Diffusion, osmosis and active transport · GCSE Biology

Diffusion

Diffusion is the spreading out of particles, resulting in a net movement from an area of higher concentration to an area of lower concentration.
It is passive: it does not need energy from respiration.
Examples: oxygen and carbon dioxide diffuse in gas exchange; urea diffuses from cells into the blood plasma to be taken to the kidneys.
The rate of diffusion increases with a bigger concentration gradient, a higher temperature and a larger membrane surface area.

Single-celled organisms have a large surface area to volume ratio, so diffusion across their surface meets their needs.
Larger organisms have a smaller surface area to volume ratio, so they need specialised exchange surfaces and transport systems.
Good exchange surfaces have a large surface area, a thin membrane (short diffusion path), an efficient blood supply in animals and ventilation for gas exchange.
Examples: alveoli in the lungs, villi in the small intestine, gills in fish, and roots and leaves in plants.

Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
If the solution outside a cell is more dilute than inside, water moves in; animal cells may burst, but plant cells become turgid because of the cell wall.
If the solution outside is more concentrated, water moves out; animal cells shrivel and plant cells become flaccid (and may become plasmolysed).

Cut equal cylinders of potato, blot them dry and measure the mass of each.
Place them in sugar or salt solutions of different concentrations, and one in distilled water, for the same time at the same temperature.
Blot and reweigh, then calculate percentage change in mass = (change in mass ÷ starting mass) × 100.
Plot percentage change against concentration; where the line crosses zero, the solution has the same concentration as the potato cells.

Active transport moves substances from a more dilute solution to a more concentrated solution, against a concentration gradient.
It needs energy from respiration, so cells that do it have many mitochondria.
Root hair cells use active transport to absorb mineral ions from very dilute solutions in the soil.
In the gut, active transport absorbs sugar molecules into the blood when the sugar concentration in the gut is lower.

Diffusion: The net movement of particles from an area of higher concentration to an area of lower concentration.
Concentration gradient: The difference in concentration between two areas.
Osmosis: The diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
Partially permeable membrane: A membrane that lets small molecules such as water through but not larger ones.
Active transport: The movement of substances against a concentration gradient, using energy from respiration.
Surface area to volume ratio: The surface area of an object divided by its volume.
Turgid: Describes a plant cell that is swollen and firm because it has taken in water.
Flaccid: Describes a plant cell that is soft because it has lost water.
Plasmolysis: When the cytoplasm of a plant cell pulls away from the cell wall because it has lost a lot of water.
Alveoli: Tiny air sacs in the lungs where gas exchange happens.