Prokaryotic Cell

‘Pro’ means ‘before’ and ‘kary’ means ‘nucleus’. Prokaryotes are simple, single-celled organisms that are the most primitive life form on earth. It includes two domains – bacteria and archaea. Prokaryotes are free-living and photosynthetic (produce their food), parasitic (living inside other organisms), or saprophytic (feeding on the dead and decaying matter).

Prokaryotes can be found almost anywhere on earth, from land to water bodies, atmosphere to hydrothermal vents, and even inside living organisms, including humans. Evidence from fossil studies shows that prokaryotes exist on earth since 3.5 billion years ago. They are supposed to have given rise to all primitive and advanced life forms present today.

The basic facts and characteristic features of a prokaryotic cell are:

• They lack a true nucleus, i.e., they do not have a nuclear membrane.
• The entire genetic material or DNA is located in a central region of the cell called the nucleoid. It typically consists of a single, circular double-stranded DNA.
• Membrane-bound organelles such as mitochondria, chloroplast, Golgi bodies, and lysosomes are absent.
• The cell wall consists of carbohydrates and amino acids.
• The plasma membrane also acts as the mitochondrial membrane.
• Reproduce asexually through binary fission and sexually by conjugation, transformation, and transduction.

The four main components found to make up a prokaryotic cell that allow them to function correctly are:

1. Cell (Plasma) Membrane: The outer covering separates the cell’s interior from the outside environment.
2. Cytoplasm: A jelly-like fluid that fills up the entire cell interior except for the organelles. All cellular structures are suspended in the cytoplasm.
3. DNA: The genetic material that helps in their reproduction.
4. Ribosome: Helps in protein synthesis.

A prokaryotic cell contains internal and external structures. They are much simple in structure and organization compared to eukaryotic cells.

How Big are Prokaryotic Cells

Typically, prokaryotic cell sizes range from 0.1 to 5.0 μm in diameter and thus are significantly smaller than eukaryotic cells. They have a surface area to volume ratio higher than eukaryotes because of their small size.   

Shapes

The three most common prokaryotic cell shapes are spiral (coiled-shaped), bacillus (rod-shaped), and coccus (spherical or oval-shaped). Other than the three basic shapes, cells can be sheathed, stalked, filamentous, star-shaped, spindle-shaped, lobed, and pleomorphic, to name a few.

Parts and Functions

1. Capsule: An additional outer covering found in some prokaryotic cells helps protect the cell from foreign invaders. The capsule is made of polysaccharides, which allow the cells to bind to various surfaces and preserves moisture within the cell.
2. Cell Wall: In bacteria, it is a covering made of carbohydrate, and lipid polymer termed peptidoglycan, found below the capsule. However, the archaeal cell wall contains no peptidoglycan and is made up of proteins and other polymers. Their cell wall provides shape to the cell while protecting the cell organelles present in the cell interior.
3. Cell (Plasma) Membrane: It is found underneath the cell wall and is made of the phospholipid bilayer. The cell membrane protects the cell while allowing the transport of essential molecules across it. 
4. Cytoplasm: A jelly-like fluid that covers the entire space present inside the cell membrane. It holds all cell organelles in place and protects the cell from internal and external shock.
5. Ribosomes: Contains 70S ribosomes, which are made of subunits 30S and 50S. They are observed in the cytosol. However, their size and number are found to vary in different prokaryotic cells. Ribosomes are the cell’s protein-synthesizing machinery.
6. Nucleoid and Plasmid: A cytoplasm region containing a single circular chromosome and small rings of extrachromosomal DNA called plasmids. The chromosome, along with plasmids, helps in cell reproduction.
7. Appendages: It includes flagella, pili (singular pilus), and fimbriae. Flagella are a tail-like structure attributed to their movement. Pili are hair-like structures found on the cell surface that help them to attach to other surfaces. They also help in their reproduction through conjugation. Fimbriae are shorter pili with similar functions to pili in helping the bacterial cell attach to external surfaces.  

There are two main types of prokaryotic cells – 1) bacterial cells and 2) archaeal cells.

1) Bacterial Cells

They are single-celled organisms found almost everywhere on earth, having diverse sizes, shapes, and structures. Their cell wall is made of peptidoglycan, a molecule made of sugar and amino acids, making the cell wall tough and thick. They contain some unique structures such as the capsule, flagella, and pili. Some bacteria form rigid and dormant structures called endospores that help them survive adverse environmental conditions such as high temperature and food scarcity.

Their genetic material consists of one circular coiled chromosome with about 160,000 base pairs (bp) to 12,200,000 bp. Bacterial cells also contain plasmids, which are smaller circular pieces of extrachromosomal DNA.

Escherichia coli (E.coli), Bacillus spp, Streptomyces spp, Clostridium spp, and Pseudomonas spp are typical bacteria examples.

2) Archaeal Cells

They are unicellular organisms that are similar in size and shape to bacteria. Archaea are commonly found in extreme environments such as springs, oceans, marshlands, and even inside other organisms, including humans.

Their cell membrane has a monolayer of phospholipid that protects the cells against harsh environments. Also, archaeal cell walls are made of pseudopeptidoglycan, made of proteins. Like the bacterial cell, archaea also contain external appendages such as flagella and pili.

Archaea have one circular chromosome, as bacteria do. The archaeal chromosome ranges between 491,000 bp to 5,700,000 bp. They can also contain plasmids.

Halobacterium spp, Thermoplasma spp, Sulfolobus spp, Desulfurococcus spp, and Acidilobus spp are typical archaea examples.

There are two modes of reproduction or cell division in prokaryotes: 1) asexual and 2) sexual.

Asexual Reproduction

It occurs by binary fission.

Binary Fission: It starts when the parent cell DNA replicates. The cell then starts to increase in size, and the cell wall extends outwards. The DNA then segregates, and a new cell wall starts to develop in the middle of the cell. Finally, the cell splits to form two individual daughter cells, completing the process of reproduction. Binary fission does not produce genetic variation.

Sexual Reproduction

It occurs through recombination, which brings genetic variation to the population. Here, genes from one prokaryote are incorporated into the other to have a genetic mix between them. Recombination is achieved through three processes:

Conjugation: Transfer of genes between cells occurs through a protein tube-like structure, called pilus, connecting the two cells.

Transformation: Bacteria take up DNA from the surrounding medium, transport it across the cell membrane, and incorporate it into the cell’s DNA.

Transduction: It involves the exchange of bacterial DNA through viruses. Bacteriophages, viruses that infect bacteria, transfer bacterial DNA from previously infected bacteria to the next bacteria they infect.