IGCSE Biology 0610: Comparing Aerobic and Anaerobic Respiration

A rowing team trains hard on a lake at dawn. Their packed lunch includes fresh bread rolls, made using yeast the day before.

Three different respiration reactions are linked to this training session: (1) aerobic respiration in a rower's leg muscles during a steady warm-up paddle, when oxygen reaches her muscle cells fast enough to meet all their energy needs; (2) anaerobic respiration in the same rower's leg muscles during an all-out sprint finish, when her muscles need energy faster than aerobic respiration alone can supply; (3) anaerobic respiration in the yeast used to make the bread rolls the day before.

(a) State the two reactants of reaction (1), the aerobic respiration during the warm-up paddle. [2]

(b) State the one carbon-containing product formed in reaction (2), the anaerobic respiration during the sprint finish. [1]

(d) Compare the amount of energy released per glucose molecule broken down in reactions (2) and (3) with the amount released in reaction (1). [2]

Part (a): The reactants of the aerobic reaction during the warm-up

During the steady warm-up paddle, oxygen reaches the rower’s muscle cells fast enough to meet all their energy needs, so reaction (1) is aerobic respiration. Aerobic respiration uses glucose and oxygen as its two reactants, breaking them down to release energy.

Part (b): The carbon-containing product of the sprint finish

During the all-out sprint, her muscles need energy faster than aerobic respiration alone can supply, so they also respire anaerobically. In human muscle, this anaerobic reaction produces only lactic acid. No carbon dioxide is formed, unlike in reaction (3).

Part (c): The products of anaerobic respiration in the yeast

The yeast used to make the bread rolls respires anaerobically, breaking glucose down without oxygen. In yeast, unlike in muscle, this produces alcohol (ethanol) and carbon dioxide.

Part (d): Comparing the energy released in each reaction

Both reaction (2), the anaerobic respiration in the sprinting muscles, and reaction (3), the anaerobic respiration in the yeast, release much less energy per glucose molecule than reaction (1), the aerobic respiration in the warmed-up muscles. This is because, without oxygen, glucose can only be partly broken down (into lactic acid in the muscles, or into alcohol and carbon dioxide in the yeast) rather than being completely broken down into carbon dioxide and water as it is during aerobic respiration.

Final answers

(a) Glucose and oxygen
(b) Lactic acid
(c) Alcohol (ethanol) and carbon dioxide
(d) Reactions (2) and (3) both release much less energy per glucose molecule than reaction (1), because glucose is only partly broken down without oxygen, rather than being completely broken down as in aerobic respiration.