Biology AQA Unit 3

Seeing as the B3 exam (AQA Board) is in less than 48 hours, I figured it would be a good idea to make sure I have a full set of revision notes. I'll place them up here, so that if anyone else wants to use them, then you can. :D

  • Created by: Bexie
  • Created on: 19-05-12 12:57

Active Transport

Active Transport - The movement of substances against a concentration gradient and/or across a cell membrane, using energy from respiration (mitochondria). Moving from a low to a high concentration gradient.

Substances can move passively in and out of cells by diffusion, until the concentration gradient on both sides of the cell membrane reaches an equilibrium. Substances can continue to move in and out of cells using active transport.

During active transport, protein carriers in the cell membrane 'pick up' particles and move them against the concentration gradient. Diffusion and Osmosis depend on a Concentration Gradient in the right direction to work.

Example of Active Transport in Humans; In the gut, nutrients are moved from the gut (a Low Concentration) to the blood (a High Concentration)

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Plasma - Liquid carries blood cells andother substances, such as glucose and carbon dioxide.

Red Blood Cells - contain haemoglobin, this binds to the oxygen creating oxyhaemoglobin. The oxygen can then be released to cells.

White Blood Cells - Make antibodies.

Placelets - Help with the clotting of the blood.

Arteries - Carry oxygenated blood away from the heart and to the organs. When the arteries run close to the surface, it feels like a pulse.

Veins - Carry low in oygen blood to the heart. Don't have a pulse but often have valves to prevent the back-flow of blood as it moves around the body.

Capilaries - In the organs of your body, they are narrow and thin (single called thick) so that Glucose and Oxygen can travel by Diffusion. 

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Breathing in other Organisms

Insects; Spiracles open in the body, to allow gases to diffuse in. They also help to prevent water loss (just like the Guard Cells of a plant). The Oxygen is delivered directly to tissues via the tracheoles. The tubes are thin and moist, and air is pumped through by the insect to maintain a Concentration Gradient.

Fish; Gills are made up of thin layers of tissue with a rick blood supply. The gills are thin, so there is only a short distance for the necessary gases to diffuse across. Surfaces are always moist.

In bony fish, the gills are contained in a gill cavity. The fish have to keep swimming, so that water is pumped over them to maintain a Concentraion Gradient. Gills can't work in air, due to not having big enough surface areas.


Whatever the organism, gas and solute exchange depends on a large surface area, moist surfaces, short diffusion distances and a large concentration gradient.

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Aerobic Respiration - Where muscles release energy.

Glycogen is a carbohydrate which can be converted rapidly into Glucose. This supplies the fuel needed to provide the energy for cellular respiration when your muscles contract.

Glucose + Oxygen ------> Carbon Dioxide + Water.

More excersise means more muscle activity, so more glucose and oxygen must be suplied and more carbon dioxide must be removed. Therefore several changes take place in the body;

1) Breathing rate increase and breathe deeper to meet the extra oxygen demand.

2) Heart Pumps faster.

3) Arteries supplying the muscles dilate.

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Anaerobic Respiration - Glucose broken down without oxygen, occurs during vigorous exercise when enough oxygen can't be supplied to the muscles.

Glucose ---------> Energy + Lactic Acid.

Allows muscles to keep working but does not release as much energy as Aerobic Respiration and build up of Lactic Acid can be painful.

This leads to Oxygen Debt, oxygen is needed after exercise to break down Lactic Acid in to Carbon Dioxide and Water.

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Alveoli have several adaptations to help make gas exchange more efficient. These are;

1) Very thin walls (single celled thick). This means shorter diffusion distances.

2) Covered by network of capillaries. This maintains a steep Concentration Gradient, by enabling gases to pass almost directly between the lungs and bloodstream.

3) Moist, so that gases can dissolve easily, and diffuse.

4) Large combined surface area, allowing large amounts of gases to be exchanged (maximises diffusion).

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The wall of the Small Intestine is lined with 'Villi'. They are finger-like projections with several adaptations to increase the speed of Diffusion. Including;

1) Rich Blood Supply, to produce a steep Concentration Gradient for efficient diffusion.

2) Large Surface area for diffusion.

3) Thin walls (only one cell thick), so that there is only a short distance for Diffusion to occur.

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Exchange in Plants

Plants have stomata which allow them to obtain Carbon Dioxide from the atmosphere by diffusion. Leaves are flat and thin, with internal air spaces, to increase the amount of surface area available.

Water moves into the roots from the soil by Osmosis.


Water moves up from the roots to the stem.


Water moves up from the stem and into the leaves.


Water is lost from the leaves by evaporation through open stomata.

(otherwise known as Transpiration)

Water moves up a plant in the transpiration system, when the water evaporates from the surface of the leaves, water is pulled up through the xylem to take its place. This movement of water particles, is known as the Transpiration Stream.

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A leaf has several layers;

1) A Waxy Cuticle helps to keep the water in.

2) Palisade Layer has cells which contain several chloroplasts to capture/absorb the light. Which can then be used for Photosynthesis.

3) Stomata on the lower surface, allow gases in and out. They can then be closed by the Guard Cells to conserve water.

4) Spongy Mesophyll layer has air spaces, to allow gases to move between the cells.


Three environmental factors, which could decrease the rate of water loss in plants are; A Moist Atmosphere, Less Wind and Less Sun.

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Layers within the Leaf


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Benefits of Exercise:

1) Improves Health. (e.g. Heart and lungs increase in size, develop a more efficient blood supply and function as effectively as possible).

2) High rate of Metabolism

3) Releases Endorphins

3) Pumps Blood around the body Faster (Increases the heart rate).

4) Muscle Growth

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What do the ribs do?

 - When breathing in, they move up and out / - When breathing out, they move down and in.

What does the diaphragm do?

 - When breathing in, it moves down. / - When breathing out, it moves up.

What does the space inside our chest do?

 - When breathing in, it gets bigger. / - When breathing out, it gets smaller.

What does the Pressure do?

 - When breathing in, it decreases. / - When breathing out, it increases.

What do the lungs do?

 - When breathing in, they inflate. / - When breathing out, they deflate.

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Double Circulatory System


The right side pumps de-oxygenated blood to the lungs, to collect Oxygen and remove any Carbon Dioxide. The left side does the opposite and pumps oxygenated  blood around the body.

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Kidneys are important for maintaining homoeostasis.

A healthy kidney produces urine by filtering the blood. It then re-absorbs all of the sugar, and the mineral ions and water needed by your body. Excess mineral ions and water, along with urea, are removed in the urine.

A high pressure forces water, urea, ions and sugar out of the blood and into the Bowmans capsule. Large molecules like proteins and blood cells can not fit through.

Useful substances, such as sugar and some dissolved ions can be actively re-absorbed against a concentration gradient. (Via Active Transport)

Remaining substances including urea, continue down nephron and down to the bladder, by collecting duct.

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Kidney Transplants and Dialysis


Advantages - Live more normal life (e.g you can eat what you want)

Disadvantages - Long waiting list. Drugs must be taken that suppress the immune system, so there is a risk of infection.



Advantages - Allows to survive. Don't need an operation. No risk of rejection.

Disadvantages - Expensive to run. Not pleasant. Several sessions a week needed.

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Yeast is a single celled microbe, which can respire with or without oxygen.


 - When respiring anaerobically, it produces ethanol and carbon dioxide in a process known as fermentation. (Sugar -------> Ethanol + Carbon Dioxide) This process can often be used in the production of beer and wine.

 - When respiring aerobically, it produces carbon dioxide and water. This process can be used to make dough rise. (Sugar + Oxygen ------> Carbon Dioxide + Water)


Fermenters are used to grow microbes on a large scale, following are required:  - Food in nutrient medium.  - Air to provide oxygen.  - Water cooled jacket, as microbes make heat by respiration.  - pH probe.  - stirrer to keep microbes in suspension and maintain an even temperature.

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Antibiotic Production

Timeline for the discovery of Penicillin:

  • Fleming was studying bacteria on petri dishes.
  • By accident, left one open before he went on holiday, leaving it to be contaminated by the air.
  • After returning, he realised that a mould had grown but near it, there was no other bacteria.
  • He named this mould Penicillin.

Commercial Production of Penicillin:

Contains sugar, amino acids, mineral salts and other nutrients. Made from soaking corn in water. fermenters with strong paddles are used to keep stirring the broth and to keep on introducing the Oxygen. Cooling jackets control the overall temperature. When the nutrients are used up, we are left with penicillin.

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Food Production using Bacteria

Bacteria is used in Yoghurt and Cheese making.

Cheese Making:

Bacteria are added to milk, which forms solid curds. The curds are then separated from the liquid whey and left to mature.

Yoghurt Making:

Milk is heated to kill any bacteria, a starter bacteria culture is then added. Bacteria ferment lactose sugar into a lactic acid. The lactic acid causes the milk to clot and then solidify.

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Biogas can be made by anaerobic fermentation of waste material in a generator. Methane is one of the largest outputs (as well as slurry, which can be used as fertiliser) and the gas can be used for cooking and heating.

- Batch generator - Gas is made in small batches, then the generator is filled with waste and cleared out at the end of each session.

- Continuous - Gas is made all of the time as waste material is continuously fed in and digested material is removed.


Biofuel: Ethanol is made from sugar cane juice or from glucose obtained from maize starch. There are several advantages from using Bioful, including;

  • Doesn't contribute to acid rain.
  • Is carbon neutral, as any Carbon Dioxide released is taken in by plants.
  • Uses cheap and readily available materials.
  • Digested material can be used as a fertiliser.
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