Prokaryote Definition (Prokaryotic cell definition)
Prokaryotic cells are cells without a true nucleus and lack membrane bound organelles. Prokaryotic organisms are called Prokaryotes – (pro means before; karyon means nucleus); Prokaryotes therefore are single-celled organisms (unicellular) which existed before the ones that have nuclei. Prokaryotic cells are therefore the simplest cells which are thought to have evolved before eukaryotic cells and they also have given rise to more complex and larger eukaryotic cells. Prokaryotic cells lack a true nucleus and have smaller (70S) ribosomes than eukaryotic cells.
Prokaryotic Cell Structure (Prokaryote structure)
Prokaryotic cells have a simple structure compared to structure of Eukaryotic cells, the simple structure of prokaryotic cells can be seen from their DNA lying freely in the cytoplasm; basic functions of prokaryotes can be carried out by simple diffusion since the size of prokaryotes are smaller (Prokaryotes are about 1000 to 10,000 times smaller in volume than Eukaryotes). The structural characteristics of Prokaryotic cells will be discussed below.
Characteristics of Prokaryotic cells
- Prokaryotes are mainly singled celled organisms (unicellular organisms)
- Average diameter of Prokaryotic cell ranges from 0.5 to 5 m .
- Prokaryotic cells lack a nucleus and also lack membrane-bound organelles
- The ribosomes of Prokaryotic ells have 70S subunits in the cytoplasm
- Some Prokaryotic cells have spiral flagella while some do not have. For those who have, the flagella are rigid
- Prokaryotic DNA is circular and lies free in the cytoplasm
- Prokaryotic cell wall contains murein which is a peptidoglycan (a polysaccharide combined with amino acids)
- Prokaryotic DNA is not complexed with histones as Histones are not found in prokaryotic cells
- Prokaryotic respiratory system is closely associated with its cell membrane
- Sexual reproduction in prokaryotic cells does not involve meiosis or mitosis but binary fission
- Ribosomes are present and occur free in the cytoplasm without attachment to endoplasmic reticulum
- Some Prokaryotes are Anaerobic organisms while others aerobic organisms
- RNA synthesis and protein synthesis occur in the same compartment in prokaryotes
Examples of Prokaryotic Cells (Prokaryotes examples)
- Cyanobacteria (Blue-green Algae)
- Archaea (or Archaebacteria)
The most studied prokaryotic cell is Escherichia coli (E. coli)
Prokaryotic cell parts and their functions
- Prokaryotic Cell wall: this is contains peptidoglycan in prokaryotes and offers protection to the organism
- Cell membrane: this controls movement of molecules in and out of the cell
- Infolding of cell surface membrane: may form a photosynthetic membrane or carry out nitrogen fixation
- Pilli: this is used for attachment to other cells or surfaces and it is also involved in sexual reproduction (it is used for exchanging genetic material during conjugation)
- Cytoplasm: it contains and supports other structures and metabolism occurs inside it
- Ribosomes: these are responsible for the production of proteins. Only the 70S subunit exist in prokaryotes
- Plasmid: small circle of DNA and there may be several plasmid
- Capsule: offers additional protection to a prokaryotic cell and enables the cell to attach to surfaces in its environment
- Circular DNA: this contains the genetic material of prokaryotes and it is sometimes referred to as the chromosome
- Flagella: structures used for movements
There are no organelles in prokaryotic cells as organelles are membrane bound structures which perform specific functions which are absent in prokaryotes but present in eukaryotes alone.
Prokaryotic Cell Diagram
Below is the diagram of a bacterium showing the prokaryotic cell parts always present and those that are sometimes present.
Prokaryotic cell parts always present
- Cell wall
- Cell membrane
- Circular DNA
The above listed parts of a prokaryotic cell are always present in all cells.
Prokaryotic cell parts that are sometimes present
- Infolding of cell surface membrane
The above listed prokaryotic cell parts are sometimes not present in some prokaryotes.