MCAT BIO/BIOCHEM: Glycolysis and Krebs Cycle Flashcards

Question 1

What is Glycolysis?

Accepted Answer

The ten-step cytoplasmic pathway that partially oxidises one glucose into two pyruvate, netting two ATP and two NADH, and functioning with or without oxygen present.

Question 2

Define Energy-Investment Phase.

Accepted Answer

The first five steps of glycolysis, which spend two ATP to phosphorylate and destabilise glucose before any energy is harvested. Also called the preparatory phase.

Question 3

Define Energy-Payoff Phase.

Accepted Answer

The last five steps of glycolysis, where two three-carbon sugars are oxidised to pyruvate, producing four ATP and two NADH so the pathway nets a positive energy return.

Question 4

What is Hexokinase?

Accepted Answer

The low-Km first enzyme of glycolysis, product-inhibited by glucose-6-phosphate; its high-Km liver isozyme glucokinase engages only when blood glucose is abundant.

Question 5

Define Phosphofructokinase-1 (PFK-1).

Accepted Answer

The main regulatory enzyme of glycolysis, allosterically switched off by ATP and citrate and switched on by AMP and fructose-2,6-bisphosphate to match sugar breakdown to energy demand.

Question 6

What is Aldolase?

Accepted Answer

The glycolytic enzyme that cleaves six-carbon fructose-1,6-bisphosphate into two interconvertible three-carbon sugars: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

Question 7

Define Glyceraldehyde-3-Phosphate (G3P).

Accepted Answer

The three-carbon sugar that is the only glycolytic intermediate to continue into the payoff phase; dihydroxyacetone phosphate must be converted into it first.

Question 8

Define Dihydroxyacetone Phosphate (DHAP).

Accepted Answer

The three-carbon ketose produced alongside glyceraldehyde-3-phosphate by aldolase; it cannot proceed through glycolysis until triose phosphate isomerase rearranges it into glyceraldehyde-3-phosphate.

Question 9

Define Triose Phosphate Isomerase.

Accepted Answer

The isomerase that interconverts dihydroxyacetone phosphate and glyceraldehyde-3-phosphate so both halves of the cleaved sugar enter the payoff phase. It is a catalytically perfect, diffusion-limited enzyme.

Question 10

Define Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH).

Accepted Answer

The only glycolytic step that reduces NAD+ to NADH, oxidising glyceraldehyde-3-phosphate and attaching inorganic phosphate to form the high-energy compound 1,3-bisphosphoglycerate.

Question 11

Define Substrate-Level Phosphorylation.

Accepted Answer

The direct handoff of a phosphate group from a high-energy substrate onto ADP or GDP by an enzyme, forming ATP or GTP with no electron transport chain and no oxygen.

Question 12

Define Phosphoenolpyruvate (PEP).

Accepted Answer

The glycolytic intermediate holding the highest phosphate-transfer potential of any biological molecule; giving up its phosphate to ADP is what drives the final, irreversible ATP-forming step.

Question 13

Define Pyruvate Kinase.

Accepted Answer

The enzyme catalysing the last, irreversible step of glycolysis, transferring phosphoenolpyruvate's phosphate to ADP to yield pyruvate and ATP. It is activated by fructose-1,6-bisphosphate.

Question 14

What is Pyruvate?

Accepted Answer

The three-carbon end product of glycolysis that sits at a metabolic crossroads: it can be oxidised to acetyl-CoA, reduced to lactate, or carboxylated to oxaloacetate.

Question 15

Define Pyruvate Dehydrogenase Complex.

Accepted Answer

A large three-enzyme assembly in the mitochondrial matrix that irreversibly decarboxylates and oxidises pyruvate, joining the remaining two carbons to coenzyme A. It is inhibited by its products NADH and acetyl-CoA.

Question 16

Define Link Reaction.

Accepted Answer

The oxidative decarboxylation of pyruvate to acetyl-CoA that bridges glycolysis and the Krebs cycle, releasing one carbon dioxide and one NADH per pyruvate. Also called the transition reaction.

Question 17

What is Acetyl-CoA?

Accepted Answer

A two-carbon acetyl group bonded to coenzyme A through a high-energy thioester; it is the common entry fuel that carbohydrates, fatty acids, and some amino acids all converge on.

Question 18

Define Krebs Cycle.

Accepted Answer

The eight-step mitochondrial pathway that fully oxidises an acetyl group to two carbon dioxide molecules while harvesting reduced electron carriers. Also known as the citric acid or tricarboxylic acid cycle.

Question 19

Define Citrate Synthase.

Accepted Answer

The enzyme opening the Krebs cycle by condensing an acetyl group with oxaloacetate; its pace is throttled by ATP, NADH, and its own product citrate when energy is plentiful.

Question 20

What is Citrate?

Accepted Answer

The six-carbon tricarboxylic acid that gives the cycle its alternative name; when it accumulates and spills into the cytoplasm it signals abundance and inhibits phosphofructokinase-1.

Question 21

What is Aconitase?

Accepted Answer

The Krebs enzyme that isomerises citrate into isocitrate by removing then re-adding water, repositioning a hydroxyl group so the molecule can next be oxidised and decarboxylated.

Question 22

Define Isocitrate Dehydrogenase.

Accepted Answer

The Krebs enzyme catalysing the first oxidative decarboxylation, releasing carbon dioxide and reducing NAD+; it is allosterically stimulated by ADP and calcium and inhibited by ATP and NADH.

Question 23

What is Alpha-Ketoglutarate?

Accepted Answer

The five-carbon Krebs intermediate formed after the first carbon dioxide is lost; it also leaves the cycle to be transaminated into the amino acid glutamate.

Question 24

Define Alpha-Ketoglutarate Dehydrogenase.

Accepted Answer

The Krebs enzyme running the second oxidative decarboxylation, converting alpha-ketoglutarate into succinyl-CoA while releasing carbon dioxide and NADH. It resembles the pyruvate dehydrogenase complex mechanically.

Question 25

Define Succinyl-CoA Synthetase.

Accepted Answer

The only Krebs enzyme performing substrate-level phosphorylation, cleaving the thioester bond of succinyl-CoA to make succinate and capture the released energy as one GTP.

Question 26

Define Succinate Dehydrogenase.

Accepted Answer

The only Krebs enzyme embedded in the inner mitochondrial membrane, and the single enzyme shared with the electron transport chain, where it acts as Complex II and reduces FAD to FADH2.

Question 27

What is Oxaloacetate?

Accepted Answer

The four-carbon acceptor that begins and ends each Krebs turn; because it is regenerated rather than consumed, tiny amounts can support the oxidation of large amounts of acetyl-CoA.

Question 28

What is FADH2?

Accepted Answer

A reduced flavin electron carrier that holds electrons at a lower energy than NADH, so each molecule yields fewer ATP when it donates them to the electron transport chain.

Question 29

What is NADH?

Accepted Answer

The principal reduced electron carrier of respiration, formed when NAD+ accepts two electrons and a proton; it later delivers them to the electron transport chain to drive ATP synthesis.

Question 30

Define Lactate Fermentation.

Accepted Answer

The anaerobic reduction of pyruvate to lactate by lactate dehydrogenase, whose real purpose is to reoxidise NADH back to NAD+ so glycolysis can keep producing ATP without oxygen.

Question 31

Define Cori Cycle.

Accepted Answer

The interorgan loop in which lactate made by anaerobic muscle travels to the liver, is rebuilt into glucose by gluconeogenesis, and returns to the muscle to be used again.

Question 32

What is Gluconeogenesis?

Accepted Answer

The pathway that builds new glucose from non-carbohydrate precursors such as lactate, glycerol, and amino acids, largely reversing glycolysis but bypassing its three irreversible steps with distinct enzymes.

Question 33

Define Anaplerotic Reaction.

Accepted Answer

A reaction that refills a depleted Krebs cycle intermediate so oxidation can continue; the main example is pyruvate carboxylase adding carbon dioxide to pyruvate to make oxaloacetate.

Question 34

Define Amphibolic Pathway.

Accepted Answer

A pathway that serves both catabolism and anabolism at once; the Krebs cycle qualifies because it both oxidises fuel for energy and supplies carbon skeletons for biosynthesis.

Question 35

Define Oxidative Phosphorylation.

Accepted Answer

The oxygen-dependent process where electrons from NADH and FADH2 pass down the electron transport chain, pumping protons whose backflow through ATP synthase makes most of a cell's ATP.

Question 36

Define Warburg Effect.

Accepted Answer

The tendency of many cancer cells to rely on glycolysis and lactate production even when oxygen is available, favouring fast ATP and biosynthetic intermediates over maximal energy yield.