Living Things and the Environment
Living Things and the Environment tests how well you describe organisms, group them, and explain their interdependence with each other and with the physical surroundings. Expect questions on the characteristics of living things, cell structure, classification, food chains and webs, energy flow, decomposition and nutrient cycling, adaptations, and human impact on ecosystems.
Living things and cells
Living things move, respire, respond to stimuli, grow, reproduce, excrete and need nutrition, while controlling internal conditions. Plant cells have a cellulose cell wall, chloroplasts and a large permanent vacuole; animal cells have none of these. Both have a nucleus, cytoplasm and a cell membrane.
Classification
Organisms are named by the binomial system: genus then species, both italicised, genus capitalised (Homo sapiens). Grouping uses shared features, moving from kingdom down to species.
Ecosystems and energy flow
A habitat is where an organism lives; a population is all members of one species in an area; a community is all the populations; an ecosystem is the community plus its abiotic factors such as light, temperature, water and soil. In a food chain the arrow points from the organism being eaten to the eater, showing the direction of energy flow. Decomposers break down dead matter and return mineral ions such as nitrates to the soil.
Worked example
Grass in a field contains 40 000 kJ of energy, and about 10 per cent passes to the next trophic level. Grass 40 000 kJ to grasshopper 4 000 kJ to lizard 400 kJ to hawk 40 kJ. Reasoning: most energy is lost at each step, as heat from respiration, in movement, and in undigested material lost as faeces, so only a small fraction is built into the consumer's body. This is why food chains rarely exceed four or five links and why top carnivores are few. The loss also explains why a pyramid of energy is always upright, even though a pyramid of numbers can be inverted, as when many insects feed on one tree.
Traps that cost marks
Drawing arrows backwards, or inserting decomposers halfway along a chain; treating a habitat as an ecosystem; explaining energy loss only as 'used up'; describing an adaptation as a deliberate choice ('the cactus grows spines to stop animals'); confusing photosynthesis with respiration when explaining the carbon cycle.