Crop yield can be determined by measuring the dry mass of plants.

Suggest how a student could find the dry mass of a plant material sample.

Suggest an advantage of using dry mass rather than fresh mass to compare crop yields.

In natural ecosystems, most light that falls on plants is not used in photosynthesis.

Suggest two reasons why.

Farming animals for humans to eat is less efficient than farming crops for humans to eat.

Use your knowledge of energy transfer to explain why.

The diagram below shows the flow of energy through an ecosystem. 

Name the organisms represented by box X in the diagram above.

Name process Y in the diagram above.

Define the terms producer and trophic level.

Farmers can increase the efficiency of energy transfer from primary consumers to humans.

Suggest three ways in which farmers can achieve this.

The diagram below shows the percentages of energy transferred from sunlight to a shrew in a woodland ecosystem.

Describe how the percentage efficiency of energy transfer between producers and primary consumers can be calculated.

Use the diagram above to calculate the percentage of sunlight energy that is used in respiration within a shrew.

Give your answer to 3 significant figures. 

In this woodland ecosystem the net productivity of the vegetation is 25,468 kJ m^-2 year^-1.

Use the diagram above to calculate the energy stores in new biomass of the shrew in kJ m^-2 year^-1.

Give your answer to 2 decimal places.

Farmers often keep their livestock indoors.

Suggest and explain why.

The diagram below shows a simplified representation of the nitrogen cycle.

Which process, A, B, or C in the diagram above, involves denitrification?

Describe the role of microorganisms in the production of nitrates from the remains of dead organisms.

Some nitrogen-fixing bacteria are found in the leaves of crops.

These bacteria only produce ammonia from nitrogen inside cells known as heterocysts.

These cells have thick walls and do not contain chlorophyll.

Heterocysts are located next to photosynthetic cells and produce anaerobic conditions inside their cells.

Suggest how the features of heterocysts allow improved efficiency of nitrogen fixation.

Farmers often use crop rotation as a method for producing high crop yields.

This method involves growing a different crop each year in the same field.

Suggest two ways in which this method may lead to higher crop yields.

An animal within an ecosystem dies and decays.

The diagram below shows what happens to some of the nitrogen-containing compounds within the animal’s body.

Name substances X and Y in the diagram above.

Name process Z in the diagram above.

Saprobionts are involved in the decay of animals.

Describe how.

Nitrogen in the atmosphere can also form nitrogen-containing compounds that can be absorbed by plants.

Describe how.

The element nitrogen is constantly cycled throughout ecosystems in a process known as the nitrogen cycle.

Name two biological molecules that contain nitrogen.

Nitrogenase catalyses the reduction of nitrogen during nitrogen fixation.

The reaction requires 16 ATP molecules to reduce each molecule of nitrogen.

Nitrogenase also catalyses reactions involving other substances.

What does this suggest about the shapes of these other substances?

When ammonia inhibits nitrogenase activity, nitrogen-fixing bacteria may benefit.

Explain how.

Farmers can use the process of genetic modification to increase the productivity of their crops.

Explain how other farming practices increase the productivity of agricultural crops.

The diagram below shows the energy transfer through a pig.

All measurements are in kJ x 10⁶ year^-1.

Use the letters E, R, U, and F to complete the equation for the energy used to produce new biomass (B).

Calculate the value of B in the diagram above.

Residual food intake (RFI) is the difference between the amount of food an animal eats and its expected food intake based on the animal’s size and rate of growth.

Some farmers selectively breed pigs with a low RFI.

Explain why pigs with a low FRI are an advantage to farmers. 

Low values of RFI are negative.

Explain why.

The diagram below shows a simplified representation of the phosphorus cycle.

Name two biological molecules that contain phosphorus.

Explain what is happening in process A in the diagram above.

Name two ways in which phosphate ions are returned from living organisms to the soil.

An increase in phosphate concentration within a lake results in a decrease in the populations of fish.

Explain how.

Farmers sometimes leave fields fallow for a year by not growing crops in it.

This leads to a decrease in the concentration of nitrate ions.

Explain why farmers have found that fewer nitrate ions are lost when grass is grown in their fields than when the fields contain only bare soil.

Leguminous crop plants have nitrogen-fixing bacteria in their root nodules.

When grown in soils with low nitrate concentrations, leguminous crops grow better than other crops.

Explain why.

Some farmers increase the concentrations of nitrate in their soil by adding fertiliser.

However, adding very high concentrations of fertiliser to soils can reduce plant growth.

Explain why.

Some nitrate from fertiliser applied to soils can enter ponds and lakes.

Explain how nitrate may cause the death of fish.

A group of scientists investigated the effect of temperature on cattle growth and the efficiency at which the cattle converted their food into new biomass.

Their results are shown in the diagram below.

Suggest two variables the scientists should control in this investigation.

Describe the effect of temperature on average growth rate of cattle.

Use the data to suggest why the scientists did not investigate temperatures higher than 35°C.

The efficiency at which cattle convert their food into new biomass is lower at 0°C compared to 15°C.

Suggest and explain why.