Structure & Function of Mitochondria (Cambridge (CIE) A Level Biology): Revision Note

Structure & function of the mitochondria

• 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 synthesise 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

Structure

• Mitochondria have two phospholipid membranes, an outer and an inner membrane

• 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

Relationship between structure and 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 due to the differing levels of metabolic activity in those cell types

  • And the resulting different levels of demand for ATP in those cells