Bioenergetics
Photosynthesis and its limiting factors, and aerobic and anaerobic respiration and the response to exercise.
Learn
Bioenergetics, explained point by point
Everything the GCSE specification expects you to be able to do, and how to actually do it - the same lesson a signed-in student studies from.
Describe photosynthesis and write its equation
Photosynthesis makes glucose in the chloroplasts of green plant cells, using light energy: carbon dioxide + water → glucose + oxygen, or 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. It is endothermic - energy is taken IN from light and stored in the glucose. The oxygen is released as a waste product that the rest of us breathe.
Explain the limiting factors that affect the rate of photosynthesis
The rate depends on light intensity, carbon dioxide concentration and temperature - whichever is in shortest supply is the limiting factor holding it back. Increase that one and the rate rises until something else runs short. Growers use this in greenhouses, adding light, warmth and CO₂ to push growth as far as it is worth paying for.
Describe how plants use the glucose they make
Glucose is a versatile building block. Plants respire some of it for energy, store some as insoluble starch, and use the rest to make cellulose for cell walls, lipids for oils and seeds, and - with nitrate from the soil - amino acids for proteins. Storing starch rather than glucose keeps water from rushing in by osmosis.
Compare aerobic and anaerobic respiration
Respiration releases energy from glucose in every living cell and is exothermic. Aerobic respiration uses oxygen and releases a lot of energy: glucose + oxygen → carbon dioxide + water. Anaerobic respiration works without oxygen but releases much less - in muscles it makes lactic acid, and in yeast it makes ethanol and carbon dioxide, which is used in brewing and baking.
Explain the body's response to exercise and oxygen debt
During exercise your muscles need more energy, so breathing and heart rate rise to deliver more oxygen and glucose and remove more carbon dioxide. If demand outstrips oxygen supply, muscles respire anaerobically and lactic acid builds up. The extra oxygen needed afterwards to break down that lactic acid is the oxygen debt - the reason you keep panting after a sprint.
Describe metabolism as the sum of reactions in a cell or body
Metabolism is all the chemical reactions happening in cells, controlled by enzymes. Some build large molecules from small ones - glucose into starch, amino acids into proteins - while others break molecules down, like respiration. Energy from respiration powers the building reactions, keeping the whole system running.
Bioenergetics key terms
The words the specification and the mark schemes use, each defined the way an examiner wants it.
- Photosynthesis
- The endothermic reaction in which plants use light energy to convert carbon dioxide and water into glucose and oxygen.
- Aerobic respiration
- Respiration using oxygen: glucose reacts with oxygen to form carbon dioxide and water, transferring energy for the cell. It happens continuously in living cells.
- Anaerobic respiration
- Respiration without oxygen. In muscles, glucose is broken down to lactic acid, transferring much less energy than aerobic respiration.
- Fermentation
- Anaerobic respiration in yeast cells, producing ethanol and carbon dioxide. It is the basis of bread-making and brewing.
- Limiting factor
- The factor in shortest supply that caps the rate of photosynthesis: light intensity, carbon dioxide concentration or temperature.
- Oxygen debt
- The extra oxygen the body needs after exercise to react with the lactic acid that built up and remove it from cells.
Practice
Try a Bioenergetics question
A GCSE-style original question from this topic. Have a go before you open the working - deciding on an answer first is what makes the working stick.
Which of these is the correct balanced symbol equation for photosynthesis?
- CO₂ + H₂O → C₆H₁₂O₆ + O₂
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
- 6CO₂ + 6H₂O → 6C₆H₁₂O₆ + O₂
Show the answer and the working
Answer: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Balancing gives six carbon dioxide and six water making one glucose and six oxygen molecules.
- Glucose, C₆H₁₂O₆, contains 6 carbon atoms, so 6 CO₂ are needed.
- Balancing the hydrogen and oxygen needs 6 H₂O and gives 6 O₂.
- So the equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.