Diffusion And Osmosis
Cells have to constantly replace substances which are used up (EG: Food and Oxygen). They also have to remove other substances (EG: CO2 and waste products)
Diffusion: Is the spreading of the particles of a gas, or any substance in solution, which results in a net movement from a region where they are at a higher concentration, to a region where they are at a lower concentration.
Oxygen required for respiration passes through cell membranes by diffusion. The greater the difference in concentration, the faster the rate of diffusion.
Osmosis: Is the movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
The membrane allows the passage of water molecules, but not solute molecules because they are too large.
Osmosis will gradually dilute the more concentrated solution. EG: At Root Hair Cells water moves from the soil into the cell by osmosis, along a concentration gradient.