Biology (B3) - Exchanges

This set is revision notes for AQA B3. For this I have used the text book by Nigel English as reference. In this sub topic of B3 there are 5 sub-sub topics and they are as followed:  Diffusion and Osmosis          Sports Drinks and Active Transport          Exchanges in Humans          Gaseous Exchange in Humans          Exchange Systems in Plants

Diffusion and Osmosis

Substances pass in and out of cells by either, diffusion, osmosis or active transport.

Molecules spread out from a high concerntration to a low concerntration. This is called diffusion, meaning the molecules are diffusing down a concerntration gradient.

This is how most molecules move in and out of human cells.

Cell membranes have specialised molecules to allow some ions and molecules through. 

When there are two solutions, one with dilute water and then a concerntrated solution, the water from the diluted water, if seperated by a semipermiable membrane, will move towards the concerntrated solution, this is called osmosis. 

Osmosis occurs when the plant absorbs water from the soil because soil water is more dilute. Osmosis is also used by animals but animal cells do not hav cell walls to protect them so if too much is absorbed, the cells burst but if the cells lose too much then the cell will shrink and will not be able to function properly.

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Sports Drinks and Active Transport

If the balance between ions and water is disturbed then cells do not work as efficiently, if we sweat we lose water but a smaller amount of ions, this can be helped by rehydrating drinks.

Most soft drinks will contain more water than ions but it is important for there to be sodium ions in the drink as this will help healthy functioning of most of the body's cells.

Sports drinks also help athletes to regain water and ions they lose through sweat and help atlethes to regain gluscose via the sugar in the drink and glucose is used up in respiration.

The best combination of water, ions and sugar is 6-8% of sugar and 120 mg/l. 

Sometimes the body needs to move substances from a low concerntration to a high concerntration so the body uses the energy from respiration to do this. 

Moving the molecules against the concerntration gradient is called active transport.

In the soil there is a low concentration of nitrate ions so the plant has to use active transport to move the nitrate ions into the roots.

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Exchanges in Humans


Gas exchange takes place in your lungs to get rid of the oxygen that you do need and the carbon dioxide that you do need, this is done through diffusion.

To carry out the gas exchange your lungs must have thin walls for the gas to diffuse for only a short distance, a good blood supply to transport the oxygen and carbon dioxide and a large surface area for difussion.

In the lungs are very small sacks called the alveoli which have all of these requirements.


The wall in your small intestine is very good at absorbing food as it has tiny folds called villi which give the small intestine a large surface area, the villi have many blood capillaries to transport absorbed food to the rest of the body, the villus are very thin so the food molecules only have to diffuse over a small distance.

The villi absorb food molecules through active transport.

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Gaseous Exchange in Humans - Structure of Breathin

If we inhale the intercostal muscles and the diaphram muscles contract. The intercostal muscles push the ribs outwards and the diaphram muscles lower the diaphram.

This change gives the thorax (the upper part of your body) more volume, expanding the lungs. This gives the gas inside less pressure causing air outside to move into the lungs, this process is known as inspiration.

When we exhale, the muscles relax, the pressure increases pushing air out, we call this expiration.

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Gaseous Exchange in Humans - Artificial Ventialtor

Spontanious breathing may stops due to illness and injury meaning the person needs help to breath. There are two devises. A machine called a ventilator or a compressed bag. 

There is a negative pressure ventilator which has nicknamed the iron lung. It encloses the whole of the subjects body except for the head and neck. A pumps removes the air in the tank causing the patients thorax to expand sucking in air. Air is then put into the vacuum to get the thorax to relax.

The positive pressure ventilators fore air into the patients lungs using a metal tube which is inserted through the patients mouth. This is used during operations where the patients muscles are deliberatly made to relax. The tube can also be placed through the neck, this is most commonly found for long-term use.

The hand-controlled ventilators are bags mainly used by paramedics for patients after accidents or drug overdoses. The air is supplied via a bag which is then controlled by hand.

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Exchange Systems in Plants

Water and minerals are absorbed through root hair cells. The root hair cells are surrounded by water and minerals so they are diffused into the roots and having a larger amount of root hair cells will increase the surface area so will speed up the diffusion.

Leaves absorb large amounts of carbon dioxide and sunlight by having a large surface area giving the leaf many air spaces known as stomata. The carbon dioxide moves through these air spaces into photosynthesising cells.

Water is lost through the leaves via the stomata, this is called transpiration.

When transpiration occours, the roots absorb more water. When the plant performs transpiration but the roots cannot absorb enough water, the plant wilts. The plant stops this by closing the stomata. 

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diaphragm* :)

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