1.8 - Diffusion

What diffusion is

• Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
• The particles spread out from where there are lots of them to where there are fewer of them.
• Because the movement is from high to low concentration, the particles are said to move down a concentration gradient.

Where and how diffusion happens

• Diffusion happens in both solutions and gases, because the particles here are free to move about randomly.
• The particles keep moving in all directions, so they actually travel both ways. But when there are many more on one side than the other, the overall result is a net movement from the crowded side to the less crowded side.
• The particles carry on spreading until they are evenly distributed.
• Example: the smell of perfume spreads through the air in a room - the perfume particles diffuse out from where they are concentrated until they spread through the whole room.

Diffusion is a passive process

• It does not need any energy from the cell to make it happen.
• The particles move on their own, driven only by the kinetic energy of their own random motion.
• The bigger the difference in concentration between the two areas, the faster diffusion takes place.

Diffusion across cell membranes

• A cell membrane is partially permeable (also called selectively permeable): it holds the cell together but still lets some substances in and out.
• The gaps between the molecules that make it up are so small that only tiny molecules can pass through.

Which molecules can cross

• Small enough to diffuse through - Oxygen, carbon dioxide, glucose, amino acids, and water.
• Too big to pass through - Large molecules such as starch and proteins.

Visking tubing as a model membrane

• Visking tubing (also called dialysis tubing) is partially permeable, so it is used in experiments as a model of a cell membrane.
• If a bag of it holding a mixture of starch and glucose solution is left in a beaker of water, glucose soon appears in the water but starch does not.
• This is because glucose molecules are small enough to pass through the tiny pores in the tubing, while starch molecules are too big.

How diffusion moves substances in and out

• Dissolved substances and gases move in and out of cells across the membrane by diffusion.
• Particles pass through from the side of higher concentration to the side of lower concentration.
• As in any diffusion, particles move randomly and travel both ways through the membrane, but when there are many more on one side there is a net movement from that side to the other.
• This lets a cell take in the substances it needs and lose the substances it must get rid of, simply by diffusing down each substance's concentration gradient.