Respiration and aerobic equations
Respiration is the chemical release of energy from glucose in EVERY living cell — it is not breathing. The energy makes ATP (adenosine triphosphate), the cell's energy currency, which powers muscle contraction, active transport, protein synthesis and keeping warm (mammals and birds).
Aerobic respiration uses oxygen, mainly in the mitochondria, and releases the most energy per glucose. Word: glucose + oxygen → carbon dioxide + water Symbol: C6H12O6 + 6O2 → 6CO2 + 6H2O
Anaerobic respiration: animals vs yeast
Anaerobic respiration releases energy WITHOUT oxygen and gives far less energy per glucose than aerobic.
Animals (e.g. exercising muscle): glucose → lactic acid (no carbon dioxide). Lactic acid builds up and causes fatigue. Afterwards, extra oxygen — the oxygen debt — breaks the lactic acid down in the liver.
Plants and yeast (fermentation): glucose → ethanol + carbon dioxide. Key split: animals make lactic acid only; yeast and plants make ethanol and carbon dioxide.
Measuring respiration rate and temperature
Practical 2.39: measure the carbon dioxide (or heat) given off by respiring seeds. Confirm CO2 with limewater (cloudy/milky) or hydrogencarbonate indicator (red → yellow), and collect the gas in a syringe to measure its volume.
Rate = volume of gas ÷ time (always state a unit, e.g. cm³/min).
Temperature: warming gives molecules more kinetic energy, so enzyme–substrate collisions are more frequent and respiration speeds up — until the enzymes denature.
Drawn from real examiner reports.
Respiration is not breathing
The most penalised error in 2f is defining respiration as "breathing in oxygen and breathing out carbon dioxide". That describes ventilation, not respiration. Respiration is a chemical process that releases energy from glucose in living cells (making ATP), and it happens in every cell, not just the lungs. Breathing is the mechanical movement of air and belongs to topic 2g.
June 2024 Paper 1BR Q3(b)(iv): asked to investigate the effect of exercise on CO2 release, many described a heart-rate experiment instead, conflating respiration with breathing.
Oil layer excludes oxygen, not "air"
In the yeast fermentation practical, the oil layer on top of the mixture excludes OXYGEN, creating anaerobic conditions. Writing that the oil "prevents air" is too vague and is not credited — you must name oxygen specifically, because it is oxygen that would otherwise allow aerobic respiration to occur.
June 2023 Paper 1BR Q4(b)(i): saying the oil layer "prevents air" rather than "prevents oxygen" was a common imprecise answer that lost the mark.
Accuracy is not reliability
Accuracy and reliability are not the same. Accuracy is how close a measurement is to the true value — improve it with better equipment, e.g. collecting the gas in a syringe instead of counting bubbles (bubble size varies). Reliability is how reproducible the result is — improve it with more repeats and a mean. A syringe improves accuracy; repeats improve reliability.
June 2023 Paper 1BR Q4(b)(vi): using a gas syringe improves accuracy, not reliability; confusing the two terms was the most frequent wrong answer.
Plateau means glucose has run out
When gas production from yeast rises then levels off (plateaus), the explanation is that the glucose has run out (been used up), so fermentation stops and no more CO2 is made. Writing only "the rate slowed" or "the bubbles stopped" is a description, not an explanation — an "explain why" question needs the cause, which is that glucose is exhausted.
June 2023 Paper 1BR Q4(b)(iv): many gave a description (the rate levelled off) rather than the explanation (glucose was used up).
Do not swap anaerobic products
Do not swap the anaerobic products. Animals (and human muscle) produce lactic acid only — no carbon dioxide. Yeast and plants produce ethanol AND carbon dioxide. Writing that animals make ethanol, or that yeast makes lactic acid, is a common mix-up. Both pathways start from glucose and both release less energy than aerobic respiration.
At 37 °C the plateau comes sooner
At 37 °C yeast respires FASTER than at 20 °C (more kinetic energy, more enzyme–substrate collisions), but the gas also stops SOONER — because the faster rate uses up the glucose more quickly, not because the enzymes have denatured. 37 °C is below the denaturing temperature, so "the enzymes denatured" is the wrong reason for the earlier plateau.
June 2023 Paper 1BR Q4(b)(v): a higher temperature gave a faster initial rate and exhausted the glucose sooner; blaming the earlier plateau on denaturation was a common error.
Memorise the four respiration equations
Memorise the four equations. Aerobic (word): glucose + oxygen → carbon dioxide + water. Aerobic (symbol): C6H12O6 + 6O2 → 6CO2 + 6H2O. Anaerobic, animals: glucose → lactic acid. Anaerobic, plants/yeast: glucose → ethanol + carbon dioxide.
Explain rate: cause to mechanism to effect
For "explain" questions, chain cause → mechanism → effect: a higher temperature gives enzymes more kinetic energy, so enzyme–substrate collisions are more frequent, so the rate rises. A graph description with no mechanism earns no "explain" marks.
Name the gas, indicator and unit
In practicals, name the specific gas (oxygen or carbon dioxide), not "air", and name the indicator — limewater (cloudy) or hydrogencarbonate indicator (red → yellow) for CO2. Keep temperature constant with a water bath, and always give a unit in any rate answer.
Respiration is the chemical process that releases energy from glucose in living cells. It is NOT breathing. It occurs in EVERY living cell, all the time -- in animals, plants, fungi, and bacteria.
The energy released is used to make adenosine triphosphate (ATP), the energy currency of the cell. ATP is then used for:
Aerobic respiration requires oxygen and releases the maximum energy per glucose molecule. It takes place mainly in the mitochondria.
Word equation:
glucose + oxygen carbon dioxide + water
Balanced symbol equation (statement 2.37):
Anaerobic respiration occurs when oxygen is unavailable or in short supply. It releases less energy per glucose molecule than aerobic respiration.
| Feature | Aerobic | Anaerobic (animals) | Anaerobic (plants and yeast) |
|---|---|---|---|
| Oxygen needed? | Yes | No | No |
| Products | + water | Lactic acid | Ethanol + |
| Energy released | High | Low | Low |
| Site | Mitochondria | Cytoplasm | Cytoplasm |
Word equation -- animals (e.g. muscle cells):
glucose lactic acid
Word equation -- plants and yeast (fermentation):
glucose ethanol + carbon dioxide
Note: plants and yeast produce carbon dioxide () as a product of anaerobic respiration; animals do not.
During intense exercise, muscles respire anaerobically and accumulate lactic acid. After exercise, extra oxygen is needed to break down this lactic acid -- this is the oxygen debt. The lactic acid is carried in the blood to the liver, where it is converted back to glucose or fully oxidised.
Define respiration (as required by the Edexcel mark scheme).
(a) Define respiration.
(b) State the word equation for aerobic respiration.
(c) State the word equation for anaerobic respiration in yeast.
(5 marks)
