Biology - plants, ecology, and the environment
What the exam tests
Plant processes, food webs, energy flow, cycles and environmental change. Expect questions interpreting food chains and explaining photosynthesis and transpiration.
Core rules
- Photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy and chlorophyll.
- Respiration occurs in plants and animals all the time, releasing energy from glucose.
- Water and minerals move in xylem; sugars move in phloem. Transpiration pulls water up and cools the leaf.
- In a food chain, arrows show energy flow. Only about 10% passes to the next trophic level; the rest is lost as heat, movement and waste.
- Decomposers recycle nutrients. Carbon cycles through photosynthesis, respiration, combustion and decomposition.
- Biodiversity increases ecosystem stability; pollution and habitat loss reduce it.
Worked example
Grass → rabbit → fox. If grass contains 10,000 kJ, the rabbit can assimilate about 1,000 kJ and the fox about 100 kJ. This is why food chains rarely have more than four or five links: too little energy remains to support another level.
Traps
- Decomposers are not usually drawn in food chains but are essential for recycling nutrients.
- Plants respire at night as well as photosynthesise in the light.
- Energy is not recycled; nutrients are.
- A food web is more realistic than a single chain because organisms eat several foods.