Bacterial Cell Structure

Bacterial cells are simple, prokaryotic cells that lack a true nucleus and membrane-bound organelles. Their structure mainly consists of a cell wall (outside the cell membrane) that provides shape and protection, and sometimes external structures like capsule, pili, and flagella for attachment or movement.

Inside the cell membrane, the cytoplasm contains essential components such as ribosomes (for protein synthesis), nucleoid (DNA region), and plasmids (extra DNA). Unlike eukaryotes, bacteria do not have mitochondria or chloroplasts, but all vital metabolic activities occur in the cytoplasm or on the cell membrane.

Cell Membrane

The phospholipid molecules are arranged in two parallel rows, called a lipid bilayer. Each phospholipid molecule contains a polar head, composed of a phosphate group and glycerol that is hydrophilic (water-loving) and soluble in water, and nonpolar tails, composed of fatty acids that are hydrophobic (water-fearing) and insoluble in water.

The polar heads are on the outer surfaces of the lipid bilayer, and the nonpolar tails are in the interior of the bilayer.

Protein molecules in the arranged at outer surface of membrane are called peripheral proteins, while some proteins are embedded inside in the membrane are called interior proteins.

Many of the proteins and some of the lipids are present on the outer surface of the plasma membrane have carbohydrates attached to them. Proteins attached to carbohydrates are called glycoproteins; lipids attached to carbohydrates are called glycolipids.

membrane proteins to move freely enough to perform their functions without destroying the structure of the membrane. This dynamic arrangement of phospholipids and proteins is referred to as the fluid mosaic model

Functions of Cell Membrane

The most important function of the plasma membrane is to serve as a selective barrier through which materials enter and exit the cell.

It is called selective permeability of the plasma membrane, through which plasma membrane allows certain molecules and ions pass through the membrane, but others are prevented from passing through it.

Plasma membranes are also important to the breakdown of nutrients and the production of energy. The plasma membranes of bacteria contain enzymes capable of catalyzing the chemical reactions that break down nutrients and produce ATP.

CELL WALL

Bacterial cell wall is a rigid structure associate with cell membrane and made of Peptidoglycane (Protein and Carbohydrates). There are two types of cell walls found in bacteria, gram negative and Gram positive cell wall.

Gram Positive cell wall has thick layer of Peptidoglycane and found over cell membrane.

Gram Negative cell wall has thin layer of peptidoglycane and this peptidoglycane layer is found between two layers of lipid cell membrane.

Gram Positive cell wall has thick layer of Peptidoglycan and found over cell membrane. Gram Negative cell wall has thin layer of peptidoglycan and this peptidoglycan layer is found between two layers of lipid cell membrane.

Functions of Cell Wall

The main function of cell wall to provide rigidity and protection to the bacterial cell. This is an rigid outer Structure over cell membrane, and prevent the cell damage due to adverse conditions of temperature, pH, Chemicals, and prevent bursting from osmotic pressure (called lysis) etc.

Teichoic acid found in cell wall also contributes to the rigidity of the cell wall. Other than this has several other functions as, wall teichoic acid fibbers participates in cell division phenomena, determination of cell shape.

Cell wall also provide the site for outer cell appendages like flagella and fimbriae.

FLAGELLA

Some prokaryotic cells have flagella, which are long filamentous appendages that helps to propel or swim bacteria in a medium towards or away from any site. The movement of bacteria from any site is called “taxis”.

A flagellum has three basic parts. The long outermost region, the filament, is constant in diameter and contains the globular (roughly spherical) protein flagellin arranged in several chains that intertwine and form a helix around a hollow core. In most bacteria, filaments are not covered by a membrane or sheath.

The filament is attached to a slightly wider hook (The second part of Flagella). The third portion of a flagellum is the basal body, which anchors the flagellum to the cell wall and plasma membrane.

Each prokaryotic flagellum moves the cell by rotating from the basal body. The movement of a prokaryotic flagellum results from rotation of its basal body and is similar to the movement of the shaft of an electric motor. The rotation of a flagellum is either clockwise or counter clockwise around its long axis.

The basal body is composed of a small central rod inserted into a series of rings. Gram-negative bacteria contain two pairs of rings; the outer pair of rings is anchored to various portions of the cell wall, and the inner pair of rings is anchored to the plasma membrane. In gram-positive bacteria, only the inner pair is present.

RIBOSOME

Ribosomes are the site for protein synthesis.

Ribosomes are tiny spherical organelles that make proteins by joining amino acids together. Ribosomes are composed of two subunits, one large and one small, that only bind together during protein synthesis. All prokaryotes have 70S (where S=Svedberg units) ribosomes while eukaryotes contain larger 80S ribosomes in their cytosol. The 70S ribosome is made up of a 50S and 30S subunits.

PLASMID

A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA.

Plasmid is extra chromosomal DNA and it is naturally exist in bacterial cells, and they also occur in some eukaryotes. Often, the genes carried in plasmids provide bacteria with genetic advantages, such as antibiotic resistance, production of some chemicals like toxins.

ENDOSPORE

When essential nutrients are depleted or in adverse conditions, certain gram-positive bacteria, of the genera Clostridium and Bacillus, form specialized cells called endospores . The process of endospore formation within a vegetative cell takes several hours and is known as sporulation or sporogenesis.

Endospores can remain dormant for thousands of years. An endospore returns to its vegetative state by a process called germination.

Germination is triggered by physical or chemical damage to the endospore's coat. The endospore's enzymes then break down the extra layers surrounding the endospore, water enters, and metabolism resumes.

Then a spore again reconvert into vegetative bacterial cell.

Functions of Endospore

Endospores help bacteria survive in extreme conditions like nutrient depletion, high temperature, or chemical exposure.