Bio 3 - Exchange of Materials - Exchange in Plants

Plants need CO2 and water for Photosynthesis to take place. They get this by diffusion thorugh their leaves

The leaves flat shape increases the surface area for diffusion, and most plants have thin leaves, which reduces the distance CO2 has to diffuse across. Finally, the many air spaces in the leaves allow CO2 to come into contact with lots of cells. This provides lots of surface area for diffusion.

Theres a problem though. Leaf cells constantly lose water by Evaporation. If CO2 could diffuse freely in and out of the leaves, water would be lost very very quickly. 

Leaf cells do not need CO2 all the time, when it is dark they don't need it as they are not Photosynthesising. 

So leaves are adapted to only allow CO2 in when it is needed. They are covered in a waxy cuticle. This is a Waterproof and Gaspfroof layer. Then all over the leaf surface are small openings called Stomata.

The Stomata are opened when the plant needs CO2, but can be closed the rest of the time to control the loss of water. This is controlled by the Guard Cells.

If you pull up a plant you will see a mesh of tiny white roots. These roots are adapted to enable plants to take water and mineral ions from the soil as efficiently as possible. These roots are thin, divided tubes with a large surface area.

The cells on the outside of the roots near the growing tips also have their own adaptations. They increase the surface area for the uptake of substances from the soil. They are known as Root Hair Cells, they have tiny projections out from the cells which push out between the soil particles.

The membranes of the Root Hair Cells increase the surface area for diffusion and osmosis. The water then only has a short distance to move across the root to the Xylem where it is moved up and around the plant.

Plant roots are also adapted to take in mineral ions by active transport. They have plenty of Mitochondria to supply the energy they need. They also have all the advantages of a large surface area and the short pathways needed for the movement of water.