How do cells release energy from glucose, and how does breathing supply the oxygen they need?
Compare aerobic and anaerobic respiration, describe the role of breathing and gas exchange in the lungs, and relate respiration to energy release
A focused answer to the O-Level Combined Science outcome on respiration. Aerobic and anaerobic respiration and their equations, energy release, breathing and gas exchange in the lungs, and the adaptations of the alveoli.
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What this dot point is asking
SEAB wants you to compare aerobic and anaerobic respiration (conditions, products and energy yield), to relate respiration to the release of energy in cells, and to describe breathing and gas exchange in the lungs, including the adaptations of the alveoli. The equations and the alveolus adaptations are reliable exam marks.
The answer
What respiration is
Respiration is the chemical process by which cells release energy from glucose. It happens in all living cells (in the mitochondria for the aerobic part) and the energy is used for movement, growth, keeping warm and other processes. Respiration is not the same as breathing: breathing is the movement of air, respiration is the release of energy.
Aerobic respiration
Aerobic respiration uses oxygen and releases a large amount of energy:
In symbols, . This is the main way human cells release energy.
Anaerobic respiration
Anaerobic respiration happens without oxygen and releases much less energy per glucose molecule. In human muscle during hard exercise it produces lactic acid:
The build-up of lactic acid causes muscle fatigue and an oxygen debt that must be repaid by deep breathing afterwards. (In yeast, anaerobic respiration produces ethanol and carbon dioxide, used in baking and brewing.)
Comparing the two
- Aerobic: uses oxygen, releases a lot of energy, products are carbon dioxide and water.
- Anaerobic (in muscle): no oxygen, releases little energy, product is lactic acid.
Breathing and gas exchange
Breathing moves air in and out of the lungs. Gas exchange happens in the alveoli (tiny air sacs): oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli to be breathed out. The alveoli are adapted for this:
- a very large surface area (millions of alveoli),
- thin walls (one cell thick) for a short diffusion distance,
- a rich blood supply to maintain a steep concentration gradient,
- a moist lining so gases dissolve before diffusing.
Examples in context
Example 1. Training and fitness. Regular exercise increases the number of mitochondria and the efficiency of the lungs and heart, so an athlete can supply oxygen faster and rely less on anaerobic respiration. This links the type of respiration to fitness and recovery times.
Example 2. Yeast in bread and beer. Yeast respiring anaerobically produces carbon dioxide that makes bread rise, and ethanol used in brewing. Comparing this with lactic acid in human muscle shows that the same lack of oxygen gives different anaerobic products in different organisms.
Try this
Q1. Write the word equation for aerobic respiration. [2 marks]
- Cue. Glucose + oxygen gives carbon dioxide + water (+ energy released).
Q2. State two differences between aerobic and anaerobic respiration in human muscle. [2 marks]
- Cue. Aerobic uses oxygen and releases a lot of energy giving carbon dioxide and water; anaerobic has no oxygen, releases little energy and produces lactic acid.
Q3. Explain how the large number of alveoli helps gas exchange. [2 marks]
- Cue. Many alveoli give a very large total surface area, so more oxygen and carbon dioxide can be exchanged at once (faster diffusion).
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 two ways in which anaerobic respiration in human muscle differs from aerobic respiration.Show worked answer →
Word equation for aerobic respiration: glucose + oxygen gives carbon dioxide + water (+ energy released).
Two differences for anaerobic respiration in human muscle:
It happens without oxygen, whereas aerobic respiration uses oxygen.
It releases much less energy from each glucose molecule than aerobic respiration, and it produces lactic acid (rather than carbon dioxide and water).
Markers reward the correct aerobic word equation, that anaerobic respiration occurs without oxygen, that it releases less energy, and that it produces lactic acid in muscle.
Original4 marksThe alveoli in the lungs are the site of gas exchange. Describe three ways the alveoli are adapted for efficient gas exchange, explaining how each adaptation helps.Show worked answer →
Large surface area: there are millions of alveoli, giving a very large total surface area, so more gas can be exchanged at once.
Thin walls (one cell thick): the wall of the alveolus and the capillary together form a very short diffusion distance, so oxygen and carbon dioxide diffuse quickly.
Rich blood supply (dense network of capillaries): blood constantly carries oxygen away and brings carbon dioxide, maintaining a steep concentration gradient for fast diffusion.
(Other valid point: the alveoli are moist, so gases dissolve before diffusing.)
Markers reward three correct adaptations, each linked to faster gas exchange (large surface area, short diffusion distance, steep concentration gradient maintained by the blood supply).
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