Cells need to obtain various substances in order to carry out life processes. Cells also need to get rid of substances that are useless and harmful to them.

Diffusion: net movement of particles from a region of higher concentration to a region of lower concentration

If there is a difference in concentration of a substance between two adjacent regions, the particles tend to spread out evenly. The net movement continues until the concentration of the substance in the two regions are the same. The difference in concentration is called a concentration gradient. Diffusion is a passive process and does not require supply of energy.

Smaller particles diffuse faster than large particles. Diffusion occurs more rapidly if the distance of diffusion is shorter. Diffusion occurs more rapidly if there is a large difference in concentration between two regions. Diffusion is faster in gaseous medium. Larger surface area promotes a higher rate of diffusion. Diffusion is faster when temperature is higher because the kinetic energy of particles increases. An example of diffusion is the exchange of gases in the air sacs of the lungs.

Osmosis: the net movement of water from higher water potential to lower water potential across a partially permeable membrane

water potential: the free energy of water, dependent on the proportion of free water molecules in a solution

Presence of solutes in a solution decreases the proportion of free water molecules and hence lowering the water potential. Presence of solute molecules will attract surrounding water molecules, hence slowing down and making them less free to move.

Tonicity: relative concentration of solutes in 2 fluids

isotonic: the cell has the same water potential as the solution

hypertonic: the cell has a higher water potential than the solution

hypotonic: the cell has a lower water potential than the solution

Cells in hypertonic solution shrink and appear to be wrinkled. Cells in hypotonic solution swell up and eventually burst. Plants cells in hypertonic solution are flaccid and eventually become plasmolysed. Cells in hypotonic solution swell and become turgid.

Active transport: the movement of particles against a concentration gradient using energy from respiration.

Active transport require special protein carriers that are embedded within the cell membrane. Energy from respiration is required to change the shape of the protein carriers so that the particles can attached to the carrier for transportation. An example of active transport is nutrients being actively transported into the blood from the intestine.