8.2.6 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 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

  

The structure of a mitochondrion

Structure

• 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

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

• Electron tomography (ET) is a novel approach able to provide three-dimensional (3D) information on cells and tissues at molecular level
• The technique is an extension of transmission electron microscopy

Transmission electron micrograph compared with electron tomography

• ET has led to new discoveries about the mitochondria, including
  • The cristae are now known to be dynamic in nature, rather than static
  • The cristae have been seen to respond physically and biochemically to a changing environment with the mitochondria

Representation of EM images showing the dynamic nature of mitochondrial cristae