Anabolism

We explain what anabolism is and its differences from catabolism. Also, importance, types of anabolism and examples.

What is anabolism?

Anabolism is the phase of metabolism in which complex molecules are generated from simpler substances within living beings. During this mechanism, cells invest chemical energy to compose complex biomolecules from simple ones. It is the opposite and complementary process of catabolism.

The term comes from the Greek Ann (“upwards”) and ballein (“throw”), since it involves the synthesis of more complex compounds from simpler ones, going “up” from the basic to the complex. To be able to do so, however, an addition of energy is required.

Like catabolism, it occurs inside cells and is driven by the action of enzymes (catalyst proteins).

Examples of anabolism can be found in both autotrophic organisms (those that make their own food) and heterotrophs (those that consume other organisms). Photosynthesis is a classic example of anabolic activity, because it involves the creation of sugars from carbon dioxide and water, using light energy.

Anabolism can have peaks of activity throughout the life cycle of an organism. So, The stages of growth and development until creating an adult individual are characterized by a marked anabolic activity.

• See also: Metabolism

How does anabolism work?

Living beings are maintained thanks to the existence of chemical reactions that occur within cells. These reactions involve creating or breaking bonds between atoms, through proteins that function as biological catalysts, called enzymes. Enzymes make chemical reactions happen within living systems.

Anabolic reactions involve the action of several enzymes that make complex molecules from simpler ones, to create new chemical bonds. To carry out this type of chemical reactions (called synthesis or construction reactions), enzymes need a supply of energy.

The basis of anabolism is synthesis reactions. In contrast, chemical bond breaking reactions are catabolic activities. In this case, instead of consuming energy, they release it.

For example, proteins are large, complex molecules that often serve structural functions within cells. They are made from smaller molecules, amino acids. Cells have a biochemical machinery, called the ribosome, which is responsible for building proteins. Inside cells, ribosomes create new bonds between amino acids, using chemical energy.

• See also: Biomolecules

Differences between anabolism and catabolism

Catabolism and anabolism are complementary, although opposite, processes. Catabolism breaks down macromolecules into simpler forms, releasing the chemical energy contained in their chemical bonds. Instead, anabolism consumes that released energy to form new bonds and new complex molecules.

Thus, one consumes energy and the other releases it; one goes from the basic to the complex and the other the other way around. When catabolism and anabolism are in balance, the cell remains stable. Often, organisms encounter particular conditions (e.g., growing in size or reproducing) that require greater anabolic activity.

• Catabolism

Autotrophic anabolism

In autotrophic organisms (those capable of making their own food) anabolism involves the transformation of inorganic molecules (such as water and carbon dioxide) into organic molecules of greater complexity and biochemical utility. This process can occur in different ways:

• Photosynthesis. It is the metabolic process of plants and organisms endowed with chlorophyll. In this process, the necessary energy is obtained from sunlight, and carbon dioxide (CO₂) and water (H₂O) are consumed to make starch (sugar) molecules.
• Chemosynthesis. This process occurs in microorganisms that live in an environment where there is no usable sunlight, but there are other types of chemical substances. For example, some bacteria and archaea can synthesize organic molecules from ammonia (NH₃).

Heterotrophic anabolism

In heterotrophic beings (which consume organic matter from other living beings to feed themselves), anabolic processes consist of the manufacture of complex molecules from simple nutrients that come from the diet.

Simple molecules (amino acids, simple sugars, trace elements) are obtained after the digestion and absorption of food. Once absorbed, heterotrophic organisms use these molecules as “bricks” to make more complex components.

This process involves a set of reactions that require an expenditure of energy. Within cells, chemical energy is available in the ATP (adenosine triphosphate) molecule.

Examples of anabolism

Examples of anabolism are metabolic pathways (chemical reactions that occur in a chain, one after the other) that result in a more complex product than the initial one, using energy. They often depend on the type of organism being treated.

For example, organisms in the kingdom Plantae are photosynthetic. That is, they can make complex sugars from water, carbon dioxide, using light energy. So, Photosynthesis is an example of anabolism in plants.

In humans, we can also find examples of anabolic reactions. For example, when we consume more food than we need, our body converts the excess nutrients into fat. That is, all the nutrients we ingest are transformed into triglycerides, chemical energy storage molecules. This process, called lipogenesis, involves metabolic pathways that expend energy. So, lipogenesis is an example of anabolism in humans.

It is common for anabolic reactions to have a peak of activity under certain conditions. For example, before an individual reaches adulthood, they go through stages of growth and development, which involve the generation of more body mass. The offspring increase in weight and height, the plants build new stems or trunks, the insects change from one stage to another, etc.

In these cases, anabolism is very active, since the manufacture of new structures requires anabolic chemical reactions to build new molecules.

Importance of anabolism

Anabolism is a vital metabolic stage, not only to provide inputs to catabolism with the aim of breaking down and releasing chemical energy, but also to:

• The storage of energy in the chemical bonds of complex molecules (such as starch in plants, or glycogen and triglycerides in animals).
• The production of components of cells and tissues, which allows the increase in muscle mass and the growth of the organism.
• The manufacture of new cells to replace old ones and replenish damaged tissues.

References

• Marieb, E. Human anatomy and physiology. 9th edition. PEARSON EDUCATION. (2008).
• De Robertis, E.Fundamentals of cellular and molecular biology. 4th Edition. El ateneo (2010).