Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview of Prokaryotic and Eukaryotic Cells

Prokaryotic and eukaryotic cells are the two fundamental cell types in microbiology. Prokaryotes include bacteria and archaea, while eukaryotes encompass organisms such as fungi, algae, protozoa, and helminths. Understanding their structural differences is essential for studying microbial physiology and classification.

Prokaryotic Cell Morphology

Shapes and Arrangements of Prokaryotic Cells

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

• Bacillus: Rod-shaped bacteria.

• Coccus: Spherical bacteria.

• Spiral: Includes vibrio (curved rod), spirillum (rigid spiral), and spirochete (flexible spiral).

• Arrangements: Cocci can form diplococci (pairs), streptococci (chains), tetrads (groups of four), sarcinae (cubical groups), and staphylococci (clusters). Bacilli can form diplobacilli (pairs), streptobacilli (chains), and coccobacilli (short rods).

Monomorphic and Pleomorphic Bacteria

Bacteria are described as monomorphic if they maintain a single shape, and pleomorphic if they can vary in shape.

Structure of a Prokaryotic Cell

General Structure

Prokaryotic cells contain several key components, each with specific functions:

• Cell wall: Provides structural support and prevents osmotic lysis.

• Plasma membrane: Regulates transport and energy transformation.

• Cytoplasm: Contains water, proteins, enzymes, and genetic material.

• Nucleoid: Region containing the bacterial chromosome (DNA).

• Plasmids: Small, circular DNA molecules with extra genetic information.

• Ribosomes: Sites of protein synthesis.

• Flagella: Used for locomotion.

• Fimbriae and pili: Used for adhesion and DNA transfer.

Glycocalyx (Sugar Coat)

The glycocalyx is a sticky, gel-like layer surrounding the cell, made of polysaccharide and/or polypeptide. It exists in two forms:

• Capsule: Neatly organized and firmly attached.

• Slime layer: Unorganized and loose.

Functions include protection from desiccation, evasion of host immune responses, and adherence to surfaces.

Cell Wall and Peptidoglycan Structure

The cell wall is composed primarily of peptidoglycan, a polymer of sugars and amino acids. Peptidoglycan consists of:

• N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) forming the carbohydrate backbone.

• Amino acid cross-bridges connecting the sugar chains.

Gram-Positive vs. Gram-Negative Cell Walls

Bacterial cell walls are classified as gram-positive or gram-negative based on their structure and staining properties:

• Gram-positive: Thick peptidoglycan layer, teichoic acids, no outer membrane.

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), periplasmic space.

These differences affect susceptibility to antibiotics and lysozyme.

Atypical Cell Walls

Some bacteria have atypical cell walls:

• Acid-fast cell walls: Contain mycolic acid, resistant to chemicals and dehydration (e.g., Mycobacterium).

• Mycoplasma: Lack cell walls, have sterols in their membranes.

Plasma Membrane Structure and Function

Composition and Function

The plasma membrane is a phospholipid bilayer with embedded proteins. It controls selective permeability and is involved in energy transformation.

Transport Across the Membrane

Substances move across the membrane by:

• Simple diffusion: Movement from high to low concentration.

• Facilitated diffusion: Movement via carrier proteins.

• Active transport: Movement against the gradient, requires energy.

Osmosis

Osmosis is the movement of water across a selectively permeable membrane, driven by solute concentration differences.

External Structures: Flagella, Fimbriae, and Pili

Flagella

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

• Monotrichous: Single flagellum.

• Lophotrichous: Tuft of flagella at one end.

• Amphitrichous: Flagella at both ends.

• Peritrichous: Flagella distributed over the entire cell.

Bacterial Motility

Bacteria move by rotating their flagella, resulting in "runs" and "tumbles" that allow directional movement.

Fimbriae

Fimbriae are fine, proteinaceous, hairlike bristles used for adhesion to surfaces and cells.

Pili

Pili are rigid tubular structures made of pilin protein. They are involved in motility and DNA transfer (conjugation).

Internal Structures

Cytoplasm

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

Nucleoid

The nucleoid region contains the bacterial chromosome, a circular thread of DNA with genetic information.

Plasmids

Plasmids are small, circular DNA molecules that carry extra genetic information and can be transferred between bacteria.

Ribosomes

Ribosomes are the sites of protein synthesis. Prokaryotic ribosomes are 70S, composed of a 50S large subunit and a 30S small subunit.

Endospores

Endospores are highly resistant, dormant structures formed by some bacteria for survival under adverse conditions. Sporulation is the process of endospore formation, and germination is the return to vegetative growth.

Eukaryotic Cell Structure

General Features

Eukaryotic cells are structurally more complex than prokaryotic cells, containing membrane-bound organelles and a defined nucleus.

Flagella and Cilia

Eukaryotic flagella and cilia are used for locomotion and are structurally distinct from prokaryotic flagella.

Cell Wall

The cell wall in eukaryotes is found in plants, algae, and fungi, and is composed of carbohydrates.

Plasma Membrane

The eukaryotic plasma membrane is a phospholipid bilayer with peripheral, integral, and transmembrane proteins, as well as sterols.

Ribosomes

Eukaryotic ribosomes are 80S (60S large subunit and 40S small subunit), while 70S ribosomes are found in mitochondria and chloroplasts.

Membrane-Bound Organelles

• Nucleus: Contains genetic material.

• Endoplasmic Reticulum (ER): Smooth and rough types, involved in protein and lipid synthesis.

• Golgi Complex: Modifies and transports proteins.

• Lysosome: Contains digestive enzymes.

• Mitochondrion: Site of cellular respiration.

• Chloroplast: Site of photosynthesis in plants and algae.

Eukaryotic Nucleus

The nucleus is surrounded by a nuclear membrane and contains the cell's genetic material.

Endoplasmic Reticulum (ER)

The ER is connected to the nuclear membrane and exists in two forms:

• Smooth ER: Synthesizes lipids.

• Rough ER: Synthesizes proteins (contains ribosomes).

Golgi Complex

The Golgi complex modifies proteins from the ER and transports them via secretory vesicles to the plasma membrane.

Mitochondria

Mitochondria are the site of ATP production and contain their own DNA and ribosomes.

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts evolved from symbiotic prokaryotes living within other prokaryotes, supported by their own DNA and ribosomes.