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Unit 2 - Diffusion
The alveoli in the lungs have a surface area equivalent to a tennis court!

Unit 2 - Diffusion

One process that needs to be understood by GCSE Biology students is diffusion. This is the passive movement of molecules in a high concentration through a membrane to an area of low concentration, and it is fundamental to life.

Diffusion is a passive process during which small particles (usually molecules) move from a region of high concentration to a region with a lower concentration. Examples of diffusing molecules are oxygen and carbon dioxide. The concentration gradient is the change in concentration from one place when compared with another place, for example comparing one side of a cell membrane with the other side. The gradient will determine whether a molecule will move and how quickly.

There are many examples of diffusion that occur in your body, for example, in your lungs, where we see the movement of oxygen and carbon dioxide molecules. When you breathe in, the concentration of oxygen molecules in your blood is lower than in the air. This means that oxygen molecules diffuse through the thin walls of your alveoli and into the capillaries on the alveoli and go into your blood. Once in your bloodstream, they combine with the red blood cells to form oxyhaemoglobin which is transported round your body. When an oxygenated red blood cell passes another cell in your body that has a low concentration of oxygen, the oxygen can leave the red blood cell and diffuse into the cell that needs more oxygen for respiration.

Carbon dioxide is one of the waste products of respiration in cells and, as it builds up in a cell, it can diffuse into the bloodstream where it dissolves in the plasma. As the blood passes through the lungs, the concentration of carbon dioxide is greater than in the air you breathed in. It therefore diffuses from the blood into the lungs where it is breathed out.

How well do you understand the passive movement of particles via diffusion. Play this quiz and test yourself.

Diffusion is the movement of particles from high concentration to which of the following?
Higher concentration
Lower concentration
Equal concentration
High concentration
The difference in concentration either side of a semi or fully permeable membrane is the concentration gradient
Diffusion is...
This means that it happens automatically, without the need for anything to cause it to happen
Which of these is a major site for gaseous exchange in the body?
Kidney tubule
The plural is alveoli
Which of these molecules is too big to diffuse through a membrane, using simple diffusion?
Carbon dioxide
If a molecule cannot fit through the holes in the cell membrane, it cannot pass into or out of a cell. Some large molecules, like proteins, have to be broken down before a cell can take them in
The rate of diffusion is increased in which of the following situations?
If there are fewer molecules
If the surface area is increased
If the temperature is decreased
If the diffusion distance is greater
More molecules can pass through the membrane at the same time
Diffusion happens faster in which of the following situations?
If the difference in concentration is bigger
If the difference in concentration is smaller
If the difference in concentration is the same
The difference in concentration makes no difference to the rate of diffusion
The bigger the difference in concentration, the faster the rate of diffusion
Which of these molecules only moves by simple diffusion?
Carbon dioxide
Small molecules diffuse more easily and more rapidly than larger molecules
Diffusion is fast in the lungs. Why is this?
Because the lungs have a huge area
Because the lungs have a huge surface area
Because the lungs have a huge volume
Because the lungs have a huge density
The alveoli in the lungs have a surface area equivalent to the area a tennis court!
Active transport is where molecules use a carrier and are moved...
with the concentration gradient
along the concentration gradient
against the concentration gradient
equally to the concentration gradient
Molecules, such as amino acids and glucose, are moved into and out of cells using active transport. This is an active process requiring energy and a carrier
Diffusion requires which of the following?
No energy
That is because it is a passive process
You can find more about this topic by visiting BBC Bitesize - Diffusion (CCEA)

Author:  Donna Maria Davidson

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