Animal Cell

We explain what an animal cell is and the theories of its origin. Also, its parts and the differences with the plant cell.

What is an animal cell?

An animal cell is a type of eukaryotic cell, that is, one that has a defined nucleus. Animal cells are those that form the different tissues of living organisms that belong to the kingdom Animalia (animals).

Since animals are complex multicellular beings, Its cells have a very high level of specialization: depending on the tissue to which they belong, they fulfill specific functions that define their morphology, their function and their needs.

See also: Multicellular organisms

Parts of an animal cell

The parts of an animal cell are:

• Core. Like all eukaryotes, animals have cells with a defined nucleus that contains almost all of their genetic material (DNA) organized into chromosomes. The nucleus is composed of an aqueous suspension called “nucleoplasm,” in which DNA and other nuclear structures are immersed, and which is wrapped in a nuclear membrane that allows the exchange of molecules with the cytoplasm.
• Plasma or cytoplasmic membrane. It is the selective lipoprotein membrane (membrane formed by macromolecular compounds made up of proteins and lipids) that covers the cell, distinguishes its interior from its exterior and allows the entry of desired substances and the exit of metabolic waste.
• Cytoplasm. It extends from the nucleus to the cytoplasmic membrane. It is composed of the cytosol and organelles (except the nucleus). It extends from the nucleus to the cytoplasmic membrane. The cytosol is the liquid part of the cytoplasm, and is composed of water, ions and biomolecules. Organelles are structures with specific shapes and functions that are immersed in the cytoplasm.

Some organelles are:

• Mitochondria. They are the organelles where most of the cell's energy (in the form of ATP) is generated. Cells can have up to thousands of mitochondria. The greater the activity of the cell, the greater the number of mitochondria it will have. Cellular respiration occurs in the mitochondria, which consists of the oxidation of organic compounds, mainly glucose, to obtain energy in the form of ATP.
• Lysosomes. They are vesicles that contain enzymes that degrade the material that enters the cell (a process called “heterophagy”) or the material produced within it (a process called “autophagy”). Lysosomes are in charge of cellular digestion and are produced by the Golgi apparatus.
• Golgi apparatus. It is made up of a series of membranes in the form of flattened sacs or cisterns. It serves to transport, modify and classify proteins that are synthesized on ribosomes in the rough endoplasmic reticulum (RER). These synthesized proteins are surrounded by a layer of RER membrane, forming vesicles.
• endoplasmic reticulum. It is a series of interconnected flattened tubules and sacs, composed of fatty acids. It is divided into two domains with different functions:
  • rough endoplasmic reticulum (RER). It is a structure that extends from the nuclear envelope. It is covered with ribosomes, on which protein synthesis occurs.
  • Smooth endoplasmic reticulum (SER). It is a structure in which fatty acids and steroids are synthesized. In addition, it plays a fundamental role in cellular detoxification, as well as in the absorption and release of calcium from the cell.
• Centriole. It is a cylindrical organelle made up of three triplets of microtubules, which are part of the cytoskeleton. These structures have an important role in the transport of organelles in the cell, in the mechanical stability of the cell and in the processes of mitosis or cell division.
• Peroxisomes. They are organelles that contain oxidases, which are enzymes that oxidize certain substances, such as toxic substances.
• Centrosome. It is made up of a pair of centrioles and pericentriolar material. The pericentriolar material contains tubulin protein complexes, which are essential in the formation of the mitotic spindle (set of microtubules that extend from the centrioles when the cell is in cell division) in cell division.
• Cilia. They are structures formed by microtubules, similar to hairs, that are located on the surface of the cell. Cilia make oar-like movements to move the fluid around the cell.
• Flagella. They are similar to cilia, but much longer. Their objective is to move the entire cell, acting as small propellants.

Differences between animal cell and plant cell

The differences between the animal cell and the plant cell can be summarized as:

• cell wall. Plant and fungal cells have a rigid cell wall, which limits their growth but makes them more compact. This wall is located outside the plasma membrane and is made up of cellulose (in the case of plants) or chitin (in the case of fungi). It is not present in animal cells.
• Size. Animal cells are smaller than plant cells, perhaps because they do not have a central vacuole filled with liquid, but rather small and numerous vesicles in their cytoplasm.
• Chloroplasts. These organelles are unique to plants, cyanobacteria and some algae. Chloroplasts contain chlorophyll, which is a fundamental biomolecule in the photosynthesis process. Animal cells lack chloroplasts, as they do not carry out photosynthesis.
• Centrioles. Animal cells have centrioles and plant cells do not. Centrioles play a very important role in the formation of the mitotic spindle, in the processes of cell division and in the formation of cilia and flagella. Plant cells do not usually have cilia or flagella, with the exception of those of some algae.

Rise of the kingdom Animals (animals)

in the kingdom Plantaewhich includes plants, several ways are known by which unicellular organisms were organized to form multicellular organisms. However, in the kingdom Animals (animals) or Metazoa (metazoans), it is not very well known how the transition from protozoa (protists) to metazoans was, that is, between unicellular life and specialized multicellular organization, since there is no evidence of it in the fossil record.

There are three theories about the origin of animals:

• colonial theory. This theory proposes that complex animal life would have emerged from colonies of choanoflagellates (flagellated protozoa), which are single-celled eukaryotic organisms. The colonies would have been hollow and spherical. There are molecular studies of ribosomal RNA and morphological studies of mitochondria and flagella that support this theory. It has been seen that the mitochondria and flagella of choanoflagellates are very similar to those of metazoans. In these colonies, organisms of the same species interact symbiotically to carry out their vital development.
• Symbiotic theory. This theory assumes that multicellular organisms originated from the symbiosis of protist organisms of different species. Symbiosis consists of the relationship, link or interaction between different organisms so that at least one of them benefits in its development.
• Cellularization theory. This theory states that multicellular organisms emerged from unicellular organisms with several nuclei, through the cellularization of these nuclei, that is, the transition from nuclei to cells.

Continue with: Histology

References

• Tortora-Derrickson, (2006). Principles of Anatomy and Physiology, 11th Edition, Editorial Médica Panamericana.
• “The cell. 4. The nucleus.THE NUCLÉOLUS” In: mmegias.webs.uvigo.es Available at: https://mmegias.webs.uvigo.es Consulted: December 31, 2022.
• “Animal cell” At: www.mineduc.gob.gt Available at: https://www.mineduc.gob.gt Consulted: December 31, 2022.
• “Animal cell” In: todomateria.com Available at: https://www.todamateria.com Consulted: December 31, 2022.