The animal cell represents the minimal unit of functionality that constitutes organisms belonging to the category of animals (Torres, 2019). This cell, in particular, is a type of eukaryotic cell that, by combining its elements and occasionally collaborating with other forms of life, such as bacteria forming the intestinal flora, contributes to the formation of tissues and functional organs, which allow the existence and survival of an animal (Torres, 2019). Álvarez (2023) mentions that, given that animals are complex multicellular organisms, the cells composing them exhibit a high level of specialization: depending on the tissue they belong to, these cells perform specific functions that define their morphology, function, and needs.
Características
All animal cells are part of the taxon of eukaryotic cells (Torres, 2019). These cells are characterized by harboring all their genetic material in a structure called the cell nucleus. Additionally, they present various organelles separated by membranes that envelop them, unlike prokaryotic cells that lack these characteristics and are smaller, with their DNA dispersed in the cytoplasm that fills their interior. Additionally, the animal cell differs from other eukaryotic cells by organizing with others to constitute multicellular organisms that are part of the animal kingdom. Animals, in turn, are living beings of both microscopic and macroscopic dimensions, characterized, among other things, by their ability to move and the presence of nerve cells. According to Torres (2019), the Animalia kingdom constitutes one of the five kingdoms present in the group of eukaryotes.
Parts of the Animal Cell
Nucleus
The nucleus is of utmost importance, as it not only houses a "instruction manual" about the molecules necessary for cell construction and regeneration but also establishes strategies for organism functioning and maintenance (Torres, 2019). In this context, in correspondence with Torres (2019), all information regarding actions inside and outside the cell is contained in this organelle.
It stores genetic material in the form of DNA (Deoxyribonucleic Acid) and coordinates various cellular activities, from growth to reproduction (Páez, 2021). Thus, the nucleus operates as a filter to regulate the entry and exit of the zone where DNA is stored, preventing its dispersion and loss (Torres, 2019). Additionally, it strives to minimize the contact of certain molecules with the chromosomes, in order to preserve genetic information (Torres, 2019). Additionally, the nucleus houses a nucleolus, an internal structure composed of the concentration of chromatin and proteins (Páez, 2021). Páez (2021) mentions that, in mammals, there are between 1 and 5 nucleoli in the cell.
Cell Membrane or Plasma Membrane
The cell membrane constitutes the outermost layer of the cell, encompassing it almost entirely and providing uniform protection to all its parts (Torres, 2019). It is mainly composed of lipids, specifically phospholipids and cholesterol, forming a double lipid layer similar to a sealed bag (Fernandes, 2021). It is worth noting that the cell membrane of animal cells, like those of other eukaryotic organisms, does not exhibit total impermeability; on the contrary, it presents entry and exit points in the form of pores that enable the exchange of substances with the external environment (Torres, 2019). Through these channels or transporters, substances necessary for metabolism enter and ions or waste products exit (Fernandes, 2021). Torres (2019) mentions that, although this process increases the risk of harmful elements entering, it is essential for maintaining homeostasis, which represents the physical-chemical balance between the cell and its environment.
Cytoplasm or Cytosol
The cytoplasm of animal cells is the space between the cytoplasmic membrane and the nucleus, surrounding all organelles (Páez, 2021). In other words, it acts as physical support for all internal components of the cell (Torres, 2019). It contributes, among other things, to ensuring the constant availability of substances necessary for cell development, regeneration, and communication (Torres, 2019). It is mainly constituted by 70% water, with the rest being a mixture of proteins, lipids, carbohydrates, and mineral salts (Páez, 2021). According to Páez (2021), this environment is essential for the development of cell activity.
Cytoskeleton
The cytoskeleton represents a set of more or less rigid filaments whose purpose is to confer shape to the cell and preserve its components in stable locations for proper cellular functioning (Torres, 2019). In correspondence with Torres (2019), through its components, such as microtubules, it facilitates the movement of certain molecules along its internal conduits.
Mitochondria
Mitochondria represent one of the most intriguing parts of the animal cell because they harbor their own DNA, which differs from the nucleus (Torres, 2019). The hypothesis suggests that this structure is, in fact, the vestige of a fusion between a cell and a bacterium (with the mitochondrion being the bacterium integrated into the cell, fused in a symbiotic relationship). During the reproduction process, the duplication of mitochondrial DNA is also carried out to transmit it to the offspring. The main function of mitochondria lies in the production of ATP, the molecule from which animal cells obtain energy, making them crucial components for metabolic processes (Torres, 2019). Their morphology is elongated, and they have two membranes: an inner one that forms crests when folded and an outer smooth one (Páez, 2021). According to Páez (2021), the quantity of mitochondria in each cell varies according to its activity (for example, muscle cells may have a high number of them).
Vacuole
The organelle, formed by the fusion of a large number of membranous vesicles, undergoes variations in its shape and structure according to the cell's needs (Fernandes, 2021). Inside it, it contains enzymes or water. In correspondence with Fernandes (2021), its main function is to preserve cellular rigidity and favor the cell's enlargement.
Golgi Apparatus
The Golgi apparatus is mainly responsible for creating molecules from raw materials that come from other parts of the animal cell (Torres, 2019). Its structure, similar to the Golgi complex present in plant cells, consists of three components: membranous sacs, tubules used for sending substances into and out of the cell, and vacuoles (Páez, 2021). Thus, it facilitates the transport, modification, and classification of proteins synthesized in the ribosomes of the rough endoplasmic reticulum (Álvarez, 2023). In accordance with Álvarez (2023), newly synthesized proteins are wrapped with a membrane layer of the rough endoplasmic reticulum, leading to the formation of vesicles.
Endoplasmic Reticulum
The endoplasmic reticulum is an organelle that adopts the form of flattened sacs and tubules stacked together, sharing the same internal space (Páez, 2021). This organelle is organized into several domains, including the rough endoplasmic reticulum, responsible for protein synthesis and characterized by flattened membranes and associated ribosomes, and the smooth endoplasmic reticulum, responsible for lipid synthesis and having a more irregular appearance, without associated ribosomes (Fernandes, 2021; Páez, 2021). Like the Golgi apparatus, the endoplasmic reticulum is also distinguished by material synthesis, but on a smaller scale (Torres, 2019). Specifically, according to Torres (2019), it is especially involved in lipid creation to maintain the integrity of the cell membrane.
Ribosomes
Ribosomes, cellular organelles present in the cytoplasm of eukaryotic and prokaryotic cells, are characterized by their globular shape and the absence of a membrane (Rothschuh Osorio, 2022). In correspondence with Fernandes (2021), these organelles are composed of RNA and proteins and play a fundamental role in protein synthesis.
Lysosomes
Lysosomes represent vesicles containing enzymes responsible for degrading material that enters the cell, called "heterophagy", or material generated internally, known as "autophagy" (Álvarez, 2023). The primary function of this organelle is to carry out cellular digestion, and they are synthesized by the Golgi apparatus (Álvarez, 2023). In short, in accordance with Torres (2019), these microscopic bodies release enzymes with the ability to "dissolve" the components of the animal cell.
Centriole
It is a cylindrical organelle composed of three triplets of microtubules, which are part of the cytoskeleton (Álvarez, 2023). In correspondence with Álvarez (2023), its function stands out in the transport of organelles within the cell, providing mechanical stability and actively participating in mitosis or cell division processes.
Centrosome
The centrosome is distinguished in the animal cell by being a cylindrical and hollow structure composed of two centrioles arranged perpendicular to each other (Páez, 2021). In the pericentriolar material, there are complexes of the tubulin protein, essential in creating the mitotic spindle, a set of microtubules that extend from the centrioles during cell division (Páez, 2021; Álvarez, 2023). From this process, Fernandes (2021) mentions that cilia and flagella originate, which are mobile structures present in some cells.
Cilia and Flagella
The cilia and flagella of the animal cell are appendages formed by microtubules, which provide mobility to the cell (Páez, 2021). These appendages are present in unicellular organisms, playing a crucial role in their locomotion, while in other cells, they perform functions such as removing substances from the environment or participating in sensory function (Páez, 2021). Cilia execute movements similar to oars to move the surrounding liquid to the cell (Álvarez, 2023). Unlike cilia, flagella are characterized by their greater length, acting as propellers that drive the movement of the entire cell (Álvarez, 2023). In terms of quantity, Páez (2021) mentions that cilia outnumber flagella.
Peroxisomes
Peroxisomes are rounded organelles, delimited by a membrane and with a diameter ranging from 0.1 to 1 micrometer (Sánchez Amador, 2021). Inside these, there are enzymes fundamental for carrying out various metabolic reactions, covering various aspects of cellular metabolism, a process by which each of these functional bodies acquires the necessary energy to carry out its activities. It is estimated that within each peroxisome, there are around 50 different enzymes capable of catalyzing various reactions, depending on the type of cell containing the organelle and its physiological state. Sánchez Amador (2021) mentions that a notable example is that these organelles contain approximately 10% of the total activity of two enzymes involved in the pentose-phosphate pathway, which are closely related to glycolysis, which is the process of glucose oxidation to obtain energy.
Types and Functions
Epithelial Cells
Epithelial cells constitute the cells found on the walls of organs, forming lining tissues (Páez, 2021). They present various specializations depending on the organ they are located in, as this specialization defines their function. Páez (2021) mentions that an example of this is evident in the cells of the epithelium of the small intestine, which develop microvilli with the purpose of increasing the surface area for nutrient absorption.
Connective Tissue Cells
These cells aim to establish an interconnected structure that, transcending the skin barrier, preserves all internal parts in their corresponding location (Torres, 2019). Torres (2019) mentions that an example is found in bone cells, which, belonging to this category, participate in bone formation, rigid structures designed to keep the rest of the elements in place.
Blood Cells
They enable all nutrients, vitamins, and molecules necessary for life to circulate through the circulatory system (Torres, 2019). Simultaneously, they prevent the spread of harmful external agents throughout the body. Thus, the activity of these cells is linked to movement (Torres, 2019). Three different types of blood cells are identified: red blood cells (or erythrocytes), white blood cells (or leukocytes), and platelets (Páez, 2021). Red blood cells stand out because they are the only cells in the human body that lack a nucleus. In correspondence with Páez (2021), these cell types primarily move through the bloodstream, performing key functions such as oxygen and CO2 transport and exchange (red blood cells), antibody production for the immune response (white blood cells), or coagulation for maintaining the circulatory system.
Muscle Cells
Three main types of muscle cells are identified: those belonging to smooth, skeletal, and cardiac muscle tissue (Páez, 2021). These cells have the ability to contract by transforming chemical energy into mechanical energy. The shape of these cells is diverse, and their function is related to the tissue to which they belong. According to Páez (2021), those of smooth muscle show an elongated shape, while those of skeletal and cardiac tissue present striations, with the latter notable for their rhythmic involuntary contraction.
Nerve Cells
One of the most characteristic types of cells in animal activity is the nerve cell, as animals are distinguished by their ability to move and process various types of information related to constant changes in their environment (Torres, 2019). Nervous tissue is composed of two types of cells: neurons and glial cells (Páez, 2021). Neurons specialize in transmitting nerve impulses through synapses, either between neurons or between a neuron and a muscle cell. On the other hand, glial cells do not transmit nerve impulses but perform support and maintenance functions for neurons. According to Páez (2021), both types have a branched or star-shaped form, thus facilitating communication between them.
Adipose Cells
Adipocytes, cells of considerable size, perform the function of storing energy in the form of fatty acids inside them (Páez, 2021). Additionally, they are responsible for secreting proteins and hormones, thus playing a crucial role in various biological functions. According to Páez (2921), these cells also serve an important purpose in the thermal and mechanical protection of the organism.
Cartilaginous Cells
Cartilaginous cells are called chondrocytes, characterized by their flattened and rounded shape, as well as the presence of microvilli (Páez, 2021). In correspondence with Páez (2021), in the human body, they are found in tissues located in the ribs, joints, nose, among other places, and together they perform a support function.
Bone Cells
Bone cells play a crucial role in the process of bone growth and breakdown (Páez, 2021). According to de Andrade (2019), in this process, they are distinguished into three main categories: osteoblasts, responsible for manufacturing the organic matrix of collagen, which is why they are located on the surface of the bone and in places where new formation is required, such as fracture sites; osteoclasts, a special group of cells responsible for bone resorption; and osteocytes, the most common type of bone cell.
References
Álvarez, D. O. (2023). Célula Animal - Concepto, Partes y Diferencias con la Vegetal. Concepto. https://concepto.de/celula-animal/
de Andrade, M. (2019). Definición de Células Óseas. Significado.com. https://significado.com/celulas-oseas/
Fernandes, A. Z. (2021, enero 22). Qué es la Célula Animal. Significados. https://www.significados.com/celula-animal/
Páez, J. C. (2021, marzo 1). Partes de la Célula Animal. Ecología Verde. https://www.ecologiaverde.com/partes-de-la-celula-animal-3279.html
Rothschuh Osorio, U. (2022, marzo 8). Ribosomas: Función y Estructura. ecologiaverde.com. https://www.ecologiaverde.com/ribosomas-funcion-y-estructura-3795.html
Sánchez Amador, S. A. (2021, abril 15). Peroxisomas: Qué son, Características y Funciones. Psicología y Mente. https://psicologiaymente.com/salud/peroxisomas
Torres, A. (2019, agosto 1). Célula Animal: Tipos, Partes y Funciones que la Caracterizan. Psicología y Mente. https://psicologiaymente.com/miscelanea/celula-animal
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