C1.2 Cell respirationIB Biology SL: Revision notes
Section 1
ATP: the energy currency of cells
ATP (adenosine triphosphate) is a nucleotide: adenine, ribose and three phosphate groups. Properties that make it suitable as the energy currency:
- it is soluble, so it moves freely in the cytoplasm;
- it is stable at the pH of cells but is hydrolysed quickly by enzymes;
- it cannot pass freely through membranes, so it stays in the cell where it was made;
- hydrolysis of one phosphate releases an amount of energy sufficient for many tasks without too much waste.
Section 2
Uses of ATP and the ATP–ADP cycle
ATP supplies energy for active transport across membranes, synthesis of macromolecules (anabolism) and movement of the whole cell or of cell components such as chromosomes.
Hydrolysis of ATP to ADP + phosphate releases energy. Synthesising ATP from ADP and phosphate requires energy, supplied by cell respiration. ATP is recycled continuously; it is not a long-term store.
Do not write that ATP 'produces energy'. Its hydrolysis releases energy that was transferred to it during respiration.
Section 3
Cell respiration vs gas exchange
Cell respiration is a system for producing ATP in cells using energy released from carbon compounds. Glucose and fatty acids are the main substrates, but many organic compounds can be used.
Gas exchange is the diffusion of oxygen into, and carbon dioxide out of, an organism or tissue. It supports aerobic respiration but is a different process.
Section 4
Aerobic vs anaerobic respiration in humans
Aerobic: glucose + oxygen → carbon dioxide + water. Needs oxygen and mitochondria; can use glucose, fatty acids and other substrates; large ATP yield; takes place in the cytoplasm and mitochondria.
Anaerobic: glucose → lactate. No oxygen needed; occurs in the cytoplasm only (no mitochondria needed); uses only glucose; small ATP yield but fast. Used when oxygen supply cannot meet demand, e.g. sprinting.
In comparison questions, cover substrate, oxygen, ATP yield, waste products and location.
Section 5
Measuring the rate of cell respiration
Rate can be measured as oxygen uptake (respirometer with an alkali such as potassium hydroxide or soda lime to absorb CO₂), CO₂ production (sensor or indicator), or loss of mass of substrate.
Volume in a capillary = distance moved × cross-sectional area. Rate = volume ÷ time, usually also ÷ mass (e.g. mm³ g⁻¹ min⁻¹).
Variables that affect rate include temperature, substrate availability, oxygen concentration and the activity of the organism. Use a control tube to correct for temperature and pressure changes.
Always give the unit of a rate, and state what you divided by.
That's the notes covered.
Carry on to the next subtopic.