AQA GCSE Biology (8461) · Paper 1
⚡ Bioenergetics
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Photosynthesis
Photosynthesis is the process by which plants make their own food. They capture light energy using chlorophyll and use it to turn carbon dioxide and water into glucose, releasing oxygen as a by-product. It is the reaction that almost every food chain on Earth starts with.
Photosynthesis is endothermic, meaning energy is taken in from the surroundings. This is the whole point of the reaction: light energy ends up stored in glucose.
The four rate factors
The rate of photosynthesis depends on light intensity, carbon dioxide concentration, temperature, and the amount of chlorophyll present. Temperature behaves differently from the others: warming speeds up the enzymes involved, until it denatures them, at which point the rate falls sharply.
Limiting factors (Higher)
At any moment, the rate of photosynthesis is set by whichever factor is in shortest supply, called the limiting factor. This is why a graph of light intensity against rate climbs and then levels off: past that point light is plentiful, and carbon dioxide or temperature has become the limiting factor instead. Adding more light beyond this point changes nothing; only increasing the limiting factor itself will raise the rate.
For Higher tier in both Biology and Combined Science: Trilogy, light intensity is proportional to 1/d², where d is distance from the source. Doubling the distance reduces the intensity to a quarter.
What the plant does with its glucose
- Some glucose is used in respiration to release energy.
- Some glucose is converted into insoluble starch for storage.
- Some glucose is used to make fats and oils for storage in seeds.
- Some glucose is used to make cellulose for cell walls.
- Some glucose is combined with nitrates to make amino acids, which are then used to build proteins.
Greenhouse growers make use of limiting factors by pumping in extra carbon dioxide and adding lighting and heating, paying for these only while the extra yield they produce is worth more than the cost. This cost-benefit trade-off is a common exam context.
Respiration
Respiration is not the same as breathing. It is the chemical reaction that happens in every living cell, all the time, transferring the energy stored in glucose so the cell can use it. Respiration is exothermic, meaning energy is transferred out to the cell.
This is aerobic respiration, meaning respiration with oxygen, and it happens mostly in the mitochondria. The energy released powers movement, through muscle contraction, warmth, by keeping body temperature steady, and the building of larger molecules from smaller ones, such as proteins from amino acids.
Anaerobic respiration
Exercise
During exercise, the muscles demand more energy, so the body increases its supply of oxygen and glucose: breathing rate and depth increase, bringing in more oxygen, and heart rate increases, delivering oxygen and glucose faster. When demand outstrips the oxygen supply, muscles switch to anaerobic respiration, and lactic acid accumulates, causing fatigue.
The oxygen debt is the extra oxygen that continues to be taken in after exercise stops, which is needed to react with the accumulated lactic acid and clear it from the muscles.
Higher content: blood transports the lactic acid to the liver, where it is converted back into glucose.
Metabolism
Metabolism is the sum of all the chemical reactions taking place in a cell or organism. Examples include converting glucose into starch or cellulose, forming lipids from glycerol and fatty acids, combining glucose and nitrates to form amino acids and then proteins, respiration itself, and breaking down excess proteins into urea. The energy released by respiration powers all of these reactions.