Question 18

College Answer

Most candidates had at least a basic understanding of mitochondral function although some 
detail was required for a pass and many did not provide this. A well labelled diagram was used 
by many candidates and scored marks. Repetition of the same information illustrated on a 
labelled diagram in subsequent text was not required and did not score additional marks. 
It was expected answers would cover basic structure (double membrane structure with cristae 
and enzymes lining the membrane and within the matrix), details of the electron transport 
chain, the citric acid cycle and beta-oxidation of long chain fatty acids and mention the 
maternal origin of DNA. Better Answers provided some information on other functions such as 
production of reactive oxygen species, role in calcium homeostasis and apoptosis, urea cycle, 
haem synthesis and heat production

Discussion

In the short span of time allowed by this MCQ, you'd be lucky to produce something as good as this (from Cellular Organelles by Bittar):

The most important features of mitochondrial structure are as follows:

• Two membranes
  • Outer membrane with pores
  • Inner membrane without pores
  • These membranes meet at contact sites (areas where membrane lipids and proteins may be exchanged with other organelles)
• Two separate compartments
  • Intermembrane space or "outer compartment"
  • Matrix space or inner compartment
• Organisation of the inner membrane into cristae (a comb of many folds) which increases its surface area
• Molecular components
  • ATP synthase molecules on the inner surface of the inner membrane 
  • Mitochondrial ribosomes in the inner compartment
  • Mitochondrial DNA in the inner compartment
  • Electron dense-granules in the inner compartment (composed of RNA)

Mitochondrial DNA characteristics include

• Maternal inheritance from oocyte mitochondrial DNA (paternal contribution is minimal)
• Mitochondrial DNA is short (16,500 base pairs) and encodes only 8 protein subunits (mainly from the electron transport chain)
• This DNA is susceptible to mutation (10 times more than nuclear DNA)

Mitochondrial function includes:

• ATP synthesis functions, including:
  • Citric acid cycle
  • Electron transport chain
  • Beta-oxidation of long chain fatty acids
• Regulatory and synthetic functions
  • Haem synthesis
  • Calcium ion storage
  • Urea cycle
  • Haem synthesis
  • Steroid synthesis
• As a byproduct of these functions
  • Heat production
  • CO₂ production
  • Production of reactive oxygen species
• Non-metabolic roles
  • Apoptosis

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