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3.2c
| Question | Answer |
|---|---|
| Many cells have surface extensions called ------ These aid in absorption, movement, and sensory processes. | microvilli, cilia, flagella, and pseudopods. |
| Microvilli (MY-cro-VIL-eye; singular, microvillus) | are extensions of the plasma membrane that serve primarily to increase a cell’s surface area (fig. 3.9 and 3.10a, b). |
| Microvilli (MY-cro-VIL-eye; singular, microvillus) | 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. |
| 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 -----, they resemble a deep-pile carpet. | electron microscope |
| 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 (SIL-ee-uh; singular, cilium) (fig. 3.10) | 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 |
| primary cilium | 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; |
| primary cilium | and in the kidney, they’re thought to monitor the flow of fluid as it is processed into urine. |
| primary cilium | 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 those nonmotile primary cilia—are sometimes responsible for birth defects and hereditary diseases called | ciliopathies |
| Cilia. | (a) Epithelium of the uterine (fallopian) tube (SEM). The short, mucus-secreting cells between the ciliated cells show bumpy microvilli on their surfaces. |
| Cilia. | (b) Three-dimensional structure of a cilium. |
| Cilia. | (c) Cross section of a few cilia and microvilli (TEM). |
| Cilia. | (d) Cross-sectional structure of a cilium. Note the relative sizes of cilia and microvilli in parts (a) and (c). |
| 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. |
| Motile cilia | 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 (fig. 3.11), propelling such materials as mucus, an egg cell, or cerebrospinal fluid. |
| Motile cilia | Each cilium bends stiffly forward and produces a power stroke that pushes along the mucus or other matter. |
| Motile cilia | After begins its power stroke, the one just ahead of it begins, and the next and the next—collectively producing a wavelike motion. |
| Motile cilia | After a cilium completes its power stroke, it pulls limply back in a recovery stroke that restores it to the upright position, ready to flex again. |
| (a) Cilia of an epithelium moving mucus along a surface layer of saline. | (b) 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. |
| How would the movement of mucus in the respiratory tract be affected if cilia were equally stiff on both their power and recovery strokes? | If cilia were stiff during both strokes, mucus would not move well in the respiratory tract, possibly causing mucus buildup and breathing problems. |
| 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. |
| Cystic Fibrosis | The significance of chloride pumps is especially evident in cystic fibrosis (CF), a hereditary disease affecting primarily white children of European descent. |
| Cystic Fibrosis | CF is usually caused by a defect in which cells make chloride pumps but fail to install them in the plasma membrane. |
| Cystic Fibrosis | Consequently, there is an inadequate saline layer on the cell surface and the mucus is dehydrated and overly sticky. |
| Cystic Fibrosis | 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. |
| Cystic Fibrosis | In the respiratory tract, the mucus clogs the cilia and prevents them from beating freely. |
| Cystic Fibrosis | 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 (ACK-so-neem) | 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 (fig. 3.10d). |
| 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 (DINE-een). |
| Dynein | , a motor protein, uses energy from ATP to crawl up the adjacent pair of microtubules. |
| Dynein | 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 | Their collective action makes the cilium bend in that direction. |
| 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 cilium, which cannot move, lacks the two central microtubules and dynein arms, but still has the nine peripheral pairs; they are said to have a 9 + 0 structure. |
| The only functional flagellum (fla-JEL-um) 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 Chapter 27. |
| Pseudopods (SOO-do-pods) are cytoplasmic-filled extensions of the cell varying in shape from fine, filamentous processes to blunt fingerlike ones (fig. 3.12). | Unlike the other three kinds of surface extensions, they change continually. |
| Pseudopods (SOO-do-pods) | 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. |
| The freshwater organism Amoeba furnishes a familiar example of pseudopods | which it uses for locomotion and food capture. |
| White blood cells called ---- 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 | neutrophils |
| 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. |
| Macrophages | 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—having both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails— |
| How does the structure of a plasma membrane depend on the amphipathic nature of phospholipids? | is fundamental to the bilayer structure. In an aqueous environment, these molecules spontaneously arrange themselves into a two-layer sandwich (bilayer). |
| How does the structure of a plasma membrane depend on the amphipathic nature of phospholipids? | The hydrophilic heads face outward, interacting with the water of the extracellular fluid and cytoplasm, while the hydrophobic tails face inward, shielded from water. This forms a stable, fluid barrier. |
| Define peripheral versus transmembrane proteins. | Transmembrane proteins pass completely through the phospholipid bilayer, with hydrophobic regions embedded in the lipid core and hydrophilic regions protruding into the intracellular and extracellular fluids. |
| Define peripheral versus transmembrane proteins. | Peripheral proteins do not penetrate the bilayer. They adhere to either the inner or outer face of the membrane, often anchored to a transmembrane protein or the cytoskeleton. |
| Receptor: | A membrane protein that binds to specific chemical messengers (like hormones) from other cells, triggering a change (like a second messenger cascade) inside the target cell. |
| Pump: | A type of carrier (transmembrane protein) that binds solutes and transfers them across the membrane, consuming ATP in the process (active transport). |
| Cell-adhesion molecule (CAM): | A membrane protein that binds cells to each other or to extracellular material, which is crucial for tissue structure and specific cellular interactions (e.g., immune cell binding). |
| Difference | A gate (or gated channel) opens and closes under specific circumstances, allowing solutes through only at certain times. Other channel proteins, called leak channels, are always open. |
| Three factors that open/close gates: | Chemical messengers (ligand-gated), Electrical potential (voltage-gated), Physical stress (mechanically gated): |
| Chemical messengers (ligand-gated) | Respond to binding of a specific molecule. |
| Electrical potential (voltage-gated) | Respond to changes in voltage across the membrane. |
| Physical stress (mechanically gated) | Respond to physical forces like stretch or pressure. |
Created by:
Russells3709