Exchange of Materials

A lot of the information was taken from the 'Nelson Thornes AQA GCSE Biology' textbook.

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  • Created on: 17-04-11 14:01
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Exchange of Materials
The net movement of particles from an area of high concentration to an area of low concentration.
Occurs in liquids and gases as their particles are free to move about.
Factors affecting the rate of diffusion:
Temperature: the higher the temperature the greater the kinetic energy of the individual particles
and hence the quicker they spread out. Diffusion occurs more rapidly in warm conditions than in
cold conditions.
Barriers: if diffusing particles meet an impermeable barrier then they will not pass through and
diffusion will stop. Most membranes are partially permeable, i.e. they let some particles through but
not others; e.g. cell membrane. These membranes select which particles pass through. Small
particles can usually pass through by diffusion quite easily but large particles cannot pass through.
The Medium: the state of medium in which particles are moving can affect their rate. Diffusion is
slowest in solids, then liquids and most rapid in gases.
Diffusion Gradient: the difference in concentration between two areas. Diffusion always occurs from
the region of high concentration to the region of low concentration. The steeper the gradient the
bigger the difference. If the concentration of particles in an area is equal then there is no gradient
and the particles will be moving equally in all directions so there is no net movement. If particles
are equally spread out they are at equilibrium.
The cell membrane is an example of diffusion in living things:
Water, glucose, amino acids and oxygen move across the cell membrane.
The movement of water molecules from a high water concentration (a dilute solution) to a low
concentration of molecules (a more concentrated solution) through a partially permeable membrane.
Examples of osmosis include:
Water moving into root hair cells of plant roots.
Water moving into plant cells around the plant to keep it upright.
Water moving from the large intestine into the blood after a meal.
The re-absorption of water in the kidneys.
Active Transport
The cellular movement of substances from an area of low
concentration to an area of low concentration.
This means substances move against the concentration gradient and
as a result cells can absorb ions from very dilute solutions.
It also makes it possible for them to move substances like sugars and
ions from one place to another through the cell membranes.
Because the substances are being transported against a gradient,
energy is required for an active transport system to carry a molecule
across the membrane and then return to its original position.
This energy comes from cellular respiration.
Cells involved in active transport usually have lots of mitochondria to
provide the energy they need:
Absorption of mineral ions by the root hair cells in roots in plants.
Absorption of glucose by epithelial cells in the small intestine.
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Cyanide is a deadly poison that smells faintly of almonds; once you have taken it, you quickly die.
It kills you because it stops the reactions of respiration in your mitochondria.
If you give individual cells cyanide all active transport stops as their energy supply dries up.
However, if you supply the cells with energy, even though the mitochondria are still poisoned, active
transport starts up again.…read more

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Exchange in the Gut
The exchange of materials in the gut is extremely important.
The food eaten is broken down in the gut to form simple sugars such
as glucose, amino acids, fatty acids and glycerol.
These nutrients need to be absorbed into the blood across the wall
of the small intestine.
They are absorbed by diffusion and active transport.…read more

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The Respiratory System in Insects
Insects have an internal respiratory system which supplies oxygen directly to their cells and removes
carbon dioxide.
They have spiracles which open when oxygen is needed but close when it isn't.
This prevents water loss.
The spiracles lead to a system of tubes which run into the cells of the tissues
themselves called tracheoles.
This is where most of the gas
exchange takes place. The tiny
tubes are freely permeable to
gases.…read more

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Uptake of Water and Mineral Ions
The roots in a plant are adapted to enable plants to take water and
minerals from the soil as efficiently as possible.
The roots are thin, divided tubes with a large surface area.
The root hair cells on the ouside of the roots are adapted to increase
surface area for the uptake of substances from the soil.…read more


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