Cells represent the smallest anatomical unit of organisms and perform various functions, which are grouped into three fundamental actions: nutrition, interaction, and reproduction (Montagud Rubio, 2020). In order to carry out these processes, Montagud Rubio (2020) mentions that it is observed that cells have organelles and other structures that facilitate their interaction with the environment, supplying energy to the organism and generating waste during this process.
What is a Cell?
The cell is the smallest anatomical unit that constitutes the structure of living beings (Montagud Rubio, 2020). Although the size of cells may vary, on average, they measure around 10 µm (micrometers) (Álvarez, 2023). The vast majority of these cells are microscopic, implying that they can only be seen through the use of a microscope. However, according to Álvarez (2023), there is an exception: the human egg, which, with its 100 µm size, can be observed with the naked eye and is comparable in magnitude to the tip of a pencil.
The fundamental areas of all cells are the nucleus, the plasma membrane, and the cytoplasm, all housing various organelles (Montagud Rubio, 2020). Thanks to the presence of these organelles, cells can carry out the three essential functions that categorize them as living beings: nutrition, interaction, and reproduction. In correspondence with Montagud Rubio (2020), these functions are achieved through specific biochemical processes that enable cell survival and functioning.
Types of Cells
The most relevant classification of cells is based on whether they have a cellular nucleus or not (Montagud Rubio, 2020). Prokaryotic cells exhibit a simple basic structure without a nuclear membrane, causing their genetic material to be dispersed in an area called the nucleoid, directly connected to the rest of the cytoplasm (Álvarez, 2023). With dimensions ranging from 1 to 5 µm, these cells are small. According to Álvarez (2023), they are recognized as the original forms of life on Earth, and to the best of our knowledge, all organisms formed by prokaryotic cells are unicellular.
In contrast, eukaryotic cells have a more complex structure (Álvarez, 2023). Their nucleus is surrounded by a nuclear membrane, confining their genetic material in the nucleus. In addition, these cells house organelles, also known as "organelles," in their cytoplasm, which can be bounded by membranes. Their size varies between 10 - 100 µm, being larger than prokaryotic cells. In the terrestrial evolution, eukaryotic cells emerged after prokaryotic cells (Álvarez, 2023). Although the differentiation between eukaryotes and prokaryotes is important, especially in the study of species evolution, the eukaryotic cell has been the most studied, with two types identified: animal and plant cells, differing in their shape and organelles (Montagud Rubio, 2020). According to Montagud Rubio (2020), animal cells are found in animals, while plant cells, besides being present in plants, can also be found in algae.
Cell Parts
Plasma Membrane
The plasma membrane, also known as the "cell membrane" or "plasmalema," delimits the interior of the cell from its surrounding environment (Montagud Rubio, 2020). This structure, with a thickness of approximately 7 nm (nanometers or millionths of a millimeter), stands out for its precise and organized arrangement that covers the entire cell (Patton & Li, 2016). According to Montagud Rubio (2020), its function lies in regulating the entry and exit of substances, facilitating the entry of nutrients, and the excretion of waste.
This structure is composed of two layers containing carbohydrates, phospholipids, and proteins, constituting a selectively permeable barrier. This means that, while maintaining the cell's stability and providing shape, it can modify its configuration to allow the entry or exit of substances (Montagud Rubio, 2020). Additionally, cholesterol, an additional lipid molecule, contributes to stabilizing the phospholipid molecules, thus preventing potential breaks in the plasma membrane (Patton & Li, 2016). According to Álvarez (2023), the main functions of this membrane include shaping and stabilizing the cell, establishing a separation between the cell's internal content and its surrounding environment, facilitating the exchange of substances to and from the cell, as well as participating in cellular interactions.
Cell Wall
This is a characteristic structure of plant cells, such as those present in plants and fungi (Montagud Rubio, 2020). Plant cells exhibit an additional wall outside the plasma membrane, providing them with rigidity and resistance. This wall is mainly composed of cellulose (Montagud Rubio, 2020). However, according to Álvarez (2023), the composition of the cell wall varies depending on the cell type. For example, in plants, it is mostly composed of cellulose, while in bacteria, it is constituted by peptidoglycan, a copolymer of sugars and amino acids.
Nucleus
The nucleus is the structure that allows the distinction between eukaryotic cells, which possess it, and prokaryotic cells, which lack it (Montagud Rubio, 2020). When observed under the microscope, the nucleus appears as a simple structure: a small sphere in the center of the cell (Patton & Li, 2016). Its main function lies in safeguarding the genetic material, which is organized into DNA chains forming genes that encode different proteins (Montagud Rubio, 2020). These genes, in turn, are grouped into chromosomes (Montagud Rubio, 2020). In addition to this protective function, Montagud Rubio (2020) and Patton & Li (2016) mention that the nucleus plays fundamental roles, such as the generation and reassembly of messenger RNA (mRNA) into proteins, the formation of pre-ribosomes (rRNA), the organization of genes into chromosomes for cell division, and the regulation of the complex process of cell reproduction.
The cell nucleus is surrounded by a nuclear membrane composed of two separate layers (Patton & Li, 2016). This envelope has extremely small nuclear pores that facilitate the entry and exit of large molecules from the nucleus. According to Patton & Li (2016), within the nucleus, there is the nucleoplasm, a special cellular substance that houses crucial structures, such as the nucleolus and chromatin granules.
Nuclear Membrane
The structure in question, like the plasma membrane that surrounds the cell, is characterized by being a double lipid membrane envelope around the nucleus (Montagud Rubio, 2020). According to Montagud Rubio (2020), this nuclear membrane plays a crucial role in regulating communication between the inside of the nucleus and the cytoplasm.
Nucleolus
Within the cell nucleus is a structure whose main function lies in the synthesis of ribosomes, from its DNA components, to generate ribosomal RNA (rRNA) (Montagud Rubio, 2020). In accordance with Montagud Rubio (2020), this activity is linked to protein synthesis. For this reason, in cells with a high rate of protein synthesis, a greater number of these nucleoli is commonly found.
Chromosomes
Structures containing genetic material are known as chromosomes, particularly visible during the process of cell division (Montagud Rubio, 2020). Humans have 22 pairs of numbered chromosomes (autosomes) and one pair of sex chromosomes (XX or XY), totaling 46 (Bates, 2024). According to Montagud Rubio (2020), each pair contains two chromosomes, one from each parent, meaning that children inherit half of their chromosomes from their mother and the other half from their father.
Chromatin
Chromatin, composed of DNA and a variety of proteins, including histones and non-histones, resides in the cell nucleus, constituting the cell's genetic material (Montagud Rubio, 2020). According to Montagud Rubio (2020), nucleosomes, identified as the fundamental units of information, are an integral part of this complex system.
Cytoplasm
The cytoplasm is recognized as the interior medium of the cell, often referred to as its body (Montagud Rubio, 2020). It consists of a liquid part known as "cytosol," which is composed of water, ions, and proteins (Álvarez, 2023). Within the cytosol, all cellular organelles are immersed (Álvarez, 2023). Many vital chemical processes for life take place in this environment (Montagud Rubio, 2020). The cytoplasm can be divided into two sections. One of them, the ectoplasm, has a gelatinous consistency, while the other, the endoplasm, is more fluid and serves as the site where organelles reside. According to Montagud Rubio (2020), this characterization is associated with the main function of the cytoplasm, which is to facilitate the movement of cellular organelles and provide them with protection.
Cytoskeleton
The cytoskeleton presents itself as an internal skeleton within the cell, providing cohesion and structure (Montagud Rubio, 2020). The cytoskeleton is composed of three types of filaments: microfilaments, intermediate filaments, and microtubules (Montagud Rubio, 2020). Microfilaments consist of fibers formed by very thin proteins, with a diameter between 3 and 6 nanometers. Actin, a contractile protein, is the main component of these microfilaments. On the other hand, intermediate filaments, approximately 10 nanometers, provide tensile strength to the cell. According to Montagud Rubio (2020), microtubules are cylindrical tubes with a diameter of 20 to 25 nanometers, composed of tubulin units, and act as the scaffold shaping the cell.
Types of Organelles
According to their designation, organelles are small organs present inside the cell (Montagud Rubio, 2020). Montagud Rubio (2020) mentions that, from a technical standpoint, the plasma membrane, cell wall, cytoplasm, and nucleus are not classified as organelles, although there could be a debate regarding whether the nucleus should be considered as such or if it is a structure that requires a representative classification.
Mitochondria
Mitochondria are organelles present in eukaryotic cells, providing the necessary energy for various activities (Montagud Rubio, 2020). In comparison to other organelles, they are considerably larger and have a globular shape. Their main function is to break down nutrients and synthesize adenosine triphosphate (ATP), a crucial substance for energy acquisition. In addition to their energy function, they exhibit reproductive capacity by possessing their own DNA, allowing them to generate more mitochondria as per the cell's needs (Montagud Rubio, 2020). The number of mitochondria in a cell can vary significantly, reaching up to thousands, depending on cellular activity (Álvarez, 2023). According to Montagud Rubio (2020), the production of ATP occurs during cellular respiration, where mitochondria use molecules from carbohydrate-rich foods to produce this vital substance.
Golgi Apparatus
The Golgi apparatus, present in all eukaryotic cells, plays an essential role in the production and transport of proteins, lipids, and lysosomes within the cell (Montagud Rubio, 2020). Its main function can be likened to a packaging plant, as it modifies vesicles originating in the endoplasmic reticulum. According to Montagud Rubio (2020), this system of endomembranes folds upon itself, creating a structure similar to a curved labyrinth, organized into flattened sacs or cisterns.
Lysosomes
Lysosomes, present in eukaryotic animal cells, are vesicles surrounded by a membrane, originating from the Golgi apparatus (Álvarez, 2023). These vesicles contain digestive and hydrolytic enzymes, which accelerate the hydrolysis of chemical bonds in various molecules. In addition to this capacity, lysosomes can carry out the digestion of other organelles within the cell, returning their components to the cytosol for subsequent reuse by the cell (a process called "autophagy"). They can also perform the complete digestion of a cell (a process called "autolysis"). When the components to be digested come from outside the cell, the process is referred to as "heterophagy" (Álvarez, 2023). According to Montagud Rubio (2020), these spherical structures are surrounded by a single membrane.
Vacuole
Plant eukaryotic cells and some animal cells contain vacuoles (Álvarez, 2023). They can also be found in some prokaryotic cells (Álvarez, 2023). These compartments, enclosed by the plasma membrane, house various fluids, water, enzymes, and can contain solids such as sugars, proteins, salts, and other nutrients (Montagud Rubio, 2020). The majority of vacuoles originate from the fusion of membranous vesicles. Their structure has no defined shape and varies according to cellular needs (Montagud Rubio, 2020). In accordance with Álvarez (2023), the main function of vacuoles is the storage of water, molecules, and nutrients.
Chloroplasts
Chloroplasts are found in eukaryotic plant cells and green algae (Álvarez, 2023). This cellular organelle consists of two membranes that house vesicles, chlorophyll, and thylakoids inside. The thylakoids are where the reaction that absorbs photons from sunlight to carry out photosynthesis takes place. Although chloroplasts are exclusive to plant and algae cells, there is a significant exception. The mollusk known as the Eastern Emerald Elysia, scientifically identified as Elysia chlorotica, feeds on chloroplasts present in the Vaucheria litorea algae. Surprisingly, according to Álvarez (2023), this mollusk is capable of photosynthesis by using the algal chloroplasts as an energy source.
Ribosomes
Ribosomes are organelles responsible for protein synthesis, an essential process for cell growth and reproduction (Montagud Rubio, 2020). They are present in both eukaryotic and prokaryotic cells but have some differences in structure and location (Álvarez, 2023). In eukaryotic cells, they are made up of two subunits that are formed separately in the nucleolus and join in the cytoplasm to synthesize proteins. Additionally, in these cells, they can be located in the nuclear membrane, the rough endoplasmic reticulum, the cytosol, mitochondria, and chloroplasts (in the case of plants). In prokaryotic cells, they are scattered throughout the cytoplasm and are smaller than those in eukaryotic cells (Álvarez, 2023). According to Montagud Rubio (2020), they are responsible for translating genetic information obtained from DNA into RNA, which is the code determining the amino acid sequence of proteins.
Endoplasmic Reticulum
The endoplasmic reticulum is a system of channels present in eukaryotic cells that is responsible for transferring or synthesizing lipids and proteins (Montagud Rubio, 2020; Álvarez, 2023). This system is distributed throughout the cytoplasm and is involved in protein synthesis (Montagud Rubio, 2020). The membranes of the endoplasmic reticulum continue with the nuclear envelope and can extend close to the plasma membrane (Montagud Rubio, 2020). There are two types of endoplasmic reticulum: rough and smooth (Montagud Rubio, 2020; Álvarez, 2023). The rough endoplasmic reticulum is a structure located below the nuclear membrane, and its surface is covered with ribosomes, organelles responsible for protein synthesis (Álvarez, 2023). On the other hand, according to Álvarez (2023), the smooth endoplasmic reticulum is a structure that extends from the rough endoplasmic reticulum, and its surface does not contain ribosomes, so proteins are not synthesized in its structure, but fatty acids and steroids are synthesized.
Cilia
Prokaryotic and eukaryotic animal cells and some algae contain extensions of the plasma membrane similar to hairs, known as cilia (Álvarez, 2023). According to Álvarez (2023), their main function is to perform movements similar to those of an oar, contributing to the movement of the surrounding fluid to the cell.
Centrosome
The centrosome is found in animal eukaryotic cells (Álvarez, 2023). This organelle is composed of centrioles and pericentriolar material, which is a set of proteins that surrounds the centrioles. According to Álvarez (2023), these tubulin complexes act as organizing centers for the growth of the mitotic spindle, which is the set of microtubules involved in cell division.
Centriole
The centriole is a cylindrical organelle composed of microtubules (Montagud Rubio, 2020). It is part of the cytoskeleton and contributes to maintaining cell shape, as well as transporting organelles and particles within the cell. When two centrioles are positioned together perpendicularly inside the cell, they are called a diplosome. This structure plays a crucial role in the movement of cilia and flagella in unicellular organisms. Additionally, according to Montagud Rubio (2020), centrioles actively participate in cell division, with each contributing to the formation of centrioles in daughter cells.
Flagella
Flagella are found in prokaryotic and eukaryotic animal cells, as well as in some algae (Álvarez, 2023). Their structure is similar to that of cilia but distinguishes itself by being longer. These flagella initiate the movement of cells as a whole, acting as small propellers that provide them with mobility (Álvarez, 2023). In accordance with Montagud Rubio (2020), this characteristic is typical of unicellular organisms or specialized cells, such as sperm cells.
Differences Between Animal and Plant Cells
Animal cells and plant cells share numerous organelles and similar structures, but they also exhibit certain details that allow for their distinction (Montagud Rubio, 2020). One of the most notable aspects is the presence of the plant cell wall, which surrounds the plasma membrane, giving it a hexagonal shape and a rigid structure. Additionally, chloroplasts stand out as a structure exclusively found in plant cells. These organelles house chlorophyll, crucial during photosynthesis. They are responsible for synthesizing sugars from carbon dioxide, water, and sunlight. Consequently, according to Montagud Rubio (2020), organisms with plant cells are classified as autotrophs since they can manufacture their own food, unlike those with animal cells, lacking chloroplasts and thus being heterotrophs.
In animal cells, energy is exclusively obtained from mitochondria, while in plant cells, both mitochondria and chloroplasts are present (Montagud Rubio, 2020). This arrangement allows plant cells to harness energy from two different organelles. This difference is crucial as plant organisms can perform both photosynthesis and cellular respiration, while animals can only carry out the latter biochemical process. Another detail, though not as relevant as the ability to perform photosynthesis, is the uniqueness of the vacuole in plant cells, which tends to be singular, centrally located, and of considerable size. In contrast, in animal cells, there are several smaller vacuoles. Additionally, according to Montagud Rubio (2020), there is the presence of centrioles in animal cells, a structure absent in plant cells.
References
Álvarez, D. O. (2023). Célula. Concepto. https://concepto.de/celula-2/
Bates, S. A. (2024). Chromosome. National Human Genome Research Institute. https://www.genome.gov/genetics-glossary/Chromosome
Montagud Rubio, N. (2020, marzo 5). Las Partes de la Célula y los Orgánulos más Importantes: Un Resumen. Psicología y Mente. https://psicologiaymente.com/salud/partes-de-celula
Patton, K. T., & Li, S. (2016). Anatomía y Estructura de la Célula: Tamaño, Composición y Funciones. En www.elsevier.com (pp. 39–41). Elsevier. https://www.elsevier.com/es-es/connect/anatomia-y-estructura-de-la-celula
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