Topic 5B: Energy Transfer and Nutrient Cycles

Question 1

How to increase the efficiency of a food web

Answer 1

Simplifying food webs:
Use pesticides to control pests.
Use biological agents to control pests.

Pests feed on crops and therefore reduce the energy available for humans.

Reducing respiratory losses:
Reduce the size of living spaces to restrict movement.
Heat the living space so less energy is wasted generating body heat.

Question 2

Mycorrhizae

Answer 2

The relationship between the roots and fungi (made of long, thin strands called hyphae). These increase the surface area of the roots to improve absorption.

Question 3

The nitrogen cycle

Answer 3

1) Nitrogen gas is converted into nitrogen compounds through nitrogen fixation carried out by Rhizobium.
2) These compounds are absorbed by plants which are then fed on by animals.
3) When these plants/animals die, these nitrogen compounds are turned into ammonium ions through ammonification.
4) These ammonium ions are then converted into nitrites and then into nitrates through nitrification.
5) These nitrates are then reabsorbed by plants, or used for Respiration in bacteria to reproduce nitrogen gas through denitrification (which occurs in anaerobic conditions).

Question 4

How does nitrogen enter an ecosystem?

Answer 4

• Lightning

* Artificial fertilisers

Question 5

The phosphorus cycle

Answer 5

1) Phosphate ions in rocks are released into the soul or seas/lakes/rivers through weathering.
2) Algae in the seas/lakes/rivers then feed on these ions before they are passed through the food chain to the birds.
3) These birds then excrete guano containing phosphate ions which then enter the soil.
4) The ions are then taken up by plants through their roots before being eaten by animals.
5) Ions are then returned to the soil through excretion or death, where saprobionts break down the plant/animal.

Question 6

Fertilisers

Answer 6

Replace lost minerals, so that more energy from the ecosystem can be used for growth, increasing the efficiency of the energy transfer.

Artificial: inorganic and contain pure chemicals.
Natural: organic matter such as manure.

Question 7

Environmental issues with fertilisers

Answer 7

If too much is applied, they leach into waterways as the plants are unable to absorb them all. This can lead to eutrophication.

Question 8

Eutrophication

Answer 8

1) Mineral ions stimulate rapid growth of algae.
2) Algae blocks light from reaching plants.
3) Plants are unable to photosynthesise and die.
4) Bacteria feed on dead plants and respire aerobically, reducing the oxygen concentration in the water.
5) Fish are unable to respire and die.