IGCSE Biology 0610: Anaerobic Respiration Word Equations in Yeast and Muscle

A home brewer adds a sachet of yeast to a sealed glass jar of sugary apple juice to make cider. The jar has an airlock fitted to its lid, which lets gas escape but stops air getting back in. Over the next few days, the brewer sees a steady stream of bubbles passing through the airlock, and the liquid slowly develops an alcoholic smell.

At the same time, a warehouse worker is rushing to load heavy boxes onto a delivery van before it leaves. Partway through, his leg muscles begin to ache, even though he is already breathing as fast and as deeply as he can.

(a) State the word equation for the reaction taking place in the yeast inside the sealed jar of cider. [2]

(b) State the word equation for the reaction taking place in the warehouse worker's aching leg muscles. [2]

(c) State one substance that is needed when the worker's muscles respire aerobically at rest, but that is NOT needed for the reactions described in (a) and (b) above. [1]

(d) State how the amount of energy released per glucose molecule broken down in the sealed jar of cider compares with the amount released per glucose molecule during aerobic respiration. [1]

Part (a): Anaerobic respiration in the yeast

The jar is sealed and fitted with an airlock that stops air (and so oxygen) getting back in, so the yeast must be respiring anaerobically. Anaerobic respiration in yeast breaks glucose down without oxygen, producing alcohol and carbon dioxide. The carbon dioxide is the gas that bubbles out through the airlock, and the alcohol is what gives the liquid its alcoholic smell.

glucose → alcohol + carbon dioxide

Part (b): Anaerobic respiration in the leg muscles

The warehouse worker’s muscles need energy faster than his aerobic respiration alone can supply it, even though he is already breathing as fast and as deeply as he can. His muscle cells respire anaerobically as well, breaking glucose down without oxygen. In muscles, unlike in yeast, this produces only lactic acid. No carbon dioxide is formed.

glucose → lactic acid

Part (c): The substance that distinguishes aerobic from anaerobic respiration

When the worker’s muscles are resting and respiring aerobically, oxygen is used to break glucose down completely. Neither the yeast’s anaerobic reaction in (a) nor the muscle’s anaerobic reaction in (b) needs oxygen. That is precisely what makes both of them anaerobic. So oxygen is the substance needed for aerobic respiration but not for either anaerobic reaction.

Part (d): Comparing the energy released

Anaerobic respiration does not fully break glucose down, so it releases much less energy per glucose molecule than aerobic respiration does. This is true both for the yeast in the sealed cider jar and for the worker’s tired muscles. In each case, the anaerobic reaction is a less efficient way of releasing energy than aerobic respiration.

Final answers

(a) glucose → alcohol + carbon dioxide
(b) glucose → lactic acid
(c) oxygen
(d) Less energy is released per glucose molecule during the anaerobic respiration in the cider jar than during aerobic respiration.