Reality → Life → Cell → Metabolism
Cell metabolism consists of catabolism, the breakdown of macromolecules with release of energy, and anabolism, the construction of new macromolecules with use of the released energy [1] . The highly complex chemical reactions involve many steps of intermediate products [2] . Among the profusion of atomic/molecular interactions, biochemists have identified various pathways [3] . At each step, highly specific enzymes, aided by inhibitors and activators, regulate the rate and amount of metabolite production in line with the needs of the cell [4] .
Catabolism ('destructive metabolism') breaks down vital macromolecules of food (such as starch, fat, and protein) into smaller components (such as glucose, fatty acids, and amino acids) from which anabolism ('constructive metabolism') builds complex biomolecules (such as glycogen, lipids and new proteins).
Some intermediate compounds (metabolites) are inputs for the construction of new macromolecules, others serve as energy storage, but most are further broken down to end products (waste), thereby releasing the energy an organism needs for internal physiological functions and interactions with the environment.
One fundamental catabolic pathway is cellular respiration to recover energy. Although the overall reaction appears to be simple (glucose is oxidized to the waste products CO2 and H2O with a release of energy), the cellular processes involved are highly complex and may be divided into three parts: (a) glycolysis, (b) the citric acid cycle, and (c) oxidative phosphorylation, with each part consisting of several chemical reactions catalyzed by enzymes. Glycolysis happens in the cell's cytoplasm and results in a split of the 6-carbon glucose molecule into two 3-carbon pyruvate molecules, with the creation of two NADH and two ATP molecules. The citric acid cycle is an anaerobic (fermenting) process in the matrix of mitochondria, decomposing pyruvate into other useful metabolites, again with the gain of some ATP. Oxidative phosphorylation, at the inner mitochondrial membrane, is a highly effective (15 times more effective than the preceding stages) process for ATP production, boosted by proton gradients created through 'pumping' by ATP synthase.
A cell's need for metabolites and energy depends on its type, age, and the constantly changing situations of neighboring cells and the organism's environment. Inhibitors and activators reduce, stop, start, or increase enzymatic activity. Inhibitors may compete with the substrate to bind with the enzyme, and activators may change the enzyme's configuration to better fit with the substrate in a 'lock and key' fashion (see also Proteins, Note 4).