AQA gcse biology unit 2

Notes on unit 2 of gcse biology

- exam board: AQA 

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  • Created by: chloe
  • Created on: 11-05-13 14:20

Animal and Plant Cells

All living things are made of cells.

Anima and plant cells, have the following parts:

  • cell membrane 
  • nucleus
  • ribosomes
  • cytoplasm 
  • mitochondria

Plant cells also have:

  • cell wall
  • permanent vacuole
  • chloroplasts 
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Cells structures and what they do

Chlorophyll - a green pigment found in chloplast which is invloved with absorbing light for photosythesis.

Permanent vacuole - filled with cell sap in plant cells

Nucleus – controls the activities of the cell ( contains DNA)

Cell wall - strengthens/supports the plant cells.

Ribosomes - where protein synthesis occurs

Cell membrane - controls the passage of substances in and out of the cell.

Chloroplasts - absorb light energy to make food in plant cells

Mitochondria - where most energy is released in respiration (ATP)

Cytoplasm – where most of the chemical reactions take place

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specialised Cells

Root hair cell

  • Absorbs water and mineral ions from the soil 
  • long ' finger-like' process with thin walls, gives it large surface area root hair cell - has a head and tail (

Sperm Cell 

  • The head contains genetic information and an enzyme to help penetrate the egg cell membrane
  • The middle section is packed with mitochondria for energy
  • long tail moves the sperm to the egg
  • Fertiles an egg - female gamete 
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Specialised Cells

Red blood cells 

  • Contians haemoglobin - to carry oxygen to the cells
  • thin outer membrane - lets oxygen diffuse easily
  • Shape - Increases surface area ( allows more oxygen to be absorbed) 
  • No nucleus - more room for haemoglobin 

Guard cell

  • Sesitive to light ( closes at night ) 
  • kidney shape 
  • thin outer wall and thick inner wall - for opening a closing 

Palisade lead cell

  • Tall - more surface area for absorbing carbon dioxide 
  • packed with cloroplast near the top - for photosythesis 
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Specialised Cells

Red blood cells 

  • Contians haemoglobin - to carry oxygen to the cells
  • thin outer membrane - lets oxygen diffuse easily
  • Shape - Increases surface area ( allows more oxygen to be absorbed) 
  • No nucleus - more room for haemoglobin 

Guard cell

  • Sesitive to light ( closes at night ) 
  • kidney shape 
  • thin outer wall and thick inner wall - for opening a closing 

Palisade lead cell

  • Tall - more surface area for absorbing carbon dioxide 
  • packed with cloroplast near the top - for photosythesis 
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Movement in and out of cells - diffusion


  • Diffusion is the movement of particles from an area of HIGH concentration to an area of LOW concentraion. 
  • happens in liquids and gases
  • only small particles can diffuse through the cell membranes 
    - glucose, oxygen and amino acids
    - big molecules like starch and proteins can't 
  • rate of diffusion depends on:
    Distance  - if not far, rate of diffusion is quicker 
    Surface area - larger SA = faster rate of diffusion
    Bigger the difference of concetration = faster the rate of diffusion 
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Movement in and out of cells - osmosis

  • A type of DIFFUSION - a passive movement of water molecules 
  • partially permable membrane - only small water molecules can PASS
  • OSMOSIS is the movement of WATER molecules across a PARTIALLY PERMERBALE MEMBRANE from a region of HIGH water concentration to a region of LOW water concentraion
  • A stronger sugar solution gets more dilute ( osmosis trys to even solution up)
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  • PHOTOSYTHESIS is the process that produces food ( glucose ) for plants 
  • Light energy is absorbed by a green substance called clorophyll 
  • photosythesis takes place in leaves of all green plants 
  • water reaches chloroplast through leaf veins and carbon dioxide diffuses into leaf
  • Four things needed for photosythesis:
    light - from the sun
    water - from soil
    carbon dioxide - diffuses from air into lead 
    chlorophll - green substance found in chloroplast  

    Limitng factors for photosythesis:  

  •  Temperature - A low temperature will limit the rate as the molecules will move less and therefore the reaction happens slower
  • Carbon dioxide - A shortage of CO2 will limit the rate as fewer molecules will be available for the reaction.
  •  Light intensity - A shortage of light means there is less energy to power the reaction
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Food chains + energy transfers

  • Radiation from the Sun is the source of energy for most communities of living organisms. 
  • Green plants capture a small part of the solar energy which reaches them
  • This energy is stored in the substances which make up the cells of the plants

Food chain = a diagram to represent feeding relationships between organisms (showing direction of energy flow)

Food web = diagram that represents all of the feeding relationships between organisms in an ecosystem

  • At each stage in a food chain, less material and less energy are contained in the biomass of the organisms 
  • Some materials and energy are always lost in the organisms’ waste materials ( faeces )
  • some energy is eventually lost as heat to the surroundings
  • some energy is lost during movement and urinating 
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Pyramid of numbers + biomass

  • Pyramids of number diagrams display the numbers of organisms at each trophic level in a food chain
  • Each block is drawn to scale
  • Pyramids of biomass 

Biomass = mass of living material

All pyramids of biomass are pyramid shaped.

The mass of living material (biomass) at each stage in a food chain is less than it was at the previous stage.

 From the bottom of the pyramid up: oak tree, caterpillar, blue ***, sparrowhawk. This is a regular pyramid shape as the base is wide and the top narrow. ( a narrow bar representing few sparrowhawks. A slightly wider bar represents a bigger number of sparrows, and catapillars are represented by the widest bar, showing they have the highest number. At the bottom of the food chain is an oak tree, and as one oak tree can feed many caterpillars, only one is needed - represented by a thin bar again. (

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Carbon Cycle

  1. Carbon enters the atmosphere as carbon dioxide from respiration and combustion.
  2. Carbon dioxide is absorbed by producers to make carbohydrates in photosynthesis.
  3. Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxide formed during respiration. The animals and plants eventually die.
  4. The dead organisms are eaten by decomposers ( e.g.  detritus feeders) and the carbon in their bodies is returned to the atmosphere as carbon dioxide. In some conditions decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion.


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Enzymes - structure and function

Enzymes are biological catalysts
- Catalysts increase the rate of chemical reactions.

  • Enzymes are protein molecules made up of long chains of amino acids.
  • These long chains are folded to produce a special shape which enables other molecules to fit into the enzyme. 
  • This shape is vital for the enzymes function.Normally only one type of molecule (the substrate) will fit into the enzyme.

The active site is the part of the enzyme which the substrate fits into


Activation Energy

  •  energy is required.In order for a chemical reaction to take place,

This is called the activation energy.

  • Enzymes reduce the activation energy of a reaction.
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Effects of temperature and pH on enzymes

In most chemical reactions as the tempature increases so does the reaction. 

  • The enzyme and substrates move around faster so they collide more often.
  • The temperature when the enzyme is working fastest is called the optimum.
  • 37˚C is the best tempertaure for enzymes to work at 
  • Higher than 40˚C and the structure of the enzyme changes
    -As a result, the active site becomes a different shape and the substrate no longer fits it is then described as denatured.

pH can also affect the shape of the active site.

  • It does this by affecting the forces that hold the enzyme molecule together
  • .A change in pH can stop the enzyme completely.
  • Different enzymes work best at different pH
    Eg. Stomach enzymes work best in acidic conditions - between 0-6 on pH scale 
  • Mouth enzymes work best in neutral conditions.

    ( (

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Respiration is the process of releasing energy from the breakdown of glucose. It takes place in every living cell, all of the time and all cells need to respire in order to produce the energy that they require.

Aerobic respiration

Aerobic respiration requires oxygen.
It happens in cells when glucose reacts with oxygen. 

glucose + oxygen    →    carbon dioxide + water (+ energy)

Anaerobic respiration

Anaerobic respiration does not need oxygen.
It happens when there is not enough oxygen for aerobic respiration. 

glucose    →    lactic acid (+ energy) 

Much less energy is released by anaerobic respiration than by aerobic respiration.

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Digestion in mouth and stomach

1. Food is chewed to create a larger surface area for the action of enzymes
2. Saliva is released which contains amylase
3. Amylase digests starch into smaller sugars (maltose)
4. Further chewing enables swallowing.
5. The food then enters the oesophagus
6. Food enters the stomach from the oesophagus
7. The walls of our stomach produce juice

This juice contains:

  • A protease enzyme – called pepsin
     This digests proteins into amino acids
  • Hydrochloric acid – this kills bacteria in our food
  • Mucus – this protects the wall of our stomach from acid and pepsin
  • The wall of our stomach is muscular, and churns our food.
  • The food remains in our stomach for a few hours. 
  • The proteins are digested.
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Digestion + absorption in the small intestine

The small intestine jobs are to :

  • absorb soluble products of digestion into the blood 
  • complete digestion of food

 Digestion in the small intestine 

3 juices are released and they are intestinal juice, pancreatic juice and bile
Intestinal juice + pancreatic juice (Both released INTO the small intestine) 
Both contain 3 main enzymes: 
Amylase to complete the digestion of starch into sugars.
  - Protease to complete the digestion of proteins into amino acids. 
  - Lipase to break down fats into fatty acids and glycerol.

 2 main things in bile:
- Alkali to neutralise the stomach acid
- Bile salts which convert large fat droplets to small fat droplets – for a large 

  • Produced by the liver
  • surface area for the enzymes to act on
  • Released into the small intestine.
  • Stored in the gall bladder
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Use of enzymes

In the home, biological detergents may contain protein-digesting and fat-digesting 

enzymes (proteases and lipases)


  • more effiecent at removing stains 
  • lowing washing temperatures - saves energy 


  • if clothes are not fully rinsed - the protease enzyme may stay in the clothes, which digests protien into the skin 
  • This can lead to allergies or irritation
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Use of enzymes ( industry )

  • Proteases are used to pre-digest the protein in some baby foods

      - This reduces how much the baby needs to digest the food

  • Carbohydrases are used to convert starch into sugar syrup

      - This is cheaper than extracting sugar from sugar cane

  • Isomerase is used to convert glucose syrup into fructose syrup

      - This is much sweeter


  • enzymes are sesitive to temperature and pH change
  • pH and temperature needs to be controlled and maintained which is expensive


  • enzymes enable indutrial reactions to take place at lower temperatures  
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Homoeostasis means keeping/ maintaing a consistant internal envrioment.

Internal condtions which are controlled are:

  • Temperature
  • ion content 
  • water content
  • blood sugar levels 

All enzymes work best at 37˚C  - Thermoregulation - maintenance of body temperature

Waste products have to be removed from the body this include:

  • Urea - Urea is produced in the liver and are a breakdown of excess amino acids, its removed by the kindneys in the urine which is then stored in the bladder.
  • Carbon dioxide - produced by respiration, leaves the body via the lungs when we breathe out.
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Thermoregulatory centre and the skin

Body temperature is monitored and controlled by the thermoregulatory centre in the brain. 
               - This centre has receptors sensitive to the temperature of blood flowing through the brain

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