Prokaryotic cell structures and viruses

In a nutshell

Cells are the fundamental structural units that make up an organism. Bacterial cells are examples of prokaryotic cells. Within every cell there are organelles that carry out specific functions and each cell type has a different internal structure that suits its function. This is known as the ultrastructure of the cell.

Prokaryotic organisms

Prokaryotes do not contain membrane-bound organelles. A bacterial cell is shown below and the organelles are described in the table below.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Prokaryotic cell structures and viruses — 1. Flagellum, 2. Mesosome, 3. Circular DNA, 4. Plasmid, 5. Capsule, 6. Cell wall, 7. Cell surface membrane, 8. Ribosomes, 9. Cytoplasm

Organelle	Structure	Function
Plasma membrane	The cell membrane is made up of lipids and proteins and it controls the movement of substances in and out of the cell.	The cell membrane contains respiratory enzymes as they don’t have mitochondria.
Cell wall	The cell wall consists of a layer of peptidoglycan made from parallel polysaccharides with cross-linked peptide chains.	The cell wall prevents the cell swelling and bursting whilst also protecting the cellular contents.
Capsule	This is also known as a slime layer and may be formed from starch, gelatin, protein or glycolipids.	The capsule protects the bacterium from phagocytes and it covers the cellular markers that the host cell will recognise. As a result, the bacterium will be less likely to be detected by the host cell’s immune system.
Plasmids	Plasmids are small circles of DNA.	The plasmid will encode for a particular part of the bacterial phenotype in addition to the genetic material encoded in the nucleoid. *Note:* Plasmids can be transferred from one bacterium to another in a type of sexual reproduction.
Nucleoid	The nucleoid is the area where the bacterial DNA is found. Prokaryotic cells don’t have a nucleus but they have a single length of DNA that is often circular and coiled.	The nucleoid contains the genetic material of the prokaryotic cell. The genetic material is what determines the phenotype of the cell.
Flagellum (plural is flagella)	Made up of a helix of the flagellin protein.	The flagellum rotates and allows the bacterium to move.
$70S$ ribosomes	Very small spherical organelles made of ribosomal RNA (rRNA) and protein. The ribosomes in prokaryotic cells are $70S$ which is smaller than those of eukaryotic cells ( $80S$ ). The ribosomes are made up of two subunits. In prokaryotic cells, the subunits are $30S$ and $50S$ .	The ribosomes are involved in protein synthesis.
Pilus (plural is pili)	Thread-like projections on the surface of bacteria. Pili are also called fimbriae. Example E. coli have pili.	Pili are used for host cell attachment and sexual reproduction. However, they also make the bacteria more susceptible to viral infections as bacteriophage can use the pili as an entry point to infect the bacterial cells.
Mesosome	Infoldings of cell surface membrane.	There is debate surrounding the function of the mesosomes. They may be the site of respiration, they may also be an artefact of the process of preparing a cell for an electron micrograph.

Viruses

Viruses are acellular, non-living particles. They are smaller than bacteria and do not have the same organelles as prokaryotes. They have nucleic acids (DNA or RNA) as their genetic material, but they can only reproduce inside the cells of other organisms. These cells are known as host cells. The virus has a protein coat called a capsid. The capsid has attachment proteins on its surface which allow the virus to attach to the host cell.

Note: Students following the Edexcel specification will require a deeper understanding of viruses. There is a separate summary dedicated to viruses.

Example

1. RNA (Nucleic acids), 2. Attachment proteins, 3. HIV, 4. Reverse transcriptase, 5. Capsid, 6. Lipid envelope