OCR A Level Biology

Revision Notes

5.7.2 Structure of the Mitochondrion

Structure of the Mitochondrion

  • Mitochondria are rod-shaped organelles 0.5 – 1.0 µm in diameter
  • They are the site of aerobic respiration in eukaryotic cells
  • The function of mitochondria is to synthesize ATP
  • Synthesis of ATP in the mitochondria occurs during the last stage of respiration called oxidative phosphorylation
    • This relies on membrane proteins that make up the ‘electron transport chain’ and the ATP synthase enzyme – the details of this are covered later in the notes


  • Mitochondria have two phospholipid membranes
  • The outer membrane is:
    • Smooth
    • Permeable to several small molecules
  • The inner membrane is:
    • Folded (cristae)
    • Less permeable
    • The site of the electron transport chain (used in oxidative phosphorylation)
    • Location of ATP synthase (used in oxidative phosphorylation)
  • The intermembrane space:
    • Has a low pH due to the high concentration of protons
    • The concentration gradient across the inner membrane is formed during oxidative phosphorylation and is essential for ATP synthesis
  • The matrix:
    • Is an aqueous solution within the inner membranes of the mitochondrion
    • Contains ribosomes, enzymes and circular mitochondrial DNA necessary for mitochondria to function

Mitochondria Structure, downloadable AS & A Level Biology revision notes

The structure of a mitochondrion.

Relationship between structure & function

  • The structure of mitochondria makes them well adapted to their function
  • They have a large surface area due to the presence of cristae (inner folds) which enables the membrane to hold many electron transport chain proteins and ATP synthase enzymes
  • More active cell types can have larger mitochondria with longer and more tightly packed cristae to enable the synthesis of more ATP because they have a larger surface area
  • The number of mitochondria in each cell can vary depending on cell activity
    • Muscle cells are more active and have more mitochondria per cell than fat cells

Exam Tip

Exam questions can sometimes ask you to explain how the structure of a mitochondrion helps it carry out its function effectively. Make sure to follow through with your answer. It is not enough to say that cristae increase the surface area of the inner membrane. You need to explain that an increased surface area of the inner membrane means there are more electron transport chain carriers and ATP synthase enzymes which results in more ATP being produced.

Be prepared to identify the different structures and locations in a mitochondrion from an electron micrograph.


Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top