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GCSE BiologyRevision notes 2020/2021
All copyright and publishing rights are owned by S-cool. First created in 2000 and updated in 2013, 2015 & 2020
,Contents
Enzymes ................................................................................................................................................. 2
Cells ........................................................................................................................................................ 3
Respiration ............................................................................................................................................. 5
Photosynthesis ...................................................................................................................................... 8
Heart and Circulation ......................................................................................................................... 10
Nutrition ............................................................................................................................................... 14
Nerves and Hormones ....................................................................................................................... 16
Defence Against Disease ................................................................................................................... 19
Homeostasis ........................................................................................................................................ 22
Drugs .................................................................................................................................................... 26
Genes and Genetics............................................................................................................................ 30
Genetic Crosses................................................................................................................................... 32
Fertilisation .......................................................................................................................................... 36
Evolution .............................................................................................................................................. 39
Environment ........................................................................................................................................ 41
Plant Growth........................................................................................................................................ 44
Environmental Problems .................................................................................................................... 46
These notes cover the main areas of this subject. Please check the specific areas you need with your exam
board. They are provided “as is” and S-cool do not guaranteed the suitability, accuracy or completeness of this
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1
All copyright and publishing rights are owned by S-cool. First created in 2000 and updated in 2013, 2015 & 2020
,Enzymes
What are enzymes?
Enzymes are biological catalysts. They speed up chemical reactions in all living things, and allow them to
occur more easily. They occur in plant cells and animal cells. Without them we would not be alive.
Enzymes are just chemical molecules, made up of proteins.
Each particular enzyme has a unique, 3-dimensional shape shared by all its molecules. Within this shape there is
an area called the active site where the chemical reactions occur.
What do enzymes do?
Some enzymes help to break down large molecules.
Others build up large molecules from small ones.
While many others help turn one molecule into another.
Probably the fastest enzyme known is called catalase. It breaks the chemical hydrogen peroxide down to water
and oxygen. Catalase is found in all cells and protects them from this dangerous waste chemical.
Optimum conditions
Each type of enzyme has its own specific optimum condition under which it works best.
Enzymes work best when they have a high enough substrate concentration for the reaction they catalyse. If too
little substrate is available the rate of the reaction is slowed and cannot increase any further.
The pH must be correct for each enzyme. If the conditions are too alkaline or acidic then the activity of the
enzyme is affected. This happens because the enzyme's shape, especially the active site, is changed. It
is denatured, and cannot hold the substrate molecule.
Temperature is a key factor too. If it is too cold the enzymes will move around too slowly to meet the substrate
molecules, so the reaction rate is slowed. Likewise, if it is too warm they do not work properly either. This is
because the extra heat energy shakes them around so much that the active sites change shape so, just like with
pH, the enzyme molecules are denatured, and can't hold the substrate.
Enzymes everywhere!
Enzymes control all kinds of reactions in all cells. For example, they help control; respiration, photosynthesis,
and our digestion, amongst many others.
Protease and lipase enzymes are used in biological washing powders to remove those stubborn stains.
Enzymes are also used in making foods and drinks. The enzyme pectinase helps to break down the cells in fruit
to release more of their juice.
2
All copyright and publishing rights are owned by S-cool. First created in 2000 and updated in 2013, 2015 & 2020
, Cells
Cell structure
Plants and animal cells share the same basic structural features, although plant cells have a few extra bits.
Animal Cells
Animal cells come in all kinds of shapes and sizes but have the same basic features.
The control centre is the nucleus; this contains all the genetic information for the cell and controls all its
activities.
The cytoplasm is like a big soup of chemicals in which the reactions occur.
Then forming the outside of the cell is the cell membrane, which acts as a barrier and controls the transfer of
materials into and out of the cell.
Plant Cells
Plant cells also come in a variety of forms but share similar features. In addition to the three basic features
found in animal cells, plant cells have some useful extra ones.
Firstly they have a rigid cell wall made of fibres of cellulose (which we use to make paper!) that gives them
shape and strength. The cell wall fits closely just outside the cell membrane like a plastic box with an inflated
balloon stuffed inside.
Secondly they have a vacuole, which stores extra water and gives extra support to the cell by pressing hard
against the cell wall.
Thirdly, most plant cells also contain small round structures called chloroplasts, which contain the green
pigment chlorophyll, which is needed for photosynthesis.
Tissues, organs and organisms
A living plant or animal is called an organism and is made up of lots of cells all working together.
Some of these cells are all of the same type; collectively they are called a tissue. They all do the same job, for
example connective tissue, which is used in animals to connect other tissues together - and stop us falling apart!
Next, some different tissues are grouped together to make up an organ such as the stomach or a leaf.
Finally, some organs work together to form an organ system such as the digestive system.
Characteristics of Life
All living things show 7 characteristics of life. All plants and animals are alive.
3
All copyright and publishing rights are owned by S-cool. First created in 2000 and updated in 2013, 2015 & 2020
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