L9. Mitochondia

L9. Mitochondia

1. Where is it found?

   • occupy a substantial portion of the cytoplasmic volume of eukaryotic cells
   • The mitochondria in some cells form long moving filaments or chains. In others they remain fixed in one position where they provide ATP directly to a site of unusually high ATP consumption—packed between adjacent myofibrils in a cardiac muscle cell, for example, or wrapped tightly around the flagellum in a sperm.

2. Importance

   • essential for the evolution of complex animals
   • Without mitochondria, present-day animal cells would be dependent on anaerobic glycolysis for all of their ATP (inefficient)

3. Composition

   • is bounded by two highly specialized membranes, which have very different functions.
     • Together they create two separate mitochondrial compartments: the internal matrix and a much narrower intermembrane space.
   • parts
     • 1. outer membrane contains many copies of a transport protein called porin, which forms large aqueous channels through the lipid bilayer. This membrane thus resembles a sieve that is permeable to all molecules of 5000 daltons or less, including small proteins.
     • 2. intermembrane space is thus chemically equivalent to the cytosol while the matrix is highly specialised.
     • 3. inner membrane
       • is highly specialized:Its lipid bilayer contains a high proportion of the “double” phospholipid cardiolipin, which has four fatty acids rather than two and may help to make the membrane especially impermeable to ions.
       • also contains a variety of transport proteins that make it selectively permeable to those small molecules that are metabolized or required by the many mitochondrial enzymes concentrated in the matrix.
       • usually highly convoluted, forming a series of infoldings, known as cristae, that project into the matrix.
         • >> increase the area of the inner membrane, so that in a liver cell, for example, it constitutes about one-third of the total cell membrane.
         • number of cristae is three times greater in the mitochondrion of a cardiac muscle cell than in the mitochondrion of a liver cell, presumably because of the greater demand for ATP in heart cells. T
         • substantial differences in the mitochondrial enzymes of different cell types

4. Origins

   • It evolved from bacteria that were engulfed by nucleated ancestral cells. They contain their own genomes, as well as their own biosynthetic machinery for making RNA and organelle proteins.
   • Mitochondria and plastids are never made from scratch, but instead arise by the growth and division of an existing mitochondrion or plastid.
     • On average, each organelle must double in mass in each cell generation and then be distributed into each daughter cell.
       • The human mitochondrial genome contains about 16,500 nucleotides and encodes 2 ribosomal RNAs, 22 transfer RNAs, and 13 different polypeptide chains
   • They seem to have evolved from bacteria that were endocytosed more than 1 billion years ago. (endosymbiont hypothesis)
     • Eucaryotic cells started out as anaerobic organisms without mitochondria and then established a stable endosymbiotic relation with a bacterium, whose oxidative phosphorylation system they subverted for their own use The endocytic event that led to the development of mitochondria is presumed to have occurred when oxygen entered the atmosphere in substantial amounts, about 1.5 × 109 years ago, before animals and plants separated.