Gas and solute exchange

L.O to learn about osmosis and learn why it is important.

All organism need to exchange substance, this is done via a number of method; diffusion, osmosis or active transport.
In order for life process, such as respiration or photosynthesis to occur, certain molecules need to enter organisms, such as water, oxygen or carbon dioxide. These substances all enter or leave via the processes mentioned above

As mentioned previously osmosis is the movement of water from an area of high water concentration to an area of low water concentration across a partially permeable membrane.
Diffusion is the passive movement of particles from an area of high concentration to an area of low concentration; therefore in many ways osmosis and diffusion are very similar.
However active transport is completely different. Active transport uses energy to transport molecules from an area of low concentration to an area of high concentration, therefore going in the opposite direction to diffusion and osmosis.

Exchange surfaces

Exchange surfaces are adapted in the following ways to make the process as efficient as possible

  • They are thin – this reduces the diffusion distance and ensures the substance diffuses across very quickly
  • A large surface area – this ensures that many substances can diffuse across at the same time

Large blood supple – this maintains the diffusion gradient


leaf cell
The under side of the leaf is the exchange surface, this part of the leaf has many holes and pores called stomata which allow substances to pass through the cell
Water and oxygen diffuses out of the cell and carbon dioxide will diffuse into cell
The stomata are controlled by guard cells which control when substance enter and leave the cell
The flattened shape of the leaf increases the area of exchange ,which makes it more efficient
There are air spaces within the leaf which allow the substances to exchange

The water is evaporate as soon as it leaves the cell which keeps the concentration gradient high, this is best in hot dry windy conditions where more water can be evaporated quickly.

Active Transport

In plants root hair cells use the process of active transport to take in minerals. Root hair cells are very long and have a very large total surface area which allows it to absorb as much water as possible

When it comes to absorbing minerals there is usually more minerals in the root hair cell than in the soil surrounding it, therefore the cell uses active transport to absorb as much minerals as possible.

Water flow through plants

Xylem tubes take water up through the plant; it is made of dead cells. The process of transpiration carries the water.
Transpiration is caused by evaporation and diffusion, so as water leaves the plant through the leaves it will leave a gap which will then be filled by the water below, therefore creating a running transpiration stream, so as water is lost ist is constantly replaced.

Phloem tubes carry food through the plant. They transport food substances such as sugars and proteins. The food can travel in both directions

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