Diffusion, osmosis and active transport as ways substances move across cell membranes, the factors that affect diffusion, the effect of osmosis on plant and animal cells, and the osmosis required practical.

What this dot point is asking

WJEC wants you to define diffusion, osmosis and active transport, describe the factors that affect diffusion, explain the effect of osmosis on plant and animal cells, and describe the required practical investigating osmosis in plant tissue.

Diffusion

Diffusion happens because particles move randomly; over time, more particles move from the crowded side to the less crowded side. Examples in biology include oxygen and carbon dioxide moving across the alveoli during gas exchange, and glucose moving from the gut into the blood.

The rate of diffusion is faster when:

• the concentration gradient is steeper (a bigger difference between the two regions),
• the temperature is higher (particles have more kinetic energy and move faster),
• the surface area is larger (more area for particles to cross),
• the distance to travel is shorter (a thinner membrane).

Osmosis

A partially permeable membrane lets water through but not larger dissolved molecules. Water always moves towards the more concentrated solution, trying to even out the concentrations. Osmosis is just diffusion of water, so it also needs no energy.

The effect of osmosis on cells

The result of osmosis depends on the solution surrounding the cell.

Plant cells. In a dilute solution, water moves into the cell by osmosis. The vacuole swells and pushes the cytoplasm against the cell wall, making the cell turgid (firm). Turgid cells support the plant. In a concentrated solution, water moves out, the cell becomes flaccid, and in extreme cases the membrane pulls away from the wall, which is plasmolysis.

Animal cells. Animal cells have no cell wall, so osmosis can damage them. In a dilute solution, water moves in and the cell may swell and burst (lysis). In a concentrated solution, water moves out and the cell shrinks and shrivels (crenation).

Active transport

Because it works against the gradient, active transport needs energy, which is why cells that carry out a lot of it (such as root hair cells) have many mitochondria. Examples include the uptake of mineral ions by root hair cells, even when minerals are more concentrated inside the root than in the soil, and the reabsorption of glucose in the kidney tubules.

Comparing the three processes

The osmosis required practical

A common required practical investigates osmosis in potato tissue. You cut potato cylinders of equal size, measure their starting mass, and place each in a different concentration of sugar or salt solution. After a set time you dry and reweigh them, then calculate the percentage change in mass:

Cylinders in dilute solutions gain mass (water enters by osmosis); those in concentrated solutions lose mass (water leaves). The concentration where there is no change in mass equals the concentration inside the potato cells.