3.1 Ecosystems
What is meant by a population? (1 mark)
All the organisms of the same species living in a particular area.
State two abiotic factors that can affect the distribution of organisms. (2 marks)
Any two: light intensity, moisture, pH or temperature.
Name the pigment that traps light energy during photosynthesis. (1 mark)
Chlorophyll.
State two ways energy can be lost between stages of a food chain. (2 marks)
Any two: heat, movement or undigested material.
Name the mineral ion supplied by fertilisers that plants use to make amino acids (1 mark)
Nitrate.
Explain the difference between a producer and a consumer. (2 marks)
A producer makes its own food, while a consumer obtains energy by eating other organisms.
A pupil wants to sample the abundance of daisies in a field. Name the equipment they should use and state how they could make the sample representative. (2 marks)
Use a quadrat and place it randomly in several locations/use many quadrats.
During the light reactions, water is split. Name the two products formed from water. (2 marks)
Hydrogen and oxygen.
What is meant by a pyramid of energy. (1 mark)
A diagram that shows the energy available at each level of a food chain.
Explain how adding nitrate fertiliser can increase crop yield. (2 marks)
Nitrates are used to make amino acids and proteins, allowing increased plant growth and therefore greater yield.
Rabbits and sheep in the same field both feed on grass. Name the type of competition occurring and explain why. (2 marks)
Interspecific competition, because different species are competing for the same limited resource.
A woodland has very few plants beneath a dense tree canopy. Suggest an abiotic factor responsible and explain its effect. (2 marks)
Light intensity. Less light reaches the ground, reducing photosynthesis and plant growth.
State the substances required for carbon fixation and name the product formed. (3 marks)
Carbon dioxide + hydrogen + ATP → sugar.
Explain why a pyramid of numbers may have an irregular shape while a pyramid of energy is always pyramid-shaped. (2 marks)
Organisms vary in size, so one large producer can support many small consumers. Energy is lost at each level, so energy always decreases up the pyramid.
State two alternatives to using pesticides to protect crops. (2 marks)
A disease causes the rabbit population to decrease greatly. Predict what will happen to plants eaten by rabbits and predators that feed on rabbits. Explain both. (4 marks)
Plants increase because there is less grazing. Predators decrease because less prey/food is available.
A pupil uses pitfall traps to estimate the abundance of ground-dwelling insects. Give two possible sources of error or limitations. (2 marks)
Any two: traps may be in unrepresentative locations, organisms may escape, predators may eat trapped organisms, or traps may flood.
Increasing light intensity initially increases photosynthesis, but eventually the rate stops increasing. Explain why. (2 marks)
Light is no longer the limiting factor. Another factor, such as CO₂ concentration or temperature, is limiting the rate.
A pupil investigates the energy content of different foods by burning the food to heat water. State two variables that should be kept constant to make the investigation valid. (2 marks)
Any two:
Explain why pesticide concentration can become high enough to kill organisms near the end of a food chain. (2 marks)
Pesticides accumulate in organisms. Predators eat many contaminated prey, causing pesticide concentration/toxicity to increase along the food chain to potentially lethal levels.
Explain why intraspecific competition is normally more intense than interspecific competition. (2 marks)
Members of the same species have the same niche and therefore require the same resources. Different species usually compete for only some resources.
Lichens are abundant at Site A but absent from Site B. Explain how lichens could be used to investigate environmental quality. (2 marks)
Lichens can act as an indicator species. Their presence or absence can indicate particular environmental conditions/pollution levels.
Describe the two stages of photosynthesis, including the roles of light, chlorophyll, water, ATP, hydrogen and carbon dioxide. (4 marks)
Light reactions: chlorophyll traps light energy, which generates ATP; water is split into hydrogen and oxygen. Carbon fixation:carbon dioxide combines with hydrogen using energy from ATP to produce sugar.
A food chain contains 80,000 kJ → 12,000 kJ → 1,800 kJ. Calculate the percentage of energy lost between the first and third levels. (2 marks)
% change = (difference ÷ original) x 100
80,000 − 1,800 = 78,200 kJ lost.
(78,200 ÷ 80,000) × 100 = 97.75% lost.
Explain how fertiliser entering freshwater can eventually cause fish to die. (4 marks)
Nitrates enter water → algal bloom → light is blocked → plants/algae die → bacteria increase as they decompose dead material → bacteria use oxygen → oxygen levels fall → fish die.