Each metabolic pathway is a series of reactions organised so the products of one reaction become the substrate of the next. As enzyme reactions are theoretically reversible, a metabolic pathway is extremely important in that biochemical reactions can proceed as equilibrium is never reached and the reaction cannot therefore begin to reverse (as stated would happen in Le Chatier’s Principle). This is vital, as if a reaction could reverse there would be very little desired product made. Another advantage of the products of one reaction becoming the substrate of the next is that the product can be ideally located to become the substrate which permits the build up of high local concentrations of substrate molecules and biochemical reactions can proceed rapidly. A pathway arranged in this way may be catalysed by a multienzyme complex.
An additional benefit of metabolic pathways is that reactants may be modified in a series of small steps, this ensures energy is released in controlled amounts and minor adjustments can be made to the structure of molecules. Each transformation within the pathway is catalysed by a specific enzyme, for example insulin converts glucose to glycogen, and each enzyme is a point of control of the overall pathway.
There are multiple metabolic pathways occurring in the body all the time. One of the most important is that of respiration. Respiration is the production of free energy in all living cells, more specifically cellular respiration is a complex metabolic pathway divided into three stages: glycolisis, the Krebs cycle and oxidative phosphorylation. This is a vital pathway as all movements and reactions that take place in living organisms require energy (ATP which is produced here). When we eat all our food must be digested and broken down; there are a number of pathways that enable this to happen. Carbohydrates, proteins and fats can all be broken down to produce glucose. Protein and fat breakdown are engaged in their own metabolic pathways, helping produce glucose by being broken down itself. Glucose uptake is stimulated by the hormone insulin from the beta cells in the pancreas, it is then taken up and quickly phosphorylated by the enzyme hexokinase, using ATP. From there, it can go several directions ending up as storage or as a source of energy.
But what happens when there is no extra food entering the body? During the starvation state, glycogen stores are depleted so there is no carbohydrate source to synthesize glucose for the brain. The body ordinarily has adequate fat stores that can supply energy for a very long time. However, fat cannot be used for gluconeogenesis (synthesising glucose). The only alternative is to break down protein and use the keto acids for gluconeogenesis. The stress of the very low glucose levels stimulates the adrenal cortex to release the hormone cortisol, which is responsible for the protein breakdown. In the end, when the fat storage is depleted, the body has no choice but to break down protein at a much higher rate to provide energy. Carbon dioxide is usually thought of as an end product of carbohydrate, protein, and fat deprivation in aerobic organisms.
Metabolic pathways are vital to living beings. They enable many efficient reactions to occur and allow for the quick breakdown or synthesising of many important molecules the body needs. Without complex path ways the metabolism would be in chaos as equilibrium would be frequently reached and very little product would be produced.
Exercise, food, and environmental temperature influence metabolism. Basal metabolism is the caloric expenditure of an organism at rest; it represents the minimum amount of energy required to maintain life at normal body temperature.
