How do cells release energy from glucose, and what changes when there is not enough oxygen?
Aerobic and anaerobic respiration as the release of energy from glucose, their word equations and products, the uses of the energy released, and oxygen debt after exercise.
A focused answer to the WJEC GCSE Biology section 1.2 topic on respiration, covering aerobic and anaerobic respiration as the release of energy from glucose, their word equations and products, the uses cells make of the energy, and oxygen debt and lactic acid after vigorous exercise.
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What this dot point is asking
WJEC wants you to define respiration, give the word equations for aerobic and anaerobic respiration, state where energy is used, and explain lactic acid and oxygen debt after exercise.
What respiration is
Respiration happens in all cells, all the time, because cells always need energy. Most respiration is aerobic (using oxygen), but cells can also respire anaerobically (without oxygen) for short periods.
Aerobic respiration
Aerobic respiration uses oxygen and takes place mainly in the mitochondria. Its word equation is:
and the balanced symbol equation is:
Aerobic respiration releases a large amount of energy from each glucose molecule, which is why it is the main way cells get energy.
Anaerobic respiration
When oxygen runs short, for example during hard exercise, muscle cells respire anaerobically. In animals (and human muscle), the word equation is:
Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, because the glucose is not fully broken down. It also produces lactic acid, which builds up in the muscles.
What the energy is used for
The energy released by respiration is used throughout the body, including for:
- muscle contraction (movement),
- active transport of substances across membranes,
- growth and cell division,
- building large molecules from smaller ones (such as proteins from amino acids),
- keeping the body at a steady temperature in mammals and birds.
Oxygen debt
During vigorous exercise, the muscles cannot get enough oxygen, so they respire anaerobically and lactic acid builds up. After the exercise stops, the body needs extra oxygen to break down this lactic acid; this extra oxygen is called the oxygen debt.
To "repay" the oxygen debt, breathing stays deep and fast and the heart keeps beating quickly for several minutes, delivering the extra oxygen to the muscles until all the lactic acid has been removed.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksCompare aerobic and anaerobic respiration in muscle cells. Give the word equation for each.Show worked answer →
A 4-mark compare question. Award marks for each equation and for the comparison.
Aerobic respiration: glucose + oxygen produces carbon dioxide + water (and releases energy). It uses oxygen and releases a large amount of energy.
Anaerobic respiration in muscle: glucose produces lactic acid (and releases energy). It happens without oxygen and releases much less energy than aerobic respiration.
Markers reward: the two correct word equations; that aerobic uses oxygen and anaerobic does not; that aerobic releases more energy. Writing oxygen on the wrong side, or forgetting that anaerobic produces lactic acid, are common errors.
WJEC style3 marksExplain why a person keeps breathing deeply and quickly for several minutes after finishing hard exercise.Show worked answer →
A 3-mark question on oxygen debt.
During hard exercise the muscles respire anaerobically, producing lactic acid, which builds up. After exercise, extra oxygen is needed to break down this lactic acid. This extra oxygen is the oxygen debt, so the person keeps breathing deeply and quickly (and the heart beats fast) to take in and deliver the extra oxygen until the lactic acid is removed.
Markers reward: anaerobic respiration produced lactic acid; extra oxygen is needed to remove it (oxygen debt); fast deep breathing supplies that oxygen. Saying simply "to get more oxygen" without the lactic acid link does not gain full marks.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)