How living things depend on each other, the difference between parasitism and mutualism, and how bacteria make nitrates available to plants in the nitrogen cycle.
Objectives
Explain interdependence in a community, including parasitism and mutualism.
Describe how nitrates are made available to plants.
Explain how fertilisers and crop rotation add nitrates to the soil.
Describe the roles of nitrogen-fixing, decomposer, nitrifying and denitrifying bacteria in the nitrogen cycle.
Specification references this topic covers: Edexcel B9.3, B9.4, B9.15.
Parasites, mutualism and the nitrogen cycle explained
Start here
No living thing lives on its own. Plants need animals to pollinate flowers. Animals need plants for food. Bacteria in the soil change nitrogen into a form that plants can use. All of these links show how organisms depend on each other.
This matters because if one species disappears, others can be affected. It also explains why farmers rotate crops and use fertilisers.
By the end you will be able to describe parasitism and mutualism, and explain how the nitrogen cycle makes nitrates for plants.
Interdependence
Interdependence means that species in a community depend on each other. They depend on each other for food, shelter, pollination and seed dispersal. For example, bees feed on the nectar of flowers, and the flowers need bees to carry pollen.
In a stable community the numbers of the species stay fairly constant. The producers make the food, the herbivores eat the plants, and the carnivores eat the herbivores. If one species is removed, other species can be affected. If a pollinator disappears, the plants may fail to reproduce.
Some of the closest links between species are parasitism and mutualism.
Parasitism
A parasite is an organism that lives on or in another organism, called the host, and takes food from it. The host is harmed. This is parasitism.
Fleas live on the skin of cats and dogs and feed on their blood. Tapeworms live in the intestines of animals, where they take in digested food. Mistletoe grows on trees and takes water and nutrients from them. The malaria parasite lives in human blood and red blood cells and causes malaria.
A parasite usually does not kill its host quickly, because it depends on the host for survival. Parasitism is a relationship in which one organism benefits and the other is harmed.
Mutualism
In mutualism both organisms benefit. Bees and flowers are an example: the bee gets nectar and the flower gets pollinated. Clownfish and sea anemones are another: the anemone protects the fish with its stinging tentacles and the fish keeps the anemone clean and feeds it with waste.
Nitrogen-fixing bacteria live in nodules on the roots of legume plants such as peas, beans and clover. The plant gives the bacteria sugars and a home. The bacteria make nitrogen compounds that the plant can use.
Gut bacteria help us to digest food and make some vitamins, and we give them a warm place with a supply of food. This is also a mutualistic relationship.
Why plants need nitrates
Plants need nitrogen to make amino acids, which are joined to make proteins. Without enough nitrogen, plants grow poorly. They may be stunted and the older leaves can turn yellow.
The air is about 78% nitrogen gas, but plants cannot use nitrogen gas. They take in nitrates, which are dissolved in the soil water, through their root hairs.
Plants take up nitrates by active transport. This needs energy from respiration. Animals get their nitrogen from eating plants or other animals.
The nitrogen cycle
Nitrogen is recycled between the air, the soil and living things. Nitrogen-fixing bacteria in the soil or in root nodules turn nitrogen gas into nitrates (nitrogen compounds) that plants can use. Lightning also turns a little nitrogen into nitrates.
Plants take in nitrates and make proteins. Animals eat plants and make their own proteins. When plants and animals die and when animals produce waste such as urea, decomposers (bacteria and fungi) break them down and release ammonia (ammonium compounds) into the soil.
Higher tier: nitrifying bacteria change ammonium compounds into nitrates, so the nitrates are available again. Denitrifying bacteria change nitrates back into nitrogen gas, which goes into the air. They are most active in waterlogged soil.
A nitrogen cycle. Nitrogen gas in the air is changed into nitrates in the soil by nitrogen-fixing bacteria. Plants take in the nitrates and animals eat the plants. Death, waste and decay release ammonium compounds. Nitrifying bacteria change these back to nitrates. Denitrifying bacteria change nitrates back into nitrogen gas.
Fertilisers and crop rotation
Crops remove nitrates from the soil, and when they are harvested the nitrogen is taken away. Farmers replace it in two ways. Fertilisers contain nitrates, phosphates and potassium compounds. They increase the yield of the crop.
Crop rotation means growing a different crop on the same field each year. One of the crops in the rotation is a legume such as clover, peas or beans. The nitrogen-fixing bacteria in the legume roots add nitrates to the soil, so the next crop has more to use. Rotation also helps to stop pests and diseases that attack one crop from building up.
Too much fertiliser can be washed into rivers and lakes. It makes algae grow quickly, which can use up oxygen and kill other organisms. This is called eutrophication. Farmers need to apply the right amount.
Recap
Interdependence: species depend on each other. In parasitism the parasite benefits and the host is harmed. In mutualism both benefit.
Plants take in nitrates to make proteins. Nitrogen-fixing bacteria in root nodules and the soil make nitrates. Decomposers release ammonium compounds.
Farmers add nitrates with fertilisers and with crop rotation that includes legumes. Higher tier: nitrifying bacteria make nitrates and denitrifying bacteria remove them.
Key points
Parasitism: parasite benefits, host harmed. Mutualism: both benefit.
Plants take in nitrates by active transport and use them to make proteins.
Nitrogen-fixing bacteria (root nodules of legumes, soil) make nitrates. Decomposers release ammonium compounds.
Higher tier: nitrifying bacteria make nitrates. Denitrifying bacteria turn nitrates into nitrogen gas.
Fertilisers and crop rotation with legumes replace nitrates. Too much fertiliser can cause eutrophication.
Worked examples
Example 1
Name the type of relationship between a tapeworm and a human, and say who benefits.
The tapeworm lives in the human intestine.
It takes digested food from the human.
This is parasitism. The tapeworm benefits and the human is harmed.
Answer: Parasitism. The tapeworm benefits and the human is harmed
Common slips
Slip: Mutualism, because both benefit. The human is harmed, so it is not mutualism.
Slip: Competition. Competition is between organisms that need the same resource. A parasite lives in the host.
Slip: Parasitism, and the human benefits. The host is harmed in parasitism.
Example 2
A farmer grows wheat in a field for several years and the yield drops. Explain how growing clover in the field for a year could improve the next wheat crop.
Wheat takes nitrates from the soil each year.
Clover is a legume with nitrogen-fixing bacteria in its root nodules.
The bacteria change nitrogen gas into nitrates.
The extra nitrates in the soil help the next wheat crop to make more protein and grow better.
Answer: Nitrogen-fixing bacteria in clover roots add nitrates to the soil, which the wheat can use to make proteins
Common slips
Slip: Clover takes all the nitrates out of the soil. Clover adds nitrates because of its nitrogen-fixing bacteria.
Slip: Clover is a fertiliser made in a factory. Clover is a plant. Its bacteria make the nitrates.
Slip: Bacteria turn nitrates into nitrogen gas. That is denitrification. The bacteria in clover roots fix nitrogen.
Slip: Wheat uses nitrogen gas from the air. Plants cannot use nitrogen gas. They need nitrates.
Example 3
Explain the roles of nitrogen-fixing bacteria, decomposers and denitrifying bacteria in the nitrogen cycle.
Nitrogen-fixing bacteria turn nitrogen gas into nitrates that plants can take in.
Decomposers break down dead plants and animals and waste, and release ammonium compounds into the soil.
Nitrifying bacteria change ammonium compounds into nitrates.
Denitrifying bacteria change nitrates into nitrogen gas, which returns to the air.
Answer: Fixing bacteria make nitrates from nitrogen gas, decomposers release ammonium compounds, and denitrifying bacteria return nitrogen gas to the air
Common slips
Slip: Decomposers make nitrates from nitrogen gas. That is nitrogen fixation. Decomposers release ammonium compounds.
Slip: Denitrifying bacteria add nitrates to the soil. They remove nitrates by changing them into nitrogen gas.
Slip: Nitrogen-fixing bacteria return nitrogen to the air. They take nitrogen from the air and fix it. Denitrifying bacteria return it.
Slip: Plants take in nitrogen gas. Plants take in nitrates.
Questions and answers
Parasites, mutualism and the nitrogen cycle practice questions for GCSE Combined Science Biology. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
In which type of relationship does only one organism benefit and the other is harmed?
Show answer
Answer: parasitism
The parasite takes from the host.
This is parasitism.
Question 2 Easy
In which type of relationship do both organisms benefit?
Show answer
Answer: mutualism
Both organisms gain.
This is mutualism.
Question 3 Easy
What do plants take in from the soil to make proteins?
Show answer
Answer: nitrates
Proteins need nitrogen.
Plants take in nitrates.
Question 4 Easy
Name a plant that has nitrogen-fixing bacteria in its root nodules.
Show answer
Answer: clover
These plants are legumes.
Clover, peas and beans are legumes.
Question 5 Easy
What gas makes up about 78% of the air?
Show answer
Answer: nitrogen
Most of the air is not oxygen.
It is nitrogen.
Question 6 Medium
Is a tapeworm in a human intestine an example of parasitism or mutualism?
Show answer
Answer: parasitism
The human is harmed.
It is parasitism.
Question 7 Medium
Are bees feeding on flowers an example of parasitism or mutualism?
Show answer
Answer: mutualism
The bee gets nectar and the flower is pollinated.
Both benefit, so it is mutualism.
Question 8 Medium
Name the type of bacteria that change nitrogen gas into nitrates.
Show answer
Answer: nitrogen-fixing bacteria
They fix nitrogen from the air.
They are nitrogen-fixing bacteria.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
(a) Name the type of relationship between a flea and a dog. [1 mark] (b) Explain how this is different from the relationship between bacteria in the root nodules of a pea plant and the plant. [3 marks]
Show answer and marking points
Answer: (a) parasitism (b) the flea benefits but the dog is harmed, but in the root nodules both the bacteria and the plant benefit because the bacteria make nitrates for the plant and the plant gives the bacteria sugars and a home
(a) parasitism (1)
(b) in parasitism one organism benefits and the host is harmed (1)
(b) in the nodules both organisms benefit (mutualism) (1)
(b) bacteria give nitrates or plant gives sugars or a home (1)
Exam-style question 24 marks
Explain how nitrates become available for plants to take up from the soil. [4 marks]
Show answer and marking points
Answer: nitrogen-fixing bacteria in the soil and root nodules turn nitrogen gas into nitrates, decomposers break down dead plants and animals and waste and release ammonium compounds, nitrifying bacteria turn these into nitrates, and fertilisers also add nitrates
nitrogen-fixing bacteria change nitrogen gas into nitrates (1)
decomposers break down dead matter or waste (1)
this releases ammonium compounds which nitrifying bacteria change to nitrates (1)
fertilisers add nitrates (1)
Common mistakes
Saying plants take in nitrogen gas
Plants take in nitrates dissolved in the soil water.
Saying a parasite and its host both benefit
In parasitism only the parasite benefits. In mutualism both benefit.
Mixing up nitrogen-fixing and denitrifying bacteria
Fixing bacteria make nitrates from nitrogen gas. Denitrifying bacteria turn nitrates into nitrogen gas.
Saying legumes take all the nitrates from the soil
Legumes add nitrates because of the bacteria in their root nodules.
Forgetting that crop rotation also controls pests
Rotation helps to stop pests and diseases of one crop from building up.
Questions students ask
Is Parasites, mutualism and the nitrogen cycle on the Foundation or Higher tier?
Parasites, mutualism and the nitrogen cycle is on both Foundation and Higher papers for GCSE Combined Science Biology, aimed at around grade 5.
What is a common mistake on Parasites, mutualism and the nitrogen cycle?
Saying plants take in nitrogen gas. Plants take in nitrates dissolved in the soil water.
Where are the answers to the Parasites, mutualism and the nitrogen cycle questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.