What is a eukaryotic cell?
Eukaryotic cells (from the Greek word Eukaryota, the union of EU – “true” and karyon “nut, nucleus”) are called all cells in whose cytoplasm a precisely defined nucleus can be found that contains most of their genetic material (DNA ). In this, they differ from prokaryotic cells, which are much more primitive and whose genetic material is distributed in the cytoplasm. In addition, unlike prokaryotes, eukaryotic cells have organelles or organelles, specialized subcellular structures that can be identified within membranes (e.g. mitochondria and chloroplasts) and are delimited by them.
The appearance of eukaryotic cells marked an important step in the evolution of life as it laid the foundation for much greater biodiversity, including the emergence of certain cells within multicellular organizations, creating higher kingdoms: protists, fungi, plants, and animals. The living things formed by eukaryotic cells are called eukaryotes.
Although the scientific community does not doubt the relevance of the appearance of eukaryotic cells, it has not yet been possible to give a very clear explanation for their formation. The most widely accepted theory raises the possible symbiogenesis between two prokaryotes, that is, a symbiotic process between a bacterium and archaea which, if coexisted in a very close way, would form the same organism over the generations, so dependent that they made each other. This theory about the origin of eukaryotic cells was proposed in 1967 by the American evolutionary biologist Lynn Margulis and is known as the endosymbiotic theory or the theory of serial endosymbiosis.
Eukaryotic cell types
There are different types of eukaryotic cells, but basically, four are recognized, each with different structures and processes:
- Plant cells. They have a cell wall (made up of cellulose and proteins) that covers their plasma membrane, giving them rigidity, protection, and durability. Also, plant cells have chloroplasts, organelles that contain the chlorophyll needed to carry out the photosynthetic process. Another feature of this cell type is the presence of a large central vacuole that maintains the cell shape and controls the movement of molecules in the cytoplasm.
- Animal cells. They don’t have chloroplasts (because they don’t photosynthesize) or a cell wall. In contrast to plant cells, however, they have centrioles (organelles that are involved in cell division) and smaller, albeit more frequent, vacuoles. Due to the lack of a cell wall, animal cells can take on a large number of variable shapes and even devour other cells.
- Cells of fungi. They are similar to animal cells but differ from them in the presence of a cell wall made of chitin (which animal cells do not have). Another differentiator is that fungal cells have a lower cellular definition than animal cells. While not the most common, there are unicellular fungi like yeast.
- Cells of unicellular eukaryotic organisms. Eukaryotic cells are often part of multicellular organisms. However, some protists are eukaryotic unicellular organisms. Although unicellular eukaryotes are simpler beings than animals and plants, the fact that they are made up of a single cell that has to perform all of the functions of the organism implies a complex organization of the cell. Also, they can reach macroscopic sizes. Some examples of this type of organism are the Euglena and the Paramecia.
Functions of the eukaryotic cell
Like prokaryotes, eukaryotic cells fulfill three essential functions: nutrition, relationship with the environment, and reproduction. Let’s see what each of them is made of.
- Nutrition. It involves the incorporation of nutrients inside the cell and their conversion into other substances that are used to form and replace cell structures, as well as to generate the energy necessary to perform all of their functions. Depending on their diet, cells can be autotrophic (they produce their food from inorganic substances through processes such as photosynthesis) or heterotrophic (they have to incorporate organic substances because they are unable to produce them).
- Relationship with the environment. Cells related to the environment around them, receive different stimuli (e.g. fluctuations in temperature, humidity, or acidity) and develop the corresponding responses to each of them (e.g. contraction or translation). This ability to respond to environmental stimuli is known as irritability.
- Reproduction. It is the process of forming new cells (or daughter cells) from an initial cell (or stem cell). There are two types of cell reproduction processes: mitosis and meiosis. Mitosis creates two identical daughter cells from one stem cell, ie with the same amount of genetic material and identical hereditary information. On the other hand, a stem cell creates four daughter cells through meiosis, which are genetically different from one another and also contain half of the genetic material of the parent cell. Mitosis intervenes in the processes of growth, tissue repair, and reproduction in living things that reproduce asexually. Meiosis has a different goal: it only happens that gametes arise.
The functions of nutrition, relationship, and reproduction are carried out by all cells belonging to both prokaryotic and eukaryotic organisms. However, these are not the only cellular functions: others depend on each cell type and the tissue or organism to which they belong. For example, neurons (which are part of nerve tissue) can communicate using electrical impulses.
Parts of a eukaryotic cell
Let’s see what are the main components that eukaryotic cells have and in what type of cell they are found:
- Cell or plasma membrane. It is a double barrier made of lipids and proteins that surrounds and limits the cell to isolate it from the environment that surrounds it. The plasma membrane has a selective permeability: it allows only the entry of necessary substances into the cytoplasm and also the expulsion of metabolic waste. This structure is present in all eukaryotic cells and even in prokaryotes.
- Cell wall. It is a rigid structure that resides outside the plasma membrane that gives shape, support, and protection to the cell. The cell wall is only present in plant and fungal cells, although its composition varies between the two cell types: in plants, it consists of cellulose and proteins, while in fungi it consists of chitin. Although this structure protects the cell, it prevents its growth and confines it to solid structures.
- Cell nucleus. It is a central organelle bounded by a double-porous membrane that allows the material to be exchanged between the cytoplasm and its interior. The nucleus houses the cell’s genetic material (DNA), which is organized into chromosomes. Also, within the nucleus, there is a specialized region called the nucleolus where ribosomes are formed. The nucleus is present in all eukaryotic cells.
- Ribosomes. They are structures that consist of RNA and proteins and in which protein synthesis takes place. Ribosomes are found in all cell types, including prokaryotes (although they are minor). Some ribosomes are free in the cytoplasm and others are attached to the rough endoplasmic reticulum.
- Cytoplasm. It is the aqueous medium in which the various organelles of the cell are located. The cytoplasm consists of the cytosol, the organelle-free aqueous part that contains dissolved substances, and the cytoskeleton, a network of filaments that form the cell.
In addition to the presence of the nucleus, one of the characteristic features of the eukaryotic cell is the presence of organelles, or subcellular compartments, surrounded by a membrane and having special functions. Let’s take a look at some of them:
- Lysosomes are a special type of vesicle that is filled with digestive enzymes and is only found in animal cells. Cell digestion processes are carried out in lysosomes, which are catalyzed by the enzymes they contain.
- Mitochondria. They are the organelles in which the cell respiration process takes place. They are surrounded by a double membrane that enables the cell to receive the energy it needs to perform its functions. Mitochondria are found in all types of eukaryotic cells and their number varies according to needs: cells with high energy requirements tend to have a larger number of mitochondria.
- Chloroplasts. They are the organelles where photosynthesis takes place and they present a complex membrane system. The basic component of these organelles is chlorophyll, a green pigment that is involved in the photosynthetic process and enables sunlight to be captured. Chloroplasts are exclusively reserved for plant cells, which is why they are present in all plants and algae whose characteristic green color is given by the presence of chlorophyll.
- Large vacuole. It is a type of large vesicle that stores water, mineral salts, and other substances and is found exclusively in plant cells. The vacuole maintains the cell shape and supports the cell in addition to participating in the intracellular movement of substances. Animal cells have vacuoles, but they are smaller and in greater numbers.
- Centrioles. They are tubular structures that only occur in animal cells. They are involved in the separation of chromosomes during the process of cell division.
- Endoplasmic reticulum. It is a membrane system that continues with the nucleus and extends through the cell. Its function is related to the synthesis of compounds intended mainly for the exterior of the cell. The endoplasmic reticulum is divided into rough and smooth depending on the presence or absence of ribosomes on its surface: the rough reticulum contains ribosomes and is mainly responsible for the synthesis of proteins for export, while the smooth reticulum is mainly related to metabolic pathways of lipids.
- Golgi apparat. It’s an organelle made up of a series of flattened discs and sacs called cisterns. The function of the Golgi apparatus is related to the modification and packaging of proteins and other biomolecules (such as carbohydrates and lipids) for their secretion or transport.
Difference between the eukaryotic cell and prokaryotic cell
We can summarize the main differences between these two cell types in the following points:
Core presence. The most important difference: In prokaryotes, the genetic material is distributed in the cytoplasm and not in the nucleus as in eukaryotes.
DNA type. Prokaryotes have a single circular DNA molecule that is not associated with proteins, which is why it is often referred to as “naked, circular DNA”. The genetic material of eukaryotes, in turn, is linear in form and is associated with proteins that make chromatin (or chromosomes, if the cell is about to start dividing). Each type of eukaryotic organism has several chromosomes that are characteristic of it.
Size. Eukaryotic cells are much larger (10-100 µm) than common prokaryotes (0.2-2.0 µm).
Constitution. The vast majority of eukaryotic organisms are multicellular while all prokaryotes are unicellular. However, it is worth remembering that there are some unicellular eukaryotic organisms such as Paramecia and yeast.
Reproduction. Prokaryotes reproduce asexually (through binary fission), while eukaryotes reproduce both sexually (through meiosis, which leads to gametes or sex cells) and asexually (through mitosis).
Cell organelles. Eukaryotic cells have organelles with specific membranes and functions such as mitochondria, lysosomes, or chloroplasts.
What is a eukaryotic cell? Types, Functions, and parts