Prokaryotic cell structure

A2.2.5 Prokaryotic Cell Structure

Prokaryotic vs. Eukaryotic Cells

• Every cell can be classified as either prokaryotic or eukaryotic, based on its structural characteristics.

• Prokaryotes: Include bacteria and archaea. They are generally simpler organisms, lacking a nucleus and membrane-bound organelles.

• Eukaryotes: Include a more complex set of organisms such as plants, animals, fungi, and protists, characterized by the presence of a nucleus and various membrane-bound organelles.

Major Differences between Prokaryotic and Eukaryotic Cells

Essential Structures Common to All Cells

• Ribosomes: Sites for protein synthesis, essential for cellular function.

• DNA: Carries genetic information necessary for cell growth and reproduction.

• Cell membrane: Regulates material transport in and out of the cell, maintaining internal homeostasis.

• Cytoplasm: Gel-like fluid that acts as a medium for metabolic reactions.

Prokaryotic Cell Structure

Key Components:

• Cell (Plasma) Membrane: A lipid bilayer that controls the movement of ions and molecules. It plays a critical role in maintaining cell integrity and transport.

• Cytoplasm: A viscous fluid that houses various organelles and is the site for important metabolic reactions including glycolysis.

• Ribosome: Small structures composed of rRNA and proteins that synthesize proteins by translating mRNA.

• Cell Wall: Composed primarily of peptidoglycan, provides structural support, shape, and protection against osmotic pressure.

• Pili: Hair-like structures that facilitate attachment to surfaces, aid in disease pathogenicity, and allow for genetic material exchange during conjugation.

• Capsule: A gelatinous layer that protects prokaryotes from desiccation and immune system attacks; enhances the ability to cause disease by facilitating adherence.

• Flagellum: A long, whip-like structure that propels the cell through its environment, aiding in motility.

Note: Some structures such as capsules and pili may not be present in all prokaryotic cells; variability exists based on species and environmental factors.

Prokaryotic Cell DNA

Locations of DNA:

• Nucleoid: The region in prokaryotic cells where the single circular DNA (not membrane-bound) is located, referred to as "naked" DNA since it is not associated with histones as found in eukaryotic DNA.

• Plasmid: A small, circular piece of DNA that is often present in prokaryotes; contains genes that can confer advantageous traits such as antibiotic resistance. Plasmids replicate independently of chromosomal DNA and can be transferred between cells.

Characteristics of Prokaryotic DNA

• Naked DNA: Unlike eukaryotic DNA, prokaryotic DNA is unbound by proteins, which allows it to replicate rapidly under favorable conditions.

Ribosomes in Prokaryotic vs. Eukaryotic Cells

• Ribosomes are crucial for protein synthesis, catalyzing polypeptide synthesis during the process of translation.

• Prokaryotic Ribosomes: Smaller (70s), composed of proteins and rRNA, facilitating rapid protein synthesis essential for cell survival in varying environments.

• Eukaryotic Ribosomes: Larger (80s) with a more complex structure, leading to more sophisticated mechanisms of protein synthesis.

• Antibiotic Selectivity: The structural differences between prokaryotic and eukaryotic ribosomes allow certain antibiotics to selectively target prokaryotic ribosomes without harming eukaryotic cells, making them effective treatments against bacterial infections.

Visual Reference

• An electron micrograph provides a visual representation of prokaryotic cell structures, including the nucleoid, cell wall, and plasma membrane.