Prokaryotes and Eukaryotes

Overview of Cell Types

Cells are classified as either prokaryotic or eukaryotic based on structural and functional differences. Prokaryotes include bacteria and archaea, while eukaryotes include plants, animals, fungi, and protists. Understanding these differences is fundamental to microbiology.

Prokaryotic Cell Morphology

Shapes and Arrangements

Prokaryotic cells exhibit a variety of shapes (morphologies) and arrangements, which are important for identification and classification.

• Coccus: Spherical shape

• Bacillus: Rod-shaped

• Spiral Forms: Includes vibrio (comma-shaped), spirillum (rigid spiral), and spirochete (flexible spiral)

• Arrangements: Cocci and bacilli can form characteristic groupings such as diplococci (pairs), streptococci (chains), staphylococci (clusters), tetrads (groups of four), and sarcinae (cubical packets of eight)

Other Terms Describing Morphology

• Monomorphic: Bacteria that maintain a single shape

• Pleomorphic: Bacteria that can alter their shape or size in response to environmental conditions

Structure of a Prokaryotic Cell

Generalized Structure

Prokaryotic cells have a simple structure lacking membrane-bound organelles. Key components include the cell wall, plasma membrane, cytoplasm, nucleoid, plasmids, ribosomes, and external structures such as flagella, fimbriae, and pili.

Glycocalyx (Capsule and Slime Layer)

The glycocalyx is a sticky, gel-like layer external to the cell wall, composed of polysaccharides and/or polypeptides. It exists in two forms:

• Capsule: Neatly organized and firmly attached to the cell wall

• Slime Layer: Unorganized and loosely attached

Functions: Protection against desiccation, phagocytosis, and aids in adherence to surfaces.

Cell Wall

The cell wall provides structural support and prevents osmotic lysis. It is primarily composed of peptidoglycan, a polymer of sugars and amino acids.

• Peptidoglycan Structure: Consists of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by peptide cross-bridges.

Gram-Positive vs. Gram-Negative Cell Walls

Bacteria are classified based on their cell wall structure, which is revealed by the Gram stain.

Atypical Cell Walls

• Acid-fast cell walls: Found in Mycobacterium and Nocardia; contain mycolic acid, making them resistant to chemicals and dehydration.

• Mycoplasma: Lack cell walls; their membranes contain sterols for stability.

Plasma (Cell) Membrane

The plasma membrane is a phospholipid bilayer with embedded proteins, responsible for selective permeability and energy transformation.

Movement Across Cell Membranes

• Simple Diffusion: Movement of solutes from high to low concentration without energy input.

• Facilitated Diffusion: Movement of solutes via specific or nonspecific carrier proteins.

• Active Transport: Movement against the concentration gradient, requiring energy and carrier proteins.

Osmosis

Osmosis is the movement of water across a selectively permeable membrane from an area of low solute concentration to high solute concentration.

External Structures of Prokaryotes

Flagella

Flagella are long, filamentous appendages used for motility. They are composed of the protein flagellin and can be arranged in various patterns:

• Monotrichous: Single flagellum at one pole

• Lophotrichous: Tuft of flagella at one pole

• Amphitrichous: Flagella at both poles

• Peritrichous: Flagella distributed over the entire cell surface

Fimbriae and Pili

• Fimbriae: Short, hairlike structures used for adhesion to surfaces and other cells.

• Pili: Longer, less numerous structures involved in motility and DNA transfer (conjugation pili/sex pili).

Internal Structures of Prokaryotes

Cytoplasm

The cytoplasm is the substance inside the plasma membrane, consisting mainly of water, proteins, enzymes, small molecules, ions, and ribosomes.

Nucleoid

The nucleoid region contains the bacterial chromosome, a single circular DNA molecule that holds the cell's genetic information.

Plasmids

Plasmids are small, circular DNA molecules that carry extra genetic information and can be transferred between bacteria, often conferring advantageous traits such as antibiotic resistance.

Ribosomes

Ribosomes are the sites of protein synthesis. Prokaryotic ribosomes are 70S, composed of 50S and 30S subunits.

Endospores

Endospores are highly resistant, dormant structures formed by some bacteria for survival in adverse conditions. They are not a means of reproduction; one cell forms one spore.

Eukaryotic Cells

General Structure

Eukaryotic cells are more complex, containing membrane-bound organelles such as the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus. They may also have cell walls (in plants, algae, fungi) and specialized structures like flagella and cilia for movement.

Cell Wall

Eukaryotic cell walls, when present, are made of carbohydrates such as cellulose (plants, algae) or chitin (fungi).

Plasma Membrane

The eukaryotic plasma membrane is a phospholipid bilayer with embedded proteins and sterols, providing structural integrity and selective permeability.

Ribosomes

• 80S ribosomes: Found in the cytoplasm and on the rough ER, composed of 60S and 40S subunits.

• 70S ribosomes: Found in mitochondria and chloroplasts, similar to prokaryotic ribosomes.

Membrane-Bound Organelles

• Nucleus: Contains the cell's genetic material (DNA) and is surrounded by a nuclear envelope.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER is studded with ribosomes and synthesizes proteins, smooth ER synthesizes lipids.

• Golgi Complex: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Mitochondria: Sites of cellular respiration and energy (ATP) production.

• Chloroplasts: Sites of photosynthesis in plants and algae.

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated from symbiotic prokaryotes living within ancestral eukaryotic cells. Evidence includes their own DNA and 70S ribosomes, similar to bacteria.

Example: Escherichia coli is a well-studied prokaryote with a typical rod shape, peritrichous flagella, and the ability to form fimbriae and pili for adhesion and conjugation.