Section Test:Basic Metabolism

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This Section Test deals with the concepts of metabolism commonly found on the MCAT.

Section Test

1. The end product of glycolysis is

NADH
acetyl-CoA
lactate
pyruvate
→ The process of glycolysis converts one molecule glucose into two molecules of pyruvate. Under aerobic conditions, this pyruvate can be further processed in the kreb cycle, however under anaerobic conditions, bacteria undergo fermentation, while eukaryotes produce lactic acid. Both NADH and ATP are produced in the process of glycolysis, however the end product is considered the pyruvate.

2. All the following metabolites are produced during the citric acid cycle except

NADPH
CO2
FADH2
ATP
→While NADPH is similar to NADH, it is NOT produced in the general metabolic processes and is generally found in very specific and unique pathways (that are not covered on the MCAT).

3. What metabolite is produced from the oxidation of pyruvate and then fed into the citric acid cycle?

glucose
acetyl-CoA
O2
CO2
→ Acetyl-CoA is the bridge between glycolysis and the citric acid cycle. Pyruvate is converted by Pyruvate Dehydrogenase to acetyl-CoA and then combines with oxaloacitate to form citrate, the initial molecule in the Krebs cycle.

4. What role does O2 play in aerobic respiration?

it plays no role
it combines with acetyl-CoA at the start of the Krebs cycle
it combines with H2O to help drive the formation of ATP
it is the final electron acceptor at the end of the electron transport chain
→ O2 is produced as H20 is split, releasing the H+ ions and thus producing the electrochemical gradient that the electron transport chain then uses to drive ATP synthesis.

5. NADH is produced during

glycolysis
the oxidation of pyruvate
the Krebs cycle
all of the above
→ All three of these processes produce NADH. Glycolysis generates 2, conversion of pyruvate to acetyl-Coa produces 1, and the Kreb cycle produces 3.

6. What substance is regenerated by fermentation?

O2
NAD+
acetyl-CoA
ATP
→ The purpose of fermentation is to reproduce NAD+ so that more glycolysis can occur. Without this step, NADH would build up in the cell (since it cannot be converted back under anaerobic conditions) resulting in the loss of any available NAD+, consequently, glycolysis would stop and no new ATP would be able to be formed!

7. The oxidation of which substance yields the most energy?

proteins
glucose
fatty acids
alcohol
→ Fatty acids store the most energy per molecule and are also the easiest to pack into a small space, thus the body preferentially stores fat for long term energy storage.

8. Under which condition would you expect the mitochondrial proton gradient to be highest?

pyruvate present, oxygen present, ATP levels high
pyruvate present, oxygen present, ATP levels low
pyruvate absent, oxygen present, ATP levels high
pyruvate absent, oxygen absent, ATP levels low
→ The electron transport chain is most effecient at producing ATP when the Kreb cycle can produce lots of NADH (because pyruvate is present) which can then be used by the electron transport chain to produce a H+ electrochemical gradient. Production of ATP with this gradient then requires O2 to be utilized. Furthermore, the pathway is regulated such that it slows if ATP is already plentiful.

9. ATP formation by glycolysis

occurs through aerobic respiration
is an extremely efficient method of acquiring energy by the cell
requires oxygen
both a and c
→ Glycolysis is NOT an efficient method of acquiring energy, this is why the Kreb cycle is also used, if possible, to further process the products of glycolysis (pyruvate) into more energy.

10. Under normal conditions, as electrons flow down the electron transport chain of the mitochondria:

the pH of the matrix increases.
the electrons lose free energy.
an electrochemical gradient is formed.
all of the above.
→ During the electron transport chain, NADH and FADH2 are oxidized such that their electrons can be transported through energy complexes to drive H+ pumping. This pumping moves H+ ions from the matrix (making it increase in pH) and thus producing an electrochemical gradient. The gradient in turn is then used to drive ATP synthesis through the flow of H+ ions back to the matrix (much like a water pumping to the top of a hill, and then a waterwheel being used when the water flows back down).

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