Cell Structure and Function in Microbiology

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Cell Structure and Function

Introduction to the Processes of Life

All living organisms share several fundamental characteristics that define life. These include growth, reproduction, responsiveness, metabolism, and cellular structure. Microbes, including bacteria, archaea, eukaryotes, and viruses, exhibit these characteristics to varying degrees.

Growth: Increase in size.
Reproduction: Increase in number.
Responsiveness: Ability to react to environmental stimuli.
Metabolism: Controlled chemical reactions of organisms.
Cellular Structure: Membrane-bound structure capable of all of the above functions.

Viruses differ from cellular life forms as they lack independent metabolism and cellular structure, relying on host cells for replication.

Types of Cells: Prokaryotes vs. Eukaryotes

Cells are classified as either prokaryotic or eukaryotic based on structural differences.

Prokaryotes: Lack a nucleus and membrane-bound organelles; include bacteria and archaea. Typically 1.0 µm or smaller.
Eukaryotes: Have a nucleus and internal membrane-bound organelles; include algae, protozoa, fungi, animals, and plants. Typically 10–100 µm in diameter.

SEM and LM images of prokaryotic and eukaryotic cells Diagram of a typical prokaryotic cell Diagram of a typical eukaryotic cell Relative sizes of virus, bacterium, protozoan, and chicken egg

External Structures of Bacterial Cells

Glycocalyces

The glycocalyx is a gelatinous, sticky substance surrounding the outside of some bacterial cells, composed of polysaccharides, polypeptides, or both. It serves as protection and aids in attachment to surfaces.

Capsule: Organized, firmly attached; may prevent recognition by host immune system.
Slime Layer: Loosely attached, water-soluble; facilitates attachment to surfaces.

TEM images of bacterial capsule and slime layer

Flagella

Flagella are long, whip-like structures responsible for bacterial motility. Not all bacteria possess flagella.

Structure: Composed of filament, hook, and basal body. The basal body anchors the flagellum to the cell wall and membrane.
Arrangement: Can be monotrichous, lophotrichous, amphitrichous, or peritrichous.

Structure of bacterial flagellum SEM images of different flagellar arrangements Diagram of flagellar arrangements

Flagellar Function and Movement

Flagella rotate to propel bacteria through their environment. Movement can be in response to stimuli (taxis), with two main types of movement:

Runs: Straight movement when flagella rotate counterclockwise as a bundle.
Tumbles: Random changes in direction when flagella rotate clockwise independently.

Diagram of bacterial run and tumble movement

Fimbriae and Pili

Fimbriae are short, bristlelike projections used for attachment to surfaces and in biofilm formation. Pili (conjugation pili) are longer than fimbriae but shorter than flagella and are involved in DNA transfer between cells (conjugation).

SEM image showing fimbriae and flagella SEM image of biofilm matrix SEM image of pilus

Bacterial Cell Walls

Structure and Function

Bacterial cell walls provide structure, shape, and protection from osmotic forces. They are primarily composed of peptidoglycan, a complex polysaccharide.

Bacterial shapes and arrangements Structures of glucose, NAG, and NAM Structure of peptidoglycan

Gram-Positive vs. Gram-Negative Cell Walls

Gram-Positive: Thick peptidoglycan layer, teichoic acids, stains purple, may contain mycolic acid in acid-fast bacteria.
Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), stains pink, Lipid A can trigger immune responses.

Gram-positive cell wall structure Gram-negative cell wall structure

Bacteria Without Cell Walls

Some bacteria lack cell walls and are often mistaken for viruses due to their small size. However, they retain other prokaryotic features such as ribosomes.

Bacterial Cytoplasmic Membranes

Structure

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

Structure of a prokaryotic cytoplasmic membrane

Function

Controls passage of substances (selectively permeable).
Maintains concentration and electrical gradients.
In photosynthetic bacteria, harvests light energy.

Membrane permeability illustration Electrical potential of a cytoplasmic membrane

Transport Across Membranes

Passive Transport: Diffusion, facilitated diffusion, osmosis (no energy required).
Active Transport: Uses ATP to move substances against gradients; includes group translocation (substance is chemically modified during transport).

Principles of diffusion Passive processes of movement across a membrane

Process	Description	Examples of Transported Substances
Diffusion	Molecules move down their electrochemical gradient through the phospholipid bilayer.	Oxygen, carbon dioxide, lipid-soluble chemicals
Facilitated diffusion	Molecules move down their gradient through channels or carrier proteins.	Glucose, fructose, urea, some vitamins
Osmosis	Water molecules move down their concentration gradient across a selectively permeable membrane.	Water
Active transport	ATP-dependent carrier proteins bring substances into cell.	Na+, K+, Ca2+, H+, Cl−
Group translocation	Substance is chemically altered during transport.	Glucose, mannose, fructose

Cytoplasm of Bacteria

Cytosol and Inclusions

The cytosol is the liquid portion of the cytoplasm, containing water, ions, and the cell's DNA in the nucleoid region. Inclusions are reserve deposits of chemicals.

Endospores

Endospores are unique, highly resistant structures formed by some bacteria as a defensive strategy against unfavorable conditions. They can survive extreme heat, radiation, and chemicals.

Archaeal Cell Structure

External Structures

Glycocalyces: Aid in biofilm formation and adherence.
Flagella: Structurally different from bacterial flagella.
Fimbriae and Hami: Hami are unique, hook-like structures for attachment.

Cell Walls and Membranes

Most archaea have cell walls made of specialized polysaccharides or proteins (not peptidoglycan).
All archaea have cytoplasmic membranes with ether-linked lipids.

Cytoplasm

Contains 70S ribosomes, fibrous cytoskeleton, and circular DNA.
Ribosomal proteins and metabolic enzymes differ from those in bacteria.

Eukaryotic Cell Structure

External Structures

Glycocalyces: Less organized than prokaryotic capsules; aid in cell recognition and protection.

Cell Walls and Membranes

Fungi, algae, plants, and some protozoa have cell walls made of cellulose, chitin, or other polysaccharides.
All eukaryotes have cytoplasmic membranes with steroid lipids and membrane rafts.

Flagella and Cilia

Flagella: Structurally distinct from prokaryotic flagella; composed of microtubules and move by undulation.
Cilia: Shorter, more numerous; move cells or substances past the cell surface.

Nonmembranous Organelles

Ribosomes: 80S in eukaryotes (except mitochondria and chloroplasts, which have 70S).
Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments for shape and movement.
Centrioles and Centrosome: Involved in cell division and formation of flagella/cilia (in animals).

Membranous Organelles

Nucleus: Contains DNA, nucleolus, and is surrounded by a nuclear envelope with pores.
Endoplasmic Reticulum (ER): SER (lipid synthesis) and RER (protein synthesis).
Golgi Body: Processes and packages molecules for export.
Lysosomes, Peroxisomes, Vacuoles, Vesicles: Storage, digestion, and detoxification.
Mitochondria: Site of ATP production; contain their own DNA and 70S ribosomes.
Chloroplasts: Photosynthetic organelles in plants and algae; contain DNA and 70S ribosomes.

Endosymbiotic Theory

This theory proposes that eukaryotic cells originated from the symbiotic union of small aerobic prokaryotes (which became mitochondria) with larger anaerobic prokaryotes. Chloroplasts are thought to have a similar origin.

Comparative Table: Prokaryotes vs. Eukaryotes vs. Archaea

Feature	Bacteria	Archaea	Eukaryotes
Nucleus	Absent	Absent	Present
Cell Wall	Peptidoglycan	Polysaccharides/proteins (no peptidoglycan)	Cellulose, chitin, or absent
Membrane Lipids	Ester-linked	Ether-linked	Ester-linked
Ribosomes	70S	70S	80S (70S in mitochondria/chloroplasts)
Flagella	Basal body, hook, filament; rotate	Basal body, hook, filament; rotate (different structure)	Microtubules; undulate
Genetic Material	Circular DNA	Circular DNA	Linear chromosomes