We have already seen in the preceding pages that autotrophic organisms (not necessarily phototrophic ones) are able to incorporate inorganic carbon as CO2 or HCO3— into hexose sugars, most commonly via the Calvin cycle. Heterotrophic organisms are unable to do this, and must convert a range of organic compounds into glucose by a series of reactions called gluconeogenesis (Figure 6.34). Many compounds such as lactate or certain amino acids can be converted to pyruvate directly or via other members of the TCA cycle, and thence to glucose. To all intents and purposes, gluconeogenesis reverses the steps of glycolysis (see above), although not all the enzymes involved are exactly the same. This is because three of the reactions are essentially irreversible, so different enzymes must be used to overcome this. These reactions are highlighted in Figure 6.34.
Once glucose or fructose has been produced, it can be converted to other hexose sugars by simple rearrangement reactions. Building up these sugars into bigger carbohydrates (polysaccharides) requires them to be in an energised form: this usually takes the form of either an ADP- or UDP-sugar, and necessitates an input of energy. Pentose sugars such as ribose are important in the synthesis of nucleotides for nucleic acids and coenzymes (see below).
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Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...