Composition of organisms

Living organisms are divided into cellular and non-cellular. Non-cellular organisms include host cell-dependent viruses.

Cell organisms can be further divided into unicellular and multicellular. As the term suggests, unicellular ones have a body made up of only one cell that performs all functions.

According to the organization of the nucleus and other cellular structures, we recognize prokaryotic cells (formed by one DNA molecule that is not separated from the cytoplasm by membrane; it does not contain organelles consisting of membranes, does not have a cytoskeleton; only ribosomes are present) and eukaryotic (contains a nucleus separated from the cytoplasm by a nuclear membrane, contains membrane organelles, a cytoskeleton present).

Prokaryotic cells form only unicellular organisms (e.g. cyanobacteria and bacteria). Eukaryotic cells form both unicellular (protozoa) and multicellular organisms (fungi, plants, animals).


The cell is the basic building and functional unit of all living organisms, which ensures nutrient uptake, its transformation into energy, performs specific functions. Many chemical reactions are required to organize all these processes. To carry out a chemical reaction in a cell, the energy that the cell is able to obtain by converting substances is needed. We are talking about substance transformation or so called metabolism.

The processes in the cell should be understood in its complexity. We cannot separate them because they are closely related; e.g. the uptake of substances into the cell takes place along with the release of substances.

Cell processes

Kinds of reactions to take place in the cells of the organism:

anabolic reactions – synthesis of products from several substrates. They are endergonic – they require energy supply because as they consume energy; use of substrates for the synthesis of substances necessary for the construction or function of the body (proteins, enzymes)..

catabolic (decomposition) reactions – degradation, cleavage of the substrate molecule into several product molecules. They are exergonic – energy is released, part of it is captured e.g. in ATP:

chemical energy – used for organism function (max 27%);
thermal energy – maintaining a constant body temperature.

In a living organism, the metabolism of substances is not separated from energy transformations. We distinguish between autotrophic and heterotrophic organisms according to the way of obtaining energy. In addition, we also recognize mixotrophic organisms (they are able to use autotrophic and heterotrophic nutrition – for example, carnivorous plants).

Autotrophic organisms are able to utilize solar energy and synthesize organic substances from simple inorganic substances in the photosynthesis process. Mainly plants are included among autotrophic organisms. Their cell structures contain chlorophyll (green dye) and the chloroplasts (organelles containing chlorophyll), which are required for process of photosynthesis.