Cells

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Cells

The cell is the smallest living unit. It forms all living organisms and the tissues of the body. The three main parts of a cell are the cell membrane, the nucleus, and the cytoplasm.

The cell membrane surrounds the cell and controls the substances that enter and exit. Inside the cell is the nucleus, which contains the nucleolus and most of the cell's DNA. It is also where almost all RNA is produced. The cytoplasm is the fluid inside the cell that contains other tiny elements with specific functions, such as the Golgi apparatus, mitochondria, and the endoplasmic reticulum. Most chemical reactions occur in the cytoplasm, and it is also where most proteins are made. The human body has more than 30 trillion cells.

Cell sizes can vary greatly. An average-sized cell measures about 10 µm (micrometers). The vast majority of cells are microscopic, meaning they can only be seen using a microscope.

On the other hand, there are cells that can be observed with the naked eye, such as the human egg, which measures 100 µm and is similar in size to the tip of a pencil.

Cell Types

Cells can be classified according to whether or not they have a membrane surrounding the nucleus, called the "nuclear membrane." According to this classification, cells can be prokaryotic or eukaryotic.

Prokaryotic Cells

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

Prokaryotic cells are small, ranging in size from 1–5 µm. They were the first life forms on Earth, and as far as is known, all living beings made up of prokaryotic cells are unicellular.

Eukaryotic Cell

Eukaryotic cells have a more complex structure than prokaryotic cells. Their nucleus is surrounded by a nuclear membrane, so their genetic material is contained within the nucleus. In addition, these cells have organelles (also called "organelles") in their cytoplasm that may be enclosed by membranes.

The size of eukaryotic cells varies between 10-100 µm, making them larger than prokaryotic cells. In Earth's evolutionary history, eukaryotic cells arose after prokaryotic cells.

Animal cell

They are made up of a plasma membrane and various organelles (Golgi complex, mitochondria, ribosomes, lysosomes, smooth and rough endoplasmic reticulum, among others).

They have lysosomes (organelles exclusive to animal cells), which are vesicles containing digestive enzymes and dedicated to degrading certain chemical compounds and cellular structures.

They have centrioles (organelles exclusive to animal cells), which play a very important role in cell division.

Plant cell

They have a nucleus surrounded by a nuclear membrane where the genetic material is found.

They have a rigid cell wall composed primarily of cellulose. This structure gives the cell its shape and provides support to the plant (plant organisms do not have skeletons like animals).

They have chloroplasts (organelles unique to plant cells), which are organelles that carry out photosynthesis, the process by which sunlight is used to synthesize organic compounds from inorganic compounds.

Parts of a Cell

Plastic or cytoplasmic membrane: It is a membrane that divides the exterior from the interior of the cell. It is formed by a continuous double layer of phospholipids and proteins interspersed or adhered to its surface. The functions of this membrane are to give shape and stability to the cell, separate the internal contents of the cell from the surrounding environment, allow the entry and exit of substances into the cell, and intervene in the interaction between cells. Present in eukaryotic and prokaryotic cells

Cell wall: It is a thick and fairly rigid layer located on the outside of the plasma membrane. It gives the cell strength and stability. Its composition varies depending on the type of cell; for example, in plants the cell wall is composed mainly of cellulose, while in bacteria it is made up of peptidoglycan (a copolymer formed by sugars and amino acids). Present in prokaryotic cells and in eukaryotic cells of plants, fungi, and algae

Nucleus: Contains almost all of the cell's genetic material (DNA) and is surrounded by a nuclear envelope containing pores. Its main functions are to store genetic information, control the activities of all organelles, and coordinate cell reproduction. Present in eukaryotic cells

Nucleoid: This is the region where DNA is found, which in prokaryotic cells is a single, closed, circular molecule. Unlike the nucleus in eukaryotic cells, the nucleoid does not have a nuclear envelope. Present in prokaryotic cells.

Cytoplasm: This is the part of the cell located between the cytoplasmic membrane and the nucleus. It consists of a liquid portion called the "cytosol," which is composed of water, ions, and proteins. All organelles are immersed in the cytosol. The main function of the cytoplasm is to serve as a support for the cell's organelles and assist in the metabolic processes that occur within it. Present in eukaryotic and prokaryotic cells

Organelles, which are structures that perform one or more specific functions, are immersed in the cytoplasm. They are called "organelles" by analogy with the term "organs." Organelles are the small organs inside the cell.

Mitochondria: They are present in eukaryotic cells of animals and plants. 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 the sites of the cells where oxygen enters. A cell can have up to thousands of mitochondria depending on its activity.

Lysosomes: They are present in eukaryotic cells of animals. They are membrane-bound vesicles that originate in the Golgi apparatus. They contain digestive and hydrolytic enzymes (enzymes that accelerate the hydrolysis of chemical bonds) that can digest a large number of molecules. They can also digest another organelle inside the cell and return its components to the cytosol to be reused by the cell (a process called "autophagy"), and digest an entire cell (a process called "autolysis"). If the components digested by lysosomes come from outside the cell, the process is called "heterophagy."

Ribosomes: Ribosomes 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 form separately in the nucleolus (a body located within the nucleus) and join together in the cytoplasm to synthesize proteins. In eukaryotic cells, these organelles are found in the nuclear membrane, the rough endoplasmic reticulum, the cytosol, the mitochondria, and the chloroplasts (in 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 by ribosomes attached to the 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 below 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. The surface of the REL does not contain ribosomes, so proteins are not synthesized within its structure, but fatty acids and steroids are.

Centrosome: It is present in eukaryotic animal cells. This organelle is made up of centrioles and pericentriolar material (a set of proteins that surrounds the centrioles). Centrioles are structures made up of microtubules surrounded by pericentriolar material, which is made up of tubulin protein complexes. Tubulin complexes are the organizing centers for the growth of the mitotic spindle (the set of microtubules involved in cell division).

Cytoskeleton: It is present in eukaryotic cells and has a similar 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 its contents. Additionally, it aids in the movement of organelles within the cell, chromosomes during cell division, and 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 perform an oar-like motion to move the fluid surrounding the cell.

Flagella: They are present in prokaryotic cells and in eukaryotic cells of animals and some algae. They have a similar structure to cilia, but are longer. Flagella move entire cells, like 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 containing vesicles, chlorophyll, and thylakoids. 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 alga Vaucheria litore. Thus, by using the alga's chloroplasts, this mollusk is capable of photosynthesis.

Vacuoles: They are present in all eukaryotic plant cells and some animal cells. They may also be present in some prokaryotic cells. These organelles are vesicles formed by the plasma membrane. Their function is to store water, molecules, and nutrients.

Chromoplates: They are present in eukaryotic plant cells. These organelles store carotenes, which are the pigments that give roots, flowers, and fruits their colors.

Leucoplasts: These are present in eukaryotic plant cells. They store slightly colored substances and contribute to the conversion of sugars into polysaccharides, fats, and proteins.

Pili: These are present in prokaryotic cells. Pili are hair-like extensions made 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:

The functions of cells are determined by the cell type. Some of their fundamental functions are:

Structural Functions: Cells can form tissues, such as adipose tissue (fat), muscle tissue, and bone tissue (bones), which support the body of animals and their organs. For example: osteoblasts are cells found in bones that form new bone tissue.

Secretory Functions: Cells can synthesize substances that they then secrete into the extracellular environment, either because these substances perform functions outside the cell or because they are waste substances. For example: acinus-secreting epithelial cells, which are found in human salivary glands and secrete the first saliva.

Metabolic Functions: Cells carry out the chemical reactions necessary to obtain energy and the substances needed to perform their various functions. In this sense, they can synthesize chemical compounds or break them down. For example, protein synthesis reactions occur in the cytosol of cells, and cellular respiration occurs in the mitochondria.

Defensive Functions: Some types of cells contribute to eliminating pathogens and fighting 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 have been 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.

Cell Reproduction

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

Mitosis

Mitosis is the process of nuclear division in somatic cells, while the division of their cytoplasm is called "cytokinesis."

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

Meiosis

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

Somatic cells (non-germ cells) are those linked to the growth of tissues and organs in multicellular organisms. They are diploid cells, meaning they have all sets of homologous chromosomes.

Germ cells can produce other germ cells through mitosis. They also produce gametes (eggs and sperm in humans, for example) through meiosis. Gametes are haploid cells, meaning they have half the chromosome pairs.

Binary Fission

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

In addition, the cytoplasm replicates, and cytoskeletal proteins organize 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 play 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 every living being, and to support this statement, it is based on the following postulates:

All living organisms are made up of cells or their secretions. Living organisms can be classified according to the number of cells that comprise them into:

Unicellular organisms: These are organisms made up of a single cell. For example: bacteria, archaea, and some fungi (such as yeast).

Multicellular organisms: These are organisms made up of several cells. The cells of these organisms are specialized to perform various functions. For example: animals, plants, and fungi.

The cell is the functional unit of all living beings because their vital functions (nutrition, growth, reproduction, response to stimuli) occur within or near the cell.

All cells come from another cell; that is, there are no cells that originate from inanimate matter.

All cells contain the hereditary information that allows them to perform and control their functions, as well as transmit genetic information to subsequent cell generations.

Activity: Answer the following questions.

1. Part of the cell that contains almost all the genetic material.

2. In what part of the cell are the organelles located?

3. Which of the following is a tissue not made up of cells?

4. Which of the following describes meiosis?

5. Which of the following parts is a thick, fairly rigid layer located on the outside of the plasma membrane?

Once you click this button, the questions will close and you won't be able to change your answer.

6. Living organisms can be classified according to the number of cells they contain into...

7. Animal and plant cells are...

8. According to the Cell Theory, the cell is a unit...

9. Is the following statement true or false: "All cells come from another cell"

10. Name the 4 types of cells.

Still have questions?

We recommend visiting the following material for greater knowledge or understanding of the topic:

Answers to open questions:

2. Álvarez, D. O. (2025, 13 marzo). Célula - Qué es, tipos, partes y funciones. Concepto. https://concepto.de/celula-2/

3. The Editors of Encyclopaedia Britannica. (2025, april 19). Mitosis | Definition, Stages, Diagram, & Facts. Encyclopedia Britannica. https://www.britannica.com/science/mitosis

5. Alberts, M, B., Bernfield, R, M., Cuffe, Michael, Lodish, F, H., Laskey, A, R., Stein, D, W., Staehelin, Andrew, L., Chow, Christopher, Slack, MW, J., Cooper, & A, J. (1999, july 26). Cell | Definition, Types, Functions, Diagram, Division, Theory, & Facts. Encyclopedia Britannica. https://www.britannica.com/science/cell-biology/Coupled-chemical-reactions