# 2.4.1 Diffusion

### Diffusion Theory

• Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration
• Molecules move down a concentration gradient, as a result of their random movement

Diffusion across the cell membrane

#### Diffusion in living organisms

• For living cells, the principle of the movement down a concentration gradient is the same, but the cell is surrounded by a cell membrane, which can restrict the free movement of the molecules
• The cell membrane is a partially permeable membrane – this means it allows some molecules to cross easily, but others with difficulty or not at all
• The simplest sort of selection is based on the size of the molecules (i.e. smaller molecules can diffuse across the membrane but larger molecules cannot)
• Diffusion helps living organisms to:
• Obtain many of their requirements
• Get rid of many of their waste products
• Carry out gas exchange for respiration

#### Examples of diffusion in living organisms

• You will need to learn examples of substances that organisms obtain by diffusion

#### Exam Tip

Remember that diffusion is a passive process, so when it occurs in a living organism, the cells of that organism do not provide the particles involved with energy to diffuse. The particles that are moving about randomly have their own kinetic energy.

### Factors that Influence Diffusion

#### Surface area to volume ratio

• The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface
• Many cells which are adapted for diffusion have increased surface area in some way – e.g. root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)

The highly folded surface of the small intestine increases its surface area

• You should be able to calculate and compare surface area to volume ratios
• You can model the effect of how increasing size affects surface area to volume ratio using simple cubes:

Calculating the surface area to volume ratio

#### Diffusion distance

• The smaller the distance molecules have to travel the faster transport will occur
• This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible

#### Temperature

• The higher the temperature, the faster molecules move as they have more energy
• This results in more collisions against the cell membrane and therefore a faster rate of movement across them

• The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
• This is because on the side with the higher concentration, more random collisions against the membrane will occur

Summary of Diffusion Factors Table

#### Exam Tip

You should have carried out investigations into the factors that influence the rate of diffusion and as so should be able to use the information above to explain experimental results in an exam.

You should also be able to plan and carry out an experiment which can investigate the effect of one of these factors.

### Author: Ruth

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.
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