Prokaryotic cells fall into a size range of about 1–5µm and hence can be observed clearly by microscopes. However, some prokaryotic cells may be larger than this.
A prokaryotic cell contains external and internal structures. Capsule, flagella, axial filaments, fimbriae, and pili are present external to the cellwall, while interior of the bacterial cell contains cytoplasm.
Flagella – Flagella are whip like structures made of protein and provide motility to the cell. Prokaryotic cells may be
- Monotrichous – Cells that have one flagellum.
- Lophotrichus – Cells that have a clump of flagella known a tuft, at one end of the cell.
- Amphitrichous – Cells that have flagella at two ends of the cell.
- Peritrichous – Cells that have flagella covering the entire cell on the surface.
Fimbriae and pili – Fimbriae are proteinaceous, sticky, projected structure used by cells to attach to each other and to objects around them, while pili are tubules that are used to transfer DNAfrom one cell to another cell.
Capsule Depending on the type of bacterium, there may be an exterior surrounding layer.such as a capsule or slime layer, made of glyocalyx
Cellwall – The prokaryotic cell’s cell wall is present outside the plasma membrane. It provides rigidity to the cell shape and structure and protects the cell from its environment. Bacterial cell wall is primarily composed of peptidoglycan and on the basis of cell wall composition the bacteria classified into gram-positive and gram negative organisms.
Cytoplasmic Membrane– The cytoplasmic membrane is a membrane that provides a selective barrier between the environment and the cell’s internal structures.
Cytoplasm is thick.aqueous, semitransparent, and elasticsmifluid present inside the prokaryotic cell. It is about 80% water and contains primarily proteins (enzymes), carbohydrates, lipids, inorganic ions, and many low- molecular-weight compounds. Inorganic ions are present in much higher concentrations in cytoplasm than in most media.
Nucleoid/Genetic material – The cytoplasm also contains a region called the nucleoid, which is where the DNA of the cell is located. The prokaryotic cell consists of a chromosome that isn’t contained within a nuclear membrane or envelope. The nucleoid or bacterial chromosome comprises a closed circle of double stranded DNA, many times the length of the cell and is highly folded and compacted.
Ribosomes – Ribosomes are the principle structure in a prokaryotic cell after the nucleoid. They are composed of a complex of protein and RNA, and are the site of protein synthesis in the cell. The prokaryotic ribosomes are 70S, comprised of sub units 50S and 30S (S stands for the sydberg coefficient which is a function of their size and shape, and determined by their rate of sedimentation in a centrifuge)
Inclusion bodies – Many granular structures known as inclusion bodies are found in the cytoplasm of certain bacteria. These contain organic compounds such as starch, glycogen or lipid and act as food reserves. Some sulphur and polyphosphate containing bodies are also found and are known as volutin or metachromatic granules.
Endospore – A number of gram-positive bacteria can form a special resistant, dormant structure called an endospore. Endospores develop within vegetative bacterial cells and are extraordinarily resistant to environmental stresses such as heat,ultraviolet radiation,gamma radiation, chemical disinfectants, and desiccation.
Prokaryotic cells are much smaller than eukaryotic cells. The main differences between prokaryotic and eukaryotic cells are described below:
|Characteristics||Prokaryotic Cells||Eukaryotic Cells|
|Cell wall||Complex composition in layers, typically contains peptidoglycan||Composition is simple, peptidoglycan not found|
|Plasma membrane||No carbohydrates or sterols||Contains carbohydrates and sterols|
|Glycocalyx||Present as capsule or slime layer||Present in cells that lack cellwall|
|Cell organelles||None. Only some inclusion bodies are present||ER, golgi body, lysosomes, mitochondria, lysosomes|
|Nucleus||Not well defined, without any nuclear membrane or nucleoli||Well defined nucleus present, with nuclear membrane and nucleus|
|Chromosome||Single circular chromosome present as nuclear material without histones||Multiple linear chromosomes found with histones|
|Cell division||Binary fission||Mitosis|
Points to Remember:
- All bacteria are prokaryotes and much simpler structurally than eukaryotes.
- Most bacteria have a cell wall outside the plasma membrane to give them shape and protect them from osmotic lysis.
- Bacterial walls are chemically complex and usually contain peptidoglycan or murein. Bacteria often are classified as either gram positive or gram negative based on differences in cell wall structure and their response to Gram staining
- The cytoplasmic matrix contains inclusion bodies and ribosomes.
- Prokaryotic genetic material is located in an area called the nucleoid and is not enclosed by a membrane.
- Structures such as capsules, flagella, and sex pili are found outside the cell wall.
- Some bacteria survive adverse environmental conditions by forming endospores, dormant structures resistant to heat, desiccation, and many chemicals.