Cellular and Acellular Organization of Microbes

The Microbial World: Classification and Overview

The microbial world encompasses a diverse array of organisms, both cellular and acellular. Microbes are classified based on their cellular organization and genetic material. Cellular microbes include prokaryotes (Bacteria and Archaea) and eukaryotes (such as fungi, algae, and protozoa), while acellular entities include viruses, viroids, and prions.

• Cellular microbes: Possess cell structure; include prokaryotes and eukaryotes.

• Acellular microbes: Lack cellular structure; include viruses (nucleic acid + protein), viroids (RNA only), and prions (protein only).

Relative Sizes of Microbes and Cells

Microbes vary greatly in size, from tiny viruses to much larger eukaryotic cells. Understanding their relative sizes is important for microscopy and laboratory techniques.

• Prokaryotes: Typically 0.1–5 μm in diameter.

• Eukaryotes: Typically 10–100 μm in diameter.

• Viruses: Range from 20–300 nm.

• Microscopy: Electron microscopes are required for viruses and small bacteria; light microscopes suffice for most cells.

Features Common to All Cells

All cellular life forms share four fundamental features: cytoplasm, chromosomes, ribosomes, and a plasma membrane. Each component plays a critical role in cell structure and function.

• Cytoplasm: Gel-like substance inside the cell where metabolic reactions occur.

• Chromosome(s): Genetic material (DNA) that encodes instructions for cellular function.

• Ribosomes: Sites of protein synthesis, composed of rRNA and proteins.

• Plasma membrane: Phospholipid bilayer that encloses the cell, controlling the movement of substances in and out.

Cytoplasm

The cytoplasm is the internal matrix of the cell, consisting of cytosol (fluid), organelles (in eukaryotes), and macromolecules. It is the site of most cellular processes.

• Prokaryotes: Cytoplasm is not compartmentalized; materials move mainly by diffusion.

• Eukaryotes: Cytoplasm contains membrane-bound organelles; intracellular transport mechanisms facilitate movement.

• Acellular microbes: Do not possess cytoplasm.

Chromosomes and Genetic Material

Chromosomes carry the genetic instructions for cellular structure and function. The organization and compaction of genetic material differ among domains of life and acellular entities.

• Prokaryotes (Bacteria & Archaea): Single, circular DNA molecule located in the nucleoid region (not membrane-bound). Compacted by cations, small proteins, and enzymes (topoisomerases in bacteria; histones in archaea).

• Eukaryotes: Multiple, linear chromosomes within a membrane-bound nucleus. DNA is wrapped around histone proteins for compaction.

• Viruses: Genetic material can be DNA or RNA, single- or double-stranded, and is enclosed in a protein capsid.

• Prions: Infectious proteins with no nucleic acid.

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis. They are composed of ribosomal RNA (rRNA) and proteins, and consist of large and small subunits.

• Prokaryotic ribosomes: 70S (50S large + 30S small subunit).

• Eukaryotic ribosomes: 80S (60S large + 40S small subunit); also 70S in mitochondria and chloroplasts.

• Location: In eukaryotes, found in cytosol, rough ER, mitochondria, and chloroplasts; in prokaryotes, found in cytosol.

• Viruses and prions: Do not possess ribosomes.

• Antibiotics: Some target prokaryotic ribosomes specifically, exploiting structural differences.

Plasma Membrane

The plasma membrane is a selectively permeable barrier composed primarily of phospholipids and proteins. It maintains cellular integrity and mediates transport, signaling, and energy processes.

• Phospholipid bilayer: Universal among archaea, bacteria, and eukaryotes; maintains fluidity and acts as a barrier.

• Saturated vs. unsaturated fatty acids: Affect membrane fluidity; unsaturated increase fluidity.

• Bilayer vs. monolayer: Some archaea have monolayer membranes for stability in extreme environments.

• Other lipids: Act as fluidity buffers (e.g., cholesterol in eukaryotes, hopanoids in bacteria, ergosterol in fungi).

• Proteins: Involved in transport (passive and active), signaling, and energy generation (especially in prokaryotes).

Functions of Plasma Membrane Proteins

• Transport: Facilitate movement of molecules across the membrane (channels, carriers, pumps).

• Signaling: Receptors detect environmental signals and initiate cellular responses.

• Energy generation: In prokaryotes, the plasma membrane is the site of respiration and photosynthesis (eukaryotes use internal organelles).

Summary Table: Features Common to All Cells