Transport across the cell membrane
Adrian Whyte and Joanna Johnson, Contributor

Grade five students from the Catherine Hall Primary School in St. James, during mathematics class, recently. - Claudine Housen Photo

Living things must be capable of transporting nutrients, wastes and gases to and from cells. The cell surface membrane is very important for this process of transport.

Small substances can pass through cell membranes as needed using the following processes: diffusion, osmosis and active transport.




Note: It is very important that persons intending to do biology exams understand fully these processes. A lot of analytical questions on the exam paper will require candidates to use their knowledge of osmosis and diffusion.

Diffusion

Particles in a mixture tend to have random movement. The movement tend to be in a direction away from where the particles are alike.

The Random Movement of Molecules.

The diagram below demonstrates the tendency for like particles to move away from each other. The particles of dye move away from each other and is dispersed in the water. This is an example of diffusion

drop of dye in water	dye dispersed in water

Osmosis

Osmosis is very similar to diffusion..... actually it is diffusion the difference here is that it describes the movement of water. Cell membranes are selectively permeable, that is they are selective to what is able to pass through them. One of the major determining factors is that the particles must be small enough to pass through the small holes in the cell membrane.


As shown in the diagrams above that the movement of water molecules is always towards the more concentrated solution or the area where there is less concentration of water molecules.

Active Transport

There are times when the cell needs to move particles against their concentration gradient. Since this is not the usual course of travel for particles energy in the form of ATP is needed to cause the particles to move from where they are in low concentration to where they are in high concentration.

Table showing real life examples of osmosis, diffusion and active transport

Type of transport	Example in plants	Example in animals
Diffusion	CO2 entering leaves for photosynthesis Irons passing from one cell to another in a plant	Products of digestion leaving alimentary canal into the walls of the intestine. O2 passing through the alveoli of the lungs into the blood capillaries.
Osmosis	Water entering the roots of plants	Water passing into the walls of the alimentary canal.
Active Transport	Mineral ions pass into the root cells from the soil this way. The loading of sucrose unto phloem tubes.	The Na+/K+ pump in nerve cells.

* Adrian Whyte and Joanna Johnson teach Biology at Ardenne High School
masterbio@gmail.com.