The Nitrogen Cycle: Demystifying this key concept in GCSE Biology

Nitrogen is a very important element found in living things as it is an essential component of many biological organisms. Proteins, DNA, ATP are some examples of biological molecules that contain nitrogen, therefore cell division/growth and metabolism also require nitrogen. The nitrogen cycle describes the way that nitrogen moves through the air, the soil, and within the bodies of living things in various forms. A lot of these processes are facilitated by bacteria in the soil. 

This topic is one of the most challenging of the GCSE course, and as a result it is often covered in higher tier/extended only or triple science only papers. In my experience as a teacher I have found that if you can label a diagram of the stages and the types of bacteria you can pick up some easy marks. The purpose of this article is to talk you through these stages and share the important information you need to know to ace your exams. 

The nitrogen cycle: a basic overview

Some of the main stages of the nitrogen cycle are controlled by bacteria in the soil. At GCSE level you do not need to know the specific names of the bacteria, however it is important to remember the categories of bacteria and what they do. 

Make sure you can name and explain the function of:

• Nitrogen-fixing bacteria

• Saprobiontic microorganisms (decomposers)

• Nitrifying bacteria

• Denitrifying bacteria

Remember that the cycle has no fixed start or end point (because it is a cycle), but when describing the stages it is common to start by describing nitrogen gas in the air being fixed by nitrogen-fixing bacteria as the ‘first’ stage and denitrification as the ‘last’. 

The stages of the nitrogen cycle

The nitrogen cycle can be broken down into the following stages:

• Nitrogen fixation: nitrogen gas from the air is converted into ammonium compounds and then into nitrates by bacteria. This can occur in the soil by free-living bacteria or by bacteria that live in the root nodules of leguminous plants. The nitrates produced by this process can be taken into the roots of plants and used for growth by the plants. 

• Nitrification: ammonia in the soil is converted into nitrites, then nitrates by nitrifying bacteria in the soil. 

• Assimilation: this is when nitrates from the soil are absorbed into the roots of plants (by active transport) and then assimilated into the biological molecules in the plant itself.

• Feeding/digestion: nitrogen containing biological molecules like DNA and proteins are passed from plants to animals and along the food chain through feeding relationships. 

• Decomposition/ammonification: dead organisms as well as urine, faeces, and waste materials like leaf litter, can be broken down by decomposing microorganisms called saprobiontic microorganisms in the soil, such as bacteria and fungi. When the nitrogen-containing biological molecules are digested it releases ammonia into the soil.

• Denitrification: this reduces the quantity of nitrogen in the soil by converting nitrates into nitrogen gas in the atmosphere. The denitrifying bacteria thrives in anaerobic condition in the soil, for example when the soil is waterlogged. Farmers often aerate their soil to avoid this. 

The importance of the nitrogen cycle

The nitrogen cycle is an essential component of many biological molecules. These include:

• All amino acids and therefore all proteins

• DNA and RNA bases (adenine, thymine, cytosine, guanine and uracil)

• ATP (needed for energy transfer from respiration)

Nitrates are an essential mineral needed for plant growth. Plants without enough nitrogen suffer from yellowed leaves, poor growth and lower yields. This means that checking and controlling the level of nitrates in the soil is important for all farmers and growers. 

Human impact on the nitrogen cycle

The nitrogen cycle has a particular impact on farming and agriculture in the UK and around the world. Farming disturbs the natural cycle by removing the stage of death and decomposition, reducing the return of nitrogen back into the soil from living organisms. As a result, most farmers will add fertilisers containing nitrates into the soil to enhance the growth of their crops. This is an expensive process and it can also be damaging to the surrounding environment, leading to issues like leaching and eutrophication.

Some farmers try to limit the damaging impact on their soil and environment by carrying out crop rotation. This is where they plant legumes like clover once every few years, which carry out nitrogen fixation, then they plow them back into the fields to increase the nitrogen content in the soil. They can also leave the field to ‘fallow’ where they leave the land unfarmed or have animals on the land in order to allow the soil to recover and gain natural fertiliser like manure.  

Understand the nitrogen cycle and breeze your Biology GCSE exams

In order to ace your exams you need to make sure that you can recognise the stages on a diagram and label them. You may also be asked to write an extended response 4-6 mark question describing the stages. As part of your revision you should practice drawing a diagram and writing an explanation to accompany it. When drawing your own cycles don’t worry about drawing fancy pictures of cows or plants - just writing a bunch of words in boxes with arrows between them is just fine! 

The nitrogen cycle is just one of many different cycles that take place in the ecosystem. Make sure you can also describe the stages of the carbon cycle.

Make sure to check out the nitrogen cycle revision note on Save My Exams. 

You can also access practice questions and flashcards covering this topic and the carbon cycle. 

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