Respiration¶

Part of 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.

What You Need to Learn¶

Living cells transfer energy from food through respiration. Compare respiration with and without oxygen, connect exercise to oxygen supply and recovery, and place these reactions within metabolism. Higher Tier includes explaining how the liver deals with lactic acid after exercise.

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

• Most aerobic respiration reactions in plant and animal cells take place in mitochondria; some initial reactions occur in the cytoplasm.
• Transfers more energy per glucose molecule than anaerobic respiration because glucose is broken down more completely.
• The carbon dioxide produced diffuses out of cells and is breathed out; the water is used or excreted.

In chemical symbols, the balanced aerobic equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. These symbols represent glucose, oxygen, carbon dioxide and water respectively.

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

• Transfers less energy per glucose molecule than aerobic respiration because glucose is only partly broken down.
• Lactic acid builds up during vigorous activity. Prolonged vigorous exercise leads to fatigue and less effective contraction; muscle pain should not be attributed simply to lactic acid alone.
• 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¶

Metabolism¶

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

• Building carbohydrates and proteins — glucose forms starch for storage in plants, glycogen for storage in animals, and cellulose for plant cell walls; amino acids join to form proteins.
• Making lipids — one glycerol molecule combines with three fatty acid molecules. Lipids are not polymers.
• Making amino acids in plants — glucose provides carbon-containing material and absorbed nitrate ions supply nitrogen, allowing protein synthesis.
• Respiration — breaking down glucose to release energy.
• Breaking down excess proteins — proteins are digested to amino acids. Excess amino acids are processed in the liver, and their nitrogen forms urea, which is excreted in urine.

Some metabolic reactions transfer energy, while others use energy supplied by respiration to build molecules. Enzymes control these reactions.

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.

• Higher Tier: blood carries lactic acid from muscles to the liver. There it can be converted back to glucose, with energy supplied by 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)¶

1. 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, physically separated from the organisms because it is corrosive.
2. Set up at a given temperature; allow the liquid in the manometer to move — its displacement shows oxygen uptake.
3. 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 an insulated flask with a cotton-wool plug can show a temperature rise as respiration releases heat energy. The plug reduces heat loss while allowing oxygen to enter. 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 energy transfer: aerobic respiration transfers more energy per glucose molecule. Anaerobic respiration transfers less because glucose breakdown is incomplete.
• 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 more energy per glucose molecule than anaerobic respiration.
• Anaerobic respiration: respiration without oxygen that releases less energy per glucose molecule than aerobic respiration.
• 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: a molecule that transfers energy between reactions in cells.
• 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 ability of muscles to contract effectively after prolonged vigorous activity.

Connected Pages¶

• Photosynthesis
• Biology Key Terms Glossary