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synthesis, and lipogenesis, respectively, and split into pyruvate for further metabolism. It can be synthesised by gluconeogenesis, or by breaking down glycogen.

Figure shows pathways of glucose metabolism inside the cell. The pathways of glucose, or carbohydrate, metabolism are shown with the key regulatory and energy-yielding steps marked. Glycolysis, or splitting of glucose, is the major top-to-bottom pathway, and it results in two pyruvate molecules; that is, fructose 1,6-bisphosphate is split, and the products are doubled.

      1.3.2.1.1 Glucose phosphorylation

      Following uptake into the cell by glucose transporters, the first step of glucose metabolism within cells is always phosphorylation to glucose 6-phosphate (G6-P), brought about by a member of a family of enzymes (hexokinases) that use ATP, again expressed in a tissue-specific manner. The form expressed in liver and pancreatic β-cells, hexokinase Type IV, is generally known as glucokinase; that expressed in skeletal muscle, Type II, is generally known simply as hexokinase. Phosphorylation ensures that the molecule does not diffuse again out of the cell, locking it into the cell, maintaining its inward concentration (and augmenting further glucose influx), and activating the molecule for further metabolism. G6-P is used by glycolysis (glucose breakdown, next section) and glycogen synthesis as well as the pentose phosphate pathway (Section 1.3.2.1.7 below); it may also be derived from glycogen breakdown (glycogenolysis) and gluconeogenesis (glucose synthesis: Section 1.3.2.1.5 below) depending on tissue and prevailing metabolic state. Thus, G6-P may be seen as lying at a major crossroads in carbohydrate metabolism (Figure 1.14).

      1.3.2.1.2 Glycolysis

      1.3.2.1.3 Lactate and ethanol metabolism

Figure with two panels labelled a and b showing Lactate and ethanol metabolism. Glycolysis produces NADH from NAD+. Panel a shows that in aerobic conditions in mammals the NAD+ is regenerated by the electron transport chain, but in anaerobic conditions NAD+ must be regenerated by lactate dehydrogenase, permitting glycolysis to continue, at the cost of accumulating lactate. Panel b shows that in yeast, pyruvate is instead reduced to ethanol in order to regenerate NAD+ and allow glycolysis to proceed.

      1.3.2.1.4 Pyruvate oxidation

      Pyruvate

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