Introduction
Eukaryotic cells are those cells that contain a nucleus and organelles enclosed by a plasma membrane. They are found in all the eukaryotic organisms. The eukaryotic organisms include four kingdoms; kingdom Protista, kingdom Fungi, kingdom Plantae and kingdom Animalia. Most of the eukaryotes are multicellular organisms having complex forms. The word eukaryotic cell is used to describe the structure of a general eukaryotic cell.
In this article, we will discuss the general structure of a eukaryotic cell as well as the differences found in the cells of the four kingdoms of eukaryotes. So, keep reading.
Structure
A eukaryotic cell consists of several membrane-bound structures in its cytoplasm. The cytoplasm of the cell is surrounded by the plasma membrane. The genetic material of the cell is present in the nucleus that is also membrane-bound. The DNA of the cell is arranged in the form of thread-like structures called chromosomes. These chromosomes become visible only during the cell division. The shape of the cell is maintained by the cytoskeleton that is made by several microtubules and micro-filaments.
Here we will discuss some organelles that are only limited to the eukaryotic cells and are not found in prokaryotes.
Nucleus
Nucleus is the main organelle that differentiates between a eukaryotic and prokaryotic cell. it is the largest organelle present in the eukaryotic cells.
Nuclear envelope
Nucleus is surrounded by two membranes that together form the nuclear envelope. At some places along the nuclear envelope, the two membranes are fused to form perforations called the nuclear pores. The exchange of materials between the nucleus and cytoplasm takes place via these pores.
Chromosomes
The DNA in the nucleus is coiled to form chromosomes. These become visible only during the process of cell division. When the cell is in the interphase, the chromosomes are in loose phase with the genes being exposed for transcription. These genes are continuously transcribed to form mRNA that leaves the nucleus via nuclear pores and guides protein synthesis.
Endoplasmic Reticulum
These are the membrane-bound organelles found only in the eukaryotic cells. They appear as an extensive system of membranes extending from the nucleus to the plasma membrane. These membranes form a system of sacs within each cell that are kept separated from the rest of the cytoplasm. Certain special processes can take place in these sacs.
There are two types of endoplasmic reticulum I eukaryotic cells;
- Rough endoplasmic reticulum
- Smooth endoplasmic reticulum
Rough Endoplasmic Reticulum
The membranes of these sacs have a rough appearance due to the presence of ribosomes that are attached in the form of dots. They are involved in the synthesis of proteins. Proteins are made by the ribosomes present on the membranes and are further processed by the enzymes present inside the rough endoplasmic reticulum.
Smooth Endoplasmic Reticulum
The membranes of these sacs lack ribosomes and thus appear smooth when vied under an electron microscope. They are involved in the synthesis of lipids, cholesterol and steroid hormones.
Golgi Complex
The term Golgi complex is used to describe the entire system of Golgi bodies present in the cytoplasm of eukaryotic cells. A Golgi body is a stack formed by membrane-bound flattened sacs. This stack is continuously being formed at one end and is broken from the other end. More than one Golgi bodies may be present within one eukaryotic cell.
The function of the Golgi bodies is to process and pack the proteins, lipids, and other molecules that are destined to be sent out of the cell. These molecules enter the Golgi body at its forming face, pass through the entire complex, and leave from the other end, packed in membrane-bound vesicles.
Lysosomes
These are the spherical membrane-bound vesicles found abundantly in the eukaryotic cells. Lysosomes contain digestive enzymes that can digest any type of substance entering the vesicle. It is necessary to keep these digestive enzymes separate from the rest of the cytoplasm to prevent autodigestion of the cells.
Lysosomes are involved in the digestion of old, worn-out organelles of the cell so that they can be recycled. They can also digest complete cell after its function has been done. This happens in the case of the mammary gland after lactation. Lysosomes present in the white blood cells protect our body from harmful pathogens by digesting them.
Mitochondria
These are one of the most important organelles found in the eukaryotic cells. Each eukaryotic cell has several mitochondria scattered in the cytoplasm.
Structure
Mitochondria are the boat-shaped organelles bound by a double membrane. Both the membranes are completely sperate form one another. The outer membrane is smooth while the inner membrane has several folds, increasing its surface area. The space between the two organelles is called inter-membranous space. The area bound by the inner membrane is called the mitochondrial matrix.
Functions
Mitochondria are called the powerhouse of the cell. They are the metabolic centers of cells having a battery of enzymes in their matrix. These enzymes metabolize the carbohydrates, fats, and lipids and release energy in the form of ATP. The electron transport chain present on the inner mitochondrial membrane is involved in oxidative phosphorylation, the major source of ATP in eukaryotic cells.
Cytoskeleton
Cytoskeleton is a framework of interlinked protein filaments present within the cytoplasm providing structural support to the cell. The cytoskeleton of eukaryotic cells is composed of three components; microfilaments, intermediate filaments, and microtubules. The three components can easily assemble and disassemble to assist in several functions performed by the cytoskeleton.
Microfilaments
These are made up of polymers of actin protein. These filaments produce force when they push against a barrier such as a membrane or an organelle. These actin filaments have binding sites for myosin filaments to form actomyosin fibers. a contractile force is generated when an actomyosin fiber is formed.
Intermediate filaments
These filaments have a diameter of around 10 nm. They are polymers of fibrous proteins. The composition of intermediate filaments varies from cell to cell. different proteins that can be found in intermediate filaments include vimentin, desmin, keratin, and lamin.
Microtubules
These are the hollow structures made up of tubulin proteins. The alpha and beta-tubulin protein join to form polymers called protofilaments. In microtubules, 13 of these protofilaments are arranged in a circular pattern. The resulting microtubules have a diameter of around 23 nanometers.
Functions
Some of the functions performed by the cytoskeleton are as follows.
- The primary function of the cytoskeleton is to provide mechanical and structural support to the cells so that their shape is maintained
- It is also responsible for connecting the cell to the extracellular connective tissue
- Cytoskeleton causes the cell to contract or elongate when needed
- The microtubules are used to form mitotic apparatus during mitosis and meiosis
- It is also responsible for intracellular transport
Cytoskeletal Projections
Many eukaryotic cells have cytoskeletal projection coming out of the plasma membrane. These are the motile structures that help the cell in locomotion. Some examples of these cytoskeletal projections are flagella and cilia.
Flagella
These are the whip-lash like structures that project from the plasma membrane of some eukaryotic cells called flagellates. Their primary function is to assist the cell in locomotion. The eukaryotic flagella are made up of microtubules. The nine pairs of microtubules are arranged around the two central single microtubules. This arrangement of microtubules makes the core of flagella found in eukaryotes.
Flagella are also found in bacteria. The prokaryotic flagella have different structures
Cilia
Cilia are very fine cytoskeletal projections that arise from the cell body. they may be motile or non-motile. The motile cilia help cell in locomotion of the cells or moving substances in the extracellular environment. The non-motile cilia are the sensory structures that can sense the chemicals present in the extracellular environment.
Differences among the Eukaryotic cells
The structure described above is seen in the case of an ideal animal cell. however, cells of other eukaryotes may have some additional components or may lack one of the organelles described above. In this section, we will discuss the difference between an animal cell and that of other eukaryotes.
Plant Cell
Here are some of the characteristics of a plant cell that differentiate it from animal cells.
Cell Wall
It is the most distinguishing characteristic of a plant cell. All the plant cells are surrounded by a very strong cell wall made up of cellulose and hemicellulose. The cell wall makes the plant cells rigid so that they cannot be deformed. This is the reason why plant cells can bear hydrostatic pressure when placed in a hypotonic environment and do not burst.
Chloroplast
These are the double membrane-bound organelles only seen in plant cells. These are the factories of photosynthesis. The photosynthetic pigments present in the chloroplasts absorb sunlight and use this energy to convert water, and carbon dioxide to glucose along with oxygen.
Vacuole
Another important difference between an animal and a plant cell is the presence of vacuole. Plant cells have a large central vacuole that stores food, water, and other nutrients. the nucleus is pushed to one side of the cell. On the other hand, animal cells have a central nucleus with various small scattered vacuoles in the cytoplasm.
Fungal cells
Following are some differences between an animal cell and a fungal cell.
Cell wall
Like plant cells, fungal cells are also surrounded by a thick and rigid cell wall that is not found in animal cells. However, the fungal cell wall is different from the one found in plant cells. The fungal cell wall is made up of chitin instead of cellulose.
Cells of Protista
It is a kingdom of eukaryotes having very diverse characteristics. The organisms of the kingdom Protista resemble prokaryotes in many ways. However, they are called eukaryotes because they have a nucleus and membrane-bound organelles.
The organisms included in the kingdom Protista are of two types, animal-like and plant-like. The following are some additional features of plant-like protists that are not seen in animal cells.
Cell wall
They have a cell wall around their cells that is made up of cellulose and other similar polysaccharides.
Chloroplasts
This photosynthetic machinery is also found in plant-like protists. It has a similar structure as that of chloroplasts seen in plant cells.
Summary
Eukaryotic cells are characterized by the presence of a nucleus and other membrane-bound organelles in their cytoplasm.
These cells are found in all eukaryotes with little difference in structure.
The nucleus is the largest organelle of a eukaryotic cell, found mostly in the center. It is surrounded by a nuclear envelope having micropores in it, called the nuclear pores. All the DNA is present in the nucleus in the form of chromosomes.
Endoplasmic reticulum is a system of membrane-bound sacks that spreads from the nucleus to the cell membrane. They are of two types;
- Smooth, having no ribosomes
- Rough, with ribosomes attached to membranes
Golgi bodies are the stacks of flattened sacks found in the eukaryotic cells. they have a forming face and a breaking face. They are involved in the processing and packaging of proteins and lipids into the vacuoles.
Mitochondria are the double membrane-bound organelles found in the eukaryotic cells. they are the powerhouse of the cells generating most of the ATP needed by them.
Cytoskeleton is a framework of fibrous proteins that provides support to the cell. The cytoskeleton of eukaryotic cells is made up of three components;
- Microfilaments
- Intermediate filaments
- Microtubules
The cytoskeleton is involved in elongation and contraction of cells, intracellular transport as well as locomotion of cells.
The cytoskeletal projections that arise from the eukaryotic cells include;
- Flagella
- Cilia
- Microvilli
- Villi
A plant cell differs from an animal cell in having;
- Large central vacuole
- Cell wall made of cellulose
- Chloroplast
A fungal cell has following different features from an animal cell;
- Cell wall made of chitin
Plant-like protists are different from animal cells in having the following;
- A cell wall similar to that of plant cells
- Chloroplast
Frequently Asked Questions
What are eukaryotic cells?
Eukaryotic cells contain DNA bounded by a well-developed nuclear envelope and other membrane-bound organelles. The genetic material of eukaryotic cells is well organised in the form of chromosomes.
What are the different types of eukaryotic cells?
Eukaryotic cells are classified into plant, animal, fungal, and protist cells.
What are the double membrane bounded organelles in eukaryotic cells?
Double membrane-bounded organelles in eukaryotic cells are the nucleus, chloroplasts and mitochondria.
What is the difference between prokaryotic and eukaryotic cells?
Unlike eukaryotic cells, prokaryotic cells are primitive cells that don’t have well-developed nuclei and other membrane-bounded organelles like mitochondria, chloroplasts, lysosomes, etc.
References
- Adl SM, Simpson AG, Lane CE, Lukeš J, Bass D, Bowser SS, et al. (September 2012). “The revised classification of eukaryotes” (PDF). The Journal of Eukaryotic Microbiology. 59 (5): 429–93. doi:10.1111/j.1550-7408.2012.00644.x. PMC 3483872. PMID 23020233.
- Nelson DL, Cox MM (2005). Lehninger Principles of Biochemistry (4th ed.). New York: W.H. Freeman. ISBN 978-0-7167-4339-2
- Hetzer MW (March 2010). “The nuclear envelope”. Cold Spring Harbor Perspectives in Biology. 2 (3): a000539. doi:10.1101/cshperspect.a000539. PMC 2829960. PMID 20300205.
- “Endoplasmic Reticulum (Rough and Smooth)”. British Society for Cell Biology.
- “Golgi Apparatus”. British Society for Cell Biology.
- “Lysosome”. British Society for Cell Biology.
- Image: https://flic.kr/p/j1wNY1