Chapter 11, pages 433-439

In the previous class period we have looked at how chloroplasts capture the energy of sunlight and convert it into chemical energy (sugar) that is usable by all living cells. We now turn our attention to the mitochondria, organelles that help cells to extract the maximum amount of energy from food.

What is the importance of mitochondria?
Mitochondria, which are sometimes described as the cell's powerhouses, are the organelles responsible for generating metabolic energy in the form of ATP by the oxidation of carbohydrates and fatty acids derived from food. The amount of energy obtainable from a given amount of food is much greater with the help of mitochondria than in the absence of mitochondria.






What do mitochondria look like?
Mitochondria are enclosed by a double-membrane system made up of an inner and an outer membrane.
The space between these two membranes is called (surprise!) the intermembrane space.
The inner membrane encloses the interior of the mitochondrion, which is known as the mitochondrial matrix. The inner membrane also forms a large number of folds, called cristae, that extend into the matrix. See Fig. 11.1






What is the function of these various parts of the mitochondrion?
The membranes of the mitochondrion serve to enclose the matrix which contains hundreds of enzymes involved in the oxidation of carbohydrates and fatty acids.
The matrix of the mitochondrion also contains the mitochondrial genome (the DNA of the mitochondrion) mitochondrial ribosomes, tRNAs, etc.
The inner membrane contains proteins that comprise the electron transport chain, the ATP synthase enzyme that makes ATP and transport proteins that are needed for allowing the passage of metabolites into and out of the matrix. Passage of molecules through the inner membrane is very tightly regulated.
The outer membrane is permeable to many molecules, including small proteins.






How come mitochondria have their own DNA?
Since mitochondria are believed to be derived from what was originally a free-living bacterium-like ancestral form, it is not surprising that they would have their own genome. Studies have shown mitochondrial DNA to be similar to the DNA of other prokaryotes, especially that of Rickettsia.






What is the mitochondrial genome like?
The mitochondrial genome is a circular molecule, like a bacterial chromosome. Mammalian mitochondrial genomes are about 16Kb (16,000 basepairs) in size. The genes identified in the mitochondrial genome encode proteins involved in electron transport and oxidative phosphorylation, as well as ribosomal and tRNAs needed for protein synthesis within the mitochondria. See Fig.11.3






Is the mitochondrion independent of the cell it is in, if its genome encodes its own proteins and its ribosomes synthesize these proteins independent of the cytoplasmic ribosomes?
No, although the mitochondrial genome encodes some of the proteins in the mitochondrion, more than 90% of the proteins in the mitochondrion are encoded by genes in the cell's nucleus. These nuclear genes are transcribed and translated by the cellular machinery to make proteins in the cytoplasm. These proteins are then sent to the mitochondria from the cytoplasm. Without these proteins there would not be functional mitochondria.






How do the proteins made in the cytosol get into the mitochondria?
Proteins destined for the mitochondria are made on free, cytoplasmic ribosomes. These proteins must be transported to the mitochondria, where, if they are targeted to the matrix, they must cross two membranes. We will discuss this below.






What is the "address label" that sends proteins to the mitochondria?
Mitochondrial proteins have at their N-terminal ends a sequence of 20-35 amino acids called a "presequence" that is their address label directing them to the mitochondria.






How do proteins with this presequence get transported to the mitochondria?
As proteins with this presequence are made and released from the cytoplasmic ribosomes, they are bound by chaperones. These chaperone proteins keep the mitochondrial proteins unfolded till they are sent into the mitochondria. The unfolded mitochondrial proteins are recognized and bound by a receptor protein on the surface of the mitochondria.






What are the steps in the transport of the protein into the mitochondria after the protein has been brought to the surface of the mitochondrion?
Proteins that are brought to the surface of the mitochondrion are sent through a channel in the mitochondrial outer membrane called the Tom complex (Tom stands for Translocase of the Outer Membrane, so don't call it the Fred or Larry complex). Once they pass through the Tom complex, the proteins get handed over to the Tim complex (it's true, Tim is the Translocase of the Inner Membrane). When the protein arrives in the mitochondrial matrix, its presequence is cleaved, and it folds into its correct conformation.

See Figure 11.4






What about proteins that are supposed to stay in the mitochondrial membranes?
Proteins that are supposed to stay in the mitochondrial outer or inner membranes have stop-transfer sequences (like the proteins targeted to the ER membrane) that prevent them from being completely passed through the Tom or Tim complex.






What exactly do the mitochondria do?
When the cell is breaking down glucose from food for energy, it first makes pyruvate through a process called glycolysis, which occurs in the cytosol. The pyruvate is then sent to the mitochondria, where it is converted to acetyl coA in the matrix of the mitochondria. Acetyl coA is further oxidized to carbon dioxide by the a series of reactions called the citric acid cycle (a.k.a. The TCA cycle or Krebs cycle). See Figure 11.2

During this process high energy electrons are generated. These electrons as they pass through the electron transfer chain in the inner mitochondrial membrane, generate a proton gradient across the inner membrane. This proton gradient is used to synthesize ATP. See Figure 11.9

Although the source of the electrons for the electron transport chain is very different in chloroplasts and mitochondria, the overall process of electron transport linked to creation of a proton gradient and the synthesis of ATP is very similar in mitochondria and chloroplasts.



How are the activities of mitochondria and chloroplasts related?
The activities of mitochondria and chloroplasts are complementary.

Chloroplasts use carbon dioxide, water and ATP to synthesize sugars, releasing oxygen in the process.

Mitochondria use oxygen and break down sugars to produce ATP, releasing carbon dioxide and water in the process.

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Copyright © 2009 Indira Rajagopal