October 21, 2007

The Mitochondrion Pt. 1 — Structure and Layout

Posted in Cell Biology at 3:59 am by D. Borst

So we learned in grade school that the mitochondria is the cell’s energy powerhouse. Lets expand upon that a little bit.

Mitochondria (along with chloroplasts and other plastids) are thought to have originally been independent entities. As such, they had their own DNA, the remains of which can be found in the mitochondria in our cells. It is thought that originally a mitochondria was endocytosed but never fully digested by a early pre-eucaryote. Over time, mitochondria have come to develop a symbiotic relationship with eucaryotic cells, to the point where all but a handful of the mitochondiral genome is now present in the host genome. The mitochondria in turn provides its host cell with incredibly large ammounts of ATP by processing Acetyl-CoA in the Krebs Cycle.

There are two views of the noble mitochondria, one in which it is a small, rigid little bean shaped organelle that is somewhat static in its shape and size, and the other, more recently supported view that the mitochondria is a dynamic and quickly changing organelle. Mitochondria are now thought to quickly change conformation, bind and split with other mitochondria, and in some case form one huge reticulated super entity. Two theories attempt to explain our earlier oversight:

  • One is that when isolating the parts of the cell, the contents were disrupted in order to get them out of the plasma membrane. However, this disruption also caused the mitochondrial membrane to be split in many places leading to smaller mitochondria to be characterized by explorers.
  • The other theory has to do with or techniques for looking at the cell, and how they are based upon actual or optical sectioning of the cell (i.e. physically sectioning or by using a particular focusing plane, which led to the observation of individual small mitochondria that were connected out of the section.
  • In any case, the new model suggests that mitochondria are much more fluid and less bean shaped than what you may see in your textbook, so remember that.

    Structure of Mitochondria

    The structure of the mitochondria is one of the important epigenetic features of the cell. Cells in which mitochondria or chloroplasts have been ”zapped” are not able to reconstruct the mitochondria. Rather the information to construct mitochondria seems to be contained within the structure of the mitochondria themselves. The mitochondria somehow, and Im not clear on the details, replicate themselves without being directly reliant upon cell machinery. Similarly, mitochondria are not regulated during the cell cycle, but it is just a matter of brownian motion that all cells splitting off get some mitochondria. The mitochondria then regulate their own replication (or expansion in the case of the reticulated mitochondrial super entity.

    Mitochondria have two membranes, an outer and an inner. The outer membrane has a whole bunch of porin transporter molecules that allow molecules of 5 kd or less to easily diffuse across the membrane. It, along with the inner membrane, forms a intermembrane space that is semicontinuous with the cytosol. This semi-continuity is important for the proton gradient that is generated across the inner membrane. The outer membrane of the mitochondria has proteins responsible for fatty acid elongation.

    The inner membrane is solid, and jam packed full of proteins. It is estimated that there is one protein for every 3 lipid molecules in the inner membrane. The inner membrane contains special lipid molecules termed ‘cardiolipids.’ These lipids have four fatty acid tails rather than the two that most membrand lipids have. It is continuous with itself, and forms an inner “matrix space” where the more important processes to energy production occur. This matrix space also contains the remains of the mitochondria’s own DNA, which because the mitochondria is highly conserved, has been useful for relatedness studies, i.e. mitochondrial eve. The inner membrane is also much longer than the outer membrane, making it fold back in upon itself to form long christae that protrude into the matrix space. This structure increases the surface area of the membrane, allowing a greater number of worker proteins to exist. ATP production is located on this inner membrane, so a greater surface area yields more possibilities for production. As will be discussed later, ATP production is driven by a proton gradient that is established across this membrane the electron transport process. Cardiolipids are thought to make this membrane more impermeable to protons because of their four tails.

    So to remember:

  • Mitochondria may have originally been free organisms that have been engulfed by proto-eucaryotes.
  • Mitochondra form dynamic entities within animal cells, quickly combining with other mitochondria to form large and irregular reticulated organelles.
  • Cells cannot make mitochondria de novo, they must inherit them from their progenitors and allow them to reproduce.
  • Mitochondria have two membranes, an outer membrane that has many porin channels within it, and a inner membrane that has special cardiolipids that make it a good barrier to protons.
  • The inner membrane of the mitochondria is the site of much of the cells ATP production.
  • That is all for this segment. The information for this post came from two sources–either Alberts’ Molecular Biology of the Cell, or the October 17 Cell Biology lecture by Dr. Todd Rosentiel at PSU.

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    39 Comments »

    1. J.P.Nema said,

      Kindly specify F0-F1 particle on inner membrane Cristae.

    2. khan said,

      very very good

    3. khan said,

      give the detail about mitochondrial membrane

    4. khalid said,

      what is the role of golgi apparatus in mitochondria

    5. khan said,

      what is the composition of F1 particals

    6. sammy said,

      ahhhh…..ok….:-l

    7. magalin said,

      i think you gave alot of info. or just the right amount that i needed to complete my work.

    8. raman said,

      thank you

    9. Ben said,

      Very useful for a cell part project im working on.

    10. Oppy said,

      Thank you, you had more in-depth analysis than my AP Bio book :)

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      EVOLUTION OF EUKARYOTIC MITCHONDRIA FROM PROKARYOTIC MITOCHONDRIA (SYMBIONTIC THEORY ) ADD PLEASE.

    12. Kavitha said,

      Thank u…it really helped me to prepare for my seminar the topic – “mt DNA”

    13. joseph said,

      not enough info on d cell death part but thanks any way

    14. nidhi said,

      the info was quiet helpful for my 3d bio project

    15. ravi kota said,

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    19. abby and licete said,

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