Scientists generally believe that the atom is the smallest indivisible portion of any material. However, the same scientists believe that the atom has components that include electrons, protons, and neutrons. The similitude of this is found in the cell. The cell theory believes that the cell is the basic unit of all living organisms. In other words, the cell is like the atom, the indivisible portion of living organisms. Just like the atom, however, every cell has its own components. These components are biologically referred to as organelles.
The organelles of a cell depend on the type of cell. In the last class, we discussed the various type of cells that exist in nature - the prokaryotic and eukaryotic cells. Out of the 5 kingdoms in which living organisms are divided according to Whittaker, only one of them - the Monerans - belongs to the prokaryotic category. Thus, a prokaryotic cell is typified by any bacterium or archaea cell.
A typical bacterium cell consists of the following organelles
A locomotive structure
Not all bacteria are motile by the way. The locomotive structure for those that are motile is the flagellum (flagella in plural form) - a microscopic, thread-like, helically shaped structure containing a protein known as flagellin that beats in a clockwise or anticlockwise direction to propel the bacterial cell about. The flagellum is usually found attached to one of either end of the bacterial cell.
A fastening structure
Just like the locomotive organelle, not all bacteria have a fastening structure. The fastening structure acts to hold the cell attached to a surface. For the cells that have it, the structure is known as the pilus (plural, pili) - a short, hair-like organelle that is usually found attached in numerous amounts to the surface of the bacterial cell. Even though this organelle is primarily concerned with fastening the cell to surfaces, it sometimes gets involved in locomotion in some bacteria while it acts as a sexual organelle in others.
The capsule
The capsule is a structure made largely of polysaccharides. It is slimy in nature that surrounds the cell and primarily serves for the clinging of cells to their neighbours and to other surfaces. The capsule also prevents the drying out of the cell during unfavourable conditions and serves a protective function against the host's immune system for the bacterial cells that are pathogenic.
The cell wall
Generally, the cell wall of the prokaryotes is a structure that houses and shapes the cell, primarily serves to protect it and acts as a permeable structure for the movement of materials in and out of the cell. The cell wall comes after the capsule layer for cells with capsules or in direct contact with the environment for cells that lack encapsulation. The structure of the cell wall differs from bacteria to bacteria but the basic component is peptidoglycan in the form of polysaccharides.
Some bacteria have cell walls that are made of a thick peptidoglycan layer with the capacity to retain the colour of a dye and are referred to as Gram-positive. Others have cell walls whose peptidoglycan layers are not so thick to the extent that retention of the colour of a dye becomes difficult and are, therefore, referred to as Gram-negative.
The cell membrane
The cell or plasma membrane is a layer that delimits the cell just after the cell wall. The organelle is made of about 60 percent phospholipid and 40 percent proteins and serves as a selectively permeable channel for the movement of materials in and out of the cell, unlike the cell wall. In addition to the selective permeability, the cell membrane doubles up as an energy-generating structure for the cell, coordination of processes such as DNA replication/transcription, and several other cellular activities.
Cytoplasm
The cytoplasm of the prokaryotic cells represents the internal portion of the cell. In other words, it is the portion delimited by the plasma membrane of the cell. It is largely made up of a liquid, gel-like portion known as the cytosol and houses the internal organelles of the cell such as the nucleoid and the ribosome. The cytosol contains macromolecules such as nucleic acids, sugar, and fats. It is within this liquid portion that most of the metabolic processes of the prokaryotic cell take place since they lack membrane-bound organelles.
Nucleoid
The nucleoid is a region containing the genetic materials of the cells of the prokaryotes in the form of DNA, proteins, and RNA. The genetic materials are not enclosed in any structure but freely lies within the cytoplasm of the cell.
Ribosome
The ribosome is a small, round organelle that functions as the site of protein synthesis. The genetic materials in the form of DNA are first transcribed to mRNA and then translated to protein through the assistance of the tRNA and the ribosome. The prokaryotic ribosome is 70S with two subunits - the 50S and 300S subunits.
The prokaryotic cell consists of bacteria and archaea as opposed to the eukaryotic that consist of fungi, plants, and animals. They are simple in structure and lack membrane-bound organelles such as the nucleus and mitochondrion. Hence, the majority of the metabolic processes take place within the matrix of the cytosol. A typical prokaryotic cell has a locomotive organelle for movement, a fastening organelle for attaching to surfaces, a surrounding capsule for sticking to neighbouring cells and fighting the host's defense system, a cell wall for protection and general permeability, a plasma membrane for selective permeability and a liquid internal portion - the cytoplasm which houses the nucleoid and the ribosome.
Next, we will be talking about the organelles of the eukaryotic cell.
Thank you all for reading.
