Cells must take in substances such as oxygen, glucose and water, and get rid of waste such as carbon dioxide. Substances cross the cell membrane in three main ways.
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. The particles move down a concentration gradient. It is a passive process, so the cell uses no energy. Oxygen, carbon dioxide and glucose can all move by diffusion. It is faster when the concentration gradient is steeper, the temperature is higher or the surface area is larger.
Osmosis is the net movement of water molecules from a dilute solution to a more concentrated solution, through a partially permeable membrane. Like diffusion it is passive. Water moves from where it is at a higher concentration to where it is at a lower concentration.
Active transport moves substances from a more dilute solution to a more concentrated solution, which is against a concentration gradient. This needs energy from respiration. Root hair cells use active transport to absorb mineral ions from the soil, and the gut uses it to absorb glucose when there is less glucose in the gut than in the blood.
