Mitochondria: labelled diagram, structure and function

A mitochondria diagram shows an oval organelle with two membranes: a smooth outer membrane and an inner membrane folded into shelves called cristae. The fluid inside is the matrix, which contains enzymes, ribosomes and a loop of DNA. Mitochondria (one is a mitochondrion) are where aerobic respiration releases most of a cell's energy as ATP.

Labelled mitochondria diagram

This is how a mitochondrion looks in an electron micrograph, sliced along its length. Mitochondria are too small to show any internal detail with a light microscope; you need an electron microscope to see the cristae.

Mitochondria structure

AQA A-level Biology notes that the DNA in mitochondria is short, circular and not associated with protein, like the DNA of prokaryotes, unlike the linear DNA in the nucleus.

What do mitochondria do?

Mitochondria are the site of the aerobic stages of respiration. Glycolysis happens in the cytoplasm; its product, pyruvate, is moved into the mitochondrial matrix, and the rest happens inside the mitochondrion:

1. The link reaction in the matrix turns pyruvate into acetyl coenzyme A, releasing carbon dioxide.
2. The Krebs cycle in the matrix releases more carbon dioxide and makes reduced coenzymes and a little ATP.
3. Oxidative phosphorylation on the inner membrane uses the reduced coenzymes to make most of the ATP, with oxygen as the final electron acceptor.

The ATP made is the cell's energy supply for muscle contraction, active transport, protein synthesis and other processes. The link reaction guide covers the first matrix stage in detail, and the cellular respiration guide gives the overall picture.

Practise the overall equation for aerobic respiration in this question from the Cell Biology course.

How the structure of a mitochondrion suits its function

• Cristae fold the inner membrane so that more of it fits inside the organelle. More membrane means more electron carriers and ATP synthase molecules, so more ATP can be made.
• The matrix holds all the enzymes for the link reaction and the Krebs cycle in one small space, close to the inner membrane that uses their products.
• Two membranes keep a narrow space between them, so a high concentration of protons can build up and drive ATP synthase.
• Its own DNA and ribosomes let it make some of its own proteins.

Cells that need a lot of energy have many mitochondria, and their mitochondria often have more densely packed cristae. Muscle cells, sperm cells and cells that do a lot of active transport, such as those lining the small intestine, are good examples.

How to draw a mitochondria diagram

1. Draw a long oval for the outer membrane, with a smooth, continuous line.
2. Draw a second line just inside it for the inner membrane, leaving a narrow gap.
3. Fold the inner line inwards in several places to make cristae, like fingers reaching into the middle. Make sure the inner membrane is one continuous line.
4. Label the outer membrane, inner membrane, cristae and matrix. At A-level, add the intermembrane space, ribosomes and circular DNA.
5. Use a ruler for label lines, keep them horizontal where possible, and don't let them cross.

Worked example: the real size of a mitochondrion

In an electron micrograph at a magnification of ×18 000, a mitochondrion is 45 mm long. How long is the real mitochondrion?

Multiply millimetres by 1000 to get micrometres (µm). You can explore organelles at different scales in the Cell Simulator.

Mitochondria vs chloroplasts

The chloroplast guide covers the other energy organelle.

Check your understanding.

Common mistakes

• Drawing only one membrane. Mitochondria have two; only the inner one is folded.
• Saying mitochondria make energy. Energy can't be made. Say they release energy from glucose and transfer it to ATP.
• Putting glycolysis in the mitochondrion. Glycolysis is in the cytoplasm.
• Calling the matrix the stroma. Stroma is the fluid in chloroplasts.
• Saying plant cells don't have mitochondria. They do, as well as chloroplasts.

Practise with a short timed paper

Cell structure comes up in GCSE Biology and in the first year of A-level Biology. The daily A-level Biology: Year 1 and GCSE Biology Higher papers are short timed papers of 8 questions, with a new one every day.

Read next

Biology · CellsChloroplast structure and function: labelled guideChloroplast structure explained: envelope, thylakoids, grana, stroma, DNA and starch, what each part does in photosynthesis, and how it is adapted.