Module 1 : Cell basic structure and function

Structural organization of prokaryotic and eukaryotic cells

What is a cell?

• A Cell is the basic structural and functional unit of all living organisms. • Types of cells: -> Prokaryotic : Bacteria and Archaea (Sometimes Archaea are preferably classified altogether as a different class and not under Prokaryotes). -> Eukaryotic : Unicellular (protists) and Multicellular (Fungi, plants and animals).

Prokaryotic vs eukaryotic cells :

CELL WALL :

The cell walls are found in prokaryotic cells and among the eukaryotes in algae, fungi and plants. A structure of great tensile strength, the cell wall is formed from fibrils of cellulose molecules, embedded in a  water-saturated matrix of polysaccharides and structural glycoproteins.

Cell walls  perform a number of important functions : • They provide protection, support, shape and rigifity to the cellular structure. • They also provides a porous medium for the circulation  and distribution of water, minerals, and other small nutrient molecules. • They contains specialized molecules that regulate growth and protect  the plant from disease.

PROKARYOTIC CELL WALLS :

1. BACTERIAL CELL WALL : Prokaryotes, especially bacteria are divided into Gram positive and Gram negative cells based on differential staining by crystal violet-iodine reagent (Gram’s reagent). Bacteria stained purple are Gram + : their cell walls are thick and composed of predominantly petidoglycan and teichoic acid polymers which lipids are conspicuousy absent. The teichoic acid polymers play key role in antigenicity of the cell. Bacteria stained pink are Gram – : their cell walls have thin peptidoglycan (sandwiched between the cell membrane and outer envelope) and lipopolysaccharides with no teichoic acid.

In Gram - positive bacteria, the purple crystal violet stain is trapped by the layer of peptidoglycan which forms the outer layer of the cell. In Gram - negative bacteria, the outer membrane of lipopolysaccharides prevents the stain from reaching the peptidoglycan layer. The outer membrane is then permeabilized by acetone treatment, and the pink safranin counterstain is trapped by the peptidoglycan layer.