Dissolved substances (within a cell)
All living things are made up of cells. The structures of different types of cells are related to their functions. To get into or out of cells, dissolved substances have to cross the cell membranes.
a) Dissolved substances can move into and out of cells by diffusion.
b) Diffusion is the spreading of the particles of a gas, or of any substance in solution, resulting in a net movement from a region where they are of a higher concentration to a region with a lower concentration. The greater the difference in concentration, the faster the rate of diffusion (please note, the same logic as 'rate of reaction' within chemistry applies here).
c) Oxygen required for respiration passes through cell membranes by diffusion.