GCSE Biology, Module B4


Note on GCSE Biology, Module B4, created by jessmitchell on 24/05/2014.
Note by jessmitchell, updated more than 1 year ago
Created by jessmitchell almost 10 years ago

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module b4

cell structure & function

plant and animal cells have similarities & differencesmost animal & plant cells have the following parts1) nucleus- contains DNA. DNA contains the instructions for making proteins, e.g. the enzymes used in the chemical respiration and photosynthesis2) Cytoplasm- gel like substance where proteins like enzymes are made. Some enzyme controlled reactions occur here, like anaerobic respiration3) Cell membrane- holds the cell together and controls what goes in and out. It lets gases and water pass through freely while acting as a barrier to other chemicals.4) Mitochondria- these are where the enzymes are needed for the reactions of aerobic respiration are found and where the reactions take place.

Plant cells have a few extra things that animal cells don't have1) rigid cell wall- made of cellulose. It supports the cell and strengthens it2) Vacuole- contains cell sap, a weak solution of sugar and salts3) Chloroplasts- these are where the reactions for photosynthesis take place. They contain a green substance called chlorophyll and the enzymes needed for photosynthesis.

YeastYeast are single celled microorganisms. They have a cell membrane, mitochondria, cytoplasm, nucleus and cell wall

Bacteria have a simple cell structureBacterial cells are a bit different to plant, animal and yeast cells. They do not have a nucleus. They have a circular molecule of  DNA which floats around in the cytoplasm. They do not have mitochondria either but they can still respire aerobically.


Enzymes are proteins produced by living things1) Living things have thousands of different chemical reactions going on inside them all the time2) these reactions need to be carefully controlled to get the right amounts of substances3) so living things produce enzymes: Enzymes are proteins that speed up chemical reactions.4) The instructions for making enzymes and other proteins are in a cell's genes

Enzymes are very specific1) chemical reactions usually involve things either being split apart or joined together.2) A substrate is a molecule that is changed in a reaction3) Every enzyme molecule has an active site- the part where a substrate joins on to the enzyme4) Enzymes only normally speed up one reaction. This is because for an enzyme to work a substrate has to be the correct shape to fit into the active site.5) This called the 'lock and key' model because the substrate fits into the enzyme just like a key fits into a lock.

Enzymes need the right temperature and pHEnzymes need to be at a specific constant temperature to work at their optimum.1) changing the temperature changes the rate of an enzyme controlled reaction2) A higher temperature increases the rate at first3) But, if it gets too hot, some of the bonds, holding the enzyme together break. This changes the shape of the enzyme's active site and so the substrate will no longer fit and the enzyme won't work anymore. It has been denatured.4) All enzymes have an optimum pH that they work best at5) If the pH is too high or too low it interferes with the bonds holding the enzyme together. This changes the shape of the active site and denatures the enzyme

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Aerobic respiration

Respiration is a series of chemical reactions that release energy by breaking large food molecules. It happens in every living cell.The energy released by respiration is used to power some of the chemical reactions that happen in cells. E.g. the reactions involved in:Movement- energy is needed to make muscles contractActive transport- this process uses energy to move some substances in and out of cellsSynthesis of large molecules- lots of large molecules are made by joining smaller molecules together- this requires energy. For example:- glucose is joined together to make things like starch and cellulose, in plant cells-in plant cells animal cells and microorganisms, glucose and nitrogen are joined together to make amino acids. The amino acids are joined to make proteins.

Aerobic respiration needs plenty of oxygen1) aerobic just means 'with oxygen'2) It releases more energy per glucose molecule than anaerobic respiration3) This is the type that is used most of the time in the body4) Aerobic respiration takes place in animal and plant cells and some microorganisms.

Glucose + oxygen -> Carbon dioxide + waterC6H12O6 + 6O2 -> 6CO2 + 6H2O

Anaerobic respiration

Anaerobic respiration doesn't use oxygen1) Anaerobic just means without oxygen2) Anaerobic respiration takes place in animal and plant cells and some microorganisms when there's very little or no oxygen. For example:Human cells- when you do really vigorous exercise your body can't supply enough oxygen to your muscle cells for aerobic respiration so they have to start respiring anaerobically.Plant cells- if the soil a plant's growing in becomes waterlogged there'll be no oxygen available for the roots- the root cells will have to respire anaerobically. Bacterial cells- Bacteria can get under your skin through puncture wounds caused by things like nails. There's very little oxygen under your skin so only bacteria that can respire anaerobically can survive there.Energy is always released during anaerobic respiration but the products of the reactions are different depending on the type of cell it happens in

Anaerobic respiration can produce lactic acid 

Glucose -> Lactic Acid + Energy Released

Anaerobic respiration can also produce ethanol and carbon dioxideIn plant cells and some microorganisms like yeast, anaerobic respiration produces ethanol and carbon dioxide:

Glucose -> Ethanol + Carbon Dioxide + Energy Released


Fermentation is when microorganisms break down sugars into other products as they respire anaerobically. Humans use fermentation to make lots of things:Biogas1) Biogas is a fuel used for things like heating and lighting2) Lots of different microorganisms are used to produce biogas3) They ferment plant and animal waste, which contains carbohydrates4) The biogas they produce is mainly made of methane and carbon dioxide.Bread-1) bread is made using yeast2) The yeast ferments the carbohydrates in the flour and release carbon dioxide- this causes the bread to riseAlcohol1) Yeast ferment sugar to form alcohol (ethanol)2) The sugar used in alcohol production comes from things like grapes (wine) and barley (beer)

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Photosynthesis is a series of chemical reactions that use energy from sunlight to produce food (The food is glucose, a sugar)

Photosynthesis happens in: The cells in green parts of plants, e.g. leaf cells, or some microorganism, e.g. phytoplankton.Chlorophyll is needed for photosynthesis to happenIt's a green substance which absorbs sunlight and allows the energy to be used to convert carbon dioxide and water into glucose. Oxygen is produced as a waste product of photosynthesis.

Carbon dioxide + water -> glucose + oxygen

6CO2 + 6H2O -> C6H12O6 + 6O2

Organisms that photosynthesize form the start of food chains. They make the energy from the sun available to other organism by converting it to glucose. The energy is transferred when the photosynthetic organisms are eaten.

Plants use glucose in 3 main ways1) Glucose is used for respiration 1) plants use some of the glucose for respiration2) this process releases energy from the glucoseGlucose is used to make chemicals for growth1) glucose is converted into cellulose for making cell walls, especially in rapidly growing plants2) Glucose is combined with nitrogen from nitrates to make amino acids, which are then made into proteins.3) Glucose is also used to help make chlorophyllGlucose is stored as starchGlucose is turned into starch and stored in roots, stems and leaves. It's used at times when the rate of photosynthesis is slower like in the winter.

Rates of photosynthesis

Three factors affect the rate of photosynthesis1) the amount of light2) the amount of CO23) temperature

Any of the three factors can become the limiting factor. This just means that it stops photosynthesis from happening any faster.Which factor is limiting at a particular time depends on the environmental conditions:-at night- light is the limiting factor

Investigating photosynthesis

Transects1) Transects are a way of investigating how something changes across an area2) To set up a transect you just run a tape measure between two fixed points 3) Then you start at one end of the transect and collect the data you want4) Then you move along the transect and collect data again5) You just keep collecting data and moving until you reach the end of the transect


Some things that help you collect data:Light meter- you need to measure the level of light, e.g. if you were comparing plants in areas with different levels of light. You could use a light meter to do this- it's a sensor that accurately measures light level.Quadrat- to make data collection quicker and easier you could use a quadrat, a square frame divided into a grid of 100 smaller squares. For example, you can estimate the percentage of cover of a plant species on the ground by counting how much of the quadrat is covered by the species.Identification key- to identify the different plant species you're looking at you could use an identification key. It's a series of questions that you can use to figure out what a plant is. 

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Diffusion, Osmosis & Active Transport

The level of certain substances needs to be controlled in order for cells to function properly.


Diffusion is simple. It's just the gradual movement of particles from places where there are lots of them, to places where there are fewer of them. It is just the natural tendency to spread out

Diffusion is the passive overall movement of particles from a region of their higher concentration to a region of their lower concentration


Osmosis is a type of diffusion- the passive movement of water molecules from an area of higher concentration to an area of lower concentration

Osmosis is the overall movement of water from an area where it is in high concentration (dilute solution) to an area where it is in low concentration (concentrated solution) through a semi-permeable membrane

1) a partially permeable membrane is just one that allows only certain substances to diffuse through it, e.g. only small molecules2) This concentrated sucrose solution gets more dilute as move water moves in. The water acts like it is trying to 'even up' the concentration on either side of the membrane.3) Plants take in water by osmosis. There's usually higher concentration of water in the soil than in the plant, so the water is drawn into the root by osmosis

Active transport

Sometimes chemicals need to be moved from an area with a lower concentration of the chemical to an area with a higher concentration across a cell membrane. This is done by active transport which uses energy from respiration.

Active transport is the overall movement of chemicals across a cell membrane from a region of lower concentration to a region of higher concentration using energy released by respiration

For example: Plants take in minerals like nitrates through their roots by active transport. The concentration of minerals in root cells is normally higher than in the soil around them. Active transport uses energy from respiration to move minerals from the soil into the root cells.

cell structure and enzymes



Diffusion, osmosis & Active transport

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