Cells are the smallest units of living things that boast all of the properties associated with life. One of these defining characteristics is metabolism, or the use of molecules or energy gathered from the environment to carry out the biochemical reactions required to remain alive and, ultimately, reproduce.

Metabolic processes, often termed metabolic pathways, can be divided into those that are anabolic, or that involve the synthesis of new molecules, and those that are catabolic, which involve the breakdown of existing molecules.

Colloquially, anabolic processes are about building a house and replacing things like windows and gutters as needed, and catabolic processes are about taking worn-out or broken pieces of the house to curb. If these are done in concert at the right pace, the house will exist in as steady a state as possible, but never passively.

Cells and the tissues they form are continually undergoing "bidirectional" metabolism, meaning that while some things are flowing in an anabolic direction, others are going in the opposite direction.

This is perhaps more evident at the level of whole organisms: If you are burning through glucose while sprinting to catch up with your dog (catabolic process), the paper cut on your hand from the day before continues to heal (anabolic process). But the same dichotomy is at work in individual cells.

Cellular reactions are catalyzed by special globular protein molecules called enzymes, which by definition participate in chemical reactions without being changed themselves in the end. They greatly speed up reactions – sometimes by a factor of well over a thousand – and thus function as catalysts.

Anabolic reactions usually require an input of energy and are therefore endothermic (loosely translated, "heat to the inside"). This makes sense; you can't grow or build muscle unless you eat, with your food intake usually scaling to the intensity and duration of a given activity.

Catabolic reactions are usually exothermic ("heat to the outside") and liberate energy, much of which is harnessed by the cell in the form of adenosine triphosphate (ATP) and used for other metabolic processes.

The main structural elements of the body and the molecules it requires for fuel plus tissue growth and replacement are composed of monomers, or small repeating units within a greater whole, called a polymer.

These units may be identical, as with the glucose molecules arranged into long chains of the storage fuel glycogen, or they may be similar and come in "flavors," as with nucleic acids and the nucleotides that make them up.

The three major macronutrient classes of macromolecules in human nutrition, called carbohydrates, proteins and fats, each consist of their own type of monomer.

Glucose is the fundamental substrate of all of life on Earth, with every living cell capable of metabolizing it for energy. As noted, glucose molecules can be linked into "chains" to form glycogen, which in humans is found primarily in muscle and the liver. Proteins consist of monomers drawn from a grab-bag of 20 different amino acids.

Fats are not polymers because they consist of three fatty acids linked to a "backbone" of the three-carbon molecule glycerol. When they grow or shrink, this occurs via the addition or removal of atoms to the ends of the fatty acid chains, rather like a capital "E" with the vertical part remaining the same size but the horizontal bars varying in length.

Consider being given a box of toy building blocks of unlimited size. Many are identical except in their color; others are different sizes, but can be joined together; still others are not meant to connect no matter the configuration you select. You can create identical constructs that include say, three to five pieces, and link these together in such a way that the junctions of these constructs are also identical.

This is essentially anabolic metabolism in action. The individual groups of three to five toy pieces represent "monomers" and the finished product is analogous to "polymer." And in cells, instead of your hands doing the work of putting the pieces together, enzymes guide the process. In both cases, the key aspect is an input of energy to generate molecules of greater complexity (and usually greater size as well).

Examples of anabolic processes include, in addition to protein synthesis, gluconeogenesis (the synthesis of glucose from various upstream substrates), the synthesis of fatty acids, lipogenesis (the synthesis of fats from fatty acids and glycerol) and the formation of urea and ketone bodies.

Most of the time, catabolic processes, at the level of individual reactions, are not simply the corresponding anabolic reactions run in reverse, although many of them are the same. Usually, different enzymes are involved.

For example, the first step in glycolysis (the catabolism of glucose) is the addition of a phosphate group to glucose, courtesy of the enzyme hexokinase, to form glucose-6-phosphate. But the final step of gluconeogenesis, the removal of the phosphate from glucose-6-phosphate to form glucose, is catalyzed by glucose-6-phosphatase.

Other vital catabolic processes going on in your body are glycogenolysis (the breakdown of glycogen in muscle or liver), lipolysis (the removal of fatty acids from glycerol), beta-oxidation (the "burning" of fatty acids), and the degradation of ketones, proteins or individual amino acids.

Keeping the body in tune with its needs in real time requires a high degree of responsiveness and coordination. The rates of anabolic and catabolic reactions can be controlled by varying the amount of enzyme or substrate mobilized to a given part of the cell, or by feedback inhibition, in which the accumulation of a product signals the reaction upstream to proceed more slowly.

Also, and importantly from the standpoint of visualizing metabolism holistically, substrates from one macronutrient pathway can be shunted into that of another as needed.

An example of this integration of pathways is that the amino acids alanine and glutamine, in addition to serving as the building blocks of proteins, can also enter gluconeogenesis. For this to happen, they need to shed their nitrogen, which is handled by enzymes called transaminases.

• Glycerol, a product of lipolysis, can also enter the gluconeogenesis pathway, which is one way to, in a loose sense, get sugar from fat. To date, however, there is no evidence that products of fatty acid oxidation can enter gluconeogenesis. 

Physical fitness is a major public concern in countries where people often have the luxury of optional exercise.

Many of the common modalities are aimed strongly in the direction of one process or another, such as lifting weights to build muscle mass (anabolic exercises) or using an elliptical trainer or treadmill for "cardio" and shedding lean or fatty body mass (or body weight) for weight loss (catabolic exercises).

One example of both systems in action is a marathon runner preparing for and running a 42.2-km (26.2-mile) race. The week before, many people intentionally load up on carbohydrate-rich foods while resting for the effort.

Because of their daily running training and the constant need to replace catabolized fuel, these athletes have high levels of activity of the enzyme glycogen synthase, which allows their muscles and liver to synthesize glycogen with unusual avidity.

During the marathon, this glycogen is converted to glucose to power the runner along for hours on end, though these athletes typically take in sources of glucose (e.g., sports drinks) throughout the event as well to prevent "hitting the wall."

• The inability of the body to generate glucose from fatty acids is the reason carbohydrates are considered critical for high-intensity, sustained exercise, as the beta-oxidation of fatty acids does not result in enough ATP to keep pace with metabolic needs.