Diffusion is the NET movement of particles down a concentration gradient, from a higher concentration to a lower concentration. It requires no energy so is a passive process.
Lipid soluble molecules (steroids) and very small molecules (H2O) diffuse directly through the bilayer.
Other charged or larger molecules have to move through the membrane via channel and carrier proteins, this is called facilitated diffusion. It also requires no energy and can only occur down a concentration gradient. Each protein is only specific to one molecule therefore giving the membrane a certain level of control.
Active transport is the movement of molecules/ions across membranes, which uses ATP to drive protein 'pumps' within the membrane. Carrier proteins are also used in this process however these protein pumps differ from the ones used in facilitated diffusion
- carry molecules in one direction
- use metabolic energy in the form of ATP
- can carry molecules at a much faster rate than diffusion
Endocytosis and exocytosis occurs when moving larger molecules (proteins) in'out of the cell in bulk. It is transported in vesicles and requires energy in the form of ATP.
Osmosis is the diffusion of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
Pure water has the highest water potential of 0kPA. The water potential of cells is therefore less than that of pure water, because of all the sugars dissolved in the cytoplasm. This means when placed in pure water, water passes into the cell by osmosis causing it to swell. In plant cells this is called turgid, and an animal cell will eventually burst and become haemolysed.
When placed in a concentrated solution water passes out of the cell by osmosis causing plant cells to become plasmolysed and animal cells to become crenated.