Questions on glycolysis, ATP and the link reaction

Question 1

Describe what happens during glycolysis, using the terms phosphorylation, lysis and oxidation, and give the net yield per molecule of glucose.

Accepted Answer

B1 phosphorylation: two molecules of ATP are hydrolysed and their phosphate groups added to glucose, giving hexose bisphosphate, which is less stable and more reactive B1 lysis: the six-carbon hexose bisphosphate splits into two three-carbon molecules of triose phosphate, so everything after this point happens twice per glucose B1 oxidation: hydrogen is removed from each triose phosphate and taken up by NAD, giving two reduced NAD per glucose, and pyruvate is what remains B1 phosphate groups are transferred directly from the intermediates onto ADP by substrate-level phosphorylation, making four ATP per glucose, so the net yield is 2 ATP, 2 reduced NAD and 2 pyruvate

Question 2

Eight molecules of glucose pass through glycolysis in a cell. Calculate the number of molecules of ATP produced, the number used, the net gain in ATP, and the number of molecules of reduced NAD formed.

Accepted Answer

M1 per molecule of glucose, glycolysis produces 4 ATP, uses 2 ATP and forms 2 reduced NAD A1 4 × 8 = 32 molecules of ATP produced A1 2 × 8 = 16 molecules of ATP used, so the net gain is 32 − 16 = 16 ATP A1 2 × 8 = 16 molecules of reduced NAD

Question 3

Describe what happens to a molecule of pyruvate during the link reaction, and give the products of the link reaction per molecule of glucose.

Accepted Answer

B1 pyruvate is decarboxylated: one carbon is removed and released as carbon dioxide, which is the first carbon dioxide respiration produces B1 it is dehydrogenated: hydrogen is removed and taken up by NAD, giving one reduced NAD B1 the two-carbon acetyl group left behind joins coenzyme A to form acetyl coenzyme A B1 the reaction runs twice per glucose, so the products are 2 CO₂, 2 reduced NAD and 2 acetyl CoA, with no ATP made

Question 4

A cell respires 6 molecules of glucose fully through glycolysis and the link reaction. Calculate the total number of molecules of reduced NAD produced by these two stages together, and calculate the total number of carbon dioxide molecules released.

Accepted Answer

M1 glycolysis produces 2 reduced NAD per glucose and the link reaction produces 2 more, one per pyruvate with two pyruvate per glucose, so 4 reduced NAD are produced per glucose by the two stages together A1 for 6 glucose: 4 × 6 = 24 reduced NAD M1 glycolysis releases no carbon dioxide, and the link reaction releases 2 CO₂ per glucose, one per pyruvate decarboxylated A1 for 6 glucose: 2 × 6 = 12 CO₂ molecules

Question 5

Explain why ATP is a suitable immediate source of energy for a cell, when the complete oxidation of glucose releases roughly 2880 kJ per mole.

Accepted Answer

B1 hydrolysis of the terminal phosphate of ATP, catalysed by ATP hydrolase, releases about 30.5 kJ per mole in a single step, so the energy is available at once with no chain of intermediates to wait for B1 that quantity is small enough to be useful for a single reaction, so little is wasted as heat, whereas releasing the 2880 kJ per mole held in glucose in one go would damage the cell B1 the reaction is easily reversed, because ATP synthase joins ADP and inorganic phosphate back together, so the same molecules are recycled about a thousand times a day rather than being stockpiled

Question 6

A mature red blood cell has neither a nucleus nor any mitochondria. Suggest how such a cell still makes ATP, and suggest what limits the amount of ATP it can obtain from each molecule of glucose.

Accepted Answer

B1 glycolysis takes place in the cytoplasm and needs neither oxygen nor a mitochondrion, so the cell can still carry it out and make ATP by substrate-level phosphorylation B1 the cell cannot carry out the link reaction, the Krebs cycle or oxidative phosphorylation, because those need the matrix and the inner membrane B1 so it gains only the net 2 ATP from glycolysis, under three per cent of the energy held in a molecule of glucose, and it must respire a great deal of glucose to meet its demand

Question 7

An adult holds roughly 50 g of ATP at any instant but turns over close to their own body mass of ATP in a day. Explain how these two facts can both be true, and explain what would happen to a cell if it stopped remaking ATP.

Accepted Answer

B1 the small standing pool of ATP is continuously hydrolysed to ADP and phosphate as energy is used, and continuously resynthesised from ADP and phosphate using energy released by respiration, so the same roughly 50 g of molecules cycles round many times a day rather than being a single-use supply B1 ATP is not stored in bulk the way glycogen or triglyceride is, so a cell has no large reserve to fall back on B1 if a cell stopped remaking ATP its entire pool would be used up within about ninety seconds, after which the energy-requiring processes ATP powers, such as active transport and muscle contraction, could not continue

Question 8

Compare glycolysis with the link reaction, referring to where each takes place in the cell and whether each needs oxygen.

Accepted Answer

B1 glycolysis takes place in the cytoplasm, whereas the link reaction takes place in the mitochondrial matrix, so pyruvate has to be transported across both mitochondrial membranes between the two stages B1 neither glycolysis nor the link reaction uses oxygen directly, since oxygen is used only at the fourth stage, oxidative phosphorylation B1 glycolysis produces a net gain of ATP by substrate-level phosphorylation, whereas the link reaction produces no ATP at all, only reduced NAD and carbon dioxide

Question 9

Before glucose can be broken down in glycolysis, it is first phosphorylated using two molecules of ATP. Explain why this step is necessary, given that it appears to use energy rather than release it.

Accepted Answer

B1 glucose is a relatively stable molecule, and adding phosphate groups to it, making hexose bisphosphate, makes it less stable and more reactive so it can be broken down in the reactions that follow B1 the phosphate groups added also carry a negative charge, which prevents the molecule leaving the cell through the surface membrane, trapping it inside the cell where it can be used B1 although this step spends 2 ATP, that spending is more than repaid later in the pathway, since 4 ATP are made by substrate-level phosphorylation from the two triose phosphate molecules, for a net gain of 2 ATP overall

Question 10

The four stages of aerobic respiration are glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. Identify the two stages that take place in the mitochondrial matrix, and identify the one stage that takes place on the inner mitochondrial membrane rather than in the matrix, and explain why it takes place there.

Accepted Answer

B1 the link reaction and the Krebs cycle both take place in the mitochondrial matrix B1 oxidative phosphorylation takes place on the inner mitochondrial membrane rather than in the matrix B1 because it is the folded inner membrane, not the matrix, that carries the electron carriers and ATP synthase molecules this stage needs

Question 11

A muscle cell respiring glucose as its only substrate is treated with a drug that specifically and completely blocks the carrier protein that moves pyruvate across the inner mitochondrial membrane, but has no other effect. Suggest what would happen to the rate of glycolysis in this cell, suggest what would happen to the rate of the link reaction, and suggest what would happen to the cell's release of carbon dioxide.

Accepted Answer

B1 glycolysis would be largely unaffected at first, since it does not require pyruvate to enter the mitochondrion and needs no oxygen or mitochondrial involvement itself, so it could continue producing ATP and reduced NAD in the cytoplasm B1 the link reaction would stop, or fall to almost nothing, because pyruvate could no longer reach the mitochondrial matrix where its enzymes are located B1 carbon dioxide release would fall to almost nothing, because glycolysis releases none: the carbon dioxide made from pyruvate, beginning with the one molecule removed from each pyruvate by decarboxylation in the link reaction, is all made in the matrix, which pyruvate can no longer reach

Question 12

Explain, using the oxidation step of glycolysis as your example, what is meant by describing a reaction as an oxidation in the context of respiration.

Accepted Answer

B1 in the context of respiration, oxidation is most easily understood as the loss of hydrogen from a molecule, rather than as a reaction with oxygen B1 in the oxidation step of glycolysis, each triose phosphate loses hydrogen and is oxidised, and that hydrogen is taken up by NAD B1 the substrate, triose phosphate, that loses the hydrogen has been oxidised, while the NAD that gains it has been reduced to reduced NAD, since the two always happen together as a redox reaction

Question 13

Compare releasing energy by hydrolysing ATP with releasing energy by completely oxidising a molecule of glucose in one reaction, referring to the quantity of energy released and how useful this is to a cell.

Accepted Answer

B1 hydrolysis of the terminal phosphate of ATP releases a small quantity of energy, about 30.5 kJ per mole, in a single, easily controlled step B1 complete oxidation of a mole of glucose releases roughly 2880 kJ, nearly a hundred times as much, and releasing this all at once in a single reaction would be far more than a cell's reactions could couple to, so most of it would be lost as heat and could damage the cell B1 respiration therefore oxidises glucose in many small steps, capturing the energy in a series of manageable packages, mostly as ATP, each similar in size to the energy a single ATP hydrolysis releases, rather than releasing the whole 2880 kJ in one uncontrolled step

Question 14

State where in a cell glycolysis takes place, and state where the link reaction takes place.

Accepted Answer

B1 glycolysis takes place in the cytoplasm of the cell B1 the link reaction takes place in the matrix of the mitochondrion

Question 15

State the three components of an ATP molecule, and state which bond is broken when ATP is hydrolysed to release usable energy.

Accepted Answer

B1 adenine, a base; ribose, a five-carbon sugar; and a chain of three phosphate groups B1 the bond to the terminal, outermost phosphate group is broken (hydrolysed), giving ADP and inorganic phosphate