Daily Newsletter

September 13, 2012 The Mitochondria

The mitochondria is known as the power house of the cell, for this is where eukaryotic cells experience oxidative phosphorylation and ATP production. We will come back to ATP in a latter newletter, but you should note that phosphorylation of proteins is a powerful activator of enzymes (allowing them to work). Everything from pump systems, cellular movement, and even muscle contraction relies on ATP.
Like the structure of the nuclear envelope, the mitochondria is a double membrane bound structure, but the origin of the nuclear envelope and mitochondria are very different. It is hypothesized that the nuclear envelope formed from the infolding (invagination) of the cell membrane. The mitochondria, in contrast, is two separate and distinct membranes.
The Endosymbiotic Theory is used to explain the development of the mitochondria. (Question: why should we consider this a theory?) Before we get to the endosymbiotic theory, we need to first look at the structure of the mitochondria:

We have an outer membrane and an inner membrane. Between the two membranes is the Intermembranous Space. The inner membrane is highly folded into Cristae (Question: why would you fold a membrane?). The inner compartment, bounded by the inner membrane, is known as the mitochondrial matrix.
With the structure in mind, how does this differ from the nuclear envelope?

The outer membrane displays eukaryotic proteins.
The inner membrane displays prokaryotic proteins.
The intermembranous space stores hydrogen ions, so is acidic.
The matrix contains a circular bacterial DNA molecule and 70s (prokaryotic) ribosomes.
The mitochondria is self-replicating (the DNA can make copies).

NOTE: Remember that phospholipid bilayer membranes allow you to create seperate fluid compartments. Here we have three fluid compartments: Intracellular, Intermembranous, matrix. Are these three fluid compartments have different chemical or electrochemical concentrations?
NOTE: The inner membrane seperates the intermembranous space (acid) from the matrix. We find that there is a H⁺ gradient across this membrane. Pumps along this membrane maintain the gradient by pumping H⁺ ions from the matrix into the intermembranous space. Pores along the inner membrane allow H⁺ ions to move back into the matrix. REMEMBER: when ions move down their electrochemical gradient across a membrane, work is done. There is a proton motive force across the inner membrane (this is the name we give to this type of H⁺ electrochemical gradient.

QUESTION: Why do we not consider the outer membrane to have an electrochemical gradient?

The endosymbiotic theory describes the mitochondria as a bacterial symbiont that was engulfed by a "proto-eukaryotic" cell. A relationship formed between the two cells, with the mitochondria taking over ATP production, and the "proto-eukaryotic" cell loosing the ability.

Genenomic analysis of the mitochondria shows that it comes from the bacterial Order Rickettsiales, which means that it is related to the intracellular parasite Rickettsia rickettsii (Rocky Mountain Spotted Fever). Mitochondrial genes are inherited matrilineally, and are the basis of human population genetics studies of the mitochondrial genome.

Like the Mitochondria, the cholorplast is an endosymbiont that exists in eukaryotic photosynthetic cells (such as plant cells). Photosynthetic cells will have both mitochondria (extract energy) and chloroplasts (build reduced organic compounds). The function of the chloroplast is to use light energy to reduce carbon (carbon fixation) in order to produce reduced carbon compounds, such as glucose. The chloroplast, like the mitochondria, has two membranes, one eukaryotic in structure and ond prokaryotic. The chloroplast is also self-replicating, and has baterial ribosomes and genophore (DNA).

Daily Challenge

Write about the mitochondria, the endosymbiotic hypothesis and human mitochondrial genetics. Explore these topics, and feel free to go deeper on any feature of the mitochondria that interests you. One question I want you to focus on is why is the mitochondrial genome reduced (smaller) than other members of the Rickettsiales? In your discussion, you must answer the question: Why do we not consider the outer membrane to have an electrochemical gradient?
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