Active transport

Active transport moves particles across a cell membrane from a region where their concentration is lower to a region where their concentration is higher.

This movement is against the concentration gradient: particles move towards the region where there are already more of them. Membrane proteins transport the particles across the membrane using energy supplied by respiration.

Mineral ion uptake by root hair cells

Root hair cells take up mineral ions dissolved in the water around soil particles, known as the soil solution. The ions enter through the root hair cells’ cell membranes.

The concentration of a mineral ion may be lower in the soil solution than inside a root hair cell. In that case, uptake moves the ion from a lower concentration to a higher concentration, against its concentration gradient. The cell achieves this by active transport.

During active transport, carrier proteins in the cell membrane move ions into the cell using energy supplied by respiration. This allows roots to obtain mineral ions needed by the plant even when the ions would not move into the cells down their concentration gradients.

Diffusion in living organisms

Cells exchange substances with their surroundings across the cell membrane. Some substances cross by diffusion when their concentration differs on the two sides of the membrane and the membrane allows them to pass.

A difference in concentration is called a concentration gradient. Particles move randomly in both directions, but more move from the region of higher concentration to the region of lower concentration. This produces a net movement down the concentration gradient. The cell does not direct the particles, and diffusion does not require the cell to supply energy.

Diffusion helps cells obtain substances they need and remove substances they produce. For example, oxygen can diffuse into a cell when its concentration is higher outside. Carbon dioxide produced by the cell can diffuse out when its concentration is higher inside.

Dissolved solutes can also diffuse across cell membranes. For example, glucose can diffuse into some cells through membrane proteins when its concentration is higher outside. Not every substance can cross every membrane: some pass through the membrane directly, some require suitable membrane proteins, and others cannot pass through it.

Osmosis

Water can move across a membrane that allows water molecules through but prevents some dissolved substances from crossing. Such a membrane is called partially permeable. The diffusion of water through it is called osmosis.

When the solutions on either side have different concentrations, there is a net movement of water from the more dilute solution to the more concentrated solution. Water molecules move in both directions, but more move towards the concentrated solution than in the opposite direction.

A cell membrane is partially permeable, so water can move into or out of a cell by osmosis. If the solution outside is more dilute than the solution inside, there is a net movement of water into the cell. If the solution outside is more concentrated, there is a net movement of water out of the cell.