Introduction

Prokaryotic cells belong to the kingdom Monera. Literally, ‘Pro’ means primitive and ‘karyos’ meaning nucleus, referring to the cell with a primitive nucleus. Prokaryotic cells are also known as the most primitive cells as millions of years ago. These cells evolved from the earliest life forms on the planet named protobionts, which were membrane-bound organic molecules. These are abundantly available from the gut of a being to the earth’s surface and constitute majorly to its biomass. Prokaryotes or Prokaryotic cells are known to be ancestors of Eukaryotes with simpler single-celled structures. The size of prokaryotes is smaller than the smallest eukaryotes but larger than a virus. The small size is due to the lesser DNA content required for the production of proteins. Due to this small size of the prokaryotic cell, they have a large surface-to-volume ratio, comparing the surface area of the cell to the volume of the cell, thus allowing easy entry of substances quickly into the cell. Examples of prokaryotic cells include Bacteria, Archaebacteria, Cyanobacteria and Mycoplasma.

    Characteristics of Prokaryotic Cells:

    Prokaryotic cells are characterised by the following:
    • Unicellular cells with size usually ranging from 0.1-5 micrometre
    • The cell wall is usually made up of amino acids and carbohydrates
    • Lack a true nucleus without a nuclear envelope and nucleolus
    • DNA is present in the nucleoid and in plasmids floating in the cytoplasm
    • Cell organelles also lack a membrane with no membrane inside the cell
    • These cells can tolerate extreme conditions and are easily adaptable
    • Few rely on photosynthesis for nutrition. The rest get it from dead or living beings
    • Most cells reproduce asexually by binary fission and sexually by conjugation
    • Unlike Eukaryotes, these cells are less complex and single-celled
    • These cells exist without a true nuclear membrane, chloroplast, Golgi bodies, mitochondria and endoplasmic reticulum.

        Prokaryotic Cell Structures and their Functions:

        The Prokaryotic cell comprises the following organelles:
        1. Cell wall: The complex cell wall is present to give shape, support, and protection to the cell, made up of amino acids and cellulose
        2. Cell membrane or plasma membrane: This layer is made up of phospholipid substances that separate the cytoplasm from the outer environment and control the entry and exit of substances into and out of the cell
        3. Cytoplasm: It has a gel-like consistency suspending all the cellular substances in it
        4. Ribosomes: These 70s ribosomes consist of 50s and 30s ribosomes and are bound to the cell membrane, helping in the synthesis of the proteins essential for cell survival
        5. Nucleoid: It contains the genetic material RNA and DNA suspended in the cytoplasm. DNA is naked without a nuclear membrane and in circular genome form
        6. Plasmid: It consists of extra or non-chromosomal small DNA floating in the cytoplasm
        7. Capsule: Outermost covering on the bacterial cell wall that protects the cell from dying in extreme conditions
        8. Vacuoles: These are the storing space of food, nutrients, and waste materials to help the cell survive in extreme conditions
        9. Cytosol: It consists of cellular materials forming the cytoskeleton of the cell
        10. Pili or pilus: these are present in a few cells and help in the transfer of external DNA structures to adjacent cells pili forming channels during sexual reproduction
        11. Cilia or Flagellum: Some cells have a single lash-like appendage responsible for cell locomotion, forming the sensory organelle
        12. Mesosomes: Folded plasma membrane containing respiratory enzymes forms mesosomes assisting respiration in a few cells

         Prokaryotic Cells types and parts:

        The following types of prokaryotic cells are present in nature:

          Bacteria:

          • Bacterial cells are present everywhere, even inside the gut of organisms and are resistant to extreme temperature, chemicals and even UV rays
          • Cell walls are made up of murein or peptidoglycans that are made up of amino acids and sugars
          • Pili, flagella are present to assist their locomotive abilities with a capsule to prevent the bacterial cell from dying
          • Plasma membrane is made up of fatty acids
          • They are of different sizes and shapes like cocci (spherical), bacilli (rod-shaped) and spirilla (spirals), and they can go into the dormant stage during starvation or unavailability of nutrients to form endospores which can reactivate in favourable conditions
          • Extrachromosomal DNA is present and reproduced by binary fission
          • Examples:
              (1) Streptococcus pneumoniae, bacteria causing pneumonia.
                (2) Lactobacillus acidophilus, the good bacteria present in curd.

              Archaebacteria:

            • Archaeal cells are unicellular cells with similar shape and size to the bacteria, hence called the ancient bacteria
            • Cell wall is not made of peptidoglycan,and the cell membrane consists of different lipids
            • Flagella is present for the locomotion
            • Plasma lipid membrane consists of fentanyl groups
            • Tend to survive in extreme conditions like hot springs, inside humans, and in acidic water, unlike bacteria
            • Examples:
                (1) Halophiles, present in highly salty environments and Thermophiles are present in high temperatures.
                  (2) Methanobrevibacter smithii, methane-producing archaebacteria present in the human gut.

                    Cyanobacteria:

                  • These are also known as the blue-green algae
                  • These cells contain chlorophyll and hence can carry out photosynthesis
                  • They contribute majorly to the environment’s oxygen supply and are considered eco-friendly

                  Reproduction In Prokaryotic Cells

                  Reproduction in Prokaryotic cells occurs every 24 hours with proper food and water supply. Reproduction can occur in either of the following ways:

                    Asexual Reproduction:

                    • Binary Fission- After DNA replication, the cell grows and expands, forming a septal ring between both the DNA and dividing the cell splits into two identical daughter cells
                    • Fragmentation- The parent cell breaks down into multiple fragments forming new daughter cells out of each fragment

                    Sexual Reproduction:

                    • Conjugation: Genes are moved from one cell to another with the help of a protein tube called a pilus
                    • Transformation: The DNA present in the outside environment is taken up by the cell and incorporated into its cell
                    • Transduction: Viruses transfer the genetic material into the cytoplasm of the bacterial cell

                    Conclusion

                    Prokaryotic cells can survive in extreme environmental conditions. They have a cell wall made up of cellulose, peptidoglycans, or murein, along with a capsule that protects the cell from dying, prokaryotic cells are much simpler as compared to Eukaryotic cells, DNA in prokaryotes is naked and does not have a nuclear membrane making it easier for DNA replication and cell division without undergoing mitosis.