The movement of substances into and out of cells

Substances can move into and out of substances by three processes:

1. Active transport
2. Diffusion
3. Osmosis

1. Active transport is the movement of substances across cell membranes against the concentration gradient. This is done by using the energy released through the process of respiration. It occurs most commonly in the roots of a plant. The mineral ions in the soil are pumped into the roots of the plant by active transport.

2. Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. An experiment to see this uses food colouring and a beaker of water. Drop the food colouring in and watch as the water slowly becomes the same colour.

3. Osmosis is the movement of water from a high water potential to a low water potential. An experiment to investigate osmosis involves two potatoes.

Weigh them using a mass balance.
 Place one in a distilled water beaker and the other in a salt-water beaker.
After a certain time, take them out at the same time and weigh them again.
The potato in the salt-water beaker should weigh less due to more water moving out. This is a result of the greater concentration gradient i.e. there was a small water potential with the salt. The salt was highly concentrated.
The potato in the distilled water beaker should have gained weight due to more water moving in. This potato had a lower water potential to begin with, in comparison to the water beaker. It was less dense and as a result, the water moved in.

The rate of movement into and out of cells is influenced by three main characteristics/ processes:

1. Surface area to volume ratio. A greater surface area means there is more surface by which the molecules can diffuse through.

2.  Temperature. A higher temperature would increase the kinetic energy of the molecules. Therefore they are more likely to collide with the membrane hence they will more likely diffuse out.

3. Concentration gradient. The steeper the concentration gradient, the more molecules will diffuse out of the cell.

Turgid cells as a means of support.

Turgid cells are a means of support because they are filled with water hence the pressure they exert on their surroundings is greater. This means the cells are stronger and push against each other, enabling the plant to grow upwards.