Substances move by diffusion, osmosis and active t
- Diffusion - particles move from an area of high concentration to an area of low concentration. Dissolved particles can diffuse in/out of cell membranes.
- Osmosis refers to water. It moves across a partially permeable membrane from an area of high water concentration to an area of low water concentration.
Sometimes substances need to move in other directions - active transport.
In life processes, gas and dissolved substances have to move through an exchange surface. The structures of these have to allow enough of the necessary substances to pass through.
Exchange surfaces are ADAPTED to maximise effectiveness.
The structure of leaves lets gases diffuse in and
- Carbon dioxide diffuses into the air spaces within the leaf, then into cells - photosunthesis occurs. Leaf's structure is adapted.
- Underneath of leaf is an exchange surface - covered in stomata where carbon dioxide diffuses in through. Water vapour (partly lost through leaf's surface) and oxygen also diffuse out through the stomata.
- Size of stomata controlled by guard cells: they close if the plant is losing water faster than it is being replaced by the roots. Without the guard cells, plant would soon wilt.
- Flattened shape of leaf increases exchange surface area - more effective.
- Walls of cells inside leaf form another exchange surface - air spaces increase surface area so there's more chance carbon dioxide can diffuse into cells.
Water vapour escapes by diffusion - alot more of it inside than outside. This is transpiration; goes quicker in hot/dry/windy conditions.