Cell

We explain what the cell is and what types of cells exist. Also, what are the parts and functions of cells.

What is a cell?

The cell is the Structural and functional unit of all living organisms. It constitutes the smallest and simplest way of biological organization, that is, the most small and the smallest orderly and living structure that is known. Examples of cells are sperm and neurons.

The cell is the smallest entity considered alive.

The size of the cells can vary greatly. An average size cell measures around 10 µm (micrometers). La gran mayoría de las células son microscópicas, es decir, solo pueden ser vistas utilizando un microscopio. On the other hand, there are cells that can be observed with the naked eye, this is the case of the human ovule, which measures 100 µm and has a size similar to the tip of a pencil.

The discovery of the cell is considered the founding step of the modern study of life (biology), since it allowed us to understand the enormous complexity of the body of living beings and allowed the emergence of numerous sciences and later disciplines.

See also: Eukaryotic cell

Cell types

The cells can be classified whether or not they have a membrane that surrounds the nucleus called “nuclear membrane”. According to this classification, cells can be prokaryotes or eukaryotes.

prokaryotic cells

prokaryotic cells have a simple basic structure, without a nuclear membrane so its genetic material is dispersed, occupying a space called nucleoid, and that is in direct contact with the rest of the cytoplasm.

prokaryotic cells are small and are between 1-5 µm. They were the first ways of life on earth and where it is known, all living beings formed by prokaryotic cells are unicellular.

See also: Prokaryotic cell

Eukaryotic cells

have the nucleus surrounded by a nuclear membrane so its genetic material is contained in the nucleus. Además, estas células poseen orgánulos (también llamados “organelas”) en su citoplasma que pueden estar delimitados por membranas.

The size of eukaryotic cells varies between 10-100 µm so they are larger than prokaryotic cells. In the evolutionary history of the Earth, eukaryotic cells emerged after prokaryotic cells.

animal cell

• They are constituted by a plasma membrane and different organelles (Golgi complex, mitochondria, ribosomes, lysosomes, smooth and rough endoplasmic reticulum, among others).
• The genetic material is found in a nucleus wrapped in a nuclear membrane.
• They have no cell wall and have very diverse forms.
• They have lysosomas (exclusive organelles of animal cells), which are vesicles that contain digestive enzymes and are dedicated to degrading certain chemical compounds and cellular structures.

plant cell

Plant cells are eukaryotic.

• They have a nucleus surrounded by a nuclear membrane where the genetic material is found.
• They have a rigid cell wall composed mainly of cellulose. This structure gives shape to the cell and provides support to the plant (plant organisms do not have skeletons like animals).
• They have chloroplasts (exclusive organelles of plant cells), which are organelles that carry out photosynthesis, process by which sunlight is used to synthesize organic compounds from inorganic compounds.

parts of the cell

• Plasma or cytoplasmic membrane (present in eukaryotic and prokaryotic cells). It is a membrane that divides the outer part of the inside of the cell. It is formed by a continuous double layer of phospholipids and proteins intercalated or adhered to its surface. The functions of this membrane are to shape and stability to the cell, separate the internal content of the cell from the surrounding environment, allow the entry and exit of substances to the cell and intervene in the interaction between cells.
• cell wall (present in prokaryotic cells and in eukaryotic cells of plants, fungi and algae). It is a thick and quite rigid layer that is located on the outside of the plasma membrane. Le confiere resistencia y estabilidad a la célula. Its composition varies according to the cell type, for example, in plants the cell wall is mainly composed of cellulose, while in bacteria it is formed by peptidoglycan (copolymer formed formed by sugars and amino acids).
• Core (present in eukaryotic cells). Contiene casi todo el material genético (ADN) de la célula y está rodeado por una envoltura nuclear que contiene poros. Its main functions are to store genetic information, control the activities of all organelles and coordinate cell reproduction.
• Nucleoid (present in prokaryotic cells). It is a region in which DNA is found, which in prokaryotic cells is a single molecule with a circular and closed shape. Unlike the nucleus in eukaryotic cells, the nucleoid does not have a nuclear envelope.
• Cytoplasm (present in eukaryotic and prokaryotic cells). It is the part of the cell that is located between the cytoplasmic membrane and the nucleus. It is made up of a liquid part called “cytosol”, which is made up of water, ions and proteins. All organelles are immersed in the cytosol. The main function of the cytoplasm is to support the cell organelles and help in the metabolic processes that occur within it.

En el citoplasma se encuentran inmersos los orgánulos, que son estructuras que cumplen una o varias funciones determinadas. Se denominan “orgánulos” por analogía con el término “órganos”. Organelles are the small organs inside the cell.

Some organelles are:

• Mitochondria. Están presentes en células eucariotas de animales y plantas. They are the structures where cellular respiration takes place, a process that allows the cell to obtain energy in the form of ATP. Mitochondria are generally located in places in cells where oxygen enters. A cell can have up to thousands of mitochondria depending on the activity it carries out.
• Lysosomes. They are present in eukaryotic cells of animals. They are membrane-surrounded vesicles that originate in the Golgi apparatus. They have digestive and hydrolytic enzymes (enzymes that accelerate the hydrolysis of chemical bonds) inside that can digest a large number of molecules. On the other hand, they can digest another organelle of the interior of the cell and return its components to the cytosol so that they are reused by the cell (process called “autophagy”, and digest an entire cell (process called “autolysis”). If the components that lysosomes digest come from outside the cell, the process is called “heterophagy.”
• Ribosomes. They are present in eukaryotic and prokaryotic cells. They are responsible for protein synthesis. In eukaryotic cells these organelles are made up of two subunits that are formed separately in the nucleolus (a body located within the nucleus) and join together in the cytoplasm to synthesize proteins. On the other hand, in eukaryotic cells, these organelles are found in the nuclear membrane, in the rough endoplasmic reticulum, in the cytosol, in the mitochondria and in the chloroplasts (in the case of plants). In prokaryotic cells, ribosomes are found in the cytosol.
• Golgi apparatus. It is present in most eukaryotic cells. It is responsible for transporting and modifying proteins that are synthesized in ribosomes attached to rough endoplasmic reticulum.
• endoplasmic reticulum. It is present in eukaryotic cells. It is a set of membranes that extends from the nuclear membrane to the cytoplasm. There are two types of endoplasmic reticulum:
  • rough endoplasmic reticulum (RER). It is a structure located after the nuclear membrane. The surface of the RER is covered with ribosomes (organelles responsible for protein synthesis).
  • Smooth endoplasmic reticulum (SER). It is a structure that extends from the RER. La superficie del REL no contiene ribosomas, por lo que no se sintetizan proteínas en su estructura, pero sí se sintetizan ácidos grasos y esteroides.
• Centrosome. It is present in eukaryotic cells of animals. This organelle is made up of centrioles and pericentriolar material (set of proteins that surrounds the centrioles). Centrioles are structures formed by microtubules surrounded by pericentriolar material, which is formed by complexes of the protein tubulin. Tubulin complexes are the organizing centers for the growth of the mitotic spindle (the set of microtubules that participate in cell division).
• Cytoskeleton. It is present in eukaryotic cells and has an analogous structure in prokaryotes. It consists of a set of protein filaments that extend through the cytosol. It serves to establish the shape of the cell and organize the contents inside. In addition, it helps the movement of organelles within the cell, of chromosomes in cell division and of entire cells.
• Cilia. They are present in prokaryotic cells and in eukaryotic cells of animals and some algae. They are extensions of the plasma membrane, similar to hairs. Cilia make an oar-like movement to move the fluid around the cell.
• Flagella. They are present in prokaryotic cells and in eukaryotes of animals and some algae. They have a similar structure to cilia, but are longer. Flagella move entire cells, as if they were small propellers that give them movement. The only cell with a flagellum in the human body is the sperm.
• Chloroplasts. They are present in eukaryotic cells of plants and green algae. They are made up of two membranes that contain vesicles, chlorophyll and thylakoids inside. The reaction that absorbs photons from sunlight to carry out photosynthesis occurs in the thylakoids. Although chloroplasts are exclusive to plant and algae cells, there is a mollusk called the emerald slug (Elysia chlorotica) that feeds on the chloroplasts of the algae Vaucheria litore. Thus, using the chloroplasts of the alga, this mollusk is capable of photosynthesis.
• Vacuoles. They are present in all eukaryotic plant cells and in some animal cells. Additionally, they may be present in some prokaryotic cells. These organelles are vesicles formed by the plasma membrane. Its function is to store water, molecules and nutrients.
• Chromoplasts. They are present in eukaryotic plant cells. These organelles store carotenes, which are the pigments that give colors to roots, flowers and fruits.
• Leucoplasts. They are present in eukaryotic plant cells. They store little colored substances and contribute to the conversion of sugars into polysaccharides, fats and proteins.
• Pili. They are present in prokaryotic cells. Pili are hair-like extensions made up of the protein pilin. They are located on the surface of certain bacteria and allow them to transfer their genetic material to other bacteria.

Functions of a cell

Cell functions are determined by the type of cell. Some of its fundamental functions are:

• Structural functions. Cells can form tissues, such as adipose tissue (fat), muscle tissue, and skeletal tissue (bones), that support the animal's body and its organs. For example: Osteoblasts are cells found in bones that form new bone tissue.
• Secretory functions. The cells can synthesize substances that then secrete to the extracellular environment, either because these substances perform functions outside the cell or because they are waste substances. For example: acinus secretory epithelial cells, which are found in the human salivary glands and secrete the first saliva.
• metabolic functions. Cells carry out chemical reactions necessary to obtain energy and the substances necessary to carry out their different functions. In this sense, they can synthesize chemical compounds or decompose them. For example: protein synthesis reactions occur in the cytosol of cells and cellular respiration occurs in the mitochondria.
• Defensive functions. Some types of cells help eliminate pathogens and fight diseases. For example: T lymphocytes are cells that specifically recognize certain antigens, which they then destroy. They also destroy the body's own cells that were infected.
• Interaction or relationship functions. Cells react to both internal and external stimuli, and generate a response. For example: thermoreceptors are skin cells that are activated by sudden changes in temperature.
• reproductive functions. Eukaryotic cells can divide by mitosis (somatic cells) and meiosis (germ cells), while prokaryotic cells can divide by binary fission. For example: sperm are cells that originate from germ cells when they divide by meiosis.

cellular reproduction

The process of cell division in eukaryotic cells can occur by mitosis or meiosis.

mitosis

Mitosis is the nuclear division process of somatic cells while the division of its cytoplasm is called “cytokinesis”.

cell division by mitosis produces two identical cells with the same type and number of chromosomes than the original cell. This type of reproduction allows the replacement of dead cells and the generation of new cells during tissue growth. It also allows the replacement of damaged cells.

meiosis

Meiosis is the process of nuclear division by which germ cells produce gametes.

Cell division by meiosis produces four cells with half as many chromosomes as the original cell. During meiosis, recombination of homologous chromosomes occurs, that is, there is an exchange of genetic information.

To take into account:

• somatic cells (non-germinal) are those linked to the growth of tissues and organs of multicellular organisms. They are diploid cells, that is, they have all sets of homologous chromosomes.
• germ cells They can produce other germ cells through mitosis. They also produce gametes (eggs and sperm in the case of humans, for example) through meiosis. Gametes are haploid cells, that is, they have half the pairs of chromosomes.

Binary fission

Prokaryotic cells divide by binary fission. Binary fission consists of the replication of prokaryotic circular DNA to generate two new identical DNA molecules, with some exceptions.

Besides, cytoplasm replicates and cytoskeletal proteins are organized to generate new cell walls and cytoplasmic membranes, giving rise to two identical daughter cells.

cell theory

The cell theory explains the role that cells have in the formation and characteristics of living beings, as well as in the constitution of life.

According to this theory, the cell is the morphological and physiological unit that forms each living being, and to substantiate this statement, it is based on the postulates:

• All living organisms are made up of cells or their secretion substances. Living organisms can be classified according to the number of cells that make them up:
  • unicellular organisms. They are organisms made up of a single cell. For example: bacteria, archaea and some fungi (such as yeast).
  • multicellular organisms. They are organisms made up of several cells. The cells of these organisms are specialized to perform various functions. For example: animals, plants and mushroom-type fungi.
• The cell is the functional unit of all living beings because their vital functions (nutrition, growth, reproduction, response to stimuli) occur within the cell or in its vicinity.
• All cells come from another cell, that is, there are no cells that originate from inanimate matter.
• All cells contain hereditary information that allows them to perform and control their functions, as well as transmit genetic information to subsequent cell generations.

Continue with: Cell theory

References

• “What is a cell?” (2021) In: gov Available at: https://medlineplus.gov. Accessed: December 27, 2022.
• Fortoul van der Goes DI(Ed.), (2017). Histology and cell biology, 3e. McGraw Hill. https://accessmedicina.mhmedical.com
• “Cell” In: /es.wikipedia.org Available at: https://es.wikipedia.org Consulted: December 27, 2022.
• “The cell. Meiosis” (2022) In: Available at: https://mmegias.webs.uvigo.es Consulted: December 27, 2022.
• Tortora-Derrickson, (2006). Principles of Anatomy and Physiology, 11^na Edition, Panamericana Medical Publishing House.