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Biology Unit 3
Leaves
Carbon dioxide diffuses into the air spaces within the leaf. It then diffuses into the cells for
photosynthesis. (6CO2 + 6H2O -> C6H12O6 + 6O2)
The underneath of the leaf is an exchange surface. It's covered with little holes called stomata which
the carbon dioxide diffuses through. Water vapour and oxygen diffuse out of the stomata. The size
of the stomata is controlled by guard cells. These close if the plant is losing too much water. The
flattened shape of the leaf increases the exchange area.

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Biology Unit 3
The area inside the leaf forms another exchange surface which increases the area of the surface so
more carbon dioxide can get into the cells.
The water vapour escapes from the stomata by transpiration which is where the water vapour
escapes by diffusion because there is a lot more in the plant than in the air outside. Therefore,
transpiration is faster in hot, dry windy conditions.
A photometer is used to measure the rate of transpiration from a plant.…read more

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Biology Unit 3
Transport of water into root hairs
1. As ions are absorbed into the root hairs by active transport, the water potential lowers
2. Water will then follow along a concentration gradient into the hairs by osmosis.
3. The water then enters the xylem which travels into the leaves for photosynthesis
4. 99% of this water may be lost by transpiration
5.…read more

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Biology Unit 3
Respiratory System:
The breathing system gets oxygen from the air into the bloodstream for respiration. It also gets rid
of carbon dioxide from the bloodstream. The lungs are in the thorax (the area above the diaphragm).
The lungs are protected by the rib cage. The air first passes though the mouth, past the epiglottis and
then into the trachea. The trachea splits into two bronchi to each lung. The bronchi split into smaller
tubes called bronchioles.…read more

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Biology Unit 3
Alveoli
They are small sacks at the end of bronchioles. They cause a massive increase in surface area. The
diffusion distance is very small (1 cell thick). The endothelium (wall of the capillary) is flattened. The
epithelium (alveolar wall) is flattened. The lining of the alveoli is moist for absorbing gases. They have
a very large blood supply.…read more

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Biology Unit 3
Absorption in the digestive system
Inside the small intestines, duodenum and ileum, there are millions and millions of tiny projections
called villi. They have a central capillary and lacteal capillary so that absorption is fast and direct. Food
is absorbed either by diffusion or active transport. The villi have a large surface area.
Gas Exchange in Insects
Insects, being larger and having a hard, chitinous and therefore impermeable exoskeleton, have a
more specialised gas exchange system.…read more

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Biology Unit 3
Gas Exchange in Fish
Fish use gills for gas exchange. Gills have numerous folds
that give them a very large surface area. The rows of gill
filaments have many protrusions called gill lamellae. The
folds are kept supported and moist by the water that is
continually pumped through the mouth and over the gills.
Fish also have an efficient transport system within the
lamellae which maintains the concentration gradient
across the lamellae.…read more

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Biology Unit 3
every cell in the body. The veins then take the used, deoxygenated blood back to the heart at low
pressure. However, the pulmonary vein is an exception.
Single circulatory systems
Blood passes once through the heart for one trip around the body. Fish, amphibians and reptiles have
a single system]. This is because they have a lower oxygen demand.…read more

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Biology Unit 3
2. The use of Stents
Stents are narrow tubes capable of supplying a blood flow equivalent to an artery. Fatty deposits
building up on the inner surface of the arteries that supply blood to the heart muscles can cause
coronary heart disease. These fatty deposits restrict the blood flow increasing blood pressure and
causing heart attacks. To combat this potential fatal condition you can insert stents into the arteries
to increase blood flow to the heart muscles.…read more

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Biology Unit 3
Capillaries
Most exchange of nutrients and gases takes place here. Small diameter, large surface area for
diffusion
Lungs > carbon dioxide is exchanged for oxygen
Tissues > O2, CO2, nutrients and wastes are exchanged
Kidneys > wastes are released to be eliminated from body
Intestine > nutrients are picked up, wastes are released
Exercise
When you exercise, your body adapts to an increased demand of oxygen and glucose.…read more

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