| MITOCHONDRIAL SYNTHESIS OF ATP FROM REDUCED COENZYMES
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| Metabolism of carbohydrates begins in the cytoplasm through the glycolytic pathway (see Chapter 11), whereas energy production from fatty acids occurs exclusively in the mitochondrion. Mitochondria are subcellular organelles, about the size of bacteria. They are essential for aerobic metabolism in eukaryotes. Their main function is to oxidize metabolic fuels and conserve free energy by synthesizing ATP.
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| EXERCISE AND MITOCHONDRIAL BIOGENESIS |
| It has long been known that exercise increases the oxidative capacity of skeletal muscle by inducing mitochondrial biogenesis. Continued exercise results in energy consumption, and AMP accumulates. AMP-activated protein kinase is a fuel sensor, and it plays a critical role in initiating the production of new mitochondria and electron transport components such as heme. Of course, such mechanisms are not only of importance in exercise training, but also in the regeneration of tissues after tissue injury, such as trauma, heart attacks and strokes. |
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| Mitochondria are bounded by a dual membrane system (Fig. 8.4). The outer membrane (OMM) contains enzyme and transport proteins, and via the pore-forming protein porin (P), it is permeable to virtually all ions, small molecules (S) and proteins less than 10000 D. Large proteins must be transported via the TOM complex (translocase in the outer mitochondrial membrane) and TIM complexes (translocases in the inner mitochondrial membrane). This is especially vital to the cell, because almost all mitochondrial proteins are nuclear encoded and must be transported into the mitochondrion. The mitochondrial genome, mtDNA, encodes 13 vital subunits of the proton pumps and ATP synthase. The inner membrane (IMM) is pleated with structures known as cristae, and is impermeable to most ions and small molecules, such as NADH, ATP, coenzymes, phosphate, and protons. Transporter proteins are required to selectively facilitate translocation of specific molecules across the inner membrane. The inner membrane also contains components of oxidative phosphorylation -the process by which reduced nucleotide coenzymes are oxidized and ATP is synthesized.
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