Diffusion - the net movement of particles from an area of high concentration to an area of lower concentration. Particles diffuse down a concentration gradient.
It is a passive process, no energy is needed for it to happen.
Particles can diffuse across plasma membranes, as long as they can move freely through the membrane.
There are three main factors that affect the rate of diffusion:
The concentration gradient - the higher it is, the faster the rate of diffusion.
The thickness of the exchange surface - the thinner the exchange surface, the faster the rate of diffusion.
The surface area - the larger the surface area, the faster the rate of diffusion.
Osmosis - diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential.
Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution.
Water molecules can diffuse easily through the plasma membrane as they are small.
Pure water has a water potential of 0. Adding solute a to pure water lowers its water potential so the water potential of any solution is negative. The more negative the water potential, the stronger the concentration of solute in the solution.
If two solutions have the same water potential they're said to be isotonic. Cells in an isotonic solution won't lose or gain any water.
Solutions with a higher water potential compared with the inside of the cell are called hypotonic. Cells in this solution would swell as water moves into them.
Solutions with a lower water potential compared with the inside of the cell are called hypertonic. Cells in this solution would shrink as water moves out of them.
Facilitated diffusion - the diffusion of particles through carrier proteins or channel proteins in the plasma membrane.
It is a passive process, it doesn't require energy.
Some larger molecules (eg amino acids and glucose) and charged atoms (eg chloride ions) can't diffuse directly through the phospholipid bilayer so transport proteins are used.
Carrier proteins - they move large molecules into or out of the cell, down their concentration gradient. The large molecule attaches to a carrier protein in the membrane, then the protein changes shape. This releases the molecule on the opposite side of the membrane
Protein channels - they form pores in the membrane for charged particles to diffuse through (down their concentration gradient). Different protein channels facilitate the diffusion of different charged particles.
Active transport - the movement of molecules and ions across plasma membranes against their concentration gradient. It requires energy.
Carrier proteins - a molecule attaches to a carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side. Energy is used, usually from ATP produced in the mitochondria, to do this against the concentration gradient.
Co-transporters - they are a type of carrier protein. They bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient. So one of the molecules moves down its concentration gradient, the other against its concentration gradient.
Example - absorption of glucose
These processes are needed to absorb the products of carbohydrate digestion across the intestinal epithelium cells. Glucose is one of these products.
At first, there is a higher concentration of glucose in the small intestine than in the blood so there's a concentration gradient. Glucose moves across the epithelial cells into the blood by diffusion. When the concentration in the lumen becomes lower than in the blood, diffusion stops.
Sodium ions are actively transported out of the small intestine epithelial cells into the blood by a sodium-potassium pump, creating a concentration gradient.
This causes sodium ions to diffuse from the small intestine lumen into the cell down their concentration gradient. They do this via the sodium-glucose co-transporter proteins. These carry glucose into the cell with the sodium, increasing the concentration in the cell.
Glucose diffuses out of the cell into the blood down its concentration gradient through a protein channel, by facilitated diffusion.