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CHEM 731/Exam 1/Dr. N. Campbell
Each question is worth 8 points for a total of 104 points. Please write legibly and neatly. I
have to be able to read and understand your writing to give you a grade.
1. The composition and architecture of membranes
(a) List the major components of membranes. (b) When a preparation of mitochondrial membranes
was treated with high salt (0.5 M NaCl), it was observed that 40% of the total protein in this
preparation was solubilized. What kind of membrane proteins are in this soluble extract, and what
forces normally hold them to the membrane? (c) What kind of proteins constitutes the insoluble 60%,
and what forces hold these proteins in the membrane?
Ans: (a) phospholipids, sterols, proteins (integral and peripheral); (b) peripheral membrane proteins,
which are associated with the membrane through ionic and hydrogen bonds between their charged
and polar side chains and the charged head groups of phospholipids; (c) integral membrane proteins
(which are held to the membrane by hydrophobic interactions between their nonpolar side chains and
the hydrophobic fatty acyl chains of phospholipids), and those peripheral membrane proteins that are
held to the membrane by a covalent lipid anchor.
2. The composition and architecture of membranes
What are the principle features of the fluid mosaic model of membranes?
Ans: The principle features of the fluid mosaic model of membranes include: (1) a lipid bilayer in
which individual lipids are free to move laterally but not across the bilayer; (2) integral membrane
proteins, which penetrate or span the bilayer, associating with lipid acyl chains by hydrophobic
interactions and exhibiting lateral mobility; (3) peripheral membrane proteins, which associate
noncovalently with the lipid head groups and protruding domains of integral membrane proteins, and
which are sometimes tethered to the membrane by a covalent lipid anchor
3. The composition and architecture of membranes
(a) Explain why phosphoglycerides are capable of spontaneously assembling into the bilayer structure
found in biological membranes but triacylglycerols are not. (b) What are the forces that drive bilayer
formation?
Ans: (a): Triacylglycerols have three fatty acyl groups in ester linkage with glycerol; they are very
hydrophobic because the carboxyl groups, which are involved in the ester linkages, cannot ionize.
Phosphoglycerides have a polar region at their head group, where a phosphate in a phosphodiester
linkage bears a full negative charge. The head group itself (serine, ethanolamine, choline, etc.) may
also be charged and is polar in any case. Thus, the phospholipid is amphipathic, having both polar
and nonpolar regions, and it forms lipid bilayers spontaneously in water. (b) These lipid bilayers are
stabilized by the energy gained from burying hydrophobic groups out of contact with water. A
hydrophobic chain in water forces the formation of a cage of immobilized water molecules around it.
When several hydrophobic regions cluster in a bilayer, the surface area exposed to water decreases,
and the water molecules in the cage are released, accompanied by a gain in entropy that drives the
formation of the bilayer.
4. The composition and architecture of membranes
A protein is found to extend all the way through the membrane of a cell. Describe this protein in
terms of the location of particular types of amino acid side chains in its structure and its ability to
move within the membrane.
Ans: This integral membrane protein associates with the lipid bilayer through hydrophobic
interactions between domains containing many hydrophobic amino acids and the fatty acyl chains of
membrane lipids. Polar and charged residues are located on portions of the protein that protrude out
of either face of the membrane. The protein is free to diffuse laterally in the plane of the membrane,
but cannot move across the lipid bilayer.
5. Membrane dynamics
The bacterium E. coli can grow at 20 °C or at 40 °C. At which growth temperature would you expect
the membrane phospholipids to have a higher ratio of saturated to unsaturated fatty acids, and why?
Ans: At 40 °C, the membranes of E. coli will contain more saturated fatty acids than at 20 °C. The
cell regulates fatty acid composition to achieve the same fluidity in its membranes, regardless of
growth temperature. Saturated fatty acids counterbalance the fluidizing effect of high temperature
6. Membrane dynamics
A plant breeder has developed a new frost-resistant variety of tomato that contains higher levels of
unsaturated fatty acids in membrane lipids than those found in standard tomato varieties. However,
when temperatures climb above 95 °F, this frost-resistant variety dies, whereas the standard variety
continues to grow. Provide a likely explanation of the biochemical basis of increased tolerance to
cold and increased susceptibility to heat of this new tomato variety.
Ans: More unsaturated fatty acids will cause an increase in membrane fluidity because unsaturated
fatty acids contain “kinks” and cannot pack as tightly as saturated fatty acids. At cold temperatures,
the fluidity increase from the extra unsaturated fatty acids counterbalances the tendency of lipids to
solidify at low temperature. At high temperatures, the fluidizing effects of the extra unsaturated fatty
acids add to the fluidizing effect of higher temperature, and the membrane of the new plant loses its
integrity.
7. Solute transport across membranes
Distinguish between simple diffusion (SD), facilitated diffusion (FD), and active transport (AT) across
a membrane. (Items to include are energy dependence, carrier protein(s), and concentration gradient).
Ans:
Simple diffusion : The movement of lipophilic molecules across the lipid bilayer from high
concentration to low concentration. Simple diffusion does not establish a concentration gradient.
Because the lipophilic molecule can “dissolve” in the lipid bilayer and the movement is from high
concentration to low concentration, this type of movement does not require energy input or carrier
proteins.
Facilitated Diffusion: The movement of polar or charged molecules across the lipid bilyer from
high concentration to low concentration. Facilitated diffusion does not establish a concentration
gradient. Because this type of movement is from high concentration to low concentration, there is no
requirement for energy input. However, this type of movement involves the crossing of the lipid
bilayer by polar or charged molecules; therefore, specific channels (carrier proteins) are formed in the
membrane to assist the movement of the polar or charged molecules across the lipid bilayer.
more constricted (3 angstrom diameter). At this point, the K+ ions must give up their water
molecules and interact directly with carbonyl oxygen atoms lining the selectivity filter. Other ions
are too big to enter the constricted portion of the channel. Na+ ions can enter but the diameter of the
dehydrated Na+ ion is too small for optimal interaction with the carbonyl oxygen atoms. There are
two potassium binding sites in the constricted regions that are crucial for the rapid ion flow. A K+
ion gives up its water molecules to enter the constricted region and have a similar type of interaction
by binding the first potassium binding site. The K+ ion can jump to the second potassium binding
site because of the similar type of interaction. Another dehydrated K+ ion can now bind in the first
potassium ion binding site. Electrostatic repulsion between the two ions will destabilize the K+ ion in
the second binding site and “push” it into solution.
11. Bioenergetics and thermodynamics
Consider the reaction: A + B C + D. If the equilibrium constant for this reaction is a large number
(say, 10,000), what do we know about the standard free-energy change ( G' °) for the reaction?
(Hint: Describe the relationship between K eq
' and G' °).
Ans: G' ° = – RT ln K eq '. If K eq ' is a large (positive) number, the term – RT ln K eq ' (and
therefore G' °) has a relatively large, negative value.
12. Bioenergetics and thermodynamics
The standard free energy change ( G' °) for ATP hydrolysis is –30.5 kJ/mol. ATP, ADP, and Pi are
mixed together at initial concentrations of 1 M of each, and then left alone until the reaction: ADP +
Pi ATP has come to equilibrium. For each species (i.e., ATP, ADP, and Pi) indicate whether the
concentration will be equal to 1 M, less than 1 M, or greater than 1 M.
Ans: At equilibrium, ATP < 1 M; ADP > 1 M; Pi > 1 M.
13. Phosphoryl group transfers and ATP
In general, when ATP hydrolysis is coupled to an energy-requiring reaction, the actual reaction often
consists of the transfer of a phosphate group from ATP to another substrate, rather than an actual
hydrolysis of the ATP. Explain.
Ans: Hydrolysis of the ATP would result in the loss of most of the free energy as heat. In a transfer
reaction, the gamma (third) phosphate of ATP is transferred to the reaction substrate to produce a
high-energy phosphorylated intermediate, which can then form the product in an exergonic reaction.
