Osmosis and active transportIB MYP Biology: Revision notes

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It happens because particles are always moving randomly. It needs no energy from the cell.

Examples: oxygen moving from the air in the alveoli into the blood; digested food moving from the gut into the blood through the villi; carbon dioxide moving into a leaf through the stomata for photosynthesis.

The rate of diffusion is faster with:

• a steeper concentration gradient
• a larger surface area
• a higher temperature (particles move faster)
• a shorter distance (a thinner membrane)

Osmosis is the net movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution) through a partially permeable membrane.

The membrane lets small water molecules pass but blocks larger solute molecules. Osmosis is a special case of diffusion and needs no energy.

In a plant cell placed in water (or a dilute solution), water enters by osmosis. The cell swells and presses on the cell wall, which stops it bursting. It is turgid, which supports the plant. In a concentrated solution water leaves, the cell becomes flaccid, and eventually the membrane pulls away from the wall: the cell is plasmolysed. A plant with flaccid cells wilts.

An animal cell has no cell wall. In pure water, red blood cells take in water and may burst. In a very concentrated solution they lose water and shrink.

Cut identical cylinders of potato, measure the mass of each, and place them in different concentrations of sugar or salt solution. After a set time, blot them dry and weigh them again.

• Independent variable: concentration of the solution.
• Dependent variable: change in mass.
• Control variables: size of the cylinders, volume of solution, time, temperature.

A gain in mass means water entered the potato; a loss means water left. The concentration where there is no change in mass is about the same as the concentration of the potato cells.

For different cylinder sizes, use the percentage change in mass: $\frac{\text{final mass} - \text{start mass}}{\text{start mass}} \times 100$.

Active transport is the movement of substances against a concentration gradient, from a lower to a higher concentration, using energy from respiration. It uses carrier proteins in the cell membrane.

Examples:

• Root hair cells take up mineral ions such as nitrate from the soil, where they are at a lower concentration than in the cell.
• In the gut glucose is absorbed into the blood against its gradient when its concentration in the gut is low.

Cells that do a lot of active transport have many mitochondria.