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Gas and Solute Exchange
Life processes need gases or dissolved substances and this happens by diffusion,
osmosis and active transport. The exchange surfaces are adapted to maximise
Diffusion ­ particles move from an area of high concentration to an area of low
Osmosis ­ the movement of water across a partially permeable membrane from a
region of high concentration to a region of low concentration.
Active transport ­ moves substance in the other direction from diffusion and osmosis.
The structure of leaves lets gases diffuse in and out of cells. The underneath is an
exchange surface, covered in stomata's which carbon dioxide diffuses in through. Water
vapour and oxygen diffuse out of the stomata. Guard cells control the size of the
stomata. They close if the plant is losing water faster than it is being replaced. The flat
shape increases the area for effectiveness. The walls of the cells in the leaf form another
exchange surface. The air spaces inside the leaf increase the area so more carbon
dioxide can get in.
Transpiration ­ when water vapour escapes by diffusion because there's a lot on the
inside and less on the outside. Happens quickest in hot, dry, windy conditions.…read more

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The Breathing System (breathing is different from respiration)
The lungs are in the thorax which is in the upper part of your
body and separated by the diaphragm from the lower part of
the body. The air breathed in goes through the trachea. This
splits into two tubes called `bronchi' which split into smaller
tubes called bronchioles which end at the alveoli where gas
exchange takes place.
Breathing in means the intercostal muscles and diaphragm
contract, the thorax volume increases and this decreases the
pressure, drawing air in.
Breathing out means intercostal muscles and diaphragm relax,
the thorax volume decreases and air is forced out.
(see pg74 for diagrams).…read more

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Diffusion Through Cell Membranes
The lungs transfer oxygen to the blood and removes
waste carbon dioxide. To do this the lungs contain
millions of alveoli. The alveoli are specialised to maximise
the diffusion of oxygen and CO. They have 1) an
enormous surface area 2) a moist lining for dissolving
gases 3) very thin walls and 4) a copious blood supply.
Inside the small intestine is covered with millions of tiny
little projections called villi. They increase the surface
area so that digested food is absorbed quickly into the
blood. They have 1) a single layer of surface cells and 2) a
very good blood supply to assist quick absorption.…read more

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Active Transport
The cells on the surface of plant roots grow into long `hairs' which stick out
into the soil giving the plant a big surface area to absorb water and mineral
ions from the soil. Most of the water and mineral ions that get into the
plant are absorbed by the root hair cells.
The concentration of minerals is usually higher in the root hair cell than in
the soil around it so normal diffusion doesn't explain how minerals are
taken up into the root hair cell. They should go the other way. Active
transport allows the plant to absorb minerals against a concentration
gradient. But active transport needs energy from respiration to make it
work. Active transport happens in humans too.
Active transport is used in the gut when there is a low concentration of
nutrients in the gut, but a high concentration of nutrients in the blood.
When there's a high concentration of glucose and amino acids in the gut
they diffuse naturally into the blood. Sometimes there's a lower
concentration. This means the concentration gradient is the wrong way.
Active transport allows nutrients to be taken into the blood, despite the
fact that the concentration gradient is the wrong way.…read more

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The Circulation System
The heart is actually two pumps. The right side pumps deoxygenated blood to the
lungs to collect oxygen and remove carbon dioxide. Then
the left side pumps this oxygenated blood around the body.
Arteries carry blood away from the heart at high pressure.
Normally, arteries carry oxygenated blood and veins carry
deoxygenated blood. The pulmonary artery and pulmonary
vein are the big exceptions to this rule.
The arteries eventually split off into thousands of tiny
capillaries which take blood to every cell in the body.
The veins then collect the `used' blood and carry it back
to the heart at low pressure to be pumped round again.
Fish only have a single circulation system and the blood
goes straight from the heart to the gills. Even more curious
circulation systems are worms who have five hearts but
don't have any circulation system.…read more

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