Form 4 · Worked answers

Cellular Respiration, worked answers

Fully worked answers for Cellular Respiration, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Writing the word equations for aerobic and anaerobic respiration, including the correct reactants and products for each pathway.
Comparing respiration in humans and yeast, including where each pathway occurs and what limits it.
Explaining an experiment on respiration or fermentation (Paper 3), including a suitable control and expected result.
Explaining how limewater or a thermometer is used to show that respiration is releasing carbon dioxide or heat.
Explaining why vigorous exercise leads to muscle fatigue and cramp in terms of lactic acid and oxygen debt.
Interpreting a graph or table of oxygen consumption, carbon dioxide production or heat released during germination or fermentation.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Write the word equation for aerobic respiration and state where in the cell most of the energy is released.

[3]

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The word equation is glucose + oxygen → carbon dioxide + water + energy. Glycolysis, the first stage, takes place in the cytoplasm, but most of the energy is released in the later aerobic stages that take place in the mitochondria. Oxygen must appear on the reactant side and both carbon dioxide and water must appear as products for the equation to be correct.

glucose + oxygencarbon dioxide + waterenergymitochondriacytoplasm

Explain why aerobic respiration releases more energy from one glucose molecule than anaerobic respiration does.

[4]

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Aerobic respiration uses oxygen to break glucose down completely into carbon dioxide and water, so almost all of the chemical energy stored in the glucose is released. Anaerobic respiration breaks glucose down only partly, into lactic acid in humans or into ethanol and carbon dioxide in yeast. These products still contain a large amount of unreleased chemical energy, so the energy yield is small. The difference is therefore due to glucose being fully broken down in aerobic respiration but only partly broken down without oxygen.

broken down completelyonly partly broken downunreleased chemical energyoxygenenergy yield

A runner's leg muscles begin to ache during a 400 m sprint. Explain, in terms of respiration, why this happens and why the runner keeps breathing heavily after crossing the finish line.

[5]

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During the sprint the muscles need energy faster than the oxygen supply can provide, so they respire anaerobically and break glucose down into lactic acid and a small amount of energy. The lactic acid builds up in the muscle tissue and causes the ache and fatigue. Because the body could not supply enough oxygen during the sprint, it has built up an oxygen debt. After the runner stops, breathing and heart rate stay high to take in the extra oxygen needed to break down the accumulated lactic acid, so the heavy breathing continues until the debt is repaid.

anaerobic respirationlactic acidoxygen debtfatiguerepaid after exercise

Compare the products of anaerobic respiration in human muscle and in yeast.

[4]

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In human muscle, glucose is broken down into lactic acid together with a small amount of energy, and no gas is produced. In yeast, glucose is broken down into ethanol and carbon dioxide together with a small amount of energy, so a gas is produced. Both processes take place without oxygen and both release only a small amount of energy because glucose is only partly broken down, but the end products are different: lactic acid in humans, and ethanol plus carbon dioxide in yeast.

lactic acidethanolcarbon dioxidewithout oxygensmall amount of energy

Describe how you would show that germinating seeds release carbon dioxide during respiration. Include a suitable control.

[6]

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Place living germinating seeds in a sealed container connected so that air can be drawn through it and then bubbled into a test tube of limewater. Over time the limewater turns cloudy, showing that carbon dioxide has been released by the seeds. Set up an identical control using seeds that have been boiled and then cooled; these seeds are dead and do not respire, so the limewater stays clear. Keeping the two set-ups at the same temperature and using the same volume of limewater makes the comparison fair, so the cloudiness in the living set-up can be attributed to respiration.

germinating seedslimewater turns cloudycarbon dioxideboiled (dead) seeds controlfair comparison

Explain why bread dough containing yeast rises when it is left in a warm place.

[4]

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The yeast respires anaerobically by fermentation, breaking down the sugar in the dough into ethanol and carbon dioxide. The carbon dioxide gas is trapped as bubbles within the stretchy dough, so the dough expands and rises. A warm place is used because the higher temperature increases the activity of the enzymes in the yeast, so fermentation and gas production are faster, up to the point where the temperature would become too high for the enzymes.

fermentationcarbon dioxidetrapped bubblesdough risesenzyme activity

State three uses in the body of the energy released by respiration.

[3]

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The energy released by respiration is used for muscle contraction to produce movement, for the active transport of ions and molecules across cell membranes, and for building large molecules such as proteins during growth and repair. It is also used to transmit nerve impulses and to generate heat that keeps body temperature stable. Any three of these uses, each stated clearly, would gain the marks.

muscle contractionactive transportbuilding proteinsnerve impulsesbody heat

Phrasing that earns marks

Energy and ATP: Respiration transfers energy stored in the chemical bonds of glucose into ATP, a small molecule that acts as the cell's immediate, usable form of energy. ATP is broken down wherever energy is needed in the cell, releasing that energy instantly, then rebuilt using more energy from respiration, so the cell is never short of an energy supply.
Aerobic respiration: Aerobic respiration completely breaks down glucose using oxygen, producing carbon dioxide, water and a large amount of energy: glucose + oxygen gives carbon dioxide + water + energy. It takes place mainly in the mitochondria and is far more efficient than any anaerobic pathway, which is why muscle and liver cells contain especially large numbers of mitochondria.
Anaerobic respiration in humans: When the oxygen supply cannot meet a muscle's demand during vigorous exercise, human cells respire anaerobically: glucose gives lactic acid + a small amount of energy. Lactic acid accumulates in the muscle, lowers its pH, and contributes to the fatigue and cramp felt during and shortly after hard exercise.
Fermentation in yeast: Yeast cells respire anaerobically by fermentation, breaking glucose down into ethanol and carbon dioxide while releasing a small amount of energy: glucose gives ethanol + carbon dioxide + energy. This reaction is exploited industrially, the carbon dioxide makes bread dough rise during baking, and the ethanol is the basis of beer and wine production.
Comparison: Aerobic respiration releases far more energy from each glucose molecule than either anaerobic pathway, because it breaks glucose down completely into carbon dioxide and water rather than only partially into lactic acid or ethanol, both of which still contain a large amount of unreleased chemical energy.
Oxygen debt: During vigorous exercise, lactic acid builds up faster than it can be removed, creating an oxygen debt. After exercise stops, breathing and heart rate stay raised so that extra oxygen can be taken in to break down the accumulated lactic acid, which is why a person keeps panting after a hard sprint has finished.
Where respiration happens: Every cell begins respiration the same way: glycolysis breaks glucose down in the cytoplasm, releasing a small amount of energy without needing oxygen. If oxygen is available, the products of glycolysis then pass into the mitochondria, where the aerobic stages take place and release most of the total energy obtained from the glucose molecule.
Uses of energy from respiration: The ATP produced by respiration powers almost every activity of a living organism, including muscle contraction for movement, active transport of substances across cell membranes, the biosynthesis of large molecules such as proteins during growth, transmission of nerve impulses, and the generation of heat that keeps body temperature stable in warm-blooded animals.

Frequently asked questions

What is the difference between respiration and breathing?

Breathing (ventilation) is the physical movement of air in and out of the lungs. Respiration is a chemical process inside every cell that releases energy from glucose. Breathing supplies the oxygen that aerobic respiration needs and removes the carbon dioxide it produces.

How does anaerobic respiration differ in humans and yeast?

In humans, glucose is broken down to lactic acid with a small energy release. In yeast, glucose is broken down to ethanol and carbon dioxide, also with a small energy release. Both happen without oxygen, but the products are different.

Why does aerobic respiration release more energy?

Aerobic respiration fully breaks glucose down to carbon dioxide and water using oxygen, releasing a large amount of energy. Anaerobic respiration only partly breaks glucose down, so much of the energy stays locked in the lactic acid or ethanol, giving a much smaller yield.