Question 1

Which bond in a phospholipid containing a phosphatidylethanolamine head group, a linoleic acid, and a palmitoleic acid tail is cleaved by phospholipase C?

Accepted Answer

The bond between the phosphate group and the head group

Question 2

Cold-water fish are highly enriched in phospholipids containing unsaturated fatty acids such as linoleic and palmitoleic acids. Why is this adaptation beneficial?

Accepted Answer

It increases membrane fluidity, allowing proper function at low temperatures.

Question 3

Which type of lipid diffusion is more favored in biological membranes, and why?

Accepted Answer

Lateral diffusion, because it does not require crossing the hydrophobic core.

Question 4

Why is the hydrolysis of ATP to ADP and $$\mathrm{P_i}$$ commonly coupled with enzyme-catalyzed reactions in metabolism?

Accepted Answer

Because ATP hydrolysis provides a favorable free energy change to drive otherwise unfavorable reactions.

Question 5

Secondary active transport is used to import glucose into intestinal cells against its concentration gradient. Which statement best describes this process?

Accepted Answer

Glucose is transported by a symporter that uses the energy from Na+ moving down its gradient, which is established by the Na+/K+ ATPase.

Question 6

Caffeine acts as a stimulant primarily by which physiological mechanism?

Accepted Answer

Blocking adenosine receptors, preventing drowsiness.

Question 7

Which regulatory mechanism ensures glycolysis and gluconeogenesis do not occur simultaneously in a cell?

Accepted Answer

Reciprocal regulation of phosphofructokinase-1 (PFK-1) and fructose-1,6-bisphosphatase by fructose-2,6-bisphosphate.

Question 8

A toxin from Vibrio cholerae catalyzes the covalent attachment of an ADP-ribose group to the α-subunit of a G protein. What is the effect on signal transduction?

Accepted Answer

The G protein becomes permanently active, leading to continuous signaling.

Question 9

Insulin stimulates dephosphorylation of PFK-2 in addition to activating PKB and PKC. Why does this occur?

Accepted Answer

Dephosphorylation of PFK-2 increases fructose-2,6-bisphosphate, activating glycolysis.

Question 10

Is cellobiose a reducing sugar? Briefly explain.

Accepted Answer

Yes, because one anomeric carbon is free and can be oxidized.

Question 11

Is cellulose a useful source of stored energy in humans? Why or why not?

Accepted Answer

No, because humans lack the enzyme to hydrolyze β-1,4-glycosidic bonds.

Question 12

Which enzyme catalyzes the committed step of glycolysis, and what is the reaction?

Accepted Answer

Phosphofructokinase-1: $$	ext{Fructose-6-phosphate} + 	ext{ATP} ightarrow 	ext{Fructose-1,6-bisphosphate} + 	ext{ADP}$$

Question 13

Why is it necessary to convert pyruvate to lactate in muscle under anaerobic conditions?

Accepted Answer

To regenerate NAD+ for glycolysis to continue.

Question 14

What are the main products of the oxidative stage of the pentose phosphate pathway, and why is this pathway important?

Accepted Answer

NADPH and ribose-5-phosphate; important for fatty acid synthesis and nucleotide biosynthesis.

Question 15

What is the functional role of the E3 component in the pyruvate dehydrogenase complex?

Accepted Answer

E3 regenerates the oxidized form of lipoamide by transferring electrons to NAD+ to form NADH.

Question 16

Which substrate and product are involved in the reaction catalyzed by complex II in the TCA cycle, and what cofactor is required?

Accepted Answer

Succinate is converted to fumarate, requiring FAD as a cofactor: $$	ext{Succinate} + 	ext{FAD} ightarrow 	ext{Fumarate} + 	ext{FADH}_2$$

Question 17

Inhibition of complex I in the electron transport chain affects which part(s) of the TCA cycle, and why?

Accepted Answer

It inhibits reactions that produce NADH, because NAD+ is not regenerated.

Question 18

Myxothiazol increases the QH2/Q ratio in respiring mitochondria. Where does it inhibit electron transfer in the electron transport chain?

Accepted Answer

At complex III, preventing electron transfer from QH2 to cytochrome c.

Question 19

Calculate the maximum number of ATP and ATP-equivalent molecules generated when one molecule of phosphoenolpyruvate (PEP) is metabolized under aerobic conditions in a human muscle cell. Show your work.

Accepted Answer

5 ATP: 1 from substrate-level phosphorylation (PEP to pyruvate), 1 NADH from pyruvate dehydrogenase (3 ATP), and 1 GTP from TCA cycle (1 ATP equivalent). $$1 + 3 + 1 = 5$$

Question 20

Calculate the maximum net gain of ATP from complete aerobic degradation of one molecule of myristic acid (C14:0). Show your work.

Accepted Answer

106 ATP: 7 cycles of β-oxidation yield 7 NADH, 7 FADH2, 8 acetyl-CoA; each acetyl-CoA yields 10 ATP in TCA cycle. $$7 	imes (2.5 + 1.5) + 8 	imes 10 = 106$$

Question 21

Why does supplementing a small amount of carbohydrate in a fat-only diet increase fatty acid metabolism?

Accepted Answer

Carbohydrate provides oxaloacetate for the TCA cycle, allowing acetyl-CoA from fatty acid oxidation to be metabolized.

Question 22

Given the concentration of Na+ inside and outside a cell are 12 mM and 145 mM, respectively, and the membrane potential is 60 mV (inside negative), calculate the free energy ($$\Delta G) of $$moving 3 moles of Na+ at 37°C. Use the equation $$\Delta G = nRT \ln \left( \frac{[	ext{Na}^+]_{	ext{out}}}{[	ext{Na}^+]_{	ext{in}}} ight) + nF\Delta \Psi$$ where $$n=3$$, $$R=8.314\ 	ext{J/mol}\cdot	ext{K}$$, $$T=310\ 	ext{K}$$, $$F=96,485\ 	ext{C/mol}$$, $$\Delta \Psi=0.060\ 	ext{V}.$$

Accepted Answer

$$\Delta G = 3 	imes 8.314 	imes 310 \ln \left( \frac{145}{12} ight) + 3 	imes 96,485 	imes 0.060 = 7,729\ 	ext{J}$$

Biochemistry Exam III Study Guidance

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