(3A) The action of cell membranes - selective diffusion transport

Although a cell membrane holds the cell together it lets substances in and out, but these substances must be dissolved in water in order pass to and fro through the cell membrane by diffusion.

You can think of a partially permeable membrane (*) as having tiny molecular sized holes in it, that only allow certain small, but NOT large, particles through.

Particles can be ions (Na⁺, Ca²⁺, K⁺, Cl^-) or molecules (H₂O, C₆H₁₂O₆).

(*) other phrases used are semi-permeable membrane or selectively permeable membrane

However, only small molecules and ions can diffuse through the cell membrane

e.g. relatively small molecules like glucose and oxygen for respiration, waste carbon dioxide from respiration, urea waste, amino acids for protein synthesis and of course water itself, as well as being the solvent.

BUT big molecules cannot get through the cell membrane e.g. starch and proteins.

In the particle model of a cell membrane on the right, the thick black dotted line represents the membrane.

Think of the grey circles as the larger molecules like proteins or starch which cannot pass from left to right through the cell membrane.

Imagine the blue circles are water - they can pass through the membrane in any direction - BUT, the net transfer is from a region of higher concentration to an area of lower concentration - in this case from right to left.

Imagine the green circles are small molecules or ions - they might be able to pass through the cell membrane in either direction, but the concentration is greater on the left than the right (remember the cell membrane controls what small molecules or ions can be transported in and out of cells).

Therefore for the green particles the diffusion gradient is from left to right and there is a net movement of the green particles (smaller molecules) from the left higher concentration to the right lower concentration passing through the cell membrane in the process.

Also bare in mind that the larger the surface area of a membrane, the faster the net rate of diffusion of a particular molecule or ion.

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(3B) Examples of diffusion in living organisms

Diffusion and the process of respiration.

The thin cell membranes allow the diffusion of small molecules in and out of cells.

Since the capillaries are thin and numerous, the diffusion distance from cells is short, so transfer of nutrients in, and waste products out, is as efficient as possible.

As the cells respire they use up oxygen/glucose, so their concentration falls in the cell. Therefore the external concentrations (e.g. in capillaries) is higher, so more oxygen/glucose will diffuse into the cell.

At the same time, the concentration of the waste product carbon dioxide builds in the cell, and so carbon dioxide will then naturally diffuse out of the cell to the lower concentration region in the capillaries.

For more details on gas exchange and other transfer systems involving diffusion see notes on:

Surface exchange of substances in animal organisms

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Keywords, phrases and learning objectives for this part on transport in organisms

Be able to describe and understand the action of cell membranes in selective diffusion in the transport ions and molecule of different sizes (size is an important factor)

Be able to describe from a diagram the membrane model for the gas exchange in respiration.

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