Brent Cornell

   
Mitochondria are the ‘powerplants’ of the cell – synthesising large amounts of ATP via aerobic respiration

• All eukaryotic cells possess mitochondria – aerobic prokaryotes use the cell membrane to perform oxidative phosphorylation

Mitochondria are thought to have once been independent prokaryotes that were internalised by eukaryotes via endosymbiosis

• They have a double membrane structure (due to vesicular coating as part of the endocytotic process)
• They have their own DNA (circular and naked) and ribosomes (70S)
• Their metabolic processes are susceptible to certain antibiotics

The structure of the mitochondrion is adapted to the function it performs:

• Outer membrane – the outer membrane contains transport proteins that enable the shuttling of pyruvate from the cytosol
• Inner membrane – contains the electron transport chain and ATP synthase (used for oxidative phosphorylation) 
• Cristae – the inner membrane is arranged into folds (cristae) that increase the SA:Vol ratio (more available surface)
• Intermembrane space – small space between membranes maximises hydrogen gradient upon proton accumulation
• Matrix – central cavity that contains appropriate enzymes and a suitable pH for the Krebs cycle to occur

Structure and Function of a Mitochondrion

   
Electron micrographs of a mitochondrion may differ in appearance depending on where the cross-section occurs

Typically, mitochondrial diagrams should display the following features:

• Usually sausage-shaped in appearance (though will appear more rounded in perpendicular cross-sections)
• Inner membrane contains many internal protrusions (cristae)
• Intermembrane space is very small (allows for a more rapid generation of a proton motive force)
• Ribosomes and mitochondrial DNA are usually not visible at standard resolutions and magnifications

Mitochondrion Diagrams

⇒  Click on the diagram to show / hide labels

     
Electron tomography is a technique by which the 3-dimensional internal structure of a sample can be modelled

• Samples are repeatedly imaged using a transmission electron microscope
• Following each image, the sample is tilted to a different angle relative to the electron beam
• The images are then compiled and used to computationally reconstruct a 3-D representation (called a tomogram)

When dealing with biological materials, samples are first prepared by either fixing and dehydrating or freezing (cryogenics)

• This stabilises the biological structures and prevents aqueous contents (i.e. water) from expanding and exploding

Using electron tomography, the following features of active mitochondria have been identified:

• The cristae are continuous with the internal mitochondrial membrane
• The intermembrane space is of a consistent width thoughout the entire mitochondrion
• The relative shape, position and volume of the cristae can change in active mitochondria

Overview of Electron Tomography