CELL TYPES: PROKARYOTIC CELLS, EUKARYOTIC CELLS - BIOLOGY - 2025

Two types of cells make up all living organisms that we can identify in nature; these are known as prokaryotes and eukaryotes. The former are typical of some microorganisms, while the latter form multicellular organisms as complex as plants and animals.

Cells represent the basic basic unit of life, which has been known more or less since 1840. It is said that they are "basic units" since within each one the same processes that we recognize in "higher" or higher organisms take place. complex.

Scheme of the two types of cells in nature: eukaryotes and prokaryotes. The main parts are shown, showing the differences between them (Source: No machine-readable author provided. Mortadelo2005 assumed (based on copyright claims). Via Wikimedia Commons)

Thus, a cell is the smallest living being that can feed, metabolize, grow and reproduce, leaving offspring (a cell can only come from another pre-existing cell).

The size of the cells can vary greatly. If we consider the size of a small bacterium, which can measure just over 100 microns, and compare it with that of the neuron of an adult human being, which can measure up to 1 meter, we will find a difference of approximately 6 orders of magnitude.

However, since the processes that occur within them are similar, the different types of cells share many characteristics. For example, all are surrounded by a membrane that separates them from the environment around them and that allows the selective passage of substances from one side to the other.

The space surrounded by this membrane contains a kind of liquid or fluid called cytosol, in which are the intracellular components that make metabolism and reproduction possible, to name a few processes.

The cytosol of all cells contains (separated by internal membranes or not) the hereditary material composed of nucleic acids; large amount of structural proteins and with enzymatic activity; ions, carbohydrates and other molecules of different chemical nature.

Some cells have a cell wall that covers their plasma membrane and that provides them with a certain rigidity, support, and mechanical and chemical resistance. In addition, both prokaryotic and eukaryotic organisms can have structures such as cilia and flagella, which serve multiple purposes.

Prokaryotic cells

Prokaryotic cells are relatively simple cells. Its name comes from the Greek "pro", which means before, and "karyon" which means nucleus, and this is used to denote organisms with a primordial or "primitive" nucleus, which lack a membranous nucleus.

Prokaryotic organisms are bacteria and archaea. Bacteria represent one of the most important groups of living beings from an ecological and practical point of view (anthropocentrically speaking), as well as with respect to their abundance (number of individuals).

Diagram of an «average» prokaryotic cell (Source: Mariana Ruiz Villarreal (LadyofHats). Spanish labels by Alejandro Porto. Via Wikimedia Commons)

Archaea, abundant like bacteria, inhabit inhospitable and hostile places such as brines, volcanic springs or highly acidic and hot sites.

There are many differences between archaea and bacteria, but only the most distinctive characteristics of bacteria will be mentioned below, since they are the best known group.

- Characteristics

Prokaryotes have highly variable sizes and shapes, which fundamentally depend on the species and the way of life considered. Bacteria, for example, are morphologically distinguished into cocci and bacilli.

Cocci are nearly spherical shapes and can associate with each other to form cell aggregates (similar to a bunch of grapes) that are characteristic of some species.

The bacilli are rod-shaped, but their width and length are highly variable; These can also be associated with each other, forming chains similar to a “string” of chorizo.

Salmonella typhimurium (red) invading human cells. Author: Rocky Mountain Laboratories, NIAID, NIH By US gov (File: SalmonellaNIAID.jpg), via Wikimedia Commons

Prokaryotic cells have a large number of structures, which are responsible for carrying out all their vital processes. One of the characteristics that differentiates a bacterium from any eukaryotic cell is the absence of internal membranous structures.

In other words, bacteria lack cytosolic organelles like those found in eukaryotes (mitochondria, nucleus, endoplasmic reticulum, etc.).

- Parts of a prokaryotic cell

A bacterium; prokaryotic cell, unicellular organisms

The parts that can be distinguished in most prokaryotes are the plasma membrane, ribosomes, inclusion bodies, nucleoid region, periplasmic space, cell wall, capsule, fimbriae, and pili and flagella.

Plasma or cell membrane

The membrane that covers bacterial cells performs various functions as an interface between them and their environment. It is composed of lipids arranged in the form of a bilayer and some associated proteins that, together, form a structure no more than 10 nm thick.

The faces of the bilayer that "face" "in" and "out" of cells contain the hydrophilic portion of lipids, while their interior is highly hydrophobic. Associated proteins can be integral or peripheral, depending on the chemical nature of their association.

Prokaryotes do not have internal membranous structures, however, their plasma membranes can form invaginations or prominent folds in their interior and these fulfill different functions.

Cytoplasm

The cytoplasm is the space between the cell membrane and the nucleus; contains the cytosol. It is quite similar to the cytoplasm of eukaryotic cells.

Cytosol

The plasma membrane encloses a liquid substance known as cytosol. There are no cytoskeletal proteins or membranous organelles within this fluid, but “regions” with defined functions and specific components can be distinguished.

A good example of some "structures" associated with the cytosol of bacteria is that of inclusion bodies, which are granules composed of organic or inorganic material embedded in the cytosolic matrix.

Ribosomes and molecular chaperones

In the cytosol of a prokaryotic cell a large number of particles can be seen (sometimes associated with the plasma membrane) that are responsible for the synthesis of cellular proteins; These are known as ribosomes and are also found in eukaryotic cells, although they are larger in the latter.

In close association with ribosomes there are also proteins called molecular chaperones, responsible for collaborating with the folding of proteins synthesized by ribosomes.

The nucleoid

Prokaryotic cells normally possess a DNA molecule that makes up a double-stranded circular chromosome. This chromosome is not enclosed within a nucleus delimited by a membrane, but rather is packaged in a defined region of the cytosol.

This region is known as the nucleoid or nuclear region. This is the one that contains all the genetic information that defines the characteristics of a bacterium and the one that replicates at the time of cell division.

The cell wall of bacteria

All bacteria have a cell wall that surrounds the plasma membrane. This structure is very important for the survival of prokaryotes, as it gives them a certain resistance against osmotic lysis.

Depending on the characteristics of the cell wall, two large groups of bacteria have been distinguished: Gram Positive and Gram Negative.

The cell wall of Gram-positive bacteria is composed of a homogeneous layer of peptidoglycan (N-acetyl glucosamine and N-acetylmuramic acid) that surrounds the plasma membrane.

Gram-negative bacteria also have a peptidoglycan cell wall on the plasma membrane, but also have an additional outer membrane that surrounds them.

The space between the cell wall and the plasma membrane of both types of bacteria is called the periplasmic space, where a large number of enzymes and other proteins with important functions are housed.

Some bacteria, in addition to the cell wall, contain a layer of polysaccharides and glycoproteins that act to protect against desiccation or attack by pathogens such as bacteriophages; it also works in cell adhesion processes.

Plasmids

Plasmids are circular structures of DNA. They are carriers of genes that are not involved in reproduction.

Capsule

It is found in some bacterial cells and helps retain moisture, assists the cell to adhere to surfaces and nutrients. It is an additional outer coating that protects the cell when it is absorbed by other organisms.

Pili

Prokaryotic cells also have external structures known as "pili" that are a kind of "hairs" on the surface of these cells and that often play important roles in the exchange of genetic information between bacteria.

Genetic material (DNA and RNA)

Prokaryotic cells have large amounts of genetic material in the form of DNA and RNA. Because prokaryotic cells lack a nucleus, the cytoplasm contains the only large circular strand of DNA that contains most of the genes necessary for cell growth, reproduction, and survival.

Eukaryotic cells

Example of a eukaryotic cell (animal cell) and its parts (Source: Alejandro Porto via Wikimedia Commons)

Eukaryotic cells make up most of the organisms we see in nature. Eukaryotes are yeasts and other single-celled fungi, giant trees like sequoias, and majestic mammals like blue whales.

Compared to prokaryotic cells, eukaryotic cells are considerably larger and more complex, as they have a large number of internal organelles and complex membranous systems embedded in their cytosol.

The word "eukaryote" comes from the Greek "eu", which means true and "karyon", which means nucleus and is used to name cells that have a "true nucleus", delimited by a membrane.

- Characteristics

Animals, plants, fungi, and some single-celled organisms such as amoebae and yeast are made up of eukaryotic cells.

With their differences, the cells that make up these organisms have a complex internal organization: they have a membranous nucleus and a great diversity of internal organelles, separate membranes.

- Parts of a eukaryotic cell

Cytoplasm

It is located between the plasma membrane and the nucleus, within it are the organelles and the cytoskeleton. The spaces that are contained by the membranes of the organelles constitute the intracellular microcompartments.

Plasma membrane

Eukaryotic cell nucleus

The nucleus is the most prominent and characteristic intracellular organelle of a eukaryotic cell. It is the "container" where the genetic material (nucleic acids) is enclosed in close association with proteins called "histones", which form eukaryotic chromosomes.

This organelle is delimited by the nuclear envelope, which corresponds to a pair of concentric membranes that separate the nuclear components from the rest of the cytosol and which has important functions from the point of view of gene expression.

Mitochondria

Mitochondria

The cytosol of a eukaryotic cell also has other very important membranous organelles, responsible for the generation of the energy that can be used by the cell: the mitochondria.

Thanks to these organelles, living organisms have the ability to live in the presence of oxygen.

Mitochondria are "rod-shaped" structures, similar to a bacterium (consult the endosymbiotic theory), they have their own genome, so they replicate almost independently of the cell that harbors them, and they have two membranes, one highly folded internal and an external one., which faces the cytosol.

A constant exchange of metabolites and information occurs between the mitochondria, the cytosol and some of the membranous organelles of eukaryotic cells, which are essential for the cell to function.

Ribosomes

They are essential structures for protein synthesis. They are made up of ribosomal RNA and proteins. Ribosomes serve to make proteins.

Chloroplasts

Chloroplast

Plants, algae and cyanobacteria, in addition to mitochondria, have organelles (plastids) specialized in photosynthesis. These contain numerous invaginations and internal membranous processes, which are rich in specific pigments and enzymes.

Rough endoplasmic reticulum (RER)

It is an area of ​​the reticulum that has ribosomes associated with the organelle membrane. In it, proteins are modified and synthesized. Its main function is to produce proteins that act outside the cell or inside a vesicle.

Smooth endoplasmic reticulum (REL)

This region of the reticulum does not have ribosomes, so its smooth appearance is responsible for synthesizing lipids and steroids.

Golgi complex or apparatus

The Golgi complex is defined as a "stack of flattened sacs" that are covered by a membrane. It is one of the modification sites of the proteins synthesized in the endoplasmic reticulum and is responsible for their distribution to other regions of the cell and to the exterior.

Endosomes

Endosomes can be described as compartments limited by a membrane that is part of endocytosis mechanisms. The main function is the classification of the proteins that are sent through the vesicles and forwarded to their final destinations, which would be various cell compartments.

Lysosomes

Lysosomes are small organelles and are responsible for the intracellular digestion of "obsolete" proteins, releasing nutritious compounds to the cytosol.

Perosixomas

Peroxisomes, on the other hand, are primarily responsible for the degradation of reactive oxygen species and are also involved in the oxidation of fatty acids.

In some parasitic microorganisms there are modified and specialized peroxisomes for glucose catabolism, which is why they are known as glycosomes.

Vacuoles

Plant cells commonly have a vacuole, which are large organelles of great importance for the growth and development of plants, since they occupy more than 80% of the total volume of cells, contain water and have a known endomembrane system like the toneplast.

Cytoskeleton

Another aspect that differentiates eukaryotic cells from prokaryotes is the presence of a network of internal filamentous proteins that form a kind of scaffold in the cytosol.

This "scaffold" contributes not only to the mechanical stability of cells, but also has important functions for intracellular communication, internal transport, and cell movements.

Microtubules

It is part of the elements of the cytoskeleton along with filaments. They can lengthen and shorten, which is known as dynamic instability.

- Cilia and flagella

As is true for bacteria, many eukaryotic, animal and plant cells, have external structures composed of microtubules that function especially in locomotion and movement.

Flagella are structures up to 1 mm long, while cilia can be 2 to 10 microns in length. These structures are abundant in microorganisms and in small multicellular organisms.

In animals and plants there are also cells with cilia and flagella. Such is the case of the flagella of sperm cells and the cilia that line the cell surfaces that make up the internal epithelia of some organs.

Centrioles

Centrioles are hollow, cylinder-shaped structures that are made up of microtubules. Its derivatives generate the basal bodies of the cilia, and they only appear in animal-type cells.

Filaments

They can be classified into actin filaments and intermediate filaments. Actin cells are flexible filaments of actin molecules and intermediates are rope-like fibers that form from different proteins.

Proteasomes

They are the protein complexes that enzymatically degrade damaged proteins.

References

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