The two essential types of cells that are found on Earth are – Prokaryotic Cells and Eukaryotic Cells.
There are numerous differences between the two, but the critical one is the presence of the nucleus, which carries the genetic material. This (nucleus) is present in a well-defined form in eukaryotic cells but is absent in prokaryotic cells.
So, in prokaryotic cells, a free-floating material is present in the cytoplasm of a cell which is nucleoid, in place of the nucleus.
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Definition of Prokaryotic cells
The most primitive, simplest, and earliest form of life present on the Earth are the Prokaryotes. These are single-celled organisms that have cytoplasm, cell membrane, cell wall, and ribosomes. But it lacks a well-defined nucleus; however, they have nucleoid. The name ‘Prokaryotes’ is a ‘Greek word, where ‘pro‘ means ‘before or primitive’ and ‘karyon‘ means ‘nucleus’.
Living beings are classified under three domains: Bacteria, Archaea, and Eukarya. Prokaryotes, which are single-celled organisms, fall under the domain of Bacteria and Archaea. The domain Eukarya are the complex one and is composed of eukaryotic organisms. Further, they can be unicellular or multicellular.
Characteristics of Prokaryotic Cells
- The size of the prokaryotic cells lies between 0.1 – 5.0 μm in diameter, which is comparatively smaller than the eukaryotic cells.
- The prokaryotic cells are basically found in different shapes like bacilli, vibrio, cocci, spherical, and spirilla. Apart from these, there can be other modifications in these shapes.
- Prokaryotic cells lack membrane-bound organelles like lysosomes, chloroplast, mitochondria, and Golgi body.
- The cell also lacks a well-defined nucleus, rather nucleoid is present, which has a single chromosome and comprises circular, double-stranded DNA.
- The histone protein is absent in prokaryotes. Further, it is majorly found in the chromosomes of eukaryotes. The cell also lacks the mitotic apparatus and nucleolus.
- The cell wall that works as the protective layer and gives shape to the cell is made up of peptidoglycans (amino acids and sugars).
- The prokaryotic cells reproduce asexually, which can be by binary fission, budding, or through conjugation methods.
- Most prokaryotes can survive under extreme conditions. This includes swamps, wetlands, hydrothermal vents, hot springs, and even inside the body of other living beings and is so-known as ‘extremophiles‘.
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Similarities Between Prokaryotes and Eukaryotes
As we know that eukaryotes have evolved from prokaryotes only and have a complex structure. But they still share a few common features;
- Cytoplasm: The fluid present within the cell, made up of salt, water, and proteins.
- Ribosomes: The protein-producing organelles.
- Genetic material (DNA): The essential component of the cell that defines the characteristic features of the living being.
- Cell membrane: The cell membrane or the plasma membrane is the outer membrane of the cell. It acts as the barrier for the cell and separates it from the outer environment. Also, it checks the material going inside and outside the cell.
Structure of the Prokaryotic cell
| Structures or Organelles | Functions |
|---|---|
| Cell Wall | The outer layer of the cell, that protects from the external environment and give the defined shape to the cell. |
| Capsule: | It is not present is all the prokaryotes, but it act as the double or additional protective layer of the cell, that provides moisture and nutrients to the cell, when it is been engulfed by the other organisms. |
| Plasma or Cell Membrane | The layer that protects the cytoplasm, it checks the entry and exit of the material, and regulates the fluidity within the cell. |
| Flagella | It helps in the cellular movements, these are whip-like long outgrowth. |
| Pili | These are the hair-like structures that help in attachment of the bacteria to the surface of the other bacteria. Short hair like structures are known as fimbriae. |
| Plasmids | These are circular-DNA structure (extra chromosomal material), that do not participate in reproduction. |
| Ribosomes | Ribosomes are the protein producing machines present in all the cellular structure. |
| Nucleoid | The part in the cytoplasm of the cell, that has the DNA molecule. |
Organelles like the endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes are absent in prokaryotes.
Examples
Escherichia Coli Bacterium (E. coli), Streptomyces Bacteria, Streptococcus Bacterium, Sulfolbus acidocaldarius archeobacterium (archaea).
