Eukaryotic Cell

We explain what a eukaryotic cell is, the types that exist, its parts and functions. Also, its differences with a prokaryotic cell.

What is a eukaryotic cell?

It is called a eukaryotic cell (from the Greek word eukaryotaunion of eu “true” and karyon “nut, kernel”) to any cell that has a defined nucleus. This nucleus contains most of your DNA and is bounded by a nuclear envelope.

This is the main difference with respect to the prokaryotic cell, which is much more primitive, and whose genetic material is organized in the cytoplasm in a region called “nucleoid”.

The eukaryotic domain includes the kingdoms Animalia (animals), Plantae (plants), Fungi (fungi), and Protistas (organisms that are not animals, plants, or animals). Living beings made up of eukaryotic cells are called eukaryotes.

Origin of eukaryotic cells

The appearance of eukaryotic cells was an important step in the evolution of life, since laid the foundation for much greater biological diversity including the emergence of specialized cells within multicellular organizations.

The scientific community has not been able to find a concrete and clear explanation of how eukaryotic cells appeared. Some theories have been put forward about the emergence of these cells:

• It is believed that eukaryotic cells arose due to the fusion between a bacteria (prokaryotic single-celled organism that has a peptidoglycan cell wall) and an archaea (prokaryotic single-celled organism that has a cell wall of glycoproteins and proteins). This is the most accepted theory, since it has been proven that in eukaryotic cells some genes come from bacteria and others from archaea. In this sense, the DNA of the nucleus of eukaryotic cells is similar to that of archaea, while the composition of the membrane and mitochondria is similar to that of bacteria.
• It is assumed that eukaryotic cells arose from archaea, but their similarities to bacteria were obtained from proto-mitochondria (an ancestor of today's mitochondria).
• It is proposed that eukaryotes and archaea arose from a modified bacteria.

It is not well understood why it took one billion years from the origin of eukaryotic cells to their specialization. It is believed that during this period of time (in which there was no evolutionary change) oxygen levels were not sufficient for the development of eukaryotes.

See also: Cellular reproduction

Types of eukaryotic cell

There are various types of eukaryotic cells:

• plant cells. They are those cells that have a cell wall (composed of cellulose and proteins) that covers their plasma membrane and gives them rigidity, protection and resistance. In addition, plant cells have chloroplasts, which are organelles that contain chlorophyll (the biomolecule necessary to carry out the photosynthesis process); and a large central vacuole, which maintains cell shape and controls the storage and degradation of substances.
• animal cells. They are those cells that do not have chloroplasts (since they do not carry out photosynthesis) or cell wall. But, unlike plant cells, they have centrioles (organelles that participate in cell division) and have smaller, but more abundant, vacuoles called vesicles. Due to the lack of a cell wall, animal cells can take on a large number of shapes.
• fungal cells. They are cells that resemble those of animals, although they differ from them due to the presence of a cell wall composed of chitin.
• Protist cells. Protists are very varied organisms: they are not animals, plants or fungi but, at the same time, they have characteristics similar to those of all these organisms. So, the cells of protists are also very varied. A characteristic of these cells is that they have a vacuole that contracts, which allows them to control the amount of water in the cell. Additionally, protist cells may contain chloroplasts and cellulose.

Vital functions of the eukaryotic cell

The vital functions of the eukaryotic cell are:

• Nutrition. It is the process by which nutrients are incorporated into the cell. The cell transforms these nutrients into other substances, which are used to form and replenish cellular structures and also to obtain the energy necessary to carry out all its functions. Organisms can be classified according to their type of nutrition into:
  • Autotrophs. They produce the organic substances they need for their development from inorganic substances. For example: plants.
  • Heterotrophs They consume organic substances from other organisms. For example: animals.
• Growth. It involves an increase in the size of an organism's individual cells, the number of cells, or both. Growth may be uniform across the various parts of an organism or may be greater in some parts than others, causing the proportions of the body to change as growth occurs.
• Response to stimuli. Cells relate to the environment that surrounds them. This relationship occurs through stimuli that generate a response. These stimuli (such as changes in temperature, changes in acidity, humidity) generate responses in cells that produce different effects in an organism (for example, sweating, tremors or contractions).
• Reproduction. It is the process of forming new cells (or daughter cells) from an initial cell (or mother cell). There are two types of cell reproduction processes: mitosis and meiosis.
  • Through mitosis a mother cell gives rise to two identical daughter cells, that is, with the same amount of genetic material and identical hereditary information. Mitosis is involved in the processes of tissue growth and repair, and in the reproduction of living beings that reproduce asexually.
  • Through meiosis a mother cell gives rise to four daughter cells that are genetically different from each other and which also have half the genetic material of the initial cell. Meiosis occurs to produce gametes (reproductive cells, eggs and sperm).
• Metabolism. Chemical reactions occur in cells that are necessary to obtain the energy that allows the performance of different cellular functions. In mitochondria, for example, cellular respiration occurs, which is the set of chemical reactions that degrade chemical compounds (such as glucose) to generate energy.

The functions of metabolism, growth, response to stimuli and reproduction are carried out by all cells belonging to both prokaryotic and eukaryotic organisms. However, these are not the only cellular functions: there are other functions depending on the specialization of each type of cell and the tissue or organism they form. For example, neurons (which are part of nervous tissue) are capable of communicating through electrical impulses, while many cells in the respiratory tract of animals sweep away foreign particles in mucus.

Parts of a eukaryotic cell

The main parts of eukaryotic cells are:

• Cell membrane, plasma or cytoplasmic. It is a membrane that surrounds the cell. It is made up of phospholipids and intercalated proteins, among other compounds. The plasma membrane serves to give shape to the cell, delimits the outside and inside of the cell and regulates the substances that enter and leave it.
• cell wall. It is a rigid layer that is found outside the plasma membrane and gives the cell shape, support and protection. The cell wall is present only in plant and fungal cells, although its composition varies between both cell types: in plants it is made up of cellulose and proteins, while in fungi it is made up of chitin. Although this structure provides protection to the cell, it prevents its growth and limits it to fixed structures.
• cell nucleus. It is a central organelle, limited by a porous envelope that allows the exchange of material between the cytoplasm and its interior. The nucleus contains the genetic material (DNA) of the cell, which is organized into chromosomes. Additionally, within the nucleus there is a specialized region called the nucleolus, where the ribosomal RNA is transcribed, which will later form part of the ribosomes. The nucleus is present in all eukaryotic cells.
• Cytoplasm . It is the aqueous medium in which the different organelles of the cell are immersed. The cytoplasm is made up of the cytosol (which is the watery part that contains dissolved substances) and organelles (which are structures that have different specialized functions).

The different organelles or organelles are immersed in the cytoplasm. Some of the main ones are:

• Lysosomes They are vesicles that contain digestive enzymes, present exclusively in animal cells. Cellular digestion processes are carried out in lysosomes, catalyzed by the enzymes they contain inside. Lysosomes can digest another organelle to reuse its individual components by the cell, called “autophagy,” or they can also digest an entire cell, called “autolysis.” These organelles are formed in the Golgi apparatus.
• Mitochondria They are the organelles where the cellular respiration process takes place. They are surrounded by a double membrane, which serves as a surface for cellular respiration reactions to occur. Mitochondria are present in all types of eukaryotic cells and their number varies depending on their needs: cells with high energy requirements usually have a greater number of mitochondria.
• Chloroplasts. They are the organelles in which photosynthesis takes place, and they have a complex system of membranes. They are mainly composed of chlorophyll, a green pigment that participates in the photosynthetic process and allows us to capture sunlight. Chloroplasts are exclusive to photosynthetic cells, which is why they are present in all plants and algae, whose characteristic green color is given by the presence of chlorophyll.
• vacuole. It is a type of large vesicle that stores water, mineral salts and other substances, and is found only in plant cells. The vacuole maintains the cell shape and provides support to the cell, in addition to participating in the intracellular movement of substances. Animal cells have vacuoles but smaller and in greater quantity.
• Centrioles. They are tubular structures found exclusively in animal cells. They participate in the separation of chromosomes during the process of cell division.
• Endoplasmic reticulum (ER). It is a system of membranes that extends from the cell nucleus. This organelle is divided into two structures:
  • rough endoplasmic reticulum (RER). It is located after the nuclear membrane. On the surface of the RER are ribosomes, which are the organelles where the synthesis of proteins occurs that are used by other organelles or exported to the outside of the cell.
  • Smooth endoplasmic reticulum (SER). Proteins are not synthesized in this organelle because it does not contain ribosomes, but fatty acids and steroids are synthesized.
• Golgi apparatus. It is an organelle composed of a set of flattened discs and sacs called “cisternae.” The function of the Golgi apparatus is related to the modification and packaging of proteins and other biomolecules (such as carbohydrates and lipids) for secretion or transport.
• Ribosomes. They are made up of two units that are formed in the nucleolus and assembled in the cytoplasm. They are the organelles where protein synthesis occurs.
• Centrosome. It is present in eukaryotic cells of animals. This organelle is made up of centrioles and pericentriolar material and is very important in the cell division process.
• Cytoskeleton. It is present in eukaryotic cells. It is formed by microfilaments composed of actin and myosin, intermediate filaments composed of keratin, and microtubules composed of tubulin. Its function is to maintain the shape of the cell, give it mechanical stability, and contribute to the movement of organelles and the cell as a whole.
• Leucoplasts. They are present in eukaryotic plant cells. Its main function is to participate in the conversion of sugars into polysaccharides, fats and proteins.

Difference between eukaryotic cell and prokaryotic cell

The main differences between eukaryotic cells and prokaryotic cells are:

References

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