Red blood cell structure and function

Red blood cells, also called erythrocytes, carry oxygen from the lungs to every respiring cell in the body. Their shape and contents are closely adapted to this single job.

A red blood cell begins as a normal cell in the bone marrow, but as it matures it loses its nucleus and most of its organelles and fills with haemoglobin. Questions on this structure usually ask you to link each feature to the transport of oxygen, so it helps to think of the cell as a container that has been stripped down and shaped for one purpose.

Parts and functions

How structure suits function

Every feature of a red blood cell links to a factor that affects how much oxygen it can carry or how fast that oxygen moves. In an exam the mark is given for the link, not for naming the feature on its own.

The biconcave disc shape gives the cell a larger surface area relative to its volume than a sphere would have, and its thin centre keeps the diffusion distance short, so oxygen enters and leaves quickly. Losing the nucleus and mitochondria frees internal space and means the cell does not consume the oxygen it is transporting, so the maximum amount reaches the tissues.

• Biconcave shape, a high surface area to volume ratio and a short diffusion distance speed up the loading and unloading of oxygen.
• No nucleus, the extra internal space is filled with haemoglobin, raising the amount of oxygen each cell can carry.
• No mitochondria, the cell does not use aerobic respiration, so it does not draw on the oxygen it is delivering.
• Haemoglobin, it binds oxygen reversibly, picking it up where oxygen is plentiful in the lungs and releasing it where oxygen is low in respiring tissue.
• Flexible membrane and small size, the cell distorts to fit through capillaries only slightly wider than itself, carrying oxygen deep into every tissue.

Related processes

In the lungs, oxygen diffuses from the alveolar air into the blood, dissolves in the plasma and binds to haemoglobin to form oxyhaemoglobin. Because oxygen is plentiful there, almost all the haemoglobin becomes saturated.

This is the loading stage, and it depends on the same diffusion that occurs across the thin, moist alveolar wall.

In respiring tissues the oxygen concentration is low, so oxyhaemoglobin breaks down and releases oxygen, which diffuses out to the cells for aerobic respiration in their mitochondria. The red blood cell then helps carry away some of the carbon dioxide produced, although most carbon dioxide travels dissolved in the plasma as hydrogen carbonate ions.

The delivery of oxygen by red blood cells is therefore the link between gas exchange in the lungs and respiration in every other cell.

Common labelling errors

Worked question

Question (in the style of Paper 2 Section A): The diagram shows a red blood cell. (a) Name the shape of the cell. [1 mark] (b) Explain two ways the cell is adapted to carry oxygen. [4 marks] (c) A person with too little haemoglobin often feels tired during exercise.

Explain why. [2 marks]

Model answer: (a) A biconcave disc. (b) The cell has no nucleus, so there is more space for haemoglobin and it can carry more oxygen.

The biconcave shape gives a large surface area to volume ratio and a short diffusion distance, so oxygen is loaded and unloaded quickly. (Alternative point: haemoglobin binds oxygen reversibly, taking it up in the lungs and releasing it in the tissues.) (c) With less haemoglobin the blood carries less oxygen to the muscles, so less aerobic respiration occurs and less energy is released; the muscles tire quickly.

Marking note: for part (b) each adaptation earns one mark for the feature and one for the linked explanation. Naming the feature alone scores half.

Source:SRC-DSKP-EN

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