How does the body get oxygen into the blood, and how do cells release energy from glucose?
The structure of the respiratory system, the mechanism of breathing in and out, gas exchange in the alveoli and their adaptations, aerobic and anaerobic respiration with their word equations, and the difference between breathing and respiration.
A focused CCEA GCSE Single Award Science answer on respiration and breathing, covering the respiratory system, the mechanism of breathing, gas exchange and alveolar adaptations, aerobic and anaerobic respiration with their equations, and breathing versus respiration.
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What this dot point is asking
CCEA wants you to label the parts of the respiratory system, describe how breathing in and out happens in terms of muscles, volume and pressure, explain how the alveoli are adapted for gas exchange, give the word equations for aerobic and anaerobic respiration, and distinguish breathing from respiration.
Structure of the respiratory system
The mechanism of breathing
Breathing works by changing the volume, and therefore the pressure, inside the chest.
- Breathing in: the intercostal muscles contract and pull the ribs up and out; the diaphragm contracts and flattens. The chest volume increases, the pressure inside falls below the outside pressure, so air flows in.
- Breathing out: the muscles relax; the ribs move down and in and the diaphragm domes up. The chest volume decreases, the pressure rises above the outside, so air is forced out.
Gas exchange in the alveoli
Alveoli are adapted by a large surface area (millions of them), thin walls one cell thick, a good blood supply and moist surfaces, all of which speed up diffusion.
Aerobic and anaerobic respiration
Anaerobic respiration happens when not enough oxygen reaches the muscles, for example during hard exercise. It releases far less energy per glucose and produces lactic acid, which builds up and causes muscle fatigue. The oxygen needed afterwards to break down the lactic acid is the oxygen debt.
Breathing versus respiration
Examples in context
Example 1. Why exercise makes you breathe harder. During exercise the muscles respire faster, using more oxygen and making more carbon dioxide. The body increases the breathing rate and depth, so more air is moved each minute, bringing in oxygen for aerobic respiration and removing carbon dioxide. It shows clearly that breathing exists to serve respiration in the cells.
Example 2. The burn during a sprint. In a hard sprint the muscles cannot get enough oxygen, so they respire anaerobically, producing lactic acid. The lactic acid builds up and causes the burning feeling and fatigue. After the sprint you keep breathing hard to repay the oxygen debt, taking in extra oxygen to break the lactic acid down. This links anaerobic respiration to a real, familiar experience.
Try this
Q1. Name the small air sacs where gas exchange takes place. [1 mark]
- Cue. The alveoli.
Q2. Give the word equation for aerobic respiration. [2 marks]
- Cue. Glucose plus oxygen gives carbon dioxide plus water.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20195 marksDescribe how air is breathed into the lungs (inhalation).Show worked answer →
Five marks for the muscles, the volume change and the pressure change in order.
The intercostal muscles between the ribs contract, pulling the ribcage up and out.
The diaphragm contracts and flattens, moving down.
Together these increase the volume of the thorax, the chest cavity.
Increasing the volume decreases the air pressure inside the lungs, so it is now lower than the air pressure outside.
Air therefore moves down the pressure gradient, from high pressure outside to low pressure in the lungs, so air flows in. Markers reward the muscles named, ribs up and out, diaphragm down and flat, volume up, pressure down, air in.
CCEA SAS 20204 marksCompare aerobic and anaerobic respiration in humans, giving the word equation for each.Show worked answer →
Four marks: two equations and two clear differences.
Aerobic respiration uses oxygen: glucose plus oxygen gives carbon dioxide plus water, releasing a lot of energy.
Anaerobic respiration in humans does not use oxygen: glucose gives lactic acid, releasing only a small amount of energy.
Differences: aerobic needs oxygen and anaerobic does not; aerobic releases much more energy per glucose; aerobic produces carbon dioxide and water, while anaerobic in muscle produces lactic acid that builds up and causes fatigue.
Markers reward both equations and the contrast in oxygen use, energy released and products.
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)