Bacterial cell - Structure and Functions

                    Bacterial cell 

              Structure and Function:

Bacterial are unicellular prokaryotic organism.

Bacterial cell have simpler internal structure. 

It lacks all membrane bound cell organelles such as mitochondria, lysosome, golgi, endoplasmic reticulum, chloroplast, peroxisome, glyoxysome, and true vacuole.

A typical bacterial cell have following structure.

A. Structure Outside cell wall

1. Capsule
2. Flagella
3. Pili
4. Cell wall
5. Cytoplasmic membrane
6. Nucleoid
7. Mesosomas
8. Ribosome
9. cytoplasm
10. Spore.

1.Capsule:

• Capsule is 0.2µm thick viscus layer outer layer to the cell wall.
• Capsule is 98% water and 2% polysaccharide or glycoprotein/ polypeptide or both.
• There are two types of capsule.
• It helps in attachments as well as it prevent the cell from desiccation and drying.
• Capsule resist phagocytosis by WBCs.

2. Flagella:

• It is 15-20 nm hair like helical structure emerges from cell wall.
• Flagella is not straight but is helical. It is composed of flagellin protein (globular protein) and known as H antigen.
• Flagella has three parts. Basal body, Hook and filament
• It helps motility of the bacteria

3. Pili or fimbriae:

• Pili are hollow filamentous and non-helical structure.
• They are numerous and shorter than flagella
• Pili is the characteristic feature of gram –ve bacteria.
• Pili is composed of pilin protein.
• Bacteria containing pili: Shigella, Proteus, Neisseria gonorrhoae, E. coli
• Attachment: pili helps the bacteria to attach the host cell surface. Most of the human pathogens of respiratory tract, urinary tract are attached with the help of pili.
• Pili (fimbriae) possess antigenic property
• Specialized function: some pili are modified for specialized function. Eg. Sex pilus (F-pili) help in transfer of DNA from donor to recipient cell during conjugation.
• F-pili also act as receptor for bacteriophage.

4. Cell wall:

• It is an important structure of a bacteria.
• It give shape to the organism.
• On the basis of cell wall composition, bacteria are classified into two major group ie. Gram Positive and gram negative.

Gram positive cell wall

Cell wall composition of gram positive bacteria.

1. Peptidoglycan
2. Lipid and Teichoic acid

Gram negative cell wall

Cell wall composition of gram negative bacteria

1. Peptidoglycan
2. Outermembrane:
   • Lipid
   • Protein
   • Lipopopysaccharide (LPS)

• Peptidoglycan layer is present in cell wall of both gram positive as well as gram negative bacteria. However, gram positive have thick layer of peptidoglycan.
• It is the major surface antigen of gram positive bacteria
• It is an additional layer present in gram negative bacteria.
• It is composed of lipid bilayer, protein and lipo-polysaccharide.

 5.Cell membrane:

• Cell membrane is the inner layer that lies inside the cell wall and encloses the cytoplasm.
• It is also known as cytoplasmic membrane or plasma membrane.
• It is about 80nm thick.
• Cell membrane of bacteria is composed of phospholipid and proteins.
• It is selectively permeable as it allows to pass selective substances such as sugar, aminoacids across it.

6. Nucleus:

• Nucleus is the most important part of the cell.
• It controls and directs all the cellular activities and stores hereditary information of cell
• Bacterial nucleus is known as nucleoid; it lacks nuclear membrane, nuceloplasm and nucleolus.
• Bacterial DNA is naked (lacked histone protein)
• It contains and stores hereditary information of the cell.
• It controls all cell activities.

7. Ribosome:

• Bacterial ribosome is of 70s type.
• Ribosomes are rounded granules found freely floating in the cytoplasm
• Ribosomes are known as universal cell organelle because it is found in both bacterial cell and eukaryotic cell.
• Chemically the ribosomes are made up of nucleic acids (particularly RNA and proteins).
• It helps in protein synthesis. 

8. Mesosome:

• Mesosome is a spherical or round sac like structure found commonly in gram positive bacteria.
• Function: It is the site for respiration in bacterial cell

9. Cytoplasm:

• It is colorless, viscus fluid present inside cell membrane.
• All the cell organelles and inclusions are found floating in cytoplasmic fluid.
• It contains proteins, lipid, minerals, nucleic acids, glycogen, water etc.
• It helps to distribute water, oxygen as other substances throughout the cell.
• Literally, all the cellular content including nucleus, and other cell organelle are floating in cytoplasm.

10. Spores (endospore):

• Spore is metabolically dormant structure produced during unfavourable condition by the process called sporulation
• Sporulation occur during late log phase or early stationary phase
• Under favourable condition spores germinate to give vegetative cell.

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