Mitochondrion structure and function

The mitochondrion is often called the "powerhouse" of the cell because it is the site of aerobic respiration, where energy stored in glucose is converted into a usable form, ATP.

Parts and functions

How structure suits function

The inner membrane is folded into finger-like cristae, which greatly increases its surface area so that more respiratory enzymes can be packed onto it, allowing more ATP to be released per mitochondrion. Cells that need a lot of energy, such as muscle cells, liver cells and sperm cells, contain far more mitochondria than less active cells, matching the number of mitochondria to the cell's energy demand.

• The inner membrane is folded into cristae, which increases its surface area so more respiratory enzymes fit onto it and more ATP is released.
• The matrix holds respiratory enzymes concentrated together, so the reactions that break glucose down further can happen quickly.
• The double membrane creates a separate internal compartment, keeping the respiration reactions and their conditions apart from the rest of the cytoplasm.
• The mitochondrion has its own DNA and ribosomes, so it can make some of its own proteins and copy itself when the cell needs more energy.
• Cells with a high energy demand, such as muscle, liver and sperm cells, contain far more mitochondria, matching the number of mitochondria to the cell's workload.

Related processes

The mitochondrion carries out aerobic respiration, the process that releases energy from glucose using oxygen. The first stage, glycolysis, happens in the cytoplasm, but the later stages take place inside the mitochondrion, where glucose is broken down completely into carbon dioxide and water.

Because oxygen is used, aerobic respiration releases far more ATP from each glucose molecule than anaerobic respiration, which happens in the cytoplasm without oxygen and gives only a small amount of ATP. The ATP made in the mitochondrion powers work throughout the cell, such as active transport, muscle contraction, cell division and protein synthesis.

Common labelling errors

Worked exam-style question

Question. The diagram shows a mitochondrion from a muscle cell. (a) Name the folded structure labelled X (the inner membrane) and the fluid labelled Y (the matrix).

(b) Explain how the folding of the inner membrane helps the mitochondrion carry out its function. (c) A liver cell contains far more mitochondria than a skin cell.

Suggest why. (d) Name the process carried out in the mitochondrion and state the gas it uses.

Model answer. (a) X is the inner membrane / cristae and Y is the matrix. (b) Folding the inner membrane into cristae increases its surface area, so more respiratory enzymes can be held on it and more ATP can be released by aerobic respiration in the same small space.

(c) The liver is a very active organ that carries out work such as making urea and storing glycogen, so its cells have a high energy demand and need a large number of mitochondria to release enough ATP; a skin cell is far less active. (d) The process is aerobic respiration, and it uses the gas oxygen.

Source:SRC-DSKP-EN

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