CELLULAR LEVEL OF ORGANIZATION: CELL TYPES AND COMPONENTS - BIOLOGY - 2025

To speak of the cellular level of organization is to speak of the structural and internal organization of the basic unit of living organisms: the cell. A cell is capable of carrying out all the characteristic processes of a living being, of which none of its isolated parts are capable.

After atoms, molecules and organic compounds, cells represent one of the basic levels of organization in multicellular organisms and the fundamental level of organization in macro and unicellular microorganisms.

Structure of an animal cell (Source: Mel23, via Wikimedia Commons)

Multicellular organisms such as animals and plants are organized in such a way that their cells are grouped together to form tissues; these tissues, when associated, give rise to organs of different types and these organs, in turn, make up what we know as systems or apparatus., which make up the entire body.

In 1839, the zoologist Theodor Schwann and the botanist Matthias Schleiden described animal and plant cells in parallel. These scientists were the first to propose the cell theory: that all living things are made up of cells.

According to evolutionary theories, all living organisms come from a common ancestor that possessed an outline of the universal machinery of all terrestrial life and different consecutive events in evolutionary history were what led to the diversification of species as we know them.

Cell types and their components

Cells are small "containers" enclosed by a membrane inside which is an aqueous solution known as cytosol or cytoplasm. They are extremely diverse, not only in size but also in ways of life, reproduction, nutrition, cover, functions, etc.

Although their basic properties are very similar, there are two cell types in nature: prokaryotes and eukaryotes. Examples of prokaryotic organisms are bacteria and archaea, while eukaryotic cells constitute the fundamental unit of animals, plants and fungi.

Prokaryotic cells

Although of variable sizes, prokaryotic cells are generally smaller than eukaryotes and prokaryotes are usually single-cell organisms, that is, they are unicellular.

Prokaryotic cells have a plasma membrane composed of a double layer of lipids and proteins, which acts as a semi-permeable barrier to different molecules and which is the only membranous system they possess, since they do not have internal organelles.

Average prokaryotic cell (Source: Mariana Ruiz Villarreal, LadyofHats via Wikimedia Commons)

Some have a gas vacuole that allows them to float in aqueous media. They have ribosomes that function in protein synthesis and inclusion bodies for the storage of carbon and other substances.

In a region known as the "nucleoid" is the genetic material in the form of deoxyribonucleic acid (DNA).

All prokaryotes have, in addition to the membrane that encloses the cytoplasm, a cell wall that gives them their shape and gives them resistance against osmotic lysis. The cell wall is usually made up of a molecule called peptidoglycan that allows one group of bacteria to be distinguished from another.

Around this wall can be a "capsule" or chalice, which helps adhere to surfaces. They may have some "appendages" such as hairs, fimbriae, and flagella, for fixation, conjugation, and movement, respectively.

Eukaryotic cells

With little difference between them, animals and plants are made up of eukaryotic cells. The distinctive feature of these cells is the presence of a nucleus that encloses the genetic material and other membranous organelles embedded in the cytoplasm.

Structure of a plant cell (Source: Mortadelo2005, via Wikimedia Commons

These cells, larger and more complex than prokaryotes, can exist as unicellular or multicellular organisms (of even more complex organization).

Plant cells invariably have a cell wall surrounding the plasma membrane, unlike animal cells.

Any eukaryotic cell is made up of common specialized structures:

-Core

-Mitochondria

-Chloroplasts (transformation of light energy into chemical energy in plant cells)

-Internal membranous system: smooth and rough endoplasmic reticulum and Golgi complex

-Cytoplasm

-Cytoskeleton

-Lysosomes

-Endosomes (in animal and fungal cells)

-Peroxisomes

-Glioxisomes (in plant cells)

-Vacuoles (store water and minerals in plant cells)

Organelles common among plants and animals

Core

It is the place where a cell's genetic (hereditary) information is stored in the form of DNA coiled on chromosomes. It is an organelle surrounded by a membrane known as the nuclear envelope.

Through structures known as "nuclear pores" that are present in the nuclear envelope, the nucleus exchanges different classes of molecules with the cytoplasm.

Inside are numerous proteins responsible for the "reading" and "transcription" of the information encoded in the genes contained in DNA.

Mitochondria

They are one of the most prominent organelles after the nucleus. They resemble a prokaryote cell, since they have a double membrane system, their own genome and a morphology similar to that of a bacterium, from which the endosymbiont theory arises.

They are organelles specialized in the generation of chemical energy in the form of ATP through oxidative phosphorylation. This process is also known as cellular respiration, since the mitochondria consume oxygen and release carbon dioxide.

Endoplasmic reticulum (smooth and rough)

It is a continuation of the outer nuclear membrane and consists of a system of membranous “sacs” and tubes that are distributed throughout much of the cytoplasm. It is the main site for the synthesis of new membranes.

The rough endoplasmic reticulum has attached ribosomes that participate in protein translation and synthesis.

Golgi complex or apparatus

It is a membranous organelle composed of piles and flattened sacks. It is located near the nucleus and is responsible for the modification, packaging and transport of proteins and lipids from the endoplasmic reticulum.

It is part of the secretory transport and communication pathway, thanks to its ability to dispatch small vesicles with various macromolecules into different compartments.

Cytosol or cytoplasm

It is the aqueous gel in which the cellular organelles surrounded by the plasma membrane are immersed. It is rich in different classes of large and small molecules and countless chemical reactions occur in it that make the continuation of cellular life possible.

Cytoskeleton

The cytoskeleton is the internal structural framework composed of filamentous proteins of different thicknesses that are responsible for the internal organization of the cell, as well as its external characteristics, especially in terms of flexibility and deformability. It is especially important in the processes of cell division.

Lysosomes and peroxisomes

They are organelles surrounded by a single membrane that are dispersed throughout the cytosol. The former are rich in digestive enzymes and are responsible for the degradation and "recycling" of different substances of internal or external origin.

Peroxisomes are responsible for the "detoxification" of cells through a series of oxidative reactions catalyzed by oxidases and catalases within them. They are responsible for the breakdown of lipids and other toxic substances.

References

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