Movement of substances
Monacia Williams, Contributor

We have looked at cells and the connections that exist among them. Now, because of the connections, substances necessary for their well- being have to move between them. The cell, for processes such as respiration, uses some of these substances, while other substances are produced as a result of processes occurring within each cell.

What causes these substances to move?

• The substances that pass in and out of living cells are made up of tiny particles which are either atoms or molecules.
• These particles are in constant motion, moving randomly.
• This causes the particles to move in order to occupy whatever space is available to them.
• For this movement to take place, that is to occupy space and become noticeable, a gradient must exist.
  Note that there must be an area where the particles are more plentiful and an area where they are less plentiful. The area where they are less plentiful is described as having a higher concentration of the particles and the area where they are less plentiful is described as having a lower concentration of the particles, that is, a gradient must exist.

Diffusion

The movement of particles is known as diffusion and the gradient is known as the diffusion gradient.

Let's see if we can use this information to put together a suitable definition for diffusion. What do we know? We know that:

• It is the movement of particles.
• These particles can either be solid, liquid or gas.
• There must be an area where the concentration is high and one where it is low.

Let's combine these now; what do we get? Diffusion is the movement of particles, solid, liquid or gas, from an area where their concentration is high to an area where their concentration is low, that is, along diffusion gradient.

That was not very hard was it? Try using this method to create other definitions. Have you ever been in a room where someone far away from you is spraying perfume or deodoriser and all of a sudden you smell it?

That's diffusion in action for you! The area where the spray came from has a lot of the spray (is more concentrated) and where you were had none (is less concentrated) so a diffusion gradient was created and the gas particles moved to occupy the space. By the way, particles will also move across membranes if the membrane lets them through, that is, permeable to them.

How does diffusion serve the living organism?

Take a deep breath, inhale, what have you done? You have taken air into your lungs. What happens to this air? Do you remember that air is a mixture of gases, one of which is oxygen? Well, the oxygen from the air is going to move into the blood capillaries surrounding the air sacs in the lungs because there is more oxygen in the sacs than in the capillaries. What does this create? You guessed it, a concentration or diffusion gradient, hence the oxygen molecules are going to move by diffusion along the diffusion gradient. The reverse happens when you exhale. This time the gas moving out is carbon dioxide.

What happens to this oxygen?

The blood takes it to the tissues where the concentration in the cells of the tissues is now lower than that in the blood, so the oxygen moves from the higher concentration in the blood to the lower concentration in the cells. This is another example of the usefulness of diffusion.

The oxygen is used for respiration, which produces energy for the cell. It also produces carbon dioxide as a waste product so the concentration of carbon dioxide in the cell increases, creating a diffusion or concentration gradient between the blood and the cell. Carbon dioxide moves out of the cell into the blood, another example of diffusion. Other waste matters, made as a result of the reactions in the cell, are removed by diffusion because of the same reason; the creation of a diffusion or concentration gradient. Urea, which goes to form urine, is an example of such waste matter.

Products of digestion

When you eat something and it is digested, the products of digestion are at high concentrations in the intestines. This creates a concentration gradient between the intestines and the blood, which transports these products. The products are thus able to move into the blood by diffusion.

So far, I have made it seem as if diffusion only occurs in animals, but this is not so. Diffusion plays a very important role in the movement of gases into and out of the air spaces of the leaves. Without it photosynthesis could not take place, because carbon dioxide would not be able to enter them.

As we go through the course we will look in greater detail at the usefulness of diffusion in the living organism. See you next week when we will look at another type of movement!

Manabu Sugimoto (right), associate professor of biology at Okayama University, smiles with graduate student, Elena Shagenardanova of Russia,who holds the latest crop of space barley, during a press conference in Tokyo. - AP

Monacia Williams teaches at Glenmuir High School.