Cell Structure and Function in Microbiology: Prokaryotic and Eukaryotic Cells

Processes of Life

Fundamental Characteristics of Living Cells

All living cells exhibit several essential processes that define life. These include growth, reproduction, responsiveness, and metabolism. Understanding these processes is foundational to microbiology.

• Growth: Increase in size or number of cells.

• Reproduction: Production of new cells or organisms.

• Responsiveness: Ability to respond to environmental stimuli.

• Metabolism: Sum of all chemical reactions within a cell.

Types of Cells

Examples of Cell Types

Cells can be classified into prokaryotic and eukaryotic types, each with distinct structural features and functions. The diversity of cell types is illustrated below.

• Prokaryotic cells: Bacteria and archaea, lacking a nucleus.

• Eukaryotic cells: Algae, protozoa, fungi, animals, and plants, possessing a nucleus and organelles.

Prokaryotic and Eukaryotic Cells: An Overview

Prokaryotes

Prokaryotic cells are characterized by their simple structure and lack of internal membrane-bound organelles. They are typically small and include bacteria and archaea.

• Lack nucleus

• Lack internal structures bound with phospholipid membranes

• Small size: ~1.0 µm in diameter

• Simple structure

• Composed of: Bacteria and archaea

Eukaryotes

Eukaryotic cells are larger and more complex, containing a nucleus and various membrane-bound organelles. They are found in algae, protozoa, fungi, animals, and plants.

• Have nucleus

• Have internal membrane-bound organelles

• Larger size: 10–100 µm in diameter

• Complex structure

• Composed of: Algae, protozoa, fungi, animals, plants

Cell Size Comparison

Approximate Size of Various Types of Cells

Microbial cells vary greatly in size, from viruses to protozoa, and even larger eukaryotic cells. Understanding these differences is important for microscopy and classification.

• Virus: ~0.3 µm diameter

• Bacterium: ~1 µm diameter

• Parasitic protozoan: ~14 µm length

• Chicken egg: ~4.7 cm diameter (47,000 µm)

External Structures of Bacterial Cells

Glycocalyces

Bacterial cells often possess a glycocalyx, a gelatinous, sticky substance surrounding the cell. It is composed of polysaccharides, polypeptides, or both, and plays a role in protection and adherence.

• Capsule: Organized, firmly attached, may prevent recognition by host.

• Slime layer: Loosely attached, water soluble, aids in surface attachment.

Flagella

Structure and Function

Flagella are long, whip-like structures responsible for bacterial motility. They consist of a filament, hook, and basal body, which anchors the flagellum to the cell wall and membrane.

• Filament: Extends outward from the cell.

• Hook: Connects filament to basal body.

• Basal body: Anchors flagellum, composed of rods and rings.

Flagella Arrangements

Bacteria exhibit various flagellar arrangements, which are important for classification and motility.

• Monotrichous: Single flagellum

• Lophotrichous: Tuft of flagella at one end

• Amphitrichous: Flagella at both ends

• Peritrichous: Flagella all over the surface

Axial Filaments (Spirochetes)

Some bacteria, such as spirochetes, possess axial filaments (endoflagella) that allow corkscrew-like movement.

• Axial filament: Located between cell membrane and outer membrane

• Function: Rotation causes cell to move in a corkscrew fashion

Fimbriae and Pili

Fimbriae

Fimbriae are short, bristle-like projections used by bacteria to adhere to surfaces, hosts, and each other. They are important in biofilm formation.

• Shorter than flagella

• Sticky, bristlelike projections

• Function: Adherence and biofilm formation

Pili

Pili are longer than fimbriae but shorter than flagella. They are typically present in small numbers and mediate the transfer of DNA between cells (conjugation).

• Composed of pilin

• Also known as conjugation pili

• Function: DNA transfer (conjugation)

Bacterial Cell Walls

Structure and Function

Bacterial cell walls provide structural support, protect against osmotic forces, and contribute to cell shape. They are composed primarily of peptidoglycan.

• Peptidoglycan: Polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

• Shapes: Cocci (spherical), bacilli (rod-shaped), etc.

Gram-Positive Cell Walls

Gram-positive bacteria have thick peptidoglycan layers and contain teichoic acids. They stain purple in the Gram stain procedure.

• Thick peptidoglycan layer

• Teichoic acids: Unique polyalcohols

• Mycolic acid: In acid-fast bacteria, aids in desiccation resistance

Gram-Negative Cell Walls

Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS). They stain pink in the Gram stain procedure.

• Thin peptidoglycan layer

• Outer membrane: Contains LPS, phospholipids, proteins

• Impediment to treatment: LPS can be toxic and hinder antibiotic entry

Bacterial Cytoplasmic Membranes

Structure

The cytoplasmic membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model.

• Phospholipid bilayer: Hydrophilic heads, hydrophobic tails

• Integral and peripheral proteins: Various functions

Function

The membrane is selectively permeable, involved in energy storage, and maintains concentration and electrical gradients.

• Energy storage

• Harvest light energy (photosynthetic bacteria)

• Selectively permeable

• Proteins allow substance transport

• Maintain gradients

Passive Processes

Substances move across membranes via passive processes such as diffusion, facilitated diffusion, and osmosis.

• Diffusion: Movement of molecules from high to low concentration

• Facilitated diffusion: Uses channel proteins

• Osmosis: Movement of water across a semipermeable membrane

Active Processes

Active transport requires energy (ATP) to move substances against concentration gradients. Group translocation chemically modifies substances during transport.

• Active transport: Uniport, antiport, symport mechanisms

• Group translocation: Substance is chemically modified during transport

Cytoplasm of Bacteria

Cytosol, Inclusions, and Endospores

The cytoplasm contains cytosol (liquid), inclusions (reserve deposits), and endospores (defensive structures).

• Cytosol: Liquid portion

• Inclusions: Storage of chemicals

• Endospores: Survival structures in harsh conditions

Nonmembranous Organelles

Bacterial cells contain ribosomes (sites of protein synthesis) and a cytoskeleton (maintains cell shape).

• Ribosomes: 70S, composed of protein and RNA

• Cytoskeleton: Simple, helical structure

Archaeal Cell Structure

External Structures

Archaea possess glycocalyces, flagella, fimbriae, and hami, with unique features compared to bacteria.

• Glycocalyces: Biofilm formation, adherence

• Flagella: Basal body, hook, filament; differences from bacterial flagella

• Fimbriae and hami: Attachment structures

Cell Walls and Cytoplasmic Membranes

Most archaea have cell walls without peptidoglycan, composed of specialized polysaccharides and proteins. All have cytoplasmic membranes.

• Cell wall: No peptidoglycan

• Cytoplasmic membrane: Maintains gradients, controls import/export

Cytoplasm of Archaea

Archaeal cytoplasm is similar to bacteria but differs in ribosomal proteins, metabolic enzymes, and genetic code.

• 70S ribosomes

• Fibrous cytoskeleton

• Circular DNA

• Differences: Ribosomal proteins, metabolic enzymes, genetic code

Eukaryotic Cell Structure

External Structures

Eukaryotic cells have glycocalyces that are less organized than prokaryotic capsules. They aid in cell anchoring, surface strengthening, dehydration protection, and cell recognition.

Cell Walls and Cytoplasmic Membranes

Cell walls are present in fungi, algae, plants, and some protozoa, composed of various polysaccharides. All eukaryotic cells have a fluid mosaic cytoplasmic membrane with steroid lipids and membrane rafts.

• Cellulose: Plant cell walls

• Chitin, glucomannan: Fungal cell walls

• Various polysaccharides: Algal cell walls

• Membrane rafts: Regions of lipids and proteins

Endocytosis

Eukaryotic cells can internalize substances via endocytosis, a process involving membrane invagination.

Flagella and Cilia

Eukaryotic flagella differ structurally and functionally from prokaryotic flagella. Cilia are shorter and more numerous, used for movement and moving substances past the cell surface.

• Flagella: Tubulin microtubules, undulate rhythmically

• Cilia: Coordinated beating, propels cells

Other Nonmembranous Organelles

Eukaryotic cells contain ribosomes (80S), cytoskeleton (tubulin, actin, intermediate filaments), centrioles, and centrosomes.

• Ribosomes: 80S (60S + 40S subunits)

• Cytoskeleton: Extensive network

• Centrioles: Role in mitosis, cytokinesis, flagella/cilia formation

• Centrosome: Region containing centrioles

Membranous Organelles

Eukaryotic cells possess several membrane-bound organelles, each with specialized functions.

• Nucleus: Contains DNA, nucleoplasm, chromatin, nucleoli, nuclear envelope, nuclear pores

• Endoplasmic reticulum: SER (lipid synthesis), RER (protein synthesis)

• Golgi body: Processes and packages molecules for export

• Lysosomes: Catabolic enzymes

• Peroxisomes: Degrade poisonous wastes

• Vacuoles and vesicles: Storage and transport

• Mitochondria: ATP production, contains 70S ribosomes and circular DNA

• Chloroplasts: Light-harvesting, contains 70S ribosomes and DNA

Endosymbiotic Theory

The endosymbiotic theory proposes that eukaryotes originated from a symbiotic relationship between small aerobic prokaryotes and larger anaerobic prokaryotes, leading to the development of mitochondria and chloroplasts.

• Small aerobic prokaryotes became internal parasites

• Larger cell became dependent on parasites for ATP production

• Aerobic prokaryotes evolved into mitochondria

• Similar scenario for chloroplasts

Additional info: The endosymbiotic theory is supported by the presence of 70S ribosomes and circular DNA in mitochondria and chloroplasts, similar to prokaryotes.