How do cells release the energy in food, and what changes when there is no oxygen?
Define respiration, write word equations for aerobic and anaerobic respiration, and compare the energy they release
A scaffolded answer to the N(A)-Level Biology outcome on respiration. What respiration is, the word equations for aerobic and anaerobic respiration in humans and yeast, and how much energy each releases.
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What this dot point is asking
This outcome wants a clear definition of respiration and the word equations for both aerobic and anaerobic respiration. You should be able to compare them: which uses oxygen, what each produces, and which releases more energy. You should know the human examples (muscles in a sprint) and the yeast example (used in baking and brewing). The marks reward correct equations and a clear comparison of the energy released.
The answer
What respiration is
Respiration is the process in which cells break down glucose to release energy. It happens in every living cell, all the time. The energy released is used for everything the body does: moving, growing, keeping warm, and building new substances. Do not confuse respiration with breathing: breathing moves air in and out of the lungs, while respiration is the chemical release of energy inside cells.
Aerobic respiration
Aerobic respiration uses oxygen. It breaks glucose down completely, releasing a large amount of energy. The word equation is:
glucose + oxygen gives carbon dioxide + water (+ energy)
Because the glucose is broken down fully, aerobic respiration releases a lot of energy from each glucose molecule. This is the main way cells get their energy when oxygen is available.
Anaerobic respiration
Anaerobic respiration releases energy from glucose without using oxygen. It only breaks the glucose down partly, so it releases much less energy. There are two versions to know:
- In human muscle: glucose gives lactic acid (+ a little energy). This happens during hard exercise when the blood cannot supply oxygen fast enough.
- In yeast (fermentation): glucose gives alcohol + carbon dioxide (+ a little energy). This is used in baking (the carbon dioxide makes bread rise) and brewing (the alcohol).
Comparing the energy released
Aerobic respiration releases far more energy than anaerobic respiration from the same amount of glucose, because aerobic respiration breaks the glucose down completely while anaerobic leaves much of the energy still locked in the products (lactic acid or alcohol).
Examples in context
Example 1. Why bread rises and beer has bubbles. Yeast respires anaerobically on sugar, producing carbon dioxide and alcohol. In bread dough the carbon dioxide gas makes the dough rise; in brewing the alcohol is the point and the carbon dioxide gives fizz. This is anaerobic respiration in yeast put to use.
Example 2. Why your muscles ache after a sprint. During a hard sprint the muscles cannot get oxygen fast enough, so they respire anaerobically and make lactic acid. The lactic acid builds up and makes the muscles ache and tire. Afterwards you keep breathing hard to get the extra oxygen needed to clear it.
Try this
Q1. Write the word equation for aerobic respiration. [2 marks]
- Cue. glucose + oxygen gives carbon dioxide + water (+ energy).
Q2. State the product of anaerobic respiration in human muscle. [1 mark]
- Cue. Lactic acid.
Q3. Explain why aerobic respiration releases more energy than anaerobic respiration. [2 marks]
- Cue. Aerobic respiration breaks glucose down completely using oxygen, while anaerobic breaks it down only partly, leaving much of the energy in the products.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksWrite the word equation for aerobic respiration and explain why aerobic respiration releases more energy than anaerobic respiration.Show worked answer →
Word equation for aerobic respiration:
glucose + oxygen gives carbon dioxide + water (+ energy)
Aerobic respiration releases more energy because the glucose is broken down completely, all the way to carbon dioxide and water, using oxygen. Anaerobic respiration does not use oxygen and only breaks the glucose down partly, so much of the energy is left locked in the products (lactic acid in humans, or alcohol in yeast).
What markers reward: the correct word equation with energy released, and the explanation that aerobic respiration breaks glucose down fully using oxygen while anaerobic does not, so aerobic gives more energy. Writing energy as a reactant is a common error; it is released, so it goes on the product side.
Original4 marksDuring a hard sprint, a runner's leg muscles start to respire anaerobically. State the word equation for anaerobic respiration in human muscle and explain why this happens during a sprint.Show worked answer →
Word equation for anaerobic respiration in human muscle:
glucose gives lactic acid (+ a little energy)
It happens during a sprint because the muscles are working so hard that the blood cannot deliver oxygen fast enough. With not enough oxygen for aerobic respiration, the muscles switch to anaerobic respiration to keep releasing some energy, even though it produces lactic acid, which builds up and causes the muscles to ache and tire.
What markers reward: the correct equation (glucose to lactic acid), and the reason that oxygen cannot be supplied fast enough during hard exercise, so the muscle respires anaerobically. Mentioning lactic acid build-up causing fatigue is a good extra.
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