Diffusion

Where it happens

Diffusion happens wherever there is a concentration gradient, in gases, liquids and across membranes, for example oxygen moving from the alveolus into the blood.

In the body, the clearest examples are the exchange surfaces. In the alveolus, oxygen diffuses from air in the alveolar space, across the one-cell-thick alveolar wall and capillary wall, into the red blood cells, while carbon dioxide diffuses the opposite way.

In the small intestine, digested food such as glucose and amino acids diffuses from the lumen into the capillaries of the villus. In a leaf, carbon dioxide diffuses through the stomata into the air spaces and then into the mesophyll cells for photosynthesis.

In a single-celled organism such as Amoeba, diffusion across the whole plasma membrane is enough to supply oxygen and remove waste, because the cell is tiny and has a high surface area to volume ratio.

Inputs and outputs

• Input: a population of particles (gas molecules, ions or small solutes) that is unevenly distributed, so a concentration gradient exists.
• Input: kinetic energy that the particles already possess because of their temperature, no ATP from respiration is needed.
• Input: a medium to move through, air, water, cytoplasm, or a partially permeable membrane with pores or channel proteins for small, soluble or lipid-soluble molecules.
• Output: a net transfer of particles from the region of higher concentration to the region of lower concentration.
• Output: an equilibrium state in which the particles are evenly spread and the concentration gradient is zero, although random movement continues.
• Output in the body: oxygen delivered into blood or cells, carbon dioxide removed, and digested food delivered into the bloodstream.

The steps

1. A concentration gradient exists: particles are more concentrated in one region than in a neighbouring region.
2. Every particle moves randomly in all directions because of its kinetic energy; the higher the temperature, the faster it moves.
3. Because there are more particles on the concentrated side, more of them happen to cross into the dilute side than cross back.
4. The result is a net movement of particles down the concentration gradient, from high to low concentration.
5. If a membrane is in the way, small or lipid-soluble molecules such as oxygen and carbon dioxide slip between the phospholipids (simple diffusion), while larger or charged particles such as glucose and ions pass through channel or carrier proteins (facilitated diffusion), still without energy.
6. As particles spread out, the gradient becomes shallower and the net rate of diffusion falls.
7. Diffusion continues until the particles are evenly distributed, dynamic equilibrium, where movement goes on in both directions at equal rates and there is no further net change.

Why it matters and how it is controlled

Diffusion moves oxygen, carbon dioxide and dissolved food in and out of cells, and it explains gas exchange in the lungs and leaves. A larger surface area, shorter distance and steeper gradient all speed it up.

The rate of diffusion is controlled by four factors, and each one appears in exam answers about exchange surfaces. A steeper concentration gradient gives a faster rate, which is why blood flow and ventilation continually carry away the substance that has just diffused, keeping the difference large.

A shorter diffusion distance gives a faster rate, which is why the alveolar wall and the capillary wall are each one cell thick. A larger surface area allows more particles to cross at once, which is why the lungs contain millions of alveoli and the ileum is lined with villi and microvilli.

A higher temperature gives particles more kinetic energy so they move faster. Cells cannot switch diffusion on or off, but they control it indirectly by adjusting the gradient, for example, mitochondria using up oxygen inside a respiring cell keep oxygen flowing in.

Diffusion also sets a limit on size. As an organism grows, its volume increases faster than its surface area, so the surface area to volume ratio falls and diffusion alone can no longer supply the inside of the body quickly enough.

This is why large multicellular organisms need specialised exchange surfaces and a transport system, while a unicellular organism manages with diffusion across its membrane alone.

Diffusion compared with osmosis and active transport

How it is examined

You may be asked to define diffusion, to give a biological example, or to compare diffusion with osmosis and active transport.

Structured questions usually give a diagram of an exchange surface or a beaker with a dye, and ask you to explain the movement using the words concentration gradient, net movement and kinetic energy. Essay-style questions ask how the alveolus, villus or leaf is adapted for efficient diffusion, and the mark scheme rewards linking each feature to one of the four rate factors.

In experiment-based papers, the diffusion of potassium manganate(VII) or ink through agar or water is a typical context: you may need to state the manipulated variable such as temperature, the responding variable such as the time taken for the colour to spread, and a constant variable such as the volume of water.

Common misconceptions

Worked exam-style question

Question. Diagram X shows an alveolus in contact with a capillary. The partial pressure of oxygen in the alveolar air is higher than in the blood arriving at the capillary.

(a) Name the process by which oxygen enters the blood. (b) Explain how three features of the alveolus increase the rate of this process.

(c) A student states that oxygen movement stops once equilibrium is reached. Comment on this statement.

Model answer. (a) Diffusion (simple diffusion, as oxygen is a small non-polar molecule). (b) The alveolar wall and capillary wall are each one cell thick, giving a short diffusion distance; the lungs contain a very large number of alveoli, giving a large surface area for oxygen to cross; a rich capillary network and continuous ventilation remove oxygenated blood and refresh alveolar air, which maintains a steep concentration gradient.

A moist lining that dissolves oxygen is also accepted. (c) The statement is partly wrong: at equilibrium net movement stops, but the oxygen molecules continue to move randomly in both directions at equal rates, a dynamic equilibrium.

In a living lung, equilibrium is not reached because blood flow keeps carrying oxygen away.

Source:SRC-DSKP-EN

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