Metabolic pathways in cells are usually either anabolic (synthetic) or catabolic (degradative). These pathways can also be described as amphibolic, a combination of two reactions in which catabolic breakdown products are subsequently used in anabolic synthetic reactions. Catabolic reactions provide the energy necessary to drive the anabolic reactions. ATP is a useful intermediate for this purpose because its terminal anhydride bond has a free energy of hydrolysis that allows ATP to serve as a donor, and ADP to serve as an acceptor, of phosphate groups. 

Most chemotrophs derive the energy needed for ATP generation from the catabolism of organic nutrients such as carbohydrates, fats and proteins. They do so either by fermentative processes in the absence of oxygen or by respiratory metabolism, which is usually, though not always, an aerobic process.

Using glucose as a prototype substrate, catabolism under both anaerobic and aerobic conditions begins with glycolysis, a 10-step pathway that converts glucose into pyruvate. In most cases, this leads to the production of two molecules of ATP per molecule of glucose. In the absence of oxygen, the reduced coenzyme NADH generated during glycolysis must be reoxidised at the expense of pyruvate, leading to fermentation end-products such as lactate or ethanol and carbon dioxide. This severely limits the extent to which the free energy content of the glucose molecule can be released, but the seven per cent or so that is available is conserved as ATP quite efficiently.

Although usually written with glucose as the starting substrate, the glycolytic sequence is also the mainstream pathway for the catabolism of a variety of related sugars, as well as for the utilisation of the glucose-1-phosphate derived by phosphorolytic cleavage of storage polysaccharides such as starch or glycogen.

Gluconeogenesis is, in a sense, the opposite of glycolysis because it is the pathway whereby some cells synthesise glucose (and other carbohydrates) from three- and four-carbon non-carbohydrate starting materials such as pyruvate. However, the gluconeogenic pathway is not just glycolysis in reverse. The two pathways share seven enzyme-catalysed reactions in common, but the three most exergonic reactions of glycolysis are bypassed in gluconeogenesis by reactions that render the pathway exergonic in the gluconeogenic direction by the input of energy from ATP and GTP.

Like other metabolic pathways, glycolysis and gluconeogenesis are highly regulated to ensure that the rate of product formation (ATP and glucose, respectively) is carefully tuned to cellular needs. The enzymes that are subject to allosteric regulation catalyse reactions unique to the respective pathways. These enzymes are regulated by one or more effectors, which include ATP, ADP and AMP, as well as acetyl CoA and citrate, key intermediates in aerobic respiration. In animal and plant cells, the most important allosteric regulator of both glycolysis and gluconeogenesis is fructose-2,6-bisphosphate, the concentration of which depends on the relative kinase and phosphatase activities of the bifunctional enzyme PFK-2. The function of PPK-2 is regulated, in turn, by the hormones glucagon and epinephrine, mediated by the intracellular concentration of cyclic AMP.

Although glycolysis may seem overly complex upon first encounter, it represents the simplest mechanism by which glucose can be degraded in dilute solution at temperatures compatible with life and with a large portion of the free energy yield conserved as ATP. Coupled to an appropriate reductive sequence to regenerate the coenzyme NAD+, glycolysis serves the cell well under anaerobic conditions, meeting energy needs despite the absence of oxygen. 

The writer is Associate Professor,Head, Department of Botany, Ananda Mohan College,Kolkata, and also fellow,Bbotanical Society of Bengal, and can be contacted at tapanmaitra59@yahoo.co.in