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Lehninger Principles of Biochemistry

David L. Nelson, Michael M. Cox

Chapter 11

Biological Membranes and Transport - all with Video Answers

Educators

Chapter Questions

02:55

Problem 1

When phospholipids are layered gently onto the surface of water, they orient at the air-water interface with their head groups in the water and their hydrophobic tails in the air. An experimental apparatus
(a) has been devised that reduces the surface area available to a layer of lipids. By measuring the force necessary to push the lipids together, it is possible to determine when the molecules are packed tightly in a continuous monolayer; as that area is approached, the force needed to further reduce the surface area increases sharply
(b). How would you usethis apparatus to determine the average area occupied by a single lipid molecule in the monolayer?

Lottie Adams
Lottie Adams
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02:25

Problem 2

In $1925,$ E. Gorter and $F .$ Grendel used an apparatus likethat described in Problem 1 to determine the surface area of a lipid monolayer formed by lipids extracted from erythrocytes of several animal species. They used a microscope to measure the dimensions of individual cells, from which they calculated the average surface area of one erythrocyte. They obtained the data shown in the table below. Were these investigators justified in concluding that "chromocytes [erythrocytes] are covered by a layer of fatty substances that is two molecules thick"(i.e., a lipid bilayer)?

Lottie Adams
Lottie Adams
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01:28

Problem 3

When a small amount of the detergent sodium dodecyl sulfate $\left(\mathrm{SDS} ; \mathrm{Na}^{+} \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{11} \mathrm{OSO}_{3}^{-}\right)$ is dissolved in water, the
detergent ions enter the solution as monomeric species. As more detergent is added, a concentration is reached (the critical micelle concentration) at which the monomers associate to form micelles. The critical micelle concentration of SDS is $8.2 \mathrm{mM}$. The micelles have an average particle weight (the sum of the molecular weights of the constituent monomers) of $18,000 .$ Calculate the number of detergent molecules in the average micelle.

Lottie Adams
Lottie Adams
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02:24

Problem 4

Lipid bilayers formed between two aqueous phases have this important property: they form two-dimensional sheets, the edges of which close on each other and undergo self-sealing to form vesicles (liposomes).
(a) What properties of lipids are responsible for this property of bilayers? Explain.
(b) What are the consequences of this property for the structure of biological membranes?

Hailey Tomashek
Hailey Tomashek
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01:11

Problem 5

The carbon-carbon bond distance for single-bonded carbons such as those in a saturated fatty acyl chain is about $1.5 \AA .$ Estimate the length of a single molecule of palmitate in its fully extended form. If two molecules of palmitate were placed end to end, how would their total length compare with the thickness of the lipid bilayer in a biological membrane?

Lottie Adams
Lottie Adams
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01:17

Problem 6

The following observations are made on an unknown membrane protein, $\mathrm{X}$. It can be extracted from disrupted erythrocyte membranes into a concentrated salt solution, and it can be cleaved into fragments by proteolytic enzymes. Treatment of erythrocytes with proteolytic enzymes followed by disruption and extraction of membrane components yields intact X. However, treatment of erythrocyte "ghosts", (which consist of just plasma membranes, produced by disrupting the cells and washing out the hemoglobin) with proteolytic enzymes, followed by disruption and extraction, yields extensively fragmented X. What do these observations indicate about the location of $X$ in the plasma membrane? Do the properties of $X$ resemble those of an integral or peripheral membrane protein?

Lottie Adams
Lottie Adams
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03:46

Problem 7

You have cloned the gene for a human erythrocyte protein, which you suspect is a membrane protein. From the nucleotide sequence of the gene, you know the amino acid sequence. From this sequence alone, how would you evaluate the possibility that the protein is an integral protein? Suppose the protein proves to be an integral protein with one transmembrane segment. Suggest biochemical or chemical experiments that might allow you to determine whether the protein is oriented with the amino terminus on the outside or the inside of the cell.

Christina Sorrentino
Christina Sorrentino
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05:12

Problem 8

$E$. coli can be induced to make about 10,000 copies of the lactose transporter $\left(M_{\mathrm{r}} 31,000\right)$ per cell. Assume that $E$. coli is a cylinder $1 \mu \mathrm{m}$ in diameter and $2 \mu \mathrm{m}$ long. What fraction of the plasma membrane surface is occupied by the lactose transporter molecules? Explain how you arrived at this conclusion.

Shazia Naz
Shazia Naz
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02:13

Problem 9

The plasma membrane of $E .$ coli is about $75 \%$ protein and $25 \%$ phospholipid by weight. How many molecules of membrane lipid are present for each molecule of membrane protein? Assume an average protein $M_{\mathrm{r}}$ of 50,000 and an average phospholipid $M_{\mathrm{r}}$ of $750 .$ What more would you need to know to estimate the fraction of the membrane surface that is covered by lipids?

Lottie Adams
Lottie Adams
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01:57

Problem 10

The experiment described in Figure $11-16$ was performed at $37^{\circ} \mathrm{C}$. If the experiment were carried out at $10^{\circ} \mathrm{C}$, what effect would you expect on the rate of diffusion? Why?

Lottie Adams
Lottie Adams
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05:23

Problem 11

Cellular membranes are self-sealing- -if they are punctured or disrupted mechanically, they quickly and automatically reseal. What properties of membranes are responsible for this important feature?

Ronald Prasad
Ronald Prasad
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01:36

Problem 12

Membrane lipids in tissue samples obtained from different parts of a reindeer's leg have different fatty acid compositions. Membrane lipids from tissue near the hooves contain a larger proportion of unsaturated fatty acids than those from tissue in the upper leg. What is the significance of this observation?

Lottie Adams
Lottie Adams
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02:48

Problem 13

The inner leaflet (monolayer) of the human erythrocyte membrane consists predominantly of phosphatidylethanolamine and phosphatidylserine. The outer leaflet consists predominantly of phosphatidylcholine and sphingomyelin. Although the phospholipid components of the membrane can diffuse in the fluid bilayer, this sidedness is preserved at all times. How?

Lottie Adams
Lottie Adams
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01:04

Problem 14

At pH $7,$ tryptophan crosses a lipid bilayer at about onethousandth the rate of indole, a closely related compound: Suggest an explanation for this observation.

Nicklas Hamilton
Nicklas Hamilton
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01:34

Problem 15

A helical wheel is a two-dimensional representation of a helix, a view along its central axis (see Fig. $11-29$ b; see also Fig. $4-4$ d) Use the helical wheel diagram shown here to determine the distribution of amino acid residues in a helical segment with the sequence -Val-Asp-Arg-Val-Phe-Ser-Asn-ValCys-Thr-His-Leu-Lys-Thr-Leu-Gln-Asp-Lys- What can you say about the surface properties of this helix? How would you expect the helix to be oriented in the tertiary structure of an integral membrane protein?

Lottie Adams
Lottie Adams
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03:06

Problem 16

Gastric juice (pH 1.5) is produced by pumping HCl from blood plasma (pH 7.4) into the stomach. Calculate the amount of free energy required to concentrate the $\mathrm{H}^{+}$ in $1 \mathrm{L}$ of gastric juice at $37^{\circ} \mathrm{C}$. Under cellular conditions, how many moles of ATP must be hydrolyzed to provide this amount of free energy? The free-energy change for ATP hydrolysis under cellular conditions is about $-58 \mathrm{kJ} / \mathrm{mol}$ (as explained in Chapter 13 ). Ignore the effects of the transmembrane electrical potential.

Lottie Adams
Lottie Adams
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02:22

Problem 17

For a typical vertebrate cell with a membrane potential of $-0.070 \mathrm{V}$ (inside negative), what is the free-energy change for transporting 1 mol of $\mathrm{Na}^{+}$ from the cell into the blood at $37^{\circ} \mathrm{C} ?$ Assume the concentration of $\mathrm{Na}^{+}$ inside the cell is $12 \mathrm{mM}$ and in blood plasma it is $145 \mathrm{mM}$

Ronald Prasad
Ronald Prasad
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01:21

Problem 18

Ouabain specifically inhibits the $\mathrm{Na}^{+} \mathrm{K}^{+}$ ATPase activity of animal tissues but is not known to inhibit any other enzyme. When ouabain is added to thin slices of living kidney tissue, it inhibits oxygen consumption by $66 \%$. Why? What does this observation tell us about the use of respiratory energy by kidney tissue?

Lottie Adams
Lottie Adams
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01:01

Problem 19

Suppose you determined experimentally that a cellular transport system for glucose, driven by symport of $\mathrm{Na}^{+},$ could accumulate glucose to concentrations 25 times greater than in the external medium, while the external $\left[\mathrm{Na}^{+}\right]$ was only 10 times greater than the intracellular $\left[\mathrm{Na}^{+}\right] .$ Would this violate the laws of thermodynamics? If not, how could you explain this observation?

Lottie Adams
Lottie Adams
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01:31

Problem 20

A bacterial lactose transporter, which is highly specific for lactose, contains a Cys residue that is essential to its transport activity. Covalent reaction of $N$ -ethylmaleimide (NEM) with this Cys residue irreversibly inactivates the transporter. A high concentration of lactose in the medium prevents inactivation by NEM, presumably by sterically protecting the Cys residue, which is in or near the lactose-binding site. You know nothing else about the transporter protein. Suggest an experiment that might allow you to determine the $M_{\mathrm{r}}$ of this Cys-containing transporter polypeptide.

Lottie Adams
Lottie Adams
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01:27

Problem 21

You are studying the uptake of L-leucine by epithelial cells of the mouse intestine. Measurements of the rates of uptake of L-leucine and several of its analogs, with and without $\mathrm{Na}^{+}$ in the assay buffer, yield the results given in the table below. What can you conclude about the properties and mechanism of the leucine transporter? Would you expect L-leucine uptake to be inhibited by ouabain?

Lottie Adams
Lottie Adams
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02:07

Problem 22

Consider the leucine transporter described in Problem $21 .$ Would $V_{\max }$ and $/$ or $K_{\mathrm{t}}$ change if you added a $\mathrm{Na}^{+}$ ionophore to the assay solution containing $\mathrm{Na}^{+}$ ? Explain.

Lottie Adams
Lottie Adams
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02:37

Problem 23

A human erythrocyte has about $2 \times 10^{5}$ AQP 1 monomers. If water molecules flow through the plasma membrane at a rate of $5 \times 10^{8}$ per AQP1 tetramer per second, and the volume of an erythrocyte is $5 \times 10^{-11} \mathrm{mL}$, how rapidly could an erythrocyte halve its volume as it encountered the high osmolarity $(1 \mathrm{M})$ in the interstitial fluid of the renal medulla? Assume that the erythrocyte consists entirely of water.

Anand Jangid
Anand Jangid
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08:44

Problem 24

Online bioinformatics tools make hydropathy analysis easy if you know the amino acid sequence of a protein. At the Protein Data Bank (www.pdb.org), the Protein Feature View displays additional information about a protein gleaned from other databases, such as UniProt and SCOP2. A simple graphical view of a hydropathy plot created using a window of 15 residues shows hydrophobic regions in red and hydrophilic regions in blue.
(a) Looking only at the displayed hydropathy plots in the Protein Feature View, what predictions would you make about the membrane topology of these proteins: glycophorin $\mathrm{A}(\mathrm{PDB} \text { ID } 1 \mathrm{AFO}),$ myoglobin $(\mathrm{PDB} \mathrm{ID} 1 \mathrm{MBO}),$ and aquaporin $(\mathrm{PDB} \mathrm{ID} 2 \mathrm{B} 6 \mathrm{O}) ?$
(b) Now, refine your information using the ProtScale tools at the ExPASy bioinformatics resource portal. Each of the PDB Protein Feature Views was created with a UniProt Knowledgebase ID. For glycophorin A, the UniProtKB ID is P02724; for myoglobin, $\quad$ P02185; and for aquaporin, Q6J8I9. Go to the ExPASy portal (http://web.expasy.org/protscale) and select the Kyte \& Doolittle hydropathy analysis option, with a window of 7 amino acids. Enter the UniProtKB ID for aquaporin (Q6J8I9, which you can also get from the PDB's Protein Feature View page), then select the option to analyze the complete chain (residues 1 to 263 ). Use the default values for the other options and click Submit to get a hydropathy plot. Save a GIF image of this plot. Now repeat the analysis using a window of 15 amino acids. Compare the results for the 7 -residue and 15 -residue window analyses. Which one gives you a better signal-to-noise ratio?
(c) Under what circumstances would it be important to use a narrower window?

Sana Riaz
Sana Riaz
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06:08

Problem 25

The receptor for the hormone epinephrine in animal cells is an integral membrane protein $\left(M_{\mathrm{r}} 64,000\right)$ that is believed to have seven membrane-spanning regions.
(a) Show that a protein of this size is capable of spanning the membrane seven times.
(b) Given the amino acid sequence of this protein, how would you predict which regions of the protein form the membrane-spanning helices?
(c) Go to the Protein Data Bank (www.pdb.org). Use the PDB identifier 1DEP to retrieve the data page for a portion of the $\beta$ -adrenergic receptor (one type of epinephrine receptor) isolated from turkey. Using JSmol to explore the structure, predict whether this portion of the receptor is located within the membrane or at the membrane surface. Explain your answer. Now use the Protein Feature View to see the hydrophobicity analysis of the sequence. Does this support your answer?
(d) Retrieve the data for a portion of another receptor, the acetylcholine receptor of neurons and myocytes, using the PDB identifier 1A11. As in (c), predict where this portion of the receptor is located and explain your answer. If you have not used the PDB, see Box $4-4$ for more information.

Sana Riaz
Sana Riaz
Numerade Educator
05:19

Problem 26

Figure $11-3$ shows the currently accepted fluid mosaic model of biological membrane structure. This model was presented in detail in a review article by $\mathrm{S}$. J. singer in $1971 .$ In the article, singer presented the three models of membrane structure that had been proposed up to that time:
A. The Davson-Danielli-Robertson Model. This was the most widely accepted model in $1971,$ when singer's review was published. In this model, the phospholipids are arranged as a bilayer. Proteins are found on both surfaces of the bilayer, attached to it by ionic interactions between the charged head groups of the phospholipids and charged groups of the proteins. Crucially, there is no protein in the interior of the bilayer.
B. The Benson Lipoprotein Subunit Model. Here the proteins are globular and the membrane is a protein-lipid mixture. The hydrophobic tails of the lipids are embedded in the hydrophobic parts of the proteins. The lipid head groups are exposed to the solvent. There is no lipid bilayer.
C. The Lipid-Globular Protein Mosaic Model. This is the model shown in Figure $11-3$ The lipids form a bilayer and proteins are embedded in it, some extending through the bilayer and others not. Proteins are anchored in the bilayer by interactions between the hydrophobic tails of the lipids and hydrophobic portions of the protein.
For the data given below, consider how each piece of information aligns with each of the three models of membrane structure. Which model(s) are supported, which are not supported, and what reservations do you have about the data or their interpretation? Explain your reasoning.
(a) When cells were fixed, stained with osmium tetroxide, and examined in the electron microscope, the membranes showed a "railroad track" appearance, with two dark-staining lines separated by a light space.
(b) The thickness of membranes in cells fixed and stained in the same way was found to be 5 to $9 \mathrm{nm}$. The thickness of a "naked" phospholipid bilayer, without proteins, was 4 to $4.5 \mathrm{nm} .$ The thickness of a single monolayer of proteins was about $1 \mathrm{nm}$
(c) singer wrote in his article: "The average amino acid composition of membrane proteins is not distinguishable from that of soluble proteins. In particular, a substantial fraction of the residues is hydrophobic" (p. 165 ).
(d) As described in Problems 1 and 2 of this chapter, researchers had extracted membranes from cells, extracted the lipids, and compared the area of the lipid monolayer with the area of the original cell membrane. The interpretation of the results was complicated by the issue illustrated in the graph of Problem 1: the area of the monolayer depended on how hard it was pushed. With very light pressures, the ratio of monolayer area to cell membrane area was about $2.0 .$ At higher pressures - thought to be more like those found in cells - the ratio was substantially lower.
(e) Circular dichroism spectroscopy uses changes in polarization of UV light to make inferences about protein secondary structure (see Fig. $4-10$ ). On average, this technique showed that membrane proteins have a large amount of $\alpha$ helix and little or no $\beta$ sheet. This finding was consistent with most membrane proteins having a globular structure.
(f) Phospholipase $C$ is an enzyme that removes the polar head group (including the phosphate) from phospholipids. In several studies, treatment of intact membranes with phospholipase $C$ removed about $70 \%$ of the head groups without disrupting the "railroad track" structure of the membrane.
(g) singer described in his article a study in which "a glycoprotein of molecular weight about 31,000 in human red blood cell membranes is cleaved by tryptic treatment of the membranes into soluble glycopeptides of about 10,000 molecular weight, while the remaining portions are quite hydrophobic" (p. 199 ). Trypsin treatment did not cause gross changes in the membranes, which remained intact. singer's review also included many more studies in this area. In the end, though, the data available in 1971 did not conclusively prove Model $C$ was correct. As more data have accumulated, this model of membrane structure has been accepted by the scientific community.

Sana Riaz
Sana Riaz
Numerade Educator