Aligned to AQA GCSE Biology (8461/8464). Wording consistent with AQA mark schemes.
Energy transfer: From sunlight to chemical stores and cellular work.
Exam Definition:
"Photosynthesis is an endothermic reaction in which energy is transferred from the environment to the chloroplasts by light to convert carbon dioxide and water into glucose."
*Chlorophyll absorbs light energy to drive the reaction.
| Use | Reason / Details |
|---|---|
| Respiration | To release energy for the plant. |
| Cellulose | To strengthen cell walls. |
| Starch | Insoluble storage form of glucose (won't affect osmosis). |
| Amino Acids | Combined with nitrate ions for protein synthesis. |
| Fats/Oils | Energy storage in seeds. |
A limiting factor stops the rate of photosynthesis from increasing. Key factors: Temperature, $CO_2$, and Light Intensity.
When a rate-of-photosynthesis graph plateaus (flattens out), the factor being tested is no longer limiting; another factor is now limiting the rate.
Important: Increasing light intensity only increases the rate if light is the limiting factor.
Exam Definition:
"Respiration is an exothermic reaction which occurs continuously in living cells to release energy for metabolic processes."
| Feature | Aerobic | Anaerobic (Animals) |
|---|---|---|
| Products | $CO_2$ + $H_2O$ | Lactic Acid |
| Energy Yield | High (Complete oxidation) | Low (Incomplete oxidation) |
| Equation | $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$ | $\text{Glucose} \rightarrow \text{Lactic Acid}$ |
$\text{Glucose} \rightarrow \text{Ethanol} + \text{Carbon Dioxide}$
In yeast, this is called fermentation and is used in bread and alcohol production.
Heart rate, breathing rate, and breath volume increase to supply more oxygenated blood to the muscles.
Lactic acid is moved to the liver. Oxygen debt is the extra oxygen needed after exercise to oxidise lactic acid to carbon dioxide and water.
Metabolism is the sum of all reactions in a cell or body, including building up (anabolic) and breaking down (catabolic) reactions.
• Units Matter: Always include units when calculating rates (e.g. bubbles/minute or $cm^3/s$).
• Starch: Insoluble, so it doesn't affect the water potential/osmosis of the cell.
• Anaerobic efficiency: Low because glucose is not fully oxidised.
• Graph check: If the line is going up, that factor is limiting. If the line is flat, something else is limiting.