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Cytology · Year 1 · Medical University of Sofia

24

Mitochondria

Free notes for topic 24 of the Cytology syllabus, open without an account. Written by a senior student against the syllabus question and checked line by line by a second student before publishing. How content is made

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Summary

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Mitochondria carry out ATP and steroid synthesis. They are 0.5 to 1 μm wide and up to 10 μm in length.

They have two membranes, and the difference between them is the thing to know. The outer membrane is 50 per cent proteins and lipids, carries porins for molecular diffusion, and has enzymes for fatty acid metabolism. The inner membrane is 80 per cent proteins and cardiolipin, carries the enzymes of oxidative phosphorylation, the electron transport system and ATP synthesis, and forms the cristae that increase its surface area.

The matrix is rich in proteins, DNA and RNA, and holds the enzymes for the Krebs cycle, lipid synthesis and protein synthesis.

Their functions are apoptosis and energy production. They are abundant in muscle, brain, kidney and liver.

1. What mitochondria do, and how big they are

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Mitochondria carry out ATP and steroid synthesis.

Their size is 0.5 to 1 μm wide, and up to 10 μm in length.

2. Structure: two membranes

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Mitochondria have two membranes, with different compositions and different jobs.

The outer membrane

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  • 50 per cent proteins and lipids.
  • It has transport proteins, the porins, that allow molecular diffusion.
  • It has enzymes for fatty acid metabolism.

The inner membrane

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  • 80 per cent proteins and cardiolipin.
  • It carries enzymes with various functions, for example oxidative phosphorylation, the electron transport system and ATP synthesis.
  • It forms cristae, which increase the membrane's surface area.

The two percentages are the exam point. The inner membrane is the one that does the chemistry, so it is the one crowded with protein, and the cristae exist to give that chemistry more room.

The matrix

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The matrix is the intercristal, or matrix, space.

  • It is rich in proteins, DNA and RNA.
  • It holds the enzymes for the Krebs cycle, lipid synthesis and protein synthesis.

That a mitochondrion holds its own DNA and RNA, and its own protein synthesis, is a fact worth noticing here and remembering later.

3. Functions

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Two functions are named.

Apoptosis

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Apoptosis is cell death.

The inner membrane releases, in times of cell stress, cytochrome, which activates proteases. Those degrade all cellular components, which results in cell death.

Energy production

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Energy production is the production of adenosine triphosphate, ATP.

It is the process of conversion in which chemical energy is stored in the form of ATP, because the phosphate is a high-energy bond, which supplies energy for most cell activities.

Some of the energy is not stored in ATP but is dissipated as heat, which maintains body temperature.

4. Defects and distribution

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Defects in mitochondrial functions produce symptoms in muscle, brain and sometimes kidney.

Mitochondria are abundant in muscle, brain, kidney and liver.

The two lists overlap for a reason: an organelle fails most visibly in the tissue that depends on it most.

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