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AQA BIOLOGY 7401/7402 AS to A2



3.5.4 Nutrient cycles

Nutrients are recycled within natural ecosystems, exemplified by the nitrogen cycle and the phosphorus cycle.

Microorganisms play a vital role in recycling chemical elements such as phosphorus and nitrogen.

• The role of saprobionts in decomposition.

• The role of mycorrhizae in facilitating the uptake of water and inorganic ions by plants.

• The role of bacteria in the nitrogen cycle in sufficient detail to illustrate the processes of saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification. (The names of individual species of bacteria are not required).

The use of natural and artificial fertilisers to replace the nitrates and phosphates lost by harvesting plants and removing livestock.

The environmental issues arising from the use of fertilisers including leaching and eutrophication


  1. Make notes on the following:

    1. The role of saprobionts in decomposition

    2. The role of mycorrhizae in facilitating the uptake of water and inorganic ions by plants.

    3. The role of bacteria in the nitrogen cycle in sufficient detail to illustrate the processes of saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification. (The names of individual species of bacteria are not required).

    4. The use of natural and artificial fertilisers to replace the nitrates and phosphates lost by harvesting plants and removing livestock.

    5. The environmental issues arising from the use of fertilisers including leaching and eutrophication.

In your final exam (Paper 3) the very last question is an essay question with a broad title that is designed to assess your scientific understanding as well as your spelling, punctuation and grammar. You should aim to draw together 5 different areas of the specification and for top marks content that is at least A Level standard but from outside of the specification.

  1. Using the guidance that follows these instructions have a go at marking the exemplar essay about the roles of water in the lives of organisms. Write notes at the end of the essay about where your marking agreed and disagreed with the suggested mark.

  2. Write an essay titled “Nutrient Cycles. It will be marked out of 25. It should be a minimum of 2 sides of print or 4 sides hand written. You should include all of the detail from points a – e from task 1. It will be marked as explained on the next few pages (see and for further guidance).

Version 1.1

A-level Biology

Paper 3 essay marking guidance

This document is a preview of the essay marking guidance that will be found in the upcoming specimen A-level Paper 3 mark scheme. The principles outlined here will apply when assessing the essay regardless of the subject content assessed. The pages related to the subject specific content of the mark scheme have been removed from this document in the interests of keeping the content of the specimen paper secure. The full version of this document will be made available with the release of the 7402/3 specimen mark scheme on the Secure Key Materials section of e-AQA.

Question 6 Level of response marking guidance

Level of response marking instructions

Level of response mark schemes are broken down into five levels, each of which has a descriptor. The descriptor for the level shows the average performance for the level. There are five marks in each level. Thus the descriptor for the level represents the mid mark in that level.

Before you apply the mark scheme to a student’s answer, read through the answer and annotate it (as instructed) to show the qualities that are being looked for. You can then apply the mark scheme.

Step 1 Determine a level

Start at the lowest level of the mark scheme and use it as a ladder to see whether the answer meets the descriptor for that level. The descriptor for the level indicates the different qualities that might be seen in the student’s answer for that level. If it meets the lowest level then go to the next one and decide if it meets this level, and so on, until you have a match between the level descriptor and the answer. With practice and familiarity you will find that for better answers you will be able to quickly skip through the lower levels of the mark scheme.

When assigning a level you should look at the overall quality of the answer and not look to pick holes in small and specific parts of the answer where the student has not performed quite as well as the rest. If the answer covers different aspects of different levels of the mark scheme you should use a best fit approach for defining the level and then use the variability of the response to help decide the mark within the level. i.e. if the response is predominantly level 3 with a small amount of level 4 material it would be placed in level 3 but be awarded a mark near the top of the level because of the level 4 content.

Step 2 Determine a mark

Once you have assigned a level you need to decide on the mark. The descriptors on how to allocate marks can help with this. The exemplar materials used during standardisation will help. There will be an answer in the standardising materials which will correspond with each level of the mark scheme. This answer will have been awarded a mark by the Lead Examiner. You can compare the student’s answer with the example to determine if it is the same standard, better or worse than the example. You can then use this to allocate a mark for the answer based on the Lead Examiner’s mark on the example.

You may well need to read back through the answer as you apply the mark scheme to clarify points and assure yourself that the level and the mark are appropriate.

Indicative content in the mark scheme is provided as a guide for examiners. It is not intended to be exhaustive and you must credit other valid points. Students do not have to cover all of the points mentioned in the indicative content to reach the highest level of the mark scheme.

An answer which contains nothing of relevance to the question must be awarded no marks.




Generalised beyond specific context

Response shows holistic approach to the question with a fully integrated answer which makes clear links between several different topics and the theme of the question.

Biology is detailed and comprehensive A-level content, uses appropriate terminology, and is very well written and always clearly explained.

No significant errors or irrelevant material.

For top marks in the band, the answer shows evidence of reading beyond specification requirements.



Integrated into a whole

Response links several topics to the main theme of the question, to form a series of interrelated points which are clearly explained.

Biology is fundamentally correct A-level content and contains some points which are detailed, though there may be some which are less well developed, with appropriate use of terminology.

Perhaps one significant error and, or, one irrelevant topic which detracts from the overall quality of the answer.



Several aspects covered but they are unrelated

Response mostly deals with suitable topics but they are not interrelated and links are not made to the theme of the question.

Biology is usually correct A-level content, though it lacks detail. It is usually clearly explained and generally uses appropriate terminology.

Some significant errors and, or, more than one irrelevant topic.



Only one or few aspects covered

Response predominantly deals with only one or two topics that relate to the question.

Biology presented shows some superficial A-level content that may be poorly explained, lacking in detail, or show limited use of appropriate terminology.

May contain a number of significant errors and, or, irrelevant topics.



Response only indirectly addresses the theme of the question and merely presents a series of biological facts which are usually descriptive in nature or poorly explained and at times may be factually incorrect.

Content and terminology is generally below A-level.

May contain a large number of errors and, or, irrelevant topics.


Nothing of relevance or no response.

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Try marking this essay ............................

The roles of water in the lives of organisms

Water is perhaps the most important molecule for the survival and life of organisms. On the surface of the planet there is obviously much more area of water than land, which shows its significance.

It is mostly due to its specific properties that water is so useful. Perhaps the most obvious is that it has a very high specific heat capacity. It is noticeable that water is a much more stable environment for organisms to live in, as it does not cool to rapidly or heats up like air. This means that organisms living in water do not have to keep re-adjust their body temperature for survival.

Water provides support for marine organisms such as jelly fish, both from the outside and inside. On the outside it allows for movement, as floating is possible, so joints are not required or hard skeleton for muscles to act upon. On the inside, it also provides support, so movement can be achieved by pushing water into the front, propagating itself along.

On earth water inside an organism also provides support. For example earthworms employ a hydrostatic skeleton, which allows muscles to be contracted against an incompressible substance.

Water is a universal solvent. It provides medium in which substances can be dissolved. It is present in cytoplasm of all organisms, so they cannot survive without it. Water is present in blood to allow digested substances such as glucose to be carried along to respiring cells and allows easier diffusion of materials between cells. Examples of this is in alveoli and cappillaries. It is a lubricant and a transport substance.

Water allows toxic materials, such as urea to be dissolved so it does not harm the organism that has produced it during metabolic activities.

Water acts as a shock absorber, for example in cavities in the brain (the serebrospinal fluid) which cushions the delicate organs against damage. It is present in joints to allow easy movement and protection against friction. In the eye to give it shape and support (as aqueous and vitreous humour). It is present in mucus in the mouth to allow easy swallowing of food, in the gut for moving the food along and dissolving enzymes to break down food and antibodies or bacteria for protection against disease.

Water is essential in homeostasis (in urine, as I have already mentioned and in control of the body temperature as specific heat capacity allows for cooling of the skin in sweat.

In plants water has significance importance. It allows for support, as water is an incompressible fluid. It fills the ceils which are surrounded by cell walls and so keep them turgid.

It is also a transport medium, just like in animals) substances, such as ions are dissolved in it and carried in the xylem. The sugars produced by photosynthesis are also dissolved in water and carried to respiring cells.

Water is an essential ingredient in photosynthesis as it is where the oxygen comes from (Hill's reaction) as water is split into oxygen and hydrogen ions (and also electrons).

Water is important in moving substances up the transpiration stream. Water evaporates from the surface of the leaf, and therefore the substances from the soil are drawn up.

Another method of water movement up the xylem is by cohesion-tension theory. The molecules of water are attracted to the walls of the xylem (adhesion) due to its polar structure, and to each other (cohesion) so it moves up the stream because the walls of the lignified xylem can take the pressure.

Fertilisation in some organisms relies on water. Fish release their eggs into the river, so that the male can then fertilize them outside the body. Some plants rely on water to transport pollen in the water.

Terrestrial animals are also dependent on the lubricating property of water. Human release mucus by their sexual organs to allow for easier penetration. Sperm is also released in a semi-liquid (containing water) this allows for easier movement to reach the egg. The mucus produced by females in the vagina, also protects against infections as water dissolves the antibodies.

One other aspect of water I have not mentioned is its insulating properties. Water expands on heating, due to its original property and structure. The ice which forms, floats on the rivers, so heat does not escape from the water below and organisms are able to survive.

Water vapour in the atmosphere also acts as a greenhouse gas, allowing for radiation to warm up the earth's surface, but not allow the heat to escape back into the atmosphere.

From all this information it is clear how many functions water has in the living organisms. Water is not only present in eyes to support and keep its shape, but to lubricate it and protect it from infection and it also helps to keep our environment warm. It makes up about 65% of human mass and up to 95% in plants. This clearly indicates its great significance.

Your score _______ /25

Suggested mark for this essay

Scientific content: the essay covers most of the material that might be expected. Some of the areas are explained well, such as specific heat capacity, fertilisation and the role of water in photosynthesis. However, there is too little development of several of the topics raised, e.g. vague references to water as a lubricant, no explanation of turgidity and an inadequate explanation of sweating. Furthermore, there are one or two confused references, such as dissolving antibodies and bacteria. (Mark: 13/16)

Breadth of knowledge: all of the main areas are addressed and there are references to both plants and animals. (Mark: 3/3)

Relevance: There are few errors and no irrelevant points. (Mark2/3)

Quality of written communication: spelling, punctuation and grammar are reasonably sound (e.g. note the mis-spelling of 'capillaries' and 'cerebrospinal fluid') and technical terms are generally used appropriately. There is an attempt at an introduction and a conclusion. However, the material is not logically presented, with a fairly disjointed structure and some repetition.

Some of the paragraphs could have been amalgamated and put into a more logical order. (Mark: 2/3)

(Total mark: 13 + 3 + 2 + 2 = 20/25)

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