Bioenergetics · GCSE Biology

Respiration

Aerobic and anaerobic respiration, mitochondria, oxygen debt, fermentation and why every living cell respires.

UNDERSTAND→RETRIEVE→REMEMBER
THE MEMORY HOOK
Respiration is not breathing. It is the chemical release of energy from glucose. Aerobic needs oxygen and transfers much more energy; anaerobic does not, and it leaves a debt.

The important bits

What you need to know

  1. 1

    Respiration is an exothermic reaction that transfers energy from glucose to the cell for movement, synthesis of large molecules, and keeping a constant temperature in mammals and birds.

  2. 2

    Aerobic respiration: glucose + oxygen → carbon dioxide + water. It happens mainly in mitochondria and transfers much more energy than anaerobic respiration.

  3. 3

    Anaerobic respiration in animal cells: glucose → lactic acid. It allows a short burst of energy when oxygen cannot be supplied fast enough, for example in sprinting.

  4. 4

    Anaerobic respiration in plants and yeast: glucose → ethanol + carbon dioxide. Yeast fermentation is used in bread (carbon dioxide makes dough rise) and alcoholic drinks.

  5. 5

    Oxygen debt is the extra oxygen needed after exercise to oxidise lactic acid, converting it back so it does not keep causing muscle fatigue.

  6. 6

    During exercise, heart rate and breathing rate rise to deliver more oxygen and glucose to muscles and to remove more carbon dioxide and lactic acid.

  7. 7

    Metabolism is the sum of all the reactions in a cell or organism, including respiration, synthesis of proteins and conversion of glucose to starch, glycogen or lipids.

Go deeper

Aerobic versus anaerobic is an energy comparison

Write both word equations until they are automatic, then attach a location and a yield. Aerobic respiration needs oxygen, produces carbon dioxide and water, and takes place largely in mitochondria. It transfers enough energy for endurance. Anaerobic respiration in animals produces lactic acid, no carbon dioxide in that equation, and far less energy, but it is fast and does not wait for oxygen. That is why a 100 m sprinter can run faster than they can supply oxygen, then pant afterwards. The panting is not “catching breath” as a vague phrase: it is delivering the oxygen debt to process lactic acid. Yeast is different again: ethanol and carbon dioxide, which is why fermenters and dough both depend on anaerobic conditions.

Go deeper

Exercise data is a delivery problem

Muscles need more energy, so they need more glucose and oxygen, and they produce more carbon dioxide. Heart rate rises to move blood faster. Breathing rate and depth rise to exchange gases faster at the alveoli. If the question shows a table of pulse rates, describe the pattern, then explain it with respiration, not with “so they can run”. After exercise the rates stay high while the oxygen debt is repaid. Glycogen in muscles and liver is a store of glucose; it is not the same as the starch store in plants, though both are polysaccharides built from glucose. Never write that energy is made. Energy is transferred from stores in glucose to the cell.

WORKED EXAMPLE

See the idea in action

A student sprints 200 m and then breathes heavily for several minutes. During the sprint, muscle cells respire anaerobically: glucose → lactic acid, because oxygen cannot be delivered fast enough. After the race, breathing stays high to supply extra oxygen. That oxygen is used to oxidise lactic acid, converting it so the oxygen debt is repaid. Aerobic respiration (glucose + oxygen → carbon dioxide + water) then covers the lower energy demand of recovery.

Exam technique

Turn knowledge into marks

Define respiration as a chemical reaction in cells, not as breathing. Give the correct equation for the organism in the question. Animals: lactic acid. Yeast and plants: ethanol and carbon dioxide. Mention mitochondria for aerobic respiration.

Common mistakes

Do not give these marks away

  1. 01

    Defining respiration as breathing, or saying energy is produced rather than transferred.

  2. 02

    Writing the yeast equation for animal anaerobic respiration, or putting lactic acid in yeast.

  3. 03

    Forgetting that plants respire all the time, including at night when photosynthesis has stopped.

QUICK RETRIEVAL

What are the products of anaerobic respiration in yeast?

ALactic acid only

BEthanol and carbon dioxide

CCarbon dioxide and water

DGlucose and oxygen

Show the answer

Ethanol and carbon dioxide. Yeast ferments glucose to ethanol and carbon dioxide. Lactic acid is the product in animal cells. Carbon dioxide and water are the products of aerobic respiration.

Quick questions

If this is the bit you searched

What is the word equation for aerobic respiration?

Glucose + oxygen → carbon dioxide + water (energy is transferred).

What is oxygen debt?

The extra oxygen required after exercise to oxidise lactic acid produced by anaerobic respiration in muscles.