Glucose energy provided

In the fed state, the only fuel used by the brain is glucose, derived from the blood. In prolonged starvation or chronic hypoglycaemia, ketone bodies are nsed which rednce the rate of utilisation of glucose by the brain bnt, even so, glucose still provides about 50% of the energy. Consequently, under all conditions, maintenance of the blood glucose concentration is essential for the function of the brain the mechanisms that are responsible for this are discnssed in Chapters 6, 12 and 16. 

Since much less glucose is required by the brain, the rate of gluconeogenesis falls and hence the rate of protein degradation falls. In the obese, the energy provided from the oxidation of glncose that has been provided by amino acids from protein degradation is as little as 5% of the total (it is much higher in the lean see below). 

In lean subjects, amino acid oxidation, via glucose formation and glucose oxidation, provides almost four times more energy than in the obese snbjects (Table 16.4). That is, the obese lose protein mnch more slowly, which may be an important factor favouring survival of the obese in starvation. This is consistent with the fact that, from the data available, obese subjects have survived starvation approximately fonr times longer than the lean (abont 300 days versns 60-70 days Table 16.5). 

Net glycogen breakdown enables the liver to secrete glucose to provide for the energy needs of much of the body, particularly the brain. 

Apart from the energy and reducing power derived from these reactions, the pathway also provides carbon skeletons for the synthesis of cellular structures. The energy provided by this pathway is used to drive the synthetic processes of the cell, and the EMP pathway is the major route for glucose metabolism. However, there are specialised alternative routes by which glucose may be metabolised and the reader is referred to specialised biochemical texts for an account of these(l7,28). 

The first part of the glycolysis process consumes energy provided by the conversion of two ATPs to ADP. It consists of five major steps in which a glucose molecule is converted to two glylceraldehyde 3-phosphate molecules with intermediate formation of glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-biphosphate, and dihydroxyacetone ... 

Several carrier systems have been shown to be present in the brain endothelium, allowing for the selective transport of a group of common substrates (Table 13.1). The most common system is the one that mediates the transport of glucose, which provides the brain with virtually all its energy. Carrier-mediated mechanisms are also responsible for the absorption of two other energy sources ketone bodies, which are derived from lipids, and lactic acid, a by-product of sugar metabolism. Carrier-mediated transport systems are also involved in the uptake of amino acids by the brain. The brain can manufacture its own small neutral and acidic amino acids however, large neutral and basic amino acids are obtained from the bloodstream. 

The catabolic oxidation of glucose, to provide cellular energy, occurs principally through three linked catabolic pathways ... 

SGLT-1 and SGLT-2 are Na+-glucose symporters they concentrate glucose (and related hexoses) inside the cell using the energy provided by co-transport of Na+ ions down their electrochemical gradient. 

A summary of the sources of ATP produced from one molecule of glucose is provided in Table 10.2. ATP production from fatty acids, the other important energy source, is discussed in Chapter 12. Several aspects of this summary require further discussion. Recall that two molecules of NADH are produced during glycolysis. When oxygen is available, the oxidation of this NADH by the ETC is preferable (in terms of energy production) to lactate formation. The inner mitochondrial membrane, however, is impermeable to NADH. Animal cells have evolved several shuttle mechanisms to transfer electrons from cytoplasmic NADH to the mitochrondrial ETC. The most prominent examples are the glycerol phosphate shuttle and the malate-aspartate shuttle. 

As already noted above, the developing mammal is highly dependent on transplacentally derived glucose to provide its energy needs. After birth, as the animal begins to adapt to a more efficient oxidative metabolism, tissues such as liver and heart show an increase in both mitochondrial number and in specific mitochondrial enzymes such as cytochrome oxidase and various enz3mies of the citric acid cycle important for oxidative phosphorylation. In the case of the developing heart, we have previously shown (Warshaw, 1972) that cytochrome oxidase activity of newborn rat heart homo-... 

Glycogen, or animal starch, is a polymer of glucose that is stored in the liver and muscle of animals. It is used in our cells at a rate that maintains the blood level of glucose and provides energy between meals. The structure of glycogen is very similar to that of amylopectin except that glycogen is more highly branched. 

The cells in our body produce carbon dioxide as a product of aerobic respiration (the oxidation of glucose to provide energy). Carbon dioxide combines with water in the blood to form a solution containing hydrogen ions. 

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