Unit2 Revisaion

unit 2 AQA biology revision 

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Unit 2
12.1 Animal and plant cells
All living things are made up of cells. The structures of different types of cells are related to their

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Page 2

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Unit 2
The chemical reactions inside cells are controlled by enzymes
Enzyme for respiration are in mitochondria
Enzyme for photosynthesis are in chloroplasts
Enzymes for protein synthesis are on the ribosomes
Some cells are specialised to carry out a particular function.
Root hair cells have a tiny hair like structure
which increases the surface area of the cell
enabling it to absorb water and ions more
Palisade cells are column-shaped cells on the
upper surface of the leaf.…read more

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Unit 2
12.2 Movement into and out of cells
To get into and out of cells, dissolved substances have to cross the cell membranes
Dissolved substances can move into and out of cells by diffusion and osmosis
Diffusion is the net movement of particles from an area of high concentration to an area of low
Diffusion is the spreading of the particles of gas, or any substance in solution, resulting in a net
movement from a region where they are of a higher concentration.…read more

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Unit 2
Osmosis is the movement of water across a partially permeable membrane from a region of
high water concentration to a region of lower water concentration.
The effect of osmosis is to gradually dilute the solution. This is what happens at root hair cells where
water moves from the soil into the cell by osmosis, along a concentration gradient.…read more

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Unit 2
Osmosis in plant cells
In hypotonic solution-distilled water
Plant cell in hypertonic solution…read more

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Unit 2
12.3 Photosynthesis
Green plants use light energy to make their own food. They obtain the raw materials they need to
make this food from the air and soil.
Four things are needed for photosynthesis to occur-
- Light from the sun -
Carbon dioxide diffused from the air
- Water from the soil -
Chlorophyll in the leaves
Two things are produced
- Glucose-for biomass and energy
- Oxygen- released into atmosphere as a by-product.…read more

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Unit 2
Plants use glucose for 6 different reasons-
For respiration-this releases energy that enables them to convert the rest of the glucose into
various other useful substances which they use to build new cells and grow.
Making fruits- glucose, along with another sugar fructose, is turned into sucrose for storing in
Making cell walls-glucose is converted into cellulose for making cell walls
Making proteins- glucose is combined with nitrates to make amino acids which are then made
into proteins.…read more

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Unit 2
Light intensity-as the light intensity s increased so does the rate of photosynthesis (only to a
certain point) this means light intensity is a limiting rate of photosynthesis.
Rise in light intensity only has effect up to some point and then begins to level out. This
means light intensity is no longer a limiting factor. Carbon dioxide or temperature must now
be the limiting factor.…read more

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Unit 2
12.4 Energy flow in ecosystems
Food chains
Radiation from the sun is the source of energy from most communities of living organisms. In green
plants, photosynthesis captures a small fraction of the solar energy which reaches them. The energy
is stored in substances which make up the cells of the plant and can be passed onto organisms which
eat the plant. This transfer of energy can be represented by a food chain.…read more

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Unit 2
In the pyramid above, the fox gets the last tiny bit of energy and biomass that is left. Only a fraction
of the suns energy is captured by the producers. Much of the biomass remains in the root system
and so does not get eaten. Rabbits run, mate, excrete, generate heat and pass on only a tenth of the
energy they get from lettuce. A lot of biomass is lost in faeces.…read more


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