Biology unit 2 - revision on limiting factors and organism distribution

This includes information and notes on limiting factors and organism distribution for both plants and animals.

I made these notes for AQA exams GCSE

Hope it's helpful :)

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How can we count the number of organisms in an are

Using a quadrat: 
It is a square shaped frame. You place it somewhere then you count the amount of organisms inside. It is best you place it randomly as then there is no chance of bias. Placing the quadrat in 10 different spots is more reliable than placing it only 3 times. Also by placing a quadrat you can estimate the percentage amount that each organism occupies in each area. Once you have collected data for about 10 different spots (depending on the size of the total area) you can calculate the mean to get an average over view of how many of that type of organism there are in the whole field.

Using a transect:
These are tapes that are laid across the ground. You can then count the amount of each type of organism that touches the tape. Once you have collected data for maybe 10 different spots you can calculate the mean to get an average over view of how many of that type of organism there are in the whole field.


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This is a quadrat -------------^

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Transect Line- this is one...


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Advantages and Disadvantages

Counting all the organisms in a field

Very Accurate      but               Takes a long time, impractical


Count all the organisms in one area

Quick and easy   but           May not represent the whole field


Count organisms in several different small areas and calculate the mean

Provides more accurate  but      Requires accurate samples
estimate of the biodiversity       to be chosen randomely
of the whole feild. 

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Organism distribution

Organism distirbution can be affected by space, competition and preditation.

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Organism distribution

Temperature:  Many species can only live in a particular temperature range. Such as camels, they have to live in warm climates as they are not adapted to colder climates.

Availability of nutrients: Very wet soils are usually short of nitrate ions so only certain plants can live there.

Light: Every plant and algae needs light for photosynthesis, if light is not present or sufficient enough the plant will die.

Water: All organisms need water. Animals adapt to where they live. Egg wrack seaweed can only live and grow in water otherwise they get too hot.

Oxygen and Carbon dioxide: Organisms need oxygen for respiration, plants need carbon dioxide for photosynthesis. 

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Limiting factors

Plants cannot grow if the conditions are not right.

Look at this graph:



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The graph

The graph clearly shows that when the light intensity is increased the rate of photosynthesis also increases. But after a while the graph peaks off.


Because light intensity is not the only limiting factor for a plant. Limiting factors also include temperature and carbon dioxide levels.

The graph shows that after a while there must be a different factor other than light intensity that is affecting the rate of photosynthesis.

This could be CO2 or temp.

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The End

I hope these notes helped with your revision, they helped me when I was making them to remember all the key information.

Please comment below if I have made any mistakes or you would like to say anything else.

Thanks :)

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its great your stuff is really helping me with revision :)

:) PurpleJaguar (: - Team GR


Thanks :)

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