Fluid mosaic model
Know this
Role of cholesterol in lipid bilayer
Stays between phospholipids and adds support. Addsa stabilization to plasma membrane. important in keeping the membrane fluid
Effects of unsaturated fatty acids on a phospholipid bilayer
Double bonds make it harder to pack lipids together due to the kink it forms and thus more fluid. This makes unsaturated substances liquid at room temperature.
peripheral proteins
Can be released from the membrane by more gentle extraction procedures that interfere with protein-protein interactions but leave the lipid bilayer intact
integral proteins
Directly attached to lipid bilayer. Can only be removed by detergents.
plasma membrane
involved in cell communication, import and export of molecules, and cell growth and motility
Water molecules are said "to reorganize into a cagelike structure" around hydrophobic compounds (e.g., see Figure 11-9). This seems paradoxical because water molecules do not interact with the hydrophobic compound. So how could they "know" about its prese
Water is a liquid, and thus hydrogen bonds between water molecules are not static; they are continually formed and broken again by thermal motion. When a water molecule happens to be next to a hydrophobic molecule, it is more restricted in motion and has
Five students in your class always sit together in the front row. This could be because
(A) they really like each other or
(B) nobody else in your class wants to sit next to them. Which explanation holds for the assembly of a lipid bilayer? Explain. Suppo
(B) is the correct analogy for lipid bilayer assembly because exclusion from water rather than attractive forces between the lipid molecules is involved. If the lipid molecules formed bonds with one another, the bilayer would be less ?uid, and might even
It seems paradoxical that a lipid bilayer can be ?uid yet asymmetrical. Explain.
The ?uidity of the bilayer is strictly con?ned to one plane: lipid molecules can diffuse laterally in their own monolayer but do not readily ?ip from one monolayer to the other. Speci?c types of lipid molecules inserted into one monolayer therefore remain
Explain why the polypeptide chain of most transmembrane proteins crosses the lipid bilayer as an alpha helix or a beta barrel.
In both an alpha helix and a betabarrel the polar peptide bonds of the polypeptide backbone can be completely shielded from the hydrophobic environment of the lipid bilayer by the hydrophobic amino acid side chains. Internal hydrogen bonds between the pep
Look carefully at the transmembrane proteins shown in Figure 11-29. What can you say about their mobility in the membrane?
Some of the molecules of the two different transmembrane proteins are anchored to the spectrin ?laments of the cell cortex. These molecules are not free to rotate or diffuse within the plane of the membrane. There is an excess of transmembrane proteins ov
Describe the different methods that cells use to restrict proteins to speci?c regions of the plasma membrane. Is a membrane with many of its proteins restricted still ?uid?
The different ways in which membrane proteins can be restricted to different regions of the membrane are summarized in Figure 11-31. The mobility of the membrane proteins is drastically reduced if they are bound to other proteins such as those of the cell
Which of the following statements are correct? Explain your answers.
A. Lipids in a lipid bilayer spin rapidly around their long axis.
B. Lipids in a lipid bilayer rapidly exchange positions with one another in their own monolayer.
C. Lipids in a lipid bi
All of the statements are correct. A, B, C, D. The lipid bilayer is ?uid because its lipid molecules can undergo these motions.
E. Glycolipids are mostly restricted to the monolayer of membranes that faces away from the cytosol. Some special glycolipids,
What is meant by the term "two-dimensional ?uid"?
In a two-dimensional ?uid, the molecules are free to move only in one plane; the molecules in a normal ?uid, in contrast, can move in three dimensions
Predict which one of the following organisms will have the highest percentage of unsaturated phospholipids in its membranes. Explain your answer.
A. Antarctic ?sh
B. Desert snake
C. Human being
D. Polar bear
E. Thermophilic bacterium that lives in hot spr
(A) Antarctic ?sh live at subzero temperatures and are cold-blooded. To keep their membranes ?uid at these temperatures, they have an unusually high percentage of unsaturated phospholipids.
Which of the three 20-amino-acid sequences listed below in the single-letter amino acid code is the most likely candidate to form a transmembrane region (alpha helix) of a transmembrane protein? Explain your answer.
A. I T L I Y F G N M S S V T Q T I L L
Sequence B is most likely to form a transmembrane helix. It is composed primarily of hydrophobic amino acids, and therefore can be stably integrated into a lipid bilayer. In contrast, sequence A contains many polar amino acids (S, T, N, Q), and sequence C
A simple enzyme reaction can be described by the equation E + S<->ES <->E + P, where E is the enzyme, S the substrate, P the product, and ES the enzyme- substrate complex.
A. Write a corresponding equation describing the workings of a transporter (T) that
A. The movement of a solute mediated by a transporter can be described by a strictly analogous equation: equation 1: T + S ? TS ? T + S
where S is the solute, S
S* is the solute on the other side of the membrane (i.e., although it is still the same molecu
A rise in the intracellular Ca2+ concentration causes muscle cells to contract. In addition to an ATPdriven Ca2+ pump, muscle cells that contract quickly and regularly, such as those of the heart, have an additional type of Ca2+ pump�an antiport that exch
If the Na+ pump is not working at full capacity because it is partially inhibited by ouabain or digitalis, the electrochemical gradient of Na+ that the pump generates is less steep than that in untreated cells. Consequently, the Ca2+-Na+ antiport works le
A transmembrane protein has the following properties: it has two binding sites, one for solute A and one for solute B. The protein can undergo a conformational change to switch between two states: either both binding sites are exposed exclusively on one s
A. The properties de?ne a transporter acting as a symport.
B. No additional properties need to be speci?ed. The important feature that provides the coupling of the two solutes is that the protein cannot switch its conformation if only one of the two solut
Figure Q12-4 (above) shows a recording from a patch-clamp experiment in which the electrical current passing across a patch of membrane is measured as a function of time. The membrane patch was plucked from the plasma membrane of a muscle cell by the tech
Each of the rectangular peaks corresponds to the opening of a single channel that allows a small current to pass. You note from the recording that the channels present in the patch of membrane open and close frequently. Each channel remains open for a ver
Explain as precisely as you can but in no more than 100 words the ionic basis of an action potential and how it is passed along an axon.
When the resting membrane potential of an axon (inside negative) rises to a threshold value, voltagegated Na+ channels in the immediate neighborhood open and allow an in?ux of Na+. This depolarizes the membrane further, causing more voltage-gated Na+ chan
In the disease myasthenia gravis, the human body makes�by mistake� antibodies to its own acetylcholine receptor molecules. These antibodies bind to and inactivate acetylcholine receptors on the plasma membrane of muscle cells. The disease leads to a devas
If the number of functional acetylcholine receptors is reduced by the antibodies, the neurotransmitter (acetylcholine) that is released from the nerve terminals cannot (or can only weakly) stimulate the muscle to contract.
When an inhibitory neurotransmitter such as GABA opens Cl- channels in the plasma membrane of a postsynaptic neuron, why does this make it harder for an excitatory neurotransmitter to excite the neuron?
Although the concentration of Cl- outside cells is much higher than inside, when transmitter-gated Cl- channels open in the plasma membrane of a postsynaptic neuron in response to an inhibitory neurotransmitter, very little Cl- enters the cell. This is be
Which of the following statements are correct? Explain your answers.
A. The plasma membrane is highly impermeable to all charged molecules.
B. Channels have speci?c binding pockets for the solute molecules they allow to pass.
C. Transporters allow solutes
A. False. The plasma membrane contains transport proteins that confer selective permeability to many but not all charged molecules. In contrast, a pure lipid bilayer lacking proteins is highly impermeable to all charged molecules.
B. False. Channels do no
List the following compounds in order of increasing lipid bilayer permeability: RNA, Ca2+, glucose, ethanol, N2, water.
The permeabilities are N2 (small and nonpolar) > ethanol (small and slightly polar) > water (small and polar) > glucose (large and polar) > Ca2+ (small and charged) > RNA (very large and charged).
Name at least one similarity and at least one difference between the following (it may help to review the de?nitions of the terms using the Glossary):
A. Symport and antiport
B. Active transport and passive transport
C. Membrane potential and electrochemi
A. Both couple the movement of two different solutes across a cell membrane. Symports transport both solutes in the same direction, whereas antiports transport the solutes in opposite directions.
B. Both are mediated by membrane transport proteins. Passiv
Discuss the following statement: "The differences between a channel and a transporter are like the differences between a bridge and a ferry.
A bridge allows vehicles to pass over water in a steady stream; the entrance can be designed to exclude, for example, oversized trucks, and it can be intermittently closed to traf?c by a gate. By analogy, gated channels allow ions to pass across a cell me
The neurotransmitter acetylcholine is made in the cytosol and then transported into synaptic vesicles, where its concentration is more than 100-fold higher than in the cytosol. When synaptic vesicles are isolated from neurons, they can take up additional
Acetylcholine is being transported into the vesicles by an H+-acetylcholine antiport in the vesicle membrane. The H+ gradient that drives the uptake is generated by an ATP-driven H+ pump in the vesicle membrane, which pumps H+ into the vesicle (hence the
Phospholipid bilayers form sealed spherical vesicles in water. Assume you have constructed lipid vesicles that contain Na+ pumps as the sole membrane protein, and assume for the sake of simplicity that each pump transports one Na+ one way and one K+ the o
A. Nothing. You require ATP to drive the Na+ pump.
B. The ATP becomes hydrolyzed, and Na+ is pumped into the vesicles, generating a concentration gradient of Na+ across the membrane. At the same time, K+ is pumped out of the vesicles, generating a concent
Name the three ways in which an ion channel can be gated.
Ion channels can be ligand-gated, voltagegated, or mechanically (stress) gated
One thousand Ca2+ channels open in the plasma membrane of a cell that is 1000 microm3 in size and has a cytosolic Ca2+ concentration of 100 nM. For how long would the channels need to stay open in order for the cytosolic Ca2+ concentration to rise to 5 mi
The cell has a volume of 10-12 liters (= 10-15 m3) and thus contains 6 � 104 calcium ions (= 6 � 1023 molecules/mole � 100 � 10-9 moles/liter � 10-12 liters). Therefore, to raise the intracellular Ca2+ concentration ?ftyfold, another 2,940,000 calcium ion
Amino acids are taken up by animal cells using a symport in the plasma membrane. What is the most likely ion whose electrochemical gradient drives the import? Is ATP consumed in the process? If so, how?
Animal cells drive most transport processes across the plasma membrane with the electrochemical gradient of Na+. ATP is needed to fuel the Na+ pump to maintain the Na+ gradient.
We will see in Chapter 15 that endosomes, which are membrane-enclosed intracellular organelles, need an acidic lumen in order to function. Acidi?cation is achieved by an H+ pump in the endosomal membrane, which also contains Cl- channels. If the channels
A. If H+ is pumped across the membrane into the endosomes, an electrochemical gradient of H+ results� composed of both an H+ concentration gradient and a membrane potential, with the interior of the vesicle positive. Both of these components add to the en
Acetylcholine-gated cation channels do not discriminate between Na+, K+, and Ca2+ ions, allowing all to pass through them freely. So why is it that when acetylcholine binds to this protein in the plasma membrane of muscle cells, the channel opens and ther
The membrane potential and the steep extracellular Na+ concentration provide a large inward electrochemical driving force and a large reservoir of Na+ ions, so that mostly Na+ ions enter the cell as acetylcholine receptors open. Ca2+ ions will also enter
The ion channels that are regulated by binding of neurotransmitters, such as acetylcholine, glutamate, GABA, or glycine, have a similar overall structure. Yet, each class of these channels consists of a very diverse set of subtypes with different transmit
The diversity of neurotransmitter-gated ion channels is a good thing for the industry, as it raises the possibility of developing new drugs speci?c for each channel type. Each of the diverse subtypes of these channels is expressed in a narrow subset of ne
Arsenate (AsO43-) is chemically very similar to phosphate (PO43-) and is used as an alternative substrate by many phosphate-requiring enzymes. In contrast to phosphate, however, an anhydride bond between arsenate and carbon is very quickly hydrolyzed none
Arsenate instead of phosphate becomes attached in step 6 of glycolysis to form 1-arseno-3phosphoglycerate (Figure A13-2). Because of its sensitivity to hydrolysis in water, the high-energy bond is destroyed before the molecule that contains it can diffuse
Looking at the chemistry detailed in Panel 13-2 (pp. 434-435), why do you suppose it is useful to link the acetyl group ?rst to another, larger carbon skeleton, oxaloacetate, before completely oxidizing both carbons to CO2?
Because the function of the citric acid cycle is to harvest the energy released during the oxidation, it is advantageous to break the overall reaction into as many steps as possible (see Figure 13-1). Using a two-carbon compound, the available chemistry w
What, if anything, is wrong with the following statement: "The oxygen consumed during the oxidation of glucose in animal cells is returned as part of CO2 to the atmosphere." How could you support your answer experimentally?
It is true that oxygen atoms are returned as part of CO2 to the atmosphere. The CO2 released from the cells, however, does not contain those speci?c oxygen atoms that were consumed as part of the oxidative phosphorylation process and converted into water.
After looking at the structures of sugars and fatty acids (discussed in Chapter 2), give an intuitive explanation as to why oxidation of a sugar yields only about half as much energy as the oxidation of an equivalent dry weight of a fatty acid.
The carbon atoms in sugar molecules are already partially oxidized, in contrast to all but the very ?rst carbon atoms in the acyl chains of fatty acids. Thus,
two carbon atoms from glucose are lost as CO2 during the conversion of pyruvate to acetyl CoA, a
The oxidation of sugar molecules by the cell takes place according to the general reaction C6H12O6 (glucose) + 6O2 ? 6CO2 + 6H2O + energy. Which of the following statements are correct? Explain your answers.
A. All of the energy produced is in the form of
A. False. If this were the case, then the reaction would be useless for the cell. No chemical energy would be harvested in a useful form (e.g., ATP) to be used for metabolic processes. (Cells would be nice and warm, though!)
B. False. No energy-conversion
Yeast cells can grow both in the presence of O2 (aerobically) and in its absence (anaerobically). Under which of the two conditions could you expect the cells to grow better? Explain your answer.
Yeast cells grow much better aerobically. Under anaerobic conditions they cannot perform oxidative phosphorylation and therefore have to produce all their ATP by glycolysis, which is less ef?cient. Whereas one glucose molecule yields a net gain of two ATP
Identical pathways that make up the complicated sequence of reactions of glycolysis, shown in Panel 13-1 (pp. 428- 429), are found in most living cells, from bacteria to humans. One could envision, however, countless alternative chemical reaction mechanis
The extreme conservation of glycolysis is some of the evidence that all present cells are derived from a single founder cell as discussed in Chapter 1. The elegant reactions of glycolysis would therefore have evolved only once, and then they would have be
Dinitrophenol (DNP) is a small molecule that renders membranes permeable to protons. In the 1940s, small amounts of this highly toxic compound were given to patients to induce weight loss. DNP was effective in melting away the pounds, especially promoting
By making membranes permeable to protons, DNP collapses�or at very small concentrations
diminishes�the proton gradient across the inner mitochondrial membrane. Cells continue to oxidize food molecules to feed high-energy electrons into the electrontranspo
Electron micrographs show that mitochondria in heart muscle have a much higher density of cristae than mitochondria in skin cells. Suggest an explanation for this observation.
The inner mitochondrial membrane is the site of oxidative phosphorylation, and it produces most of the cell's ATP. Cristae are portions of the mitochondrial inner membrane that are folded inward. Mitochondria that have a higher density of cristae have a l
When the drug dinitrophenol (DNP) is added to mitochondria, the inner membrane becomes permeable to protons (H+). In contrast, when the drug nigericin is added to mitochondria, the inner membrane becomes permeable to K+.
(A) How does the electrochemical p
A. The DNP collapses the electrochemical proton gradient completely. H+ ions that are pumped to one side of the membrane ?ow back freely, and therefore no energy to drive ATP synthesis can be stored across the membrane. B. An electrochemical gradient is m
Two different diffusible electron carriers, ubiquinone and cytochrome c, shuttle electrons between the three protein complexes of the electron-transport chain. Could the same diffusible carrier, in principle, be used for both steps? Explain your answer.
It would not be productive to use the same carrier in two steps. If ubiquinone, for example, could transfer electrons directly to the cytochrome c oxidase, the cytochrome c reductase complex would often be skipped when electrons are collected from NADH de
Which of the following statements are correct? Explain your answers.
A. Many, but not all, electron-transfer reactions involve metal ions.
B. The electron-transport chain generates an electrical potential across the membrane because it moves electrons fro
A. True. NAD+ and quinones are examples of compounds that do not have metal ions but can participate in electron transfer.
B. False. The potential is due to protons (H+) that are pumped across the membrane from the matrix to the intermembrane space. Elect
In the following statement, choose the correct one of the alternatives in italics and justify your answer. "If no O2 is available, all components of the mitochondrial electrontransport chain will accumulate in their reduced/oxidized form. If O2 is suddenl
If no O2 is available, all components of the mitochondrial electron-transport chain will accumulate in their reduced form. This is the case because electrons derived from NADH enter the chain but cannot be transferred to O2. The electron-transport chain t
As your ?rst experiment in the laboratory, your adviser asks you to reconstitute puri?ed bacteriorhodopsin, a light-driven H+ pump from the plasma membrane of photosynthetic bacteria, and puri?ed ATP synthase from ox-heart mitochondria together into the s
A. When the vesicles are exposed to light, H+ ions (derived from H2O) pumped into the vesicles by the bacteriorhodopsin ?ow back out through the ATP synthase, causing ATP to be made in the solution surrounding the vesicles in response to light.
B. If the
Some bacteria have become specialized to live in an environment of high pH (pH ~10). Do you suppose that these bacteria use a proton gradient across their plasma membrane to produce their ATP? (Hint: all cells must maintain their cytoplasm at a pH close t
If these bacteria used a proton gradient to make their ATP in a fashion analogous to that in other bacteria (that is, fewer protons inside than outside), they would need to raise their cytoplasmic pH even higher than that of their environment (pH 10). Cel
A manuscript has been submitted for publication to a prestigious scienti?c journal. In the paper, the authors describe an experiment in which they have succeeded in trapping an individual ATP synthase molecule and then mechanically rotating its head by ap
This experiment would suggest a two-step model for ATP synthase function. According to this model, the ?ow of protons through the base of the synthase drives rotation of the head, which in turn causes ATP synthesis. In their experiment, the authors have s