Movement of substances (cont'd)
Monacia Williams, Contributor

Last week we looked at the factors that affected the movement of substances. We also looked at one type of movement - diffusion. This week we will continue our study as we look at other types of movement, the first of which is known as osmosis.

Osmosis

Osmosis, like diffusion, requires the presence of a concentration gradient, that is, there must be an area where there is a greater concentration of water molecules and an area where the concentration of water molecules is less. They differ in that while there may or may not be a selectively permeable membrane for diffusion to occur, there must be one for osmosis to take place. The most important difference, however, is that only the movement of one type of particle is considered in osmosis. This particle is the water molecule.

We can, therefore, sum up the differences between osmosis and diffusion as follows:

• In osmosis, a selectively permeable membrane must be present.
• Only the movement of water is considered.

There are some new words that you will need to learn to help you describe osmosis. These words are used when two solutions, usually one internal and one external, are being compared. These words are:

• Hypotonic - this is used to describe the solution that has the greater concentration of water molecules.
• Isotonic - this is used when the two solutions have the same concentration of water molecules.
• Hypertonic - this is used to describe the solution that has the lower concentration of water molecules.

Do you notice that I have avoided the use of the terms dilute and concentrated? I hope that you will do the same, because these words can cause confusion at times.

Cell membrane

Do you remember that one of the structures present in the cell is the cell membrane and that this is present in both the plant and animal cell? Remember also that in the plant cell, the cell wall is found on the outside of the cell membrane? Do you also remember that one of the properties of the cell membrane is that it is selectively permeable? Well, the cell membrane encloses the cytoplasm with the cytosol, which is a solution of substances in water. Outside of the cell is another type of solution, the tissue fluid. If the concentration of water molecules in both solutions is different, then osmosis will occur. When the cytosol is hypotonic (with a greater concentration of water molecules) to the tissue fluid, water will leave the cell and enter the tissue fluid. When it is hypertonic (with a lower concentration of water molecules) to the tissue fluid, water will leave the tissue fluid and enter the cell. If the concentration of water molecules in both is the same (isotonic), the movement of water into and out of the cell will be equal and the cell remains unchanged.

When animal cells are placed in a hypotonic solution, osmosis occurs and water enters the cells. This will cause the cell to swell, putting a strain on the cell membrane, causing it to burst. This phenomenon is known as lysis. When the cells are put into a hypertonic solution, osmosis occurs and the cells lose water. This will cause the cell to shrink since as the cell loses water, it shrivels. This phenomenon is known as crenation.

Plant cells

The same processes occur in plant cells, but the results are different. This difference is due to the presence of the cell wall. The cell wall prevents the cell from undergoing crenation and lysis. When placed in a hypotonic solution, a plant cell will swell as water enters the vacuole and the cytoplasm by osmosis, but it will not burst. Instead, it becomes turgid as the contents of the cytoplasm press against the wall. This is an important feature in plants with soft stems, because it helps the plant to stay upright and the leaves to be firm. When the plant cell is placed in a hypertonic solution, osmosis occurs as water leaves the cell. The cell will not become crenated, but as the water leaves, the solution in the cytoplasm becomes less and less and the cytoplasm shrinks away from the cell wall. The cell is no longer turgid and is now said to be plasmolysed.

You must always remember to use the correct terms to describe these phenomena in the plant and animal cells!

Active transport

A third type of movement in living tissue is known as active transport. This occurs when substances move against their concentration gradient. This requires that the cell has to use energy to carry out this type of movement. Hence, it is known as active transport. This is the type of transport that gets mineral ions into the roots of plants and helps to maintain the concentration of sodium and potassium ions required for the transmission of nerve impulses in animals. Remember that in osmosis and diffusion, substance move along their concentration gradient so energy is not required, hence, this type of movement is said to be passive.

I guess that from the number of times that the word 'remember' has been used in this lesson you will realise that this is an important topic. See you next week!

Dr Audia Barnett (centre), executive director of the Scientific Research Council (SRC), speaks with Dr Artnel Henry (right), chairman of the Jamaica Bureau of Standards. Merline Bardowell (left), executive director of the National Commission on Science and Technology (NCST), Dr Alfred Sangster, a commissioner of the NCST, and SRC chairman, James Moss-Solomon, look on, during the National Quality and Innovation Awards ceremony, held at The Jamaica Pegasus hotel in New Kingston recently. - Colin Hamilton/Freelance Photographer

Monacia Williams teaches at Glenmuir High School.