📌Definition Table

Term	Definition
Prokaryote	A unicellular organism lacking a true nucleus and membrane-bound organelles.
Nucleoid	Region in a prokaryotic cell containing the circular DNA molecule.
Plasmid	Small, circular DNA molecule in prokaryotes, often carrying extra genes.
Binary Fission	A method of asexual reproduction in prokaryotes where the cell divides into two identical cells.
Pili	Hair-like structures on bacteria used for attachment or DNA transfer.
Flagella	Long, whip-like structures used for movement in some prokaryotes.

📌Introduction

Prokaryotic cells are the simplest and oldest forms of life, found in domains Bacteria and Archaea. They are small, typically 0.1–5.0 μm, and lack a nucleus and membrane-bound organelles. Despite their simplicity, prokaryotes display a diverse range of adaptations, enabling them to survive in almost every environment on Earth.

❤️ CAS Link: Create a public awareness campaign about biofilms and their role in medical device infections.

📌 General Characteristics of Prokaryotes

• Unicellular with simple internal organisation.
• DNA is located in the nucleoid region, not enclosed by a membrane.
• Contain 70S ribosomes for protein synthesis.
• Reproduce mainly through binary fission.
• Some possess additional DNA in plasmids, which can be transferred between cells.
• Cell wall composition varies — Gram-positive vs Gram-negative bacteria.

🧠 Examiner Tip: Always state “70S ribosomes” for prokaryotes and “80S ribosomes” for eukaryotes — mixing them up is a common error in exams.

📌 Cell Wall and Plasma Membrane

• Cell wall maintains shape, protects against osmotic pressure, and provides structural support.
• Peptidoglycan is the main component in bacterial cell walls.
• Gram-positive bacteria have thick peptidoglycan layers; Gram-negative have thinner layers and an outer membrane.
• Plasma membrane controls substance exchange with the environment.
• Some bacteria have a capsule for protection and attachment to surfaces.
• Archaea have cell walls without peptidoglycan (pseudopeptidoglycan instead).

🌍 Real-World Connection: Antibiotics like penicillin target bacterial cell wall synthesis, making them ineffective against Archaea.

📌 Genetic Material: Nucleoid and Plasmids

• Nucleoid contains a single circular chromosome made of DNA.
• DNA is supercoiled to fit into the cell.
• Plasmids carry extra genes, often for antibiotic resistance.
• Plasmids can be transferred between bacteria via conjugation (pili-mediated).
• Chromosome replication is linked to binary fission.
• No histones in bacteria, but Archaea have histone-like proteins.

📌 Movement and Surface Structures

• Flagella provide motility, enabling movement towards nutrients or away from toxins.
• Flagellar movement is driven by a rotary motor powered by proton gradients.
• Pili aid in surface attachment and genetic exchange.
• Fimbriae are shorter and more numerous, mainly for adhesion.
• Some prokaryotes secrete a slime layer to aid movement on surfaces.
• Motility and adhesion are important for colonisation in both pathogenic and symbiotic bacteria.

📌 Binary Fission

• DNA is replicated beginning at the origin of replication.
• Chromosomes attach to the cell membrane and are pulled apart as the cell elongates.
• The plasma membrane pinches inwards to divide the cell.
• Produces two genetically identical daughter cells.
• Occurs rapidly under favourable conditions — some bacteria divide every 20 minutes.
• Allows for fast population growth but limits genetic diversity unless horizontal gene transfer occurs.

📝 Paper 2: Data Response Tip: If asked to draw or label a prokaryotic cell, always include: cell wall, plasma membrane, cytoplasm, nucleoid, plasmid, 70S ribosomes, pili, and flagella — missing any of these may cost marks.