The eukaryotic cells are structural components of a broad line of organisms characterized by having cells with a core delimited by a membrane and having a set of organelles.
Among the most prominent organelles of eukaryotes we have the mitochondria , responsible for cellular respiration and other pathways related to the generation of energy, and chloroplasts , found in plants and responsible for the photosynthetic process.
In addition, there are other structures limited by membranes such as the Golgi apparatus , the endoplasmic reticulum , vacuoles , lysosomes , peroxisomes , among others, that are unique to eukaryotes.
The organisms that are part of eukaryotes are quite heterogeneous, both in size and morphology. The group ranges from unicellular protozoa and microscopic yeasts to plants and large animals that inhabit the deep sea.
Eukaryotes differ from prokaryotes mainly by the presence of the nucleus and other internal organelles, in addition to having a high organization of genetic material. It can be said that eukaryotes are much more complex in different aspects, both structural and functional.
The most important characteristics that define a eukaryotic cell are: the presence of a defined nucleus with the genetic material ( DNA ) inside, the subcellular organelles that perform specific tasks, and the cytoskeleton .
Thus, some lineages have special characteristics. For example, plants have chloroplasts, a large vacuole, and a thick wall of cellulose. In fungi, the chitin wall is characteristic. Lastly, animal cells have centrioles .
Similarly, there are eukaryotic unicellular organisms within protists and fungi.
One of the distinctive characteristics of eukaryotes is the presence of organelles or subcellular compartments surrounded by a membrane. Among the most conspicuous we have:
The nucleus is the most conspicuous structure in eukaryotic cells. It is delimited by a double porous lipid membrane that allows the exchange of substances between the cytoplasm and the nuclear interior.
It is the organelle in charge of coordinating all cellular processes, since it contains all the necessary instructions in the DNA that allows an immense variety of processes to be carried out.
The nucleus is not a perfectly spherical and static organelle with DNA randomly dispersed within it. It is a structure of exquisite complexity with different components such as: the nuclear envelope, the chromatin and the nucleolus .
There are also other bodies inside the nucleus such as the Cajal bodies and the PML bodies ( promyelocytic leukemia ).
Mitochondria are organelles surrounded by a double membrane system and are found in both plants and animals. The number of mitochondria per cell varies according to its needs: in cells with high energy requirements the number is relatively higher.
The metabolic pathways that take place in the mitochondria are: the citric acid cycle, electron transport and oxidative phosphorylation, beta oxidation of fatty acids, and the breakdown of amino acids .
Chloroplasts are typical organelles of plants and algae, presenting complex membrane systems. The most important constituent is chlorophyll, a green pigment that participates directly in photosynthesis.
In addition to the reactions associated with photosynthesis, chloroplasts can generate ATP, synthesize amino acids, fatty acids, among others. Recent studies have shown that this compartment is related to the production of substances against pathogens.
Like mitochondria, chloroplasts have their own genetic material, in a circular shape. From the evolutionary point of view, this fact is evidence that supports the theory of the possible endosymbiotic process that gave rise to mitochondria and chloroplasts.
The reticulum is a system of membranes that continues with the nucleus and that extends throughout the cell in the form of a labyrinth.
It is divided into smooth endoplasmic reticulum and rough endoplasmic reticulum , depending on the presence of ribosomes on it. The rough reticulum is primarily responsible for protein synthesis – thanks to anchored ribosomes. The smooth, for its part, is related to the metabolic pathways of lipids
It consists of a series of flattened discs called “Golgian cisterns.” It is related to the secretion and modification of proteins. It also participates in the synthesis of other biomolecules , such as lipids and carbohydrates .
In 1980, the researcher Carl Woese and collaborators managed to establish the relationships between living beings using molecular techniques. Through a series of pioneering experiments, they succeeded in establishing three domains (also called “super realms”) leaving behind the traditional view of the five realms.
According to Woese’s results, we can classify the living forms of the earth into three conspicuous groups: Archaea, Eubacteria and Eukarya.
In the Eukarya domain are the organisms that we know as eukaryotes. This lineage is widely diverse and encompasses a number of both unicellular and multicellular organisms.
Unicellular eukaryotes are extremely complex organisms, since they must possess in a single cell all the typical functions of a eukaryote. Protozoa are historically classified as rhizopods, ciliates, flagellates, and sporozoans.
As most prominent examples we have the euglena: photosynthetic species capable of moving through a flagellum.
There are also ciliated eukaryotes, such as the famous paramecia belonging to the genus Paramecium. These have a typical slipper shape and move thanks to the presence of numerous cilia.
In this group there are also pathogenic species of humans and other animals, such as the genus Trypanosoma. This group of parasites is characterized by having an elongated body and a typical flagellum. They are the cause of Chagas disease ( Trypanosoma cruzi ) and sleeping sickness ( Trypanosoma brucei ).
The genus Plasmodium is the causative agent of malaria or malaria in humans. This disease can be fatal.
There are also unicellular fungi, but the most outstanding characteristics of this group will be described in later sections.
All the great complexity of plants that we observe daily belongs to the eukaryotic lineage, from grasses and grasses to complex and large trees.
The cells of these individuals are characterized by having a cell wall composed of cellulose, which gives rigidity to the structure. In addition, they have chloroplasts that contain all the biochemical elements necessary for the photosynthetic process to occur.
Plants represent a highly diverse group of organisms, with complex life cycles that would be impossible to encompass in just a few characteristics.
The term “fungus” is used to designate different organisms such as molds, yeasts and individuals that are capable of producing mushrooms.
Depending on the species, they can reproduce sexually or asexually. They are characterized mainly by the production of spores: small latent structures that can develop when environmental conditions are suitable.
You might think that they are similar to plants, since both are characterized by leading a sessile way of life, that is, they do not move. However, fungi lack chloroplasts and do not have the necessary enzymatic machinery to carry out photosynthesis.
Their way of feeding is heterotrophic , like most animals, so they must look for an energy source.
The animals represent a group made up of almost a million correctly cataloged and classified species, although zoologists estimate that the true value could be closer to 7 or 8 million. They are as diverse a group as those mentioned above.
They are characterized by being heterotrophic (they look for their own food) and have a remarkable mobility that allows them to move. For this task they have a series of varied locomotion mechanisms that allow them to move on land, water and air.
Regarding their morphology, we find incredibly heterogeneous groups. Although we could make a division into invertebrates and vertebrates , where the characteristic that distinguishes them is the presence of the vertebral column and the notochord.
Among the invertebrates we have porifers, cnidarians, annelids, nematodes, flatworms, arthropods, mollusks and echinoderms. While vertebrates include better known groups such as fish, amphibians, reptiles, birds, and mammals.
Eukaryotic cell types
There is a great diversity of eukaryotic cells. Although you might think that the most complex are found in animals and plants, this is incorrect. The greatest complexity is observed in protist organisms, which must have all the elements required for life confined within a single cell.
The evolutionary path that led to the appearance of multicellular organisms brought with it the need to distribute tasks within the individual, which is known as cell differentiation . Thus, each cell is responsible for a series of limited activities and has a morphology that allows it to carry them out.
As the process of gamete fusion or fertilization occurs, the resulting zygote undergoes a series of subsequent cell divisions that will lead to the formation of more than 250 cell types.
In animals, the differentiation pathways followed by the embryo are directed by signals it receives from the environment and largely depends on its position in the developing organism. Among the most prominent cell types we have:
The neurons or cells specialized in the conduction of the nervous impulse that are part of the nervous system .
Skeletal muscle cells that possess contractile properties and are aligned in a network of filaments. These allow the typical movements of animals such as running or walking.
Cartilage cells specialize in support. For this reason they are surrounded by a matrix that has collagen.
The cellular components of blood are red and white blood cells , and platelets. The former are disc-shaped, lack a nucleus when mature and have the function of transporting hemoglobin. White blood cells participate in the immune response and platelets in the blood clotting process.
Eukaryotes present a series of metabolic pathways such as glycolysis , pentose phosphate pathways, beta oxidation of fatty acids, among others, organized in specific cellular compartments. For example, ATP is generated in the mitochondria.
Plant cells have a characteristic metabolism, since they have the enzymatic machinery necessary to take in sunlight and generate organic compounds. This process is photosynthesis and turns them into autotrophic organisms that can synthesize the energetic components required by their metabolism.
Plants have a specific pathway called the glyoxylate cycle that occurs in the glyoxysome and is responsible for the conversion of lipids into carbohydrates.
The animals and fungi are characterized by heterotrophs. These lineages are unable to produce their own food, so they must actively seek it out and degrade it.
Differences with prokaryotes
The crucial difference between a eukaryote and a prokaryote is the presence of a nucleus bounded by a membrane and defined in the first group of organisms.
We can reach this conclusion by examining the etymology of both terms: prokaryote comes from the roots pro which means “before” and karyon which is nucleus; while eukaryote refer to the presence of a “true nucleus” ( eu meaning “true” and karyon meaning nucleus)
However, we find unicellular eukaryotes (that is, the entire organism is a single cell) such as the well-known Paramecium or yeasts. In the same way, we find multicellular eukaryotic organisms (made up of more than one cell) like animals, including humans.
According to the fossil record, it has been possible to conclude that eukaryotes evolved from prokaryotes. Therefore, it is logical to assume that both groups have similar characteristics such as the presence of a cell membrane , common metabolic pathways, among others. The most conspicuous differences between the two groups will be described below:
Eukaryotic organisms are usually larger in size than prokaryotes, since they are much more complex and with more cellular elements.
On average, the diameter of a prokaryote is between 1 and 3 µm, while a eukaryotic cell can be in the order of 10 to 100 µm. Although there are notable exceptions to this rule.
Presence of organelles
In prokaryotic organisms there are no structures delimited by a cell membrane. These are extremely simple and lack these internal bodies.
Normally, the only membranes that prokaryotes have are in charge of delimiting the organism with the external environment (note that this membrane is also present in eukaryotes).
As mentioned above, the presence of a nucleus is a key element to discriminate between both groups. In prokaryotes, the genetic material is not delimited by any type of biological membrane.
In contrast, eukaryotes are cells with a complex interior structure and, depending on the cell type, present the specific organelles that were described in detail in the previous section. These cells usually have a single nucleus with two copies of each gene – as in most cells in humans.
In eukaryotes, DNA (deoxyribonucleic acids) is highly organized at different levels. This long molecule is associated with proteins, called histones, and is compacted to such a level that it is able to enter a small nucleus, which can be observed at a certain point in cell division as chromosomes.
Prokaryotes do not have such sophisticated levels of organization. Generally, genetic material occurs as a single circular molecule that can adhere to the biomembrane that surrounds the cell.
However, the DNA molecule is not randomly distributed. Although it is not wrapped in a membrane, the genetic material is located in a region called the nucleoid.
Mitochondria and chloroplasts
In the specific case of mitochondria, these are cellular organelles where the proteins necessary for cellular respiration processes are found. Prokaryotes – which must contain these enzymes for oxidative reactions – are anchored in the plasma membrane.
Likewise, in such a case that the prokaryotic organism is photosynthetic, the process takes place in the chromatophores.
Ribosomes are the structures responsible for translating the messenger RNA into the proteins that the molecule encodes. They are quite abundant, for example a common bacterium, such as Escherichia coli, can possess up to 15,000 ribosomes.
Two units that make up the ribosome can be distinguished : a major and a minor. The prokaryotic lineage is characterized by presenting 70S ribosomes, composed of the large 50S subunit and the small 30S subunit. In contrast, in eukaryotes they are composed of a large 60S and a small 40S subunit.
In prokaryotes, ribosomes are scattered throughout the cytoplasm. While in eukaryotes they are anchored to membranes, as in the rough endoplasmic reticulum.
The cytoplasm in prokaryotic organisms has a mostly granular appearance, thanks to the presence of ribosomes. In prokaryotes, DNA synthesis occurs in the cytoplasm.
Cell wall presence
Both prokaryotic and eukaryotic organisms are delimited from their external environment by a double lipidic biological membrane. However, the cell wall is a structure that surrounds the cell and is only present in the prokaryotic lineage, in plants and in fungi.
This wall is rigid and the most intuitive general function is to protect the cell from environmental stress and possible osmotic changes. However, at the compositional level this wall is totally different in these three groups.
The wall of bacteria is composed of a compound called peptidoglycan, formed by two structural blocks linked by β-1,4 type bonds: N-acetyl-glucosamine and N-acetylmuramic acid.
In plants and fungi – both eukaryotes – the composition of the wall also varies. In the first group it is cellulose, a polymer formed by repeating units of the sugar glucose, while fungi have walls of chitin and other elements such as glycoproteins and glycans. Note that not all fungi have a cell wall.
The genetic material between eukaryotes and prokaryotes varies not only in the way it is compacted, but also in its structure and quantity.
Prokaryotes are characterized by having low amounts of DNA, between 600,000 base pairs up to 8 million. That is, they can code from 500 to a few thousand proteins.
Introns (DNA sequences that do not code for proteins and are disrupting genes) are present in eukaryotes and not in prokaryotes.
Horizontal gene transfer is a significant process in prokaryotes, while in eukaryotes it is practically absent.
Cell division processes
In both groups, the cell volume increases until it reaches an adequate size. Eukaryotes carry out division by a complex process of mitosis , which results in two daughter cells of similar size.
The function of mitosis is to ensure an appropriate number of chromosomes after each cell division.
An exception to this process is the cell division of yeasts, particularly of the genus Saccharomyces , where the division leads to the generation of a smaller daughter cell, since it is formed by means of a “bulge”.
Prokaryotic cells do not undergo mitosis cell division – an intrinsic consequence of the lack of a nucleus. In these organisms the division occurs by binary division. Thus, the cell grows and divides into two equal parts.
There are certain elements that participate in cell division in eukaryotes, such as centromeres. In the case of prokaryotes, there are no analogues to these and only a few species of bacteria have microtubules . Reproduction of the sexual type is common in eukaryotes and rare in prokaryotes.
Eukaryotes have a very complex organization at the cytoskeleton level. This system is made up of three types of filaments classified by their diameter into microfilaments, intermediate filaments and microtubules. In addition, there are proteins with motor properties associated with this system.
Eukaryotes have a series of processes that allow the cell to move in its environment. These are the flagella, whose shape is reminiscent of a whip and the movement is different in eukaryotes and prokaryotes. Cilia are shorter and generally present in large numbers.
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