Energy and Respiration: Biology 9700 (Cambridge International AS & A Level)

All living cells need a continuous supply of energy, and ATP (adenosine triphosphate) acts as the universal energy currency that transfers this energy to wherever it is needed in the cell.

Which statement best explains why ATP, rather than glucose, is used directly to supply energy for most cellular reactions?

Aerobic respiration in a eukaryotic cell is often described as occurring in four stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation.

Which option correctly identifies where each of these four stages takes place?

Aerobic respiration begins with the breakdown of glucose in the cytoplasm and continues inside the mitochondrion, releasing carbon dioxide and reducing coenzymes before a final stage generates most of the cell's ATP.

(a) State where in the cell glycolysis takes place, and describe the products formed from one molecule of glucose during glycolysis, including the net change in ATP and the reduced coenzyme produced. [4] (b) Pyruvate produced by glycolysis is transported into the mitochondrion to take part in the link reaction. Describe what happens to a molecule of pyruvate during the link reaction, name the products formed, and state where in the mitochondrion this reaction takes place. [3] (c) Describe the Krebs cycle, including where it takes place in the cell, how the 2-carbon fragment from acetyl coenzyme A is incorporated into the cycle, and the total products (carbon dioxide, reduced coenzymes and ATP) formed from the two molecules of acetyl coenzyme A derived from one molecule of glucose. [4]

Oxidative phosphorylation takes place on the inner mitochondrial membrane and, through chemiosmosis, generates far more ATP than the earlier stages of respiration combined.

(a) Describe how the reduced NAD and reduced FAD formed earlier in respiration are used during oxidative phosphorylation to build up a proton gradient across the inner mitochondrial membrane, and explain how this gradient is then used to synthesise ATP. [5] (b) State the role of oxygen in oxidative phosphorylation, and explain what would happen to the electron transport chain and to ATP production if no oxygen were available. [3] (c) Explain how the structure of a mitochondrion is adapted to its function in aerobic respiration. [3]

When oxygen cannot be supplied quickly enough to sustain aerobic respiration, cells can respire anaerobically instead, and the type of respiratory substrate used also affects the gases exchanged during respiration.

(a) Compare anaerobic respiration in mammalian muscle cells with anaerobic respiration in yeast cells, naming the products formed from pyruvate in each case, and explaining why both pathways are needed to allow glycolysis to continue. [4] (b) The respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in during respiration. State the equation for the aerobic respiration of glucose, use it to give the RQ for carbohydrate, and explain why the RQ for a lipid respiratory substrate is lower than this. [4]

Living cells constantly need a supply of energy from the hydrolysis of ATP to drive processes that would not otherwise happen, while some other processes in a cell take place without any direct input of energy from ATP.

Which of the following processes in a cell requires energy directly from the hydrolysis of ATP?

Glycolysis can be divided into an earlier phosphorylation phase, in which ATP is used, and a later oxidation phase, in which ATP is generated by substrate-level phosphorylation.

(a) Describe what happens to a glucose molecule during the phosphorylation phase of glycolysis, naming the products formed, and state why two molecules of ATP are used at this stage. [3] (b) Describe what happens during the oxidation phase of glycolysis, and use your answer to explain how glycolysis has a net gain of only 2 ATP molecules per glucose molecule despite 4 molecules of ATP being generated by substrate-level phosphorylation. [3]

Reduced NAD and reduced FAD both deliver hydrogen (as protons and electrons) to the electron transport chain during oxidative phosphorylation, but oxidation of one molecule of reduced FAD ultimately results in less ATP being synthesised than oxidation of one molecule of reduced NAD.

Which statement correctly explains this difference?

During vigorous exercise, oxygen cannot always be delivered to muscle cells quickly enough to sustain fully aerobic respiration, so some ATP is generated anaerobically. After exercise has stopped, a person continues to breathe more deeply and rapidly than at rest for some time.

(a) Explain, in terms of the availability of NAD, why muscle cells respire anaerobically when oxygen delivery cannot keep pace with demand, and state what happens to the pyruvate produced by glycolysis under these conditions. [3] (b) Explain, in terms of the fate of the substance formed in (a), why extra oxygen (an "oxygen debt") is needed for some time after exercise has finished, even though the exercise itself has ended. [3]

A respirometer can be used to measure the volume of oxygen taken up by living organisms and, together with a suitable control, to calculate a respiratory quotient (RQ).

(a) Describe how a respirometer, set up with germinating seeds and soda lime, can be used to measure the volume of oxygen taken up by the seeds over a given time, and explain why a second, identical respirometer set up without any living organism is also needed. [3] (b) In one investigation, a respirometer containing soda lime and germinating seeds showed a decrease in gas volume of 0.36 cm³ over 10 minutes. An identical respirometer, set up with the same seeds but without soda lime (so any carbon dioxide produced remained in the system), showed a decrease in gas volume of 0.09 cm³ over the same 10 minutes. Calculate the respiratory quotient (RQ) of the seeds, showing your working. [3] (c) The seeds used were oil-storing seeds. Using your answer to (b), suggest what this RQ value indicates about the main respiratory substrate being used by the germinating seeds. [2]