1. What is "biomass" and what is the source of biomass on Earth ?
Answer
Biomass is the mass of living organisms (in a specific area at a specific time). Photosynthetic organisms (green plants and algae) are the producers of biomass.
2. Describe what happens to the numbers of predators and prey in a stable community.
Answer
Numbers rise and fall in cycles: as the number of prey increases, the number of predators also increases (after a short lag phase). As the number of predators increases, the number of prey decreases because they are being eaten. Eventually, the reduced number of prey causes the number of predators to fall, since there is not enough food.
3. Give two examples of processes that release carbon into the atmosphere in the carbon cycle.
Answer
Respiration and combustion.
4. Name the fuel produced by biogas generators.
Answer
Methane gas.
5. Name the prey organism and the secondary consumer in the food chain: grass ➜ hare ➜ lynx.
Answer
Hare is the prey and lynx is the secondary consumer.
6. The graph in the figure below shows the changes in the populations of the lynx and the hare between 1890 and 1920. Suggest an explanation for the changes in the hare population between 1910 and 1915.
Answer
Population increased because the number of predators was relatively low and then decreased because the number of predators doubled between 1910 and 1912, so more hares were eaten.
7. Name some environmental changes that can affect the distribution of species.
Answer
Temperature, availability of water, human activity, etc.
8. The figure below shows the water cycle. Name the processes labelled A and B.
Answer
9. Explain how the carbon in dead plants can become part of a living plant.
Answer
Carbon compounds in the dead plants are broken down by decomposers such as bacteria and fungi. The decomposers respire and release carbon dioxide back into the atmosphere and mineral ions back into the soil. Carbon dioxide diffuses into the leaf of a plant and is used in photosynthesis. Glucose produced in photosynthesis is used to synthesise proteins and cellulose (for cell walls) for growth in the living plant.
10. Explain what gardeners and farmers use compost for.
Answer
As a natural fertiliser for growing garden plants and crops.
11. As milk decays, microorganisms break down lactose in milk to lactic acid, and the pH of the milk decreases. A student carries out an experiment to investigate the effect of temperature on the rate of decay of fresh milk. Explain why the rate of pH change would be faster at 40°C and slower at 60°C.
Answer
At 40°C the process is faster because microorganisms reproduce more rapidly at higher temperatures, increasing their numbers to break down lactose, or because this temperature is closer to the optimum for enzyme activity, so lactose is digested more quickly.
At 60°C the process is slower because microorganisms are killed by the high temperature, leaving no bacteria to break down lactose, or because enzymes are denatured so their active sites no longer fit the lactose.