Diffusion, osmosis and active transport

Substances move into and out of cells through structures you already know:

• Cytoplasm — the jelly-like substance where dissolved substances travel between organelles.
• Nucleus — controls the cell and holds the genetic material.
• Cell membrane — a partially permeable barrier that controls what enters and leaves the cell.
• Mitochondria — release the energy (from respiration) needed for active transport.
• Ribosomes — build the transport proteins found in the cell membrane.
• Chloroplast — uses substances absorbed by the cell, such as carbon dioxide and water, for photosynthesis.
• Cell wall — a rigid, freely permeable layer that supports a plant cell and stops it bursting.
• Vacuole — a plant cell's store of cell sap, which affects how much water enters by osmosis.

Diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. It happens because particles move randomly, and needs no energy input from the cell.

The rate of diffusion increases with: a bigger difference in concentration (steeper gradient), a higher temperature (particles move faster), and a larger surface area (more space for particles to cross). Oxygen diffusing from air into blood across the thin walls of alveoli is a key GCSE example.

Osmosis is the diffusion of water molecules from a dilute solution to a more concentrated solution, through a partially permeable membrane — a membrane that lets small molecules like water through but not larger solute molecules.

In plant cells, water entering by osmosis makes the cell turgid (firm) as the vacuole swells against the cell wall. In animal cells, too much water entering can make a cell burst, since there is no cell wall to resist the pressure.

If a plant cell is placed in a more concentrated solution, water leaves by osmosis and the cell becomes flaccid (soft). If too much water leaves, the cell membrane pulls away from the cell wall — this is called plasmolysis.

Active transport moves particles from a lower to a higher concentration — against the concentration gradient. Because this is the opposite direction to diffusion, it requires energy, which is released by respiration in the mitochondria.

Two key examples: root hair cells absorbing mineral ions from dilute soil water, and cells lining the small intestine absorbing glucose from the gut even when glucose concentration is higher inside the blood than in the gut.