Bacteria structure is extremely small, microscopic and unicellular. They were discovered by Anton von Leeuwenhoek in the year 1676. Their size varies between 1 to 5 microns in length, but some of them are as big as 80 microns in length.
Bacteria cell structure
Bacteria is a prokaryotic cell. It is enveloped by three layers like the capsule, cell wall and cell membrane. The membrane encloses protoplasm which has nuclear materials, ribosomes, food materials, etc. The cell structure is composed of
- Cell wall
- Cell membrane
- Nuclear material
- Flagella and
Capsule: This is a mucilage sheet secreted by the cell membrane. It is a slimy layer which is loosely arranged around the cell wall. This is present only in some bacteria during the adverse environmental conditions. It protects the cell from desiccation in conditions like high temperature and drought. This capsule is digested and consumed by the bacteria when the conditions get normal.
Cell wall: It is a hard envelop present around the bacterial cell. It gives definite shape and protection to the entire cell.
The cell wall is a nonliving cover made up of mucopolysaccharides and mucopeptides. This character of bacteria is similar to plant cells having a cell wall. The muco-polypeptide is a polymer of N-Acetyl muramic acid (NAM) and N-acetylglucosamine (NAG). Unlike the plants, bacterial cell wall lack cellulose in it.
This cell wall chemistry varies significantly and bacteria are differentiated as Gram-positive and gram-negative strains.
The gram +ve bacteria have a thick wall made of single layers of mucopeptide. On the other side, the Gram -ve have three layers like lipoprotein, lipopolysaccharide and mucopeptide.
Cell membrane: This is the outermost cell organelle lying immediately below the cell wall. It is a living membrane. This cell membrane forms an external lining layer to the protoplasm.
It is made of phospholipids and proteins and hence it is semipermeable. It regulates the movement of substances into and out of the cell.
Interestingly, it has numerous respiratory enzymes in it. The ATP is formed by oxidative phosphorylation here with some of the TCA cycle enzymes present in the protoplasm. The lack of mitochondria in the cell is substituted by this cell membrane.
So, the cell membrane is useful for transport of substances and also respiration.
Protoplasm: It is a viscous mixture present inside the cell membrane. It consists of carbohydrates, lipids, proteins, water, minerals, glycogen, etc. Most cell organelles of higher organisms like the mitochondria, Golgi complex, plastids, endoplasmic reticulum are absent in it.
Mesosomes: These are spherical invaginations formed by the infoldings of the cell membrane.
They are found to be involved in the replication of genetic material and cell division. Further, they are involved in the formation of cell wall and excretion of exo-enzymes.
Ribosomes: The bacterial cell structure has 70 s ribosomes. It consists of two sub-units as the 50S and 30S ribosomes.
The ribosomes are present in free-flowing form are grouped into polysomes.
These ribosomes are useful in protein synthesis. But, for humans, they are the target of an antibiotic attack in the treatment of bacterial infections.
Nuclear material: As mentioned before, bacteria lack a true nucleus. The genetic material is not enclosed by a nuclear membrane. The nuclear material consists of a single circular chromosome. The DNA is present a single-stranded circular molecule. Additional rings of DNA called plasmid are present in the cytoplasm. These plasmids are exchanged in between bacteria during sexual reproduction.
Flagella: These are the organs of locomotion present in motile bacteria. This is a hair or whip-like structure arising out of basal granule.
They are made up of protein flagellin which is arranged in eight parallel chains. A bacteria cell can consist of single, double or multiple flagella. Based on flagella arrangement, bacteria are classified into different types. Like
Atrichous = no flagella, Monotrichous= Single flagella, lophotrichous= flagella at one side, amphi trichous= Flagella as tuft on two sides, peritrochous=flagella all around the cell.
Pili: These are also called as fimbriae and are small tubular outgrowths present in Gram-negative bacteria. They are helpful in attachment to another bacterial cell during sexual reproduction by conjugation.
Besides the above, bacteria structure also consists of food stored as glycogen, proteins and oil drops. Further, the photosynthetic bacteria have green pigment distributed in protoplasm lamellae.