Prokaryotic Cell Structure I Function

Prokaryotic Cell: Structure and Function

Introduction

Prokaryotic cells are the simplest and most primitive type of cells, lacking a true nucleus and membrane-bound organelles. They belong to the domains Bacteria and Archaea and are unicellular organisms. Despite their simplicity, prokaryotic cells carry out all essential life processes, including metabolism, growth, and reproduction.

Structure of Prokaryotic Cells

Prokaryotic cells have a well-organized structure that allows them to survive in diverse environments. The major components of a prokaryotic cell include:

[1] Cell Envelope

The cell envelope consists of three layers that provide protection and structural integrity to the cell.

(a) Glycocalyx (Capsule/Slime Layer)

• It is an outermost layer made of polysaccharides.
• If it is well-organized and firmly attached, it is called a capsule (provides protection against phagocytosis).
• If it is loosely attached, it is called a slime layer (helps in adhesion and biofilm formation).

(b) Cell Wall

• Composed of peptidoglycan in bacteria, providing shape and rigidity.
• Gram-positive bacteria have a thick peptidoglycan layer.
• Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides (LPS).

(c) Plasma Membrane

• A selectively permeable phospholipid bilayer with embedded proteins.
• It controls the exchange of materials and contains enzymes for respiration and ATP production.

 

[2] Cytoplasm

• A semi-fluid matrix where all cellular activities occur.
• Contains water, enzymes, nutrients, ribosomes, and genetic material.

 

[3] Genetic Material (Nucleoid Region)

• Prokaryotes lack a true nucleus; their genetic material (DNA) is present in a nucleoid.
• They have a single circular chromosome without histones.
• Some bacteria have plasmids—small, circular DNA molecules that provide resistance to antibiotics and other survival advantages.

 

[4] Ribosomes (70S Type)

• Smaller than eukaryotic ribosomes (80S).
• Composed of 30S (small subunit) and 50S (large subunit).
• Function: Protein synthesis.

 

[5] Appendages

Prokaryotic cells have external structures that help in movement, attachment, and DNA transfer.

(a) Flagella

• Long, whip-like structures made of flagellin protein.
• Provide motility using a rotary motion.
• Types of flagellar arrangements:
  • Monotrichous – single flagellum.
  • Lophotrichous – multiple flagella at one pole.
  • Amphitrichous – flagella at both poles.
  • Peritrichous – flagella all around the cell.

(b) Pili (Fimbriae)

• Short, hair-like structures made of protein.
• Help in attachment to surfaces and bacterial conjugation (DNA transfer).

 

[6] Inclusions (Storage Granules)

• Granules that store nutrients like glycogen, sulfur, phosphate, and polyhydroxybutyrate.
• Gas vacuoles help in buoyancy regulation (e.g., in cyanobacteria).

 

Table: Functions of Prokaryotic Cell Components