Darkthorn’s Blog

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Carbohydrate Catabolism

1. During the preparatory stage of glycolysis, the molecule of glucose is phosphorylated, and converted into two molecules of glyceraldehyde-3-phosphate. This requires the use of two ATP molecules.

In the payoff phase, the glyceraldehyde-3-phosphate is converted to pyruvate. This releases four ATP molecules, and produces two NADH molecules.

1. In the preparatory phase the phosphate groups are obtained from 2 ATP and attached in place of the OH groups, producing glyceraldehyde-3-phosphate, and leaving 2 ADP molecules behind.

In the pay-off phase, a molecule of Pi replaces the H group, releasing electrons to form NADH and 1,3-biphosphoglycerate. The phosphate group is then removed by ADP to form ATP and 3-phosphoglycerate.

1. Substrate-level phosphorylation is the transfer of a high energy phosphate group from 1,3-biphosphoylate/phosphoenolpyruvate to ADP to produce ATP and a molecule of 3-phosphoglycerate/pyruvate.

1. Although the conversion of glucose to glucose-6-phosphate is endergonic (requires energy, in the form of 2 ATP molecules), the overall process of glycolysis still proceeds. This is because the overall reaction is exogonic, and the later stages have excess energy to help the first reactions proceed.

1. For humans, pyruvate has two possible metabolic fates. In anaerobic conditions, such as during a sprint, pyruvate undergoes fermentation (in active skeletal muscle, the retina or red blood cells) to produce lactate and ATP. This method of producing energy is very inefficient compared to the energy produced under aerobic conditions. In aerobic conditions, during everyday cellular respiration, pyruvate is further oxidised to form the eventual products of carbon dioxide and water.

Lipid Catabolism

1. There are six steps in the digestion and transport of dietary lipids to adipose tissue.
   1. In the small intestine fat is emulsified by bile salts secreted from the gall bladder to produce mixed micelles.
   2. Triacylglycerols within the mixed micelles are degraded by intestinal lipases into monoacylglycerols, diacylglycerols, free fatty acids and glycerol.
   3. These products then diffuse into the epithelium cells lining the intestinal surface where they are reconverted into triacylglycerols.
   4. The insoluable triacylglycerols are packaged with phospholipids, proteins and cholesterol to form chylomicrons. Chylomicrons then move from the epithelial cells into the lymphatic system, where they can enter the bloodstream.
   5. The chylomicrons move through the bloodstream until the reach adipose tissue. They are then degraded into their original parts (releasing free fatty acids and glycerol) within the capillaries of the adipose tissue.
   6. The freed fatty acids are taken up by the cells, where they are reesterified for storage as triacylglycerols.

The mobilisation of stored lipids maintains the body’s energy levels during starvation (both long term and between meals). Low glucose levels in the blood cause the release of hormones which activate the enzyme triacylglycerol lipase. This stimulates the breakdown of stored fatty acids into fatty acids and glycerol – the fatty acids are transported by serum albumin to the muscles and the glycerol is recycled back to the liver. This is basically the reverse of the fats original breakdown.

1. The CoA pool in the intermembrane space provides the CoA required to generate the first stage Fatty Acyl-CoA from a fatty acid. CoA is then replaced by carnitine so that the fatty acid can pass across the membrane. The second pool of CoA releases the carnitine (to pass back into the intermembrane space) and regenerates Fatty Acyl-CoA. The carnitine prevents the two pools from interacting because it is the only thing allowed across the membrane.
2. The four reactions of B-oxidation are: oxidation, hydration, another oxidation and thiolytic cleavage.
3. Reduced electron carriers are generated by reaction 1 (FAD à FADH) and reaction 3 (NAD+ à NADH).