ANIMAL CELL: PARTS, FUNCTIONS, ORGANELLES [WITH PICTURES] - BIOLOGY - 2024

The animal cell is the type of eukaryotic cell that all animals in the biosphere are composed of, both the tiny ones that we cannot see and the protozoa, since they are microscopic, like whales and elephants, which are colossal mammals.

The fact that animal cells are eukaryotic cells implies that they have intracellular organelles that are separated from the rest of the cytosolic components thanks to the presence of lipid membranes and, in addition, it implies that their genetic material is enclosed within a specialized structure known as core.

Diagram of an animal cell and its parts (Source: Alejandro Porto via Wikimedia Commons) Animal cells present a wide diversity of organelles immersed in the cell interior. Some of these structures are also present in its counterpart: the plant cell. However, some are unique to animals, such as centrioles.

This class of cell is very diverse in terms of its shape and function, which is easily evident when observing and detailing any animal tissue under a microscope. It is estimated that there are on average 200 different types of animal cells.

Characteristics of the animal cell

- Just as it is true for plant cells and for bacteria and other cellular organisms, animal cells represent for animals the main structural blocks that make up their bodies.

- They are eukaryotic cells, that is, their hereditary material is enclosed by a membrane within the cytosol.

- They are heterotrophic cells, which means that they must obtain energy to carry out their functions from the environment that surrounds them.

- They differ from plant cells and many bacteria, in that they do not have a rigid cell wall that protects them from highly fluctuating environmental conditions.

- Like some "lower" plants, animal cells have structures called " centrosomes ", made up of a pair of " centrioles ", which participate in cell division and in the organization of cytoskeletal microtubules.

Here is an animation of a human animal cell, where you can easily see the nucleus:

Organelles of the animal cell and their functions

If the reader were to observe an animal cell through a microscope, at an initial glance it is likely that the presence of a structure that delimits a quantity of volume from the surrounding medium would catch his eye.

Within what this structure contains, it is possible to appreciate a kind of liquid in which a sphere with a denser and more opaque appearance is suspended. It is, then, the plasma membrane, the cytosol and the cell nucleus, which are, perhaps, the most obvious structures.

Magnification by microscope 430 times. You can see the nucleus with the genetic material and various organelles, such as the endoplasmic reticulum. Jlipuma1 It will be necessary to increase the magnification of the microscope objective and pay careful attention to what is observed in order to verify the presence of many other organelles embedded in the cytosol of the cell in question.

If you had to make a list of the various organelles that make up the cytosol of an "average" animal cell such as the hypothetical cell that the reader is looking at under the microscope, it would look something like this:

- Plasma and organellar membrane

- Cytosol and cytoskeleton

- Core

- Nucleolus

- Endoplasmic reticulum

- Golgi complex

- Lysosomes

- Peroxisomes

- Centrosomes

- Mitochondria

- Cilia and flagella

Cell or plasma membrane

The plasma membrane is indicated in the lower right

Membranes are, without a doubt, one of the most important structures, not only for the existence of animal cells, but also for plant cells, bacteria and archaea.

The plasma membrane exercises the transcendental function of separating the cellular content from the environment that surrounds it, serving, in turn, as a selective permeability barrier, since it has associated specific proteins that mediate the passage of substances from one side of the cell to the other. itself.

Organellar membranes

The membranes that surround the internal organelles (organellar membranes) allow the separation of the different compartments that make up the cells, including the nucleus, which somehow allows the "optimization" of resources and the division of internal tasks.

Composition and structure

Structure of the plasma membrane. The extracellular medium is indicated and the bottom part is the intracellular medium

All biological membranes, including those of animal cells, are made up of lipid bilayers that are organized in such a way that the fatty acids of the lipid molecules face each other in the "center" of the bilayer, while the heads polar ones "look" towards the aqueous medium that surrounds them (intra- and extracellularly speaking).

The structural and molecular characteristics of the lipids that make up the membranes of animal cells largely depend on the type of cell in question, as well as the type of organelle.

Both the plasma membrane of an animal cell and the membranes that surround its organelles are associated with proteins that serve different functions. These can be integral (those that cross the membrane and are strongly associated with it) or peripheral (that are associated with one of the two faces of the membrane and do not cross it).

Cytosol and cytoskeleton

The cytosol is the semi-gelatinous medium in which all the internal components of a cell are embedded in an organized manner. Its composition is relatively stable and is characterized by the presence of water and all the nutrients and signaling molecules that the animal cell needs to survive.

The cytoskeleton, on the other hand, is a complex network of protein filaments that is distributed and extends throughout the cytosol.

Part of its function is to give each cell its characteristic shape, to organize its internal components in a specific region of the cytosol, and to allow the cell to carry out coordinated movements. It also participates in numerous intracellular signaling and communication processes, vital to all cells.

Cytosol filaments

Cytoskeleton: network of filamentous proteins. Alice Avelino This architectural framework inside cells is made up of three types of filamentous proteins known as intermediate filaments, microtubules, and actin filaments; each with specific properties and functions.

The intermediate filaments of the cytosol can be of several types: keratin filaments, vimentin filaments and related to vimentin and neurofilaments. At the core these are known as the nuclear laminae.

Microtubules are made up of a protein called tubulin and in animals they are formed from structures known as centrosomes; while actin filaments are made up of the protein for which they were named, and are thin and flexible structures.

Centrosomes

They are the main centers of organization of microtubules. They are located on the periphery of the nucleus when the cell divides and is made up of centrioles joined at right angles, each of which is made up of nine triplets of microtubules arranged cylindrically.

Core

The cell nucleus (Source: BruceBlaus. When using this image in external sources it can be cited as: Blausen.com staff (2014). «Medical gallery of Blausen Medical 2014». WikiJournal of Medicine 1 (2). DOI: 10.15347 / wjm / 2014.010. ISSN 2002-4436. via Wikimedia Commons) This is the organelle that differentiates prokaryotic cells from eukaryotes. Its main function is to contain the genetic material (DNA) inside, thus controlling basically all cellular functions.

Complex processes take place inside it, such as DNA replication during cell division, gene transcription and an important part of the processing of the resulting messenger RNAs, which are exported to the cytosol for translation into proteins or to exercise their regulatory functions..

The nucleus is surrounded by a double membrane known as the nuclear envelope, which, like the plasma membrane, represents a selective permeability barrier, since it prevents the free passage of molecules to both sides of each other.

The communication of the nucleus with the rest of the cytosol and its components occurs through structures of the nuclear envelope called nuclear pore complexes, which are capable of recognizing specific signals or labels in the molecules that are imported or exported through their inside.

Between the two membranes of the nuclear envelope there is a space that has been called the perinuclear space and it is important to note that the outer portion of the nuclear envelope continues with the membrane of the endoplasmic reticulum, connecting the perinuclear space with the lumen of the latter organelle..

The interior of the nucleus is surprisingly organized, which is possible thanks to the existence of proteins that function as a "nucleoskeleton", which provide it with some structural support. In addition, the chromosomes in which the nuclear DNA is organized are located in specific regions of the organelle.

Nucleolus

Nucleolus or nucleolus at the top

The nucleolus is found inside the nucleus and is the site where the transcription and processing of ribosomal RNAs occurs, as well as the assembly of ribosomes, which are the structures responsible for the translation of messenger RNAs into protein sequences.

It is not a nuclear organelle, that is, it is not surrounded by a membrane, it is simply composed of the regions of the chromosomes where ribosomal genes are encoded and by the protein machinery in charge of their transcription and enzymatic processing (RNA polymerases, mainly).

Endoplasmic reticulum

It is a kind of "network" of sacs or cisterns and tubules surrounded by a membrane that is continuous with the outer membrane of the nuclear envelope. Some authors consider that it is the largest organelle of most cells, since in some cases it can represent up to 10% of the cell.

If viewed under the microscope, it can be seen that there is a rough endoplasmic reticulum and another with a smooth appearance. While the rough endoplasmic reticulum has hundreds of ribosomes embedded in its outer surface (which are responsible for the translation of membrane proteins), the smooth portion is related to lipid metabolism.

The smooth and rough endoplasmic reticulum (Source: OpenStax via Wikimedia Commons) The function of this organelle has to do with the processing and distribution of cellular proteins, especially those that are associated with lipid membranes, in other words, it participates in the first station of the secretory route.

It is also one of the main protein glycosylation sites, which is the addition of carbohydrate moieties to specific regions of the peptide chain of a protein.

Golgi complex

The Golgi complex or apparatus is another organelle specialized in the processing and distribution of proteins from the endoplasmic reticulum to their final destinations, which can be lysosomes, secretory vesicles or the plasma membrane.

Inside it, glycolipid synthesis and protein glycosylation also take place.

It is, therefore, a complex made up of flattened “bags” or cisterns covered by a membrane, which are associated with a large number of transport vesicles that detach from themselves.

It has a polarity, which is why a cis face (oriented towards the endoplasmic reticulum) and a trans face (which is where the vesicles exit) are recognized.

Lysosomes

The lysosome degrades the materials that enter the cell and recycles intracellular materials. Step 1-Material that enters the food vacuole through the plasma membrane. Step 2-A lysosome within an active hydrolytic enzyme appears as the food vacuole moves away from the plasma membrane. Step 3-Fusion of the lysosome with the food vacuole and hydrolytic enzymes. Step 4-Hydrolytic enzymes digest food particles. Jordan hawes They are organelles surrounded by a membrane and responsible for the degradation of different types of large organic molecules such as proteins, lipids, carbohydrates and nucleic acids, for which they have specialized hydrolase enzymes.

They act as the cell's "purification" system and are the recycling center for obsolete components, including defective or unnecessary cytosolic organelles.

They have the appearance of spherical vacuoles and are relatively dense in content, but their shape and size varies from cell to cell.

Peroxisomes

Graphic representation of a peroxisome.

Source: Rock 'n Roll These small organelles function in many reactions of the energy metabolism of animals; They have up to 50 different types of enzymes and are involved in:

- Production of hydrogen peroxide and elimination of free radicals

- The degradation of fatty acids, amino acids and other organic acids

- The biosynthesis of lipids (especially cholesterol and dolichol)

- The synthesis of bile acids derived from cholesterol

- The synthesis of plasmalogens (essential for the heart and brain tissue), etc.

Mitochondria

Mitochondria

Mitochondria are the main energy-producing organelles in the form of ATP in animal cells with aerobic metabolism. They are morphologically similar to a bacterium and have their own genome, so they multiply independently of the cell.

These organelles have an "integrative" function in the intermediary metabolism of different metabolic pathways, especially with regard to oxidative phosphorylation, fatty acid oxidation, the Krebs cycle, the urea cycle, ketogenesis and gluconeogenesis.

Cilia and flagella

Many animal cells have cilia or flagella that give them the ability to move, examples of these are sperm, flagellate parasites such as trypanosomatids or hair cells present in respiratory epithelia.

Cilia and flagella are essentially composed of more or less stable arrangements of microtubules and project from the cytosol towards the plasma membrane.

The cilia are shorter, similar to hairs, while the flagella, as their name might indicate, are longer and thinner, specialized in cell movement.

Animal cell examples

There are multiple examples of animal cells in nature, among which are:

- Neurons, an example of a large neuron is the giant squid axon, which can measure up to 1 meter long and 1 millimeter wide.

Nerve cell (Source: User: Dhp1080 via Wikimedia Commons)

- The eggs that we consume, for example, are a good example of the largest cells, especially if we consider an ostrich egg.

- The skin cells, which make up the different layers of the dermis.

- All unicellular animals, such as flagellated protozoa that cause numerous diseases in man.

- The sperm cells of animals that have sexual reproduction, which have a head and a tail and have directed movements.

- Red blood cells, which are cells without a nucleus, or the rest of the blood cells, such as white blood cells. In the following image you can see red blood cells on a slide:

Animal cell types

In animals there is a wide cellular diversity. Next we will mention the most relevant types:

Blood cells

In the blood we find two types of specialized cells. Red blood cells or erythrocytes are responsible for the transport of oxygen to the different organs of the body. One of the most relevant characteristics of red blood cells is that, when mature, the cell nucleus disappears.

Inside the red blood cells is hemoglobin, a molecule capable of binding oxygen and transporting it. Erythrocytes are shaped like a disk. They are round and flat. Its cell membrane is flexible enough to allow these cells to cross narrow blood vessels.

The second cell type is white blood cells or leukocytes. Its function is completely different. They are involved in defending against infection, disease, and germs. They are an important component of the immune system.

Muscle cells

Muscles are made up of three cell types: skeletal, smooth, and cardiac. These cells allow movement in animals. As its name implies, skeletal muscle is attached to bones and contributes to their movements. The cells of these structures are characterized by being long like a fiber and by having more than one nucleus (polynucleated).

They are made up of two types of proteins: actin and myosin. Both can be visualized under the microscope as "bands". Because of these characteristics, they are also called striated muscle cells.

Mitochondria are an important organelle in muscle cells and are found in high proportions. Roughly in the hundreds.

For its part, smooth muscle constitutes the walls of the organs. Compared to skeletal muscle cells, they are smaller in size and have a single nucleus.

Finally, the cardiac cells are found in the heart. These are responsible for the beats. They have one or more nuclei and their structure is branched.

Epithelial cells

Epithelial cells cover the outer surfaces of the body and the surfaces of organs. These cells are flat and generally irregular in shape. Typical structures in animals, such as claws, hair and nails, are made up of clusters of epithelial cells. They are classified into three types: squamous, columnar, and cubic.

- The first type, the scaly, protects the body from the entry of germs, creating several layers on the skin. They are also present in the blood vessels and in the esophagus.

- The columnar is present in the stomach, intestines, pharynx and larynx.

- The cubic is found in the thyroid gland and in the kidneys.

Nerve cells

Nerve cells or neurons are the fundamental unit of the nervous system. Its function is the transmission of the nervous impulse. These cells have the peculiarity of communicating with each other. Three types of neurons can be distinguished: sensory, association and motor neurons.

Neurons are typically made up of dendrites, structures that give this cell type a tree-like appearance. The cell body is the area of ​​the neuron where the cell organelles are found.

The axons are the processes that extend throughout the body. They can reach quite long lengths: from centimeters to meters. The set of axons of various neurons make up the nerves.

Differences between animal cells and plant cells

There are certain key aspects that differentiate an animal cell from a plant. The main differences are related to the presence of cell walls, vacuoles, chloroplasts and centrioles.

Cellular wall

Cell wall structure

One of the most notable differences between the two eukaryotic cells is the presence of a cell wall in plants, a structure absent in animals. The main component of the cell wall is cellulose.

However, the cell wall is not unique to plants. It is also found in fungi and bacteria, although the chemical composition varies between groups.

In contrast, animal cells are bounded by a cell membrane. This characteristic makes animal cells much more flexible than plant cells. In fact, animal cells can take different forms, whereas cells in plants are rigid.

Vacuoles

Vacuoles are a kind of sacks filled with water, salts, debris or pigments. In animal cells, vacuoles are usually quite numerous and small.

In plant cells there is only a single large vacuole. This "sac" determines cell turgor. When filled with water, the plant looks plump. When the vacuole empties, the plant loses rigidity and withers.

Chloroplasts

Chloroplasts are membranous organelles present only in plants. Chloroplasts contain a pigment called chlorophyll. This molecule captures light and is responsible for the green color of plants.

A key plant process occurs in chloroplasts: photosynthesis. Thanks to this organelle, the plant can take sunlight and, through biochemical reactions, transform it into organic molecules that serve as food for the plant.

Animals do not have this organelle. For food they require an external carbon source found in food. Therefore, plants are autotrophs and animals heterotrophs. Like mitochondria, the origin of chloroplasts is thought to be endosymbiotic.

Centrioles

Centrioles are absent in plant cells. These structures are barrel-shaped and are involved in cell division processes. Microtubules are born from the centrioles, responsible for the distribution of chromosomes in daughter cells.

References

1. Alberts, B., Bray, D., Hopkin, K., Johnson, AD, Lewis, J., Raff, M.,… & Walter, P. (2013). Essential cell biology. Garland Science.
2. Cooper, GM, Hausman, RE, & Hausman, RE (2000). The cell: a molecular approach (Vol. 10). Washington, DC: ASM press.
3. Gartner, LP, & Hiatt, JL (2006). Color textbook of histology ebook. Elsevier Health Sciences.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). New York: McGraw-Hill.
5. Villanueva, JR (1970). The living cell.