AQA Biology Unit 2 (B2) - Cells, Transport, Plants, Food Chains & Energy Flow

Notes on cell structure, specialized cells, diffusion, active transport, photosynthesis, limiting factors, plants and minerals, biomass, energy flow and pyramids of biomass/number.

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Additional Biology (AQA
Structure of Cells
The two main types of cells you need to know for this exam are animal cells and plant cells and
their structure. The following are features that both plant and animal cells share:
A nucleus which controls cell activity and holds genetic information
Cytoplasm where chemical reactions take place
A cell membrane that controls what goes in and out of the cell
Ribosomes where protein synthesis happens (ie the creation of proteins)
Mitochondria where energy is released during respiration
The nucleus is in the centre of the cell, surrounded by cytoplasm which contains the ribosomes and
mitochondria. It is the cell membrane that goes around the whole cell, like a protective wall (note that
this is different from a cell wall, which is a feature of a plant cell).
Plant cells, however, have some exclusive features which aren't present in animal cells. These
A cellulose cell wall to strengthen the plant cell
A permanent vacuole that is filled with cell sap. This is found in the cytoplasm and keeps
the cell turgid.
Green chloroplasts that are needed for photosynthesis.

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There are a few cells that are specialized for a certain function. These cells normally have
adaptations which help them carry out their specialized function effectively.…read more

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Table from BBC Bitesize.
Transport of Substances in and out of Cells
There are two different ways (for additional biology) in which substances travel in and out of cells.
These are diffusion and osmosis.
Diffusion is defined as the net movement of particles from an area of high concentration to an area
of lower concentration.…read more

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Diffusion is down a concentration
gradient, meaning that it will only occur when there is an area of lower concentration to diffuse to.
The diagram shows that eventually there will be a near enough equal amount of particles on either
side of the membrane. Diffusion happens in cells across the partially permeable cell membrane.
Remember that the greater the difference in concentration, the faster the rate of diffusion.
Osmosis is basically diffusion of water.…read more

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Plants are adapted for photosynthesis in a number of ways:
The leaves are broad, creating a large surface area for light to be absorbed
Leaves are thin, so carbon dioxide does not have to go far to reach the cells of the leaf
A complex root system brings water into the various parts of the plant efficiently
Leaves also contain stomata which open and close to let water out and oxygen in, based on
a number of factors including current level of water and oxygen,…read more

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Doing this ensures the maximum yield for their crops, as photosynthesis can occur at an
optimum rate.
Plants and Minerals
A number of different elements are required by plants so they can stay alive, which include carbon,
hydrogen and oxygen. Carbon and oxygen can be acquired from carbon dioxide in the air, while
hydrogen is obtained from water in soil. Minerals are also needed to stay alive. The main ones
needed are nitrates and magnesium.…read more

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Diagram from GCSE Revision 101
In a pyramid of numbers, the size of each trophic level (stage of the pyramid going up) reflects,
generally, how many of the certain type of organisms are involved on this part of the food chain. As
you can see, there is only one oak tree compared to the ten thousand aphids, so the trophic level for
the oak tree is very small.…read more


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