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Department of Biological and Environmental Sciences

Cell & Molecular Biology
Dr. David A. Johnson
Biol 405    4 Credits   Spring 2017  MWF 11:45-12:50 AM   PH

<<<  Mitochondria  >>>
Chapter 11

Mitochondria are organelles specialized for aerobic respiration. As covered earlier, they are believed to have arisen by  endosymbiosis of a prokaryote in an ancestral eukaryotic cell. (Mitochondria video)
  • The Structure of a Mitochondrion: A mitochondrion has a double membrane (inner and outer) with the inner membrane folded, creating cristae. Between the two membranes is the intermembrane space and inside the inner membrane is the matrix. The outer membrane is highly permeable to small molecules due to channels formed by porin proteins. However, the inner membrane is impermeable to most ions and small molecules.

    • The Genetic Material of  Mitochondria: The genetic material of mitochondria is circular DNA (like the chromosomes of bacteria) which for most animals is about 16 kb in length. (Plant mitochondrial DNAs are considerable longer.) There are many copies of this circular molecule in each mitochondrion. Mitochondrial genes in animals include those for a few of the proteins needed for oxidative phosphorylation (the rest are encoded by nuclear genes)(2010 article), mitochondrial rRNA genes, and most of the mitochondrial tRNA genes (mt tRNAs).
      • The Human Mitochondrial Genome: In humans, the mitochondrial DNA is about 16 kb and encodes 13 proteins* that are embedded in the inner mitochondrial membrane and are involved in oxidative phosphorylation. It also encodes the 16S and a 12S rRNA, and the 22 tRNAs. The 16S rRNA is the only rRNA in the large ribosomal subunit  (39S)  and the 12S rRNA is the only rRNA in the small ribosomal subunit (28S). The mitochondrial genetic code is and example of a deviation from the universal code. The ribosomal proteins found in mitochondrial are all coded for by nuclear genes and these proteins are imported into the mitochodrion. The large subunit has 48 proteins, 28 of which are similar to E. coli ribosomal proteins. The small subunit has about 30 proteins. There is a lower RNA/protein ratio in mitochondrial versus E. coli. *But, wait a minute..."For decades, scientists thought they had a handle on the mitochondrial chromosome: 13 genes for proteins, two for rRNAs and 22 for tRNAs, all tightly packed with no introns." ... but now "it appears to contain small open reading frames that are hidden inside the other genes."
      • Mitochondrial Gene Inheritance: Mitochondria are usually passed to the zygote only via the egg and not the sperm. Therefore, mitochondrial gene inheritance follows a unique maternal inheritance pattern. Mitochondrial genes also seem to have a higher mutation rate and are therefore useful in revealing genetic differences between closely related organisms. (See "Mutation rate in various species" from that infallible source Wikipedia.) Since there are multiple copies of each mitochondrial gene in every cell, this means that the recovery of mitochondrial genetic material from minute samples is easier than the recovery of nuclear genetic material.
      • Endosymbiosis and Mitochondria: Evidence for the endosymbiotic origin of mitochondria include 1) a circular chromosome with one origin of replication; 2) formylmethionine used in the initiation of protein synthesis; 3) similarity of rRNAs.
    • Other Mitochondrial Components: The vast majority of mitochondrial proteins are imported from the cytosol as described earlier. At least about 1000 different proteins are encoded by nuclear genes and imported into the mitochondria. Intermembrane proteins need only cross the outer membrane, while matrix proteins must cross both membranes. This involves a specific import system similar to that required for proteins crossing other membranes.

    • Oxidative Phosphorylation: During the breakdown of energy-containing molecules, the majority of the energy captured in the form of ATP comes from oxidative phosphorylation. For example, the breakdown of a molecule of glucose yields 4 ATPs directly, but also generates 10 NADHs and 2 FADH2s (molecules that have been reduced, receiving high energy electrons). These electrons are then passed through various electron transport complexes that are embedded in the inner mitochondrial membrane. With each transfer, energy is released and this energy is used to pump protons (H+) into the intermembrane space. (Some is capture directly by the synthesis of ATP.) The proton gradient (difference in concentration and difference in charge) created represents potential energy and this energy is used to manufacture ATP from ADP and P. (The impermeability of the inner membrane maintains this gradient.) FADH2 enters this electron transport system a little further downstream and therefore does not produce as great a proton gradient (fewer ATPs). The  enzyme (also in the same membrane) that catalyzed production ATP reaction is ATP synthase. In order for this synthesis to work, it is necessary to move ATP out of, and ADP into the matrix. This is accomplished by a membrane protein that exchanges one ADP in the intermembrane space for one ATP in the matrix. The energy to drive this transport comes from the charge gradient (+ in intermembrane space due to the high concentration of protons). Since ATP is more negatively charged that ADP, this gradient favors ATP leaving the matrix. (Electron Transport Chain movie; ATP Synthase movie)
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