Respiration¶

Part of 4.4 Bioenergetics.

Respiration releases energy from glucose so cells can do useful work. The key contrasts are between aerobic and anaerobic conditions, and between resting demand and exercise demand.

Learning Objectives¶

ID	Official specification wording	Main teaching sections
`4.4.2-lo-1`	4.4.2.1 Students should be able to describe cellular respiration as an exothermic reaction which is continuously occurring in living cells. 4.4.2.1 The energy transferred supplies all the energy needed for living processes. 4.4.2.1 Respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen), to transfer energy. 4.4.2.1 Students should be able to compare the processes of aerobic and anaerobic respiration with regard to the need for oxygen, the differing products and the relative amounts of energy transferred. 4.4.2.1 Organisms need energy for: 4.4.2.1 • chemical reactions to build larger molecules 4.4.2.1 • movement 4.4.2.1 • keeping warm. 4.4.2.1 Aerobic respiration is represented by the equation: 4.4.2.1 glucose +oxygen carbon dioxide +water Students should recognise the chemical symbols: 4.4.2.1 C H O , O , CO and H O. 4.4.2.1 6 12 6 2 2 2 Anaerobic respiration in muscles is represented by the equation: 4.4.2.1 glucose lactic acid As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration. 4.4.2.1 Anaerobic respiration in plant and yeast cells is represented by the equation: 4.4.2.1 glucose ethanol + carbon dioxide Anaerobic respiration in yeast cells is called fermentation and has economic importance in the manufacture of bread and alcoholic drinks.	What is Respiration?, Aerobic Respiration, Anaerobic Respiration, Aerobic vs Anaerobic: Comparison
`4.4.2-lo-2`	4.4.2.2 During exercise the human body reacts to the increased demand for energy. 4.4.2.2 The heart rate, breathing rate and breath volume increase during exercise to supply the muscles with more oxygenated blood. 4.4.2.2 If insufficient oxygen is supplied anaerobic respiration takes place in muscles. The incomplete oxidation of glucose causes a build up of lactic acid and creates an oxygen debt. During long periods of vigorous activity muscles become fatigued and stop contracting efficiently.	Response to Exercise, Practical Investigations
`4.4.2-lo-3`	4.4.2.2 (HT only) Blood flowing through the muscles transports the lactic acid to the liver where it is converted back into glucose. Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells.	Oxygen Debt and Recovery
`4.4.2-lo-4`	4.4.2.3 Students should be able to explain the importance of sugars, amino acids, fatty acids and glycerol in the synthesis and breakdown of carbohydrates, proteins and lipids. 4.4.2.3 Metabolism is the sum of all the reactions in a cell or the body. 4.4.2.3 The energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesise new molecules. 4.4.2.3 Metabolism includes: 4.4.2.3 • conversion of glucose to starch, glycogen and cellulose 4.4.2.3 • the formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids 4.4.2.3 • the use of glucose and nitrate ions to form amino acids which in turn are used to synthesise proteins 4.4.2.3 • respiration 4.4.2.3 • breakdown of excess proteins to form urea for excretion. 4.4.2.3 All of these aspects are covered in more detail in the relevant specification section but are linked together here.	Metabolism

What is Respiration?¶

Respiration is a chemical process in which living cells break down glucose to release energy, stored in a molecule called ATP. It is an exothermic reaction — energy is released as heat.

Energy from respiration is used for: - Building larger molecules (e.g. synthesising proteins from amino acids) - Muscle contraction (movement) - Maintaining body temperature - Digestion (smooth muscle contractions) - Breathing (intercostal muscles and diaphragm) - Cell division, active transport, growth, and nerve impulse transmission

Note: respiration is not the same as breathing. Breathing brings oxygen into the body; respiration is the chemical reaction inside cells that uses that oxygen.

Aerobic Respiration¶

Aerobic respiration uses oxygen and releases a large amount of energy.

Word equation: glucose + oxygen → carbon dioxide + water

Takes place continuously in the mitochondria of cells.
Produces approximately 32 ATP molecules per glucose molecule.
The carbon dioxide produced diffuses out of cells and is breathed out; the water is used or excreted.

Anaerobic Respiration¶

Anaerobic respiration occurs when cells cannot get enough oxygen to meet demand. It produces far less energy but allows cells to keep functioning temporarily.

In animal muscle cells:

Word equation: glucose → lactic acid

Only 2 ATP molecules are produced per glucose molecule — much less efficient than aerobic respiration.
Lactic acid accumulates in muscles, causing pain and fatigue. This is the "burn" felt during intense exercise.
The build-up of lactic acid is linked to the concept of oxygen debt.

In plant cells and yeast (fermentation):

Word equation: glucose → ethanol + carbon dioxide

Also called fermentation.
The carbon dioxide produced makes bread dough rise when yeast is added.
The ethanol is the alcohol in beer, wine, and other fermented drinks.

Aerobic vs Anaerobic: Comparison¶

Feature	Aerobic	Anaerobic (animals)	Anaerobic (yeast/plants)
Oxygen needed?	Yes	No	No
Products	CO₂ + water	Lactic acid	Ethanol + CO₂
ATP produced	~32	~2	~2
Efficiency	High	Low	Low

Metabolism¶

Metabolism is the total of all chemical reactions occurring in a cell or body. It includes:

Building polymers from monomers — e.g. joining glucose molecules to form starch; joining amino acids to form proteins; joining glycerol and fatty acids to form lipids.
Respiration — breaking down glucose to release energy.
Breaking down excess proteins — proteins that are not needed are broken down into urea, which is excreted in urine.

All metabolic reactions require energy, which comes from respiration. Many are catalysed by enzymes.

Response to Exercise¶

During exercise, muscles need more energy. The body responds by increasing the rate of aerobic respiration, which requires more oxygen and glucose delivery.

Breathing rate increases — to bring more oxygen into the blood and remove more CO₂.
Breath volume (tidal volume) increases — each breath is deeper, moving more air.
Heart rate increases — to deliver oxygenated blood to muscles faster.

During intense exercise, oxygen cannot be delivered fast enough, so anaerobic respiration begins in the muscles, producing lactic acid.

Oxygen Debt and Recovery¶

Oxygen debt is the extra oxygen needed after exercise to oxidise (remove) the lactic acid that accumulated.

Lactic acid is transported in the blood to the liver, where it is converted back to glucose using aerobic respiration.
Until lactic acid levels fall, heart rate and breathing rate remain elevated.
Once the lactic acid has been cleared, the oxygen debt is repaid and heart/breathing rates return to resting levels.

Muscle fatigue: if anaerobic exercise continues for too long, muscles tire and their contractions weaken.

Cardiac Output¶

Cardiac output = heart rate × stroke volume

Cardiac output: volume of blood pumped from a ventricle per minute (cm³/min).
Stroke volume: volume pumped per beat.

Example: if cardiac output is 5,000 cm³/min and heart rate is 100 beats/min, stroke volume = 5000 ÷ 100 = 50 cm³.

Practical Investigations¶

Investigating the Effect of Temperature on Respiration Rate (Respirometer)¶

Place a small organism (e.g. woodlice) in one tube of a respirometer; place inert beads of equal mass in a control tube. Add sodium hydroxide (absorbs CO₂) to both tubes.
Set up at a given temperature; allow the liquid in the manometer to move — its displacement shows oxygen uptake.
Repeat at different temperatures to show how temperature affects respiration rate.

Detecting CO₂ from Germinating Beans¶

Germinating beans release CO₂ as they respire. Using hydrogencarbonate indicator (orange → yellow when CO₂ rises), the production of CO₂ by living beans can be compared with a control of dead (boiled) beans.

Detecting Heat from Germinating Beans¶

Germinating beans in a sealed flask show a temperature rise over several days as respiration releases heat energy. Dead beans do not show this rise.

Common Confusions¶

Respiration vs breathing: respiration is a chemical reaction in cells; breathing is the physical movement of air in and out of the lungs. Do not confuse the two.
Aerobic vs anaerobic efficiency: aerobic produces ~32 ATP; anaerobic only ~2. Anaerobic is not just "slower" — it is fundamentally less complete.
Lactic acid in plants: plants and yeast do not produce lactic acid anaerobically — they produce ethanol and CO₂. This is a very common error.
Oxygen debt: the oxygen debt is not about breathing — it is the oxygen needed to break down lactic acid in the liver and restore normal conditions.

Key Terms¶

Respiration: the process that transfers energy from glucose in cells.
Aerobic respiration: respiration that uses oxygen and releases a large amount of energy (≈32 ATP per glucose).
Anaerobic respiration: respiration without oxygen that releases less energy (≈2 ATP per glucose).
Lactic acid: a product of anaerobic respiration in animal muscles; accumulates during intense exercise.
Oxygen debt: the extra oxygen needed after exercise to break down accumulated lactic acid and restore normal conditions.
Metabolism: the sum of all the chemical reactions in a cell or body.
ATP: the molecule that stores and transfers energy within cells; produced during respiration.
Fermentation: anaerobic respiration in yeast and plants; produces ethanol and carbon dioxide.
Cardiac output: the volume of blood pumped from the heart per minute; calculated as heart rate × stroke volume.
Muscle fatigue: reduced effectiveness of muscle contractions after prolonged anaerobic exercise due to lactic acid build-up.