune system to tell which cells belong to one’s body and which are foreign invaders. |
| Cells adhere to one another and to extracellular material through membrane proteins called cell-adhesion molecules (CAMs). With few exceptions (such as blood cells and metastasizing cancer cells), | cells don’t survive and grow normally unless they’re mechanically linked to the extracellular material. Special events such as sperm–egg binding and the binding of an immune cell to a cancer cell also require CAMs. |
| (a) Receptor | A receptor that binds to chemical messengers such as hormones sent by other cells. |
| (b) Enzyme | An enzyme that breaks down a chemical messenger by altering its shape or structure. |
| (c) Channel | A channel protein that is constantly open and allows solutes to pass into and out of the cell. |
| (d) Gated channel | A gate that opens and closes to allow only certain ions or molecules through. |
| (e) Cell-identity marker | A glycoprotein acting as a cell identity marker. |
| (f) 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, the second messenger (see fig. 2.29b). |
| Cyclic AMP then activates cytoplasmic enzymes called kinases (KY-nace-es), 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. |
| Is a 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 |
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