Introduction to Prokaryotic vs Eukaryotic
Living organisms are divided into two groups based on the primary differences in their cellular structures; prokaryotes and eukaryotes. It has been believed that prokaryotes were the first organisms to appear on Earth. The rest of the higher organisms evolved from these simple prokaryotic organisms.
They differ in terms of cellular structures, body forms, habitat, modes of reproduction, cellular metabolism, and many others. In this article, we will discuss the major differences between the two types of cells.
Organisms
Before discussing the differences between prokaryotes and eukaryotes, let us first know the organisms included in the two groups. The latest classification divides all known living organisms into five kingdoms.
- Kingdom Monera, includes all the prokaryotic organisms, true bacteria as well as ancient bacteria
- Kingdom Protista, includes eukaryotes that are different from the organisms in the rest of the four kingdoms
- The other three kingdoms, Fungi, Plantae, and Animalia include all the eukaryotic organisms
Cellular Structures
In this heading, we will discuss the major differentiating cellular features of eukaryotes and prokaryotes.
Cell Wall
The cell wall is found in all the prokaryotes except the members of mycoplasma. It is also present in all the eukaryotic cells except the cells of animals and animal-like protists. However, some major differences are found in the prokaryotic and eukaryotic cell walls.
Prokaryotic Cell Wall
The bacterial or prokaryotic cell wall is majorly made up of peptidoglycan. It is composed of two membranes with an intervening space called periplasmic space. Lipoproteins along with molecules of teichoic acid and lipoteichoic acid are also present in prokaryotic cell walls. Three types of cell walls can be seen in the prokaryotic cells;
- Gram-positive cell wall
- Gram-negative cell wall
- Cell wall of Mycobacteria
Eukaryotic Cell Wall
The eukaryotic cell wall is found in plants, fungi, and some protists. It is different from the prokaryotic cell wall in many aspects.
- It is made up of a single membrane
- Periplasmic space is absent
- Teichoic acid and lipoteichoic acid molecules are not present
- It does not give different staining like gram-positive and gram-negative cell walls
Eukaryotes have different cell walls in fungi and plant cells. the fungal cell wall is majorly made up of chitin while the plant cell wall is made up of cellulose.
Read more about the Five Kingdoms Classification System
Cellular Capsule
A true capsule is found around some prokaryotic cells. such bacteria are called capsular bacteria. The capsule is largely made up of polysaccharides. It is the outermost component of the bacterial cell envelope. It protects the cell from decomposition and plays a major role in the virulence of certain bacteria.
Although the eukaryotic cells lack a true capsule, a similar structure is made up of glycoproteins and glycolipids called the glycocalyx. It is also found in prokaryotic cells.
Genetic Material
The genetic material in both eukaryotes and prokaryotes is composed of DNA. However, there is a difference in the way it is stored in the cell. Eukaryotic cells have a nucleus while prokaryotic cells have a nucleoid in the center of the cell.
Eukaryotic Nucleus
It is a double membrane-bound organelle found in all eukaryotic cells. The genetic material is contained in the nucleus in the form of chromosomes. Different substances can enter or leave the nucleus by passing through certain pores in the nuclear envelope called nuclear pores.
Prokaryotic Nucleoid
The genetic material in the prokaryotes is present in the form of a large central nucleoid. The DNA is not condensed into chromosomes. Rather, it is present in the form of a large central molecule called the nucleoid. There is no barrier between the genetic material and the cytoplasm of the cell.
Ribosomes
Ribosomes are the factories of protein synthesis present in both eukaryotes and prokaryotes. The function of these organelles is essentially the same in both types of cells. however, there is a difference between their size in eukaryotes and prokaryotes.
Eukaryotic Ribosomes
The eukaryotic ribosomes are larger than the prokaryotic ribosomes. The larger subunit sediments are at 60S while the smaller subunit sediments are at 40S. Both these subunits join to form a single particle that sediments at the 80S.
(S stands for Svedberg, a unit that measures particle size based on their sedimentation rate during ultracentrifugation)
Most of the eukaryotic ribosomes are found in association with the rough endoplasmic reticulum. Some ribosomes are also scattered in the cytoplasm.
Prokaryotic Ribosomes
These are the smaller particles. The larger subunit sediments are at 50S while the smaller subunit sediments are at 30S. The particle formed by the combination of two subunits sediments at the 70S.
Prokaryotic cells lack endoplasmic reticulum. Their ribosomes are found scattered in the cytoplasm of the cell.
Cellular Processes
Different types of cellular processes emerge from the cell membrane of prokaryotic and eukaryotic cells.
Flagella
Flagella is the common cellular process found in both types of cells. Its function is to assist the cell in locomotion. It also helps in detecting certain chemicals present in the surrounding environment of the cell.
Pili
These are the small cellular appendages found only on prokaryotic cells. They are made up of pilin protein. Their function is to assist in the bacterial conjugation process, a process by which genetic material is transferred among the prokaryotic cells.
Cilia
These are the thin cellular process found only on eukaryotic cells. their diameter is less than flagella. Cilia also assist in locomotion and detection of chemicals found in the extracellular environment. The beating of cilia can also remove the particles attached to the cell surface.
Villi
These are the cellular appendages found only in the eukaryotic cells. They serve to increase the surface area of the cell and help in the absorption of different substances.
Cytoskeleton
It is a framework of microtubules, microfilaments, and intermediate filaments that provides structural support to the cells. It maintains the cell shape and assists in the intracellular transport of different substances. It is only found in the eukaryotic cells.
Other organelles
Prokaryotic cells lack membrane-bound organelles like mitochondria, Golgi apparatus, endoplasmic reticulum, etc. All these organelles are present in the eukaryotic cells.
Cell Division
The prokaryotic and eukaryotic cells also adopt different methods of cell division. Prokaryotes divide via binary fission while eukaryotes divide either by mitosis or meiosis.
Binary Fission
It is the oldest method of cell division found in prokaryotic cells. The genetic material is first duplicated by the process of DNA replication. The cell elongates and then divides into two halves giving rise to new cells called daughter cells.
These daughter cells can grow and undergo binary fission again, giving rise to new prokaryotic cells. After each cell cycle, two new cells originate. It is also called the duplication process. The duplication time is different for different bacterial species.
Binary fission is also seen in some single-celled eukaryotes like Amoeba, Euglena, and other members of the kingdom Protista.
Mitosis and Meiosis
These are two advanced methods of cell division found only in eukaryotes. Each method consists of two processes; the division of the nucleus called karyokinesis, and the division of cytoplasm called cytokinesis.
The nuclear material is first duplicated before the beginning of cell division.
Mitosis results in daughter cells having the same number of chromosomes as present in the parent cells. It involves only one cycle of cell division. All the somatic cells are divided via mitosis.
Meiosis involves two cycles of cell division following one DNA replication. It results in the generation of four daughter cells from one parent cell, having half the number of chromosomes as were present in the parent cell.
Cellular Metabolism
The eukaryotic and prokaryotic cells show differences among the various metabolic processes taking place in the cell. The eukaryotic cells have membrane-bound organelles that divide the cell into compartments. Different metabolic processes are confined to certain cellular compartments. However, this is not the case in prokaryotes that lack membrane-bound organelles.
Here, we will discuss how some metabolic processes are carried out differently in two types of cells.
Photosynthesis
It is the process by which organisms can make their food from raw material using the energy provided by sunlight. The solar energy is used to excite electrons that are then passed through the electron transport chain. The energy released by these electrons is used to make glucose from water and carbon dioxide and oxygen are released as a by-product.
Chlorophyll, the light-absorbing pigment plays a major role in photosynthesis. It is contained within the chloroplast of eukaryotic cells. The chloroplast is a membrane-bound organelle found only in eukaryotes. Prokaryotic cells have infoldings of the plasma membrane that act as sites for photosynthesis. The chlorophyll is attached to these membrane infoldings.
Cellular Respiration
It is the process by which the food material is broken down into simpler substances and energy is released. The released energy is used for various other cellular processes. Different types of food molecules that can be used include glucose, fats, lipids, amino acids, etc.
Mitochondria present in the eukaryotic cells contain enzymes for most of the reactions of cellular respiration. They remove the energy from various food materials and use it to make ATP. Mitochondria are the double-membrane bound organelles and are absent in the prokaryotic cells.
Prokaryotic cells have all the enzymes in the cytoplasm of the cells. All the reactions of cellular respiration take place in the cytoplasm of prokaryotes.
Gene Expression
It is the process by which the information present in the DNA is expressed in the form of proteins that perform various functions in the cell. It involves two steps;
- Transcription, synthesis of RNA copies from the DNA
- Translation, synthesis of proteins by using the information copied from the gene in RNA
The DNA in the eukaryotic cells is present in the nucleus. The transcription process in these cells takes place in the nucleus and the RNA molecules are then transferred to the cytoplasm. The synthesis of proteins occurs on the ribosomes present in the cytoplasm of the cell.
Prokaryotic cells lack a nucleus. Thus, transcription and translation take place in the cytoplasm of the cell. The two processes are often coupled and are going on side by side in the prokaryotes while this is not the case in the eukaryotes.
Cellular Organization
Prokaryotes are simple unicellular organisms. Each cell lives its own life, independent of the other cells. they are not organized into higher structures. The prokaryotic cells may form colonies. However, each cell in the colony is responsible for its survival independent of the other cells.
Most of the eukaryotes are complex multicellular organisms. The eukaryotic cells are organized to form tissue, organs, and organ systems. The cells in multicellular complex bodies live in harmony with one another and influence their functions and survival.
Summary
Cells of all the known living organisms are divided into eukaryotes or prokaryotes based on their distinct structural features.
Prokaryotic organisms are included in the kingdom Monera while the rest of the four kingdoms include eukaryotic organisms.
The major difference between the two cell types is the nucleus. The DNA in eukaryotic cells is present in the nucleus while it lies as a single circular molecule in prokaryotic cells.
The cell wall in eukaryotes is made up of cellulose or chitin while prokaryotes have peptidoglycan in their cell walls.
Some prokaryotic cells are covered by a polysaccharide capsule. Eukaryotes have glycocalyx that serves a similar function.
The ribosomes in eukaryotic cells sediment at the 80S while those in the prokaryotic cells sediment at 70S.
The cellular processes are also different in the two types of cells.
- Flagella are present in both eukaryotes and prokaryotes
- Pili are present only in prokaryotes
- Cilia and villi are present only in prokaryotic cells
The prokaryotic cells divide by binary fission while eukaryotic cells undergo either meiosis or mitosis.
Cellular processes in eukaryotes take place in membrane-bound organelles while they occur in the cytoplasm in the case of prokaryotic cells.
The two steps of gene expression take place simultaneously in prokaryotes while they occur separately in eukaryotes.
Prokaryotes are unicellular organisms while most eukaryotes are multicellular organisms with cells organized to form tissues, organs, and organ systems.
Frequently Asked Questions
What are the three differences between prokaryotic and eukaryotic cells?
Eukaryotic cells have a membrane-bound nucleus while prokaryotic cells lack it. Membrane-bound organelles are present in eukaryotic cells but not in prokaryotic cells.
What are some examples of prokaryotic cells?
Various bacteria are examples of prokaryotic cells such as E. coli, S. pneumonia, Salmonella typhi, N. meningitides, etc.
What are some examples of eukaryotic cells?
Examples of eukaryotic cells include animal and plant cells, such as human cells, dog cells, cat cells, etc.
Does a prokaryotic cell have DNA?
DNA is present in prokaryotic cells. But unlike eukaryotic cells, it is present as a free-floating circular molecule in the cytoplasm of the cell.
References
- NC State University. “Prokaryotes: Single-celled Organisms”.
- Campbell, N. “Biology: Concepts & Connections”. Pearson Education. San Francisco: 2003.
- Coté G, De Tullio M (2010). “Beyond Prokaryotes and Eukaryotes: Planctomycetes and Cell Organization”. Nature.
- https://en.wikipedia.org/wiki/Cell_wall
- https://simple.wikipedia.org/wiki/File:Flagellum-beating.png