This mini-lesson covers the CCEA topic Breathing & the respiratory system: aerobic and anaerobic respiration (in muscle and yeast), the breathing system and how we ventilate the lungs, gas exchange at the alveoli, and simple breathing-rate calculations.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Respiration · what it is
Respiration is not breathing
Respiration is the chemical process that releases energy from glucose. It happens in all living cells, all the time — the aerobic form takes place mainly in the mitochondria.
Breathing (ventilation) is completely different: it is the movement of air in and out of the lungs. Breathing supplies the oxygen and removes the carbon dioxide that respiration uses and produces — but breathing itself releases no energy.
The energy released by respiration is used for movement, growth, keeping warm, active transport and reproduction.
Watch out: respiration releases energy — it does not "make" or "create" energy. And it happens in every living cell, not just in the lungs.
Respiration · aerobic
Aerobic respiration
Aerobic respiration uses oxygen to fully break down glucose. It releases a large amount of energy per glucose molecule.
Energy: a lot — glucose is completely broken down.
Balanced symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Notice there are 6 carbons, 12 hydrogens and 18 oxygens on each side — it balances.
Quick check
What comes out?
?In aerobic respiration, glucose reacts with oxygen. What are the two products formed?
Respiration · anaerobic
Anaerobic respiration
Anaerobic respiration is respiration without oxygen. Glucose is only partly broken down, so it releases much less energy per glucose than aerobic respiration. CCEA needs both of these word equations:
glucose → lactic acidin mammalian muscle during hard exercise (a little energy released)
glucose → ethanol + carbon dioxidein yeast — called fermentation (a little energy released)
In muscle: during vigorous exercise, oxygen can't reach the muscles fast enough, so they respire anaerobically and make lactic acid.
The lactic acid builds up, causing an oxygen debt and muscle fatigue. Extra oxygen is needed afterwards to break it down.
In yeast: yeast respires anaerobically to make ethanol + carbon dioxide — used in brewing (alcohol) and baking (the CO₂ makes bread rise).
Key difference: anaerobic respiration releases far less energy per glucose than aerobic, because the glucose is not fully broken down.
Sort it
Which kind of respiration?
Tap a statement, then tap the box it belongs to.
🫁 Aerobic
💪 Anaerobic (muscle)
🍺 Anaerobic (yeast)
Quick check
The sprinter's problem
?A sprinter's leg muscles run out of oxygen during a hard race. What substance builds up when muscle respires anaerobically?
Quick check
Yeast & energy
?Which statement is correct about respiration?
Breathing · the respiratory system
The breathing system
Air travels from the nasal cavity down the trachea, which splits into two bronchi, then into smaller bronchioles, ending in tiny air sacs called alveoli — where gas exchange happens.
Air path: nasal cavity → trachea → bronchi → bronchioles → alveoli. The ribs, intercostal muscles and diaphragm move air in and out.
Also learn: the lungs sit inside the ribs and are protected by pleural membranes with a thin layer of pleural fluid between them, which reduces friction as the lungs move.
Breathing · ventilation
How we breathe in and out
Breathing works by changing the volume of the thorax, which changes the pressure inside. Air always moves from high pressure to low pressure.
Breathing IN (inhalation)
Intercostal muscles contract → ribs move up and out.
Diaphragm contracts and flattens (moves down).
Thorax volume increases → pressure inside decreases → air moves IN.
Breathing OUT (exhalation)
Intercostal muscles relax → ribs move down and in.
Diaphragm relaxes and domes up.
Thorax volume decreases → pressure inside increases → air is pushed OUT.
Remember the chain: muscles → volume → pressure → air movement. Breathing in and breathing out are the exact opposites of each other.
Quick check
What does the diaphragm do?
?When you breathe in, what happens to the diaphragm?
Breathing · gas exchange
Gas exchange at the alveoli
At the alveoli, gases move by diffusion down their concentration gradients:
Oxygen diffuses from the alveolus into the blood.
Carbon dioxide diffuses from the blood into the alveolus to be breathed out.
Exchange surfaces are adapted to make diffusion fast: a large surface area, thin walls (short diffusion distance), moist, permeable, a good blood supply, and a steep concentration gradient kept up by breathing.
Exhaled air has less oxygen, more carbon dioxide and more water vapour than inhaled air. Nitrogen (~78%) is unchanged.Quick check
Breathe out
?Compared with the air you breathe in, which gas is present in a higher amount in the air you breathe out?
Calculate
Your turn — breathing rate
1At rest, a student takes 30 breaths in 2 minutes. Calculate their breathing rate in breaths per minute.