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.

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.

Mineral ion uptake by root hairs

Root hair cells absorb mineral ions dissolved in the water surrounding soil particles. Plants need these mineral ions for healthy growth.

Sometimes the concentration of a mineral ion is lower in the soil solution than inside a root hair cell. To absorb more of that ion, the cell must move it from a lower concentration to a higher concentration, against its concentration gradient.

This movement is called active transport. It requires energy released by respiration in the root hair cell. Active transport allows the plant to absorb mineral ions even when the soil solution is very dilute.

Sugar absorption in the small intestine

Sugar molecules such as glucose are absorbed when they move from the contents of the small intestine, across its lining, into the blood.

When the sugar concentration in the intestine is lower than in the blood, sugar can still be absorbed by active transport. This moves sugar overall from a lower concentration to a higher concentration, against the concentration gradient. Active transport requires energy released by respiration in the cells of the intestinal lining.

Once absorbed, sugar is carried in the blood to body cells. The cells can use it in respiration to release energy.

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.