lectures19+20 - ICU3 Lectures 19 20 Bioenergetics[1[2 1 ATP is a very common metabolite and is present in high concentration(~6mM in most cells 2 ATP is

lectures19+20 - ICU3 Lectures 19 20 Bioenergetics[1[2 1 ATP...

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ICU3 Lectures 19 & 20: Bioenergetics [1] & [2] 1. ATP is a very common metabolite and is present in high concentration (~6mM) in most cells. 2. ATP is often described as a “high energy” compound, but hydrolysis of ATP to ADP or AMP yields only modest amounts of energy. Although small, this additional energy tips the balance and is sufficient to drive many unfavourable processes in the required direction. 3. ATP carries small packets of energy from place to place. It is the energy currency of the cell. If glucose molecules were £5 notes, then ATP is small change. 4. There is roughly 75g of ATP in the average human. A reasonably active person (12MJ diet) turns over about 75 kilos of ATP every day, so a typical ATP molecule is broken down to ADP and resynthesised 1000 times each day. In rapidly metabolising tissues the lifetime of each ATP molecule is only a few seconds. 5. The concentration of free ADP is normally much lower than that of ATP – about 200 times lower in the cytosol of eukaryotic cells. [This situation is clouded by large amounts of bound ADP permanently stuck to the actin cytoskeleton.] A low concentration of free cytosolic ADP is essential for metabolism to work properly. Diffusion rate is proportional to concentration, making it difficult to recycle ADP quickly in rapidly metabolising tissues such as cardiac muscle. Shuttle systems have evolved which accelerate ADP transport within cells: 6. About half the total energy available from food oxidation is “captured” in the form of ATP. Of this ATP, about half the free energy can be converted into useful work, such as muscle contraction. Engineering / industrial processes such as electricity generation and motor cars achieve comparable overall efficiencies. 7. Under aerobic conditions 95% of the cellular ATP is produced within the mitochondria, which actively scavenge the cytosol for ADP, exporting ATP in exchange. 8. The energy available from splitting ATP depends on how far the hydrolysis reaction is displaced from equilibrium. The lower the ADP concentration, the more energy is available. Eukaryotic cells increase the energy yield from each molecule of ATP by transporting ATP and ADP between cell compartments. Visit our website at
9. Mitochondria have two membranes, which are very different. The outer membrane is smooth and highly permeable. It contains porin, an integral membrane protein that self- assembles into “grommets” with a central hole. Molecules under 5000 daltons have free passage. There are no ionic or electrical gradients across the mitochondrial outer membrane. 10. The inner membrane thrown into folds called cristae which increase the surface area. It is selectively permeable to small molecules. About a dozen metabolites can cross the inner membrane using highly specific protein carriers, but all other movements are blocked. It has a unique lipid composition which is an excellent electrical insulator. There are very large ionic

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