- The movement of molecules from regions of higher concentration to regions of lower concentration is called diffusion.
- Movement by diffusion is passive.
- Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, temperature, and pressure.
- Facilitated diffusion is the passive transport of molecules along their concentration gradient across a biological membrane with the help of special proteins.
- The proteins form channels in the membrane for molecules to pass through.
- The porins are proteins that form huge pores in the membrane allowing molecules up to the size of small proteins to pass through. Example- pores in the outer membranes of the plastids, mitochondria and some bacteria.
- Aquaporins are integral membrane proteins that form water channels, which rotate and release the extracellular molecule bound to the transport protein inside the cell.
- Some carrier or transport proteins allow diffusion only if two types of molecules move together.
Fig. Facilitated diffusion
- In a symport, both molecules cross the membrane in the same direction
- In an antiport, both the molecules move in opposite directions.
- When a molecule moves across a membrane independent of other molecules, the process is called uniport.
Fig. Uniport, symport, antiport
- The movement of molecules against a concentration gradient, from the region of lower concentration to the region of higher concentration, with the help of ATP is called active transport.
- Active transport is an up-hill transport.
Fig. Active transport