Cell Structure and Function: Microbiology Study Guide

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Cell Structure and Function

Major Characteristics of Life in Microorganisms

Microorganisms exhibit fundamental characteristics of life, which distinguish them from non-living entities such as viruses. These characteristics include growth, reproduction, responsiveness, metabolism, and cellular structure.

Growth: Increase in size; occurs in bacteria, archaea, and eukaryotes, but not in viruses.
Reproduction: Increase in number; all cellular microbes reproduce, while viruses rely on host cells for replication.
Responsiveness: Ability to react to environmental stimuli; present in all cellular microbes, limited in viruses.
Metabolism: Controlled chemical reactions; all cellular microbes metabolize, viruses use host metabolism.
Cellular Structure: Membrane-bound structure capable of all above functions; absent in viruses.

Example: Bacteria such as Escherichia coli grow, reproduce, respond to stimuli, and metabolize nutrients independently.

Microbial Classification and Cell Types

Microbiologists classify microorganisms based on cell structure and function. The two primary cell types are prokaryotic and eukaryotic cells.

Prokaryotes: Lack a nucleus and membrane-bound organelles; include bacteria and archaea.
Eukaryotes: Possess a nucleus and internal membrane-bound organelles; include algae, protozoa, fungi, animals, and plants.

Example: E. coli (prokaryote) and Paramecium (eukaryote) differ in complexity and size.

SEM image of E. coli and LM image of Paramecium

Prokaryotic Cell Structure

Prokaryotic cells are typically small (1.0 µm or less), lack a nucleus, and have a simple internal structure. They can simultaneously read DNA and synthesize proteins.

TEM image of a typical prokaryotic cell

Eukaryotic Cell Structure

Eukaryotic cells are larger (10–100 µm), have a nucleus, and contain complex internal organelles. Their structure allows for compartmentalization of cellular functions.

TEM image of a typical eukaryotic cell

Cell Size Comparison

Microbial cells vary greatly in size, from viruses (0.3 µm) to protozoa (14 µm) and multicellular organisms.

Size comparison of chicken egg, virus, bacterium, and protozoan

External Structures of Bacterial Cells

Glycocalyces

Bacterial cells may possess a glycocalyx, a gelatinous, sticky substance surrounding the cell. It is composed of polysaccharides, polypeptides, or both, and serves protective and adhesive functions.

TEM images of bacterial glycocalyx: capsule and slime layer

Types of Glycocalyces

Capsule: Organized, firmly attached; protects against host recognition (e.g., Streptococcus pneumoniae).
Slime Layer: Loosely attached, water-soluble; aids in surface attachment (e.g., dental plaque).

Flagella

Flagella are long, whip-like structures responsible for bacterial motility. They consist of a filament, hook, and basal body, and their arrangement varies among species.

Structure of bacterial flagella SEM images of basic arrangements of bacterial flagella Diagram of flagella arrangements: monotrichous, amphitrichous, lophotrichous, peritrichous

Axial Filament

Some bacteria, such as spirochetes, possess axial filaments that enable corkscrew motion.

Spirochetes and axial filament structure

Flagellar Function

Rotation: Propels bacteria; can be clockwise or counterclockwise.
Taxis: Movement in response to stimuli; includes runs and tumbles.

Motion of a peritrichous bacterium: runs and tumbles

Fimbriae and Pili

Fimbriae are short, bristle-like projections used for adhesion and biofilm formation. Pili are longer, specialized fimbriae involved in DNA transfer (conjugation).

SEM image showing flagella and fimbriae SEM image of biofilm matrix and bacteria SEM image of pilus connecting two bacterial cells

Comparison Table: Flagella, Fimbriae, and Pili

Feature	Flagella	Fimbriae	Pili
Main function	Movement (motility)	Attachment (adhesion)	DNA transfer (conjugation)
Appearance	Long, whip-like	Short, bristle-like	Longer than fimbriae, shorter than flagella
Number	Varies	Numerous	One or two per cell
Main role	Motility	Adhesion	Genetic exchange
Important process	Taxis	Biofilm formation	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, a polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by tetrapeptides.

Diagram of cocci and bacilli shapes Biomolecules overview: carbohydrates, proteins, nucleic acids, lipids Structure of peptidoglycan: NAG, NAM, and tetrapeptides

Gram-Positive vs. Gram-Negative Cell Walls

Gram-Positive: Thick peptidoglycan layer, teichoic acids, stains purple.
Gram-Negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), stains pink.

Structure of Gram-positive cell wall Structure of Gram-negative cell wall

Bacterial Cytoplasmic Membrane

Structure

The cytoplasmic membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model. It separates the cell interior from the environment and is essential for selective permeability.

Structure of a phospholipid bilayer

Function

Selective Permeability: Controls passage of substances.
Transport: Facilitates movement of molecules via passive and active processes.
Gradient Maintenance: Maintains concentration and electrical gradients.

Membrane permeability Electrical potential of a cytoplasmic membrane

Transport Mechanisms

Passive Transport

Diffusion: Movement of molecules down their gradient.
Facilitated Diffusion: Uses channels or carrier proteins.
Osmosis: Diffusion of water across a semipermeable membrane.

Principles of diffusion Passive transport mechanisms

Active Transport

Active Transport: ATP-dependent movement against gradient.
Group Translocation: Substance is chemically modified during transport.

Transport Processes Table

Process	Description	Examples
Diffusion	Molecules move down gradient through bilayer	Oxygen, CO2, lipid-soluble chemicals
Facilitated diffusion	Through channels/carrier proteins	Glucose, urea, vitamins
Osmosis	Water moves down gradient	Water
Active transport	ATP-dependent carrier proteins	Na+, K+, Ca2+, H+, Cl-
Group translocation	Chemical modification during transport	Glucose, mannose, fructose

Cytoplasm of Bacteria

Cytosol and Inclusions

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

Endospores

Endospores are highly resistant structures formed by some bacteria as a defense against unfavorable conditions. They withstand heat, radiation, and chemicals.

Nonmembranous Organelles

Ribosomes: Sites of protein synthesis; prokaryotes have 70S ribosomes.
Cytoskeleton: Protein fibers involved in cell shape, division, and DNA segregation.

Archaeal Cell Structure

External Structures

Glycocalyces: Aid in biofilm formation and adhesion.
Flagella: Structurally distinct from bacterial flagella.
Fimbriae and Hami: Attachment structures; hami are unique to archaea.

Cell Walls and Membranes

Cell Walls: Composed of polysaccharides or proteins, not peptidoglycan.
Cytoplasmic Membranes: Maintain gradients and control transport.

Cytoplasm

Similar to bacteria: 70S ribosomes, fibrous cytoskeleton, circular DNA.
Differences: Ribosomal proteins and metabolic enzymes more similar to eukaryotes.

Comparison Table: Prokaryotic Structural Features

Feature	Archaea	Bacteria
Glycocalyx	Polypeptide/polysaccharide	Polypeptide/polysaccharide
Flagella	Present in some; grow at base	Present in some; grow at tip
Fimbriae	Attachment, biofilms	Attachment, motility, biofilms
Pili	None discovered	Present in some; DNA exchange
Hami	Present in some	Absent
Cell Walls	Polysaccharides/proteins	Peptidoglycan
Cytoplasmic Membrane	Ether linkages	Ester linkages
Cytoplasm	Circular DNA, 70S ribosomes	Circular DNA, 70S ribosomes

Eukaryotic Cell Structure

External Structures

Glycocalyces: Less organized; anchor cells, strengthen surface, protect against dehydration, and aid in recognition.

Cell Walls and Cytoplasmic Membranes

Cell Walls: Found in fungi, algae, plants, some protozoa; composed of cellulose, chitin, glucomannan, or other polysaccharides.
Cytoplasmic Membranes: Fluid mosaic of phospholipids and proteins; contain steroids for fluidity and membrane rafts.

Eukaryotic cell wall structure Eukaryotic cytoplasmic membrane

Transport Processes Unique to Eukaryotes

Endocytosis: Uptake of substances via pseudopods; includes phagocytosis (solids) and pinocytosis (liquids).
Exocytosis: Release of substances via vesicle fusion.

Cytoplasm of Eukaryotes

Flagella and Cilia

Flagella: Tubulin-based, "9+2" microtubule arrangement, undulate for movement.
Cilia: Shorter, more numerous, coordinated beating for movement and substance transport.

Eukaryotic flagella and cilia structure Movement of eukaryotic flagella and cilia Eukaryotic cilia structure Movement of eukaryotic cilia

Other Nonmembranous Organelles

Ribosomes: Larger (80S) than prokaryotic ribosomes; composed of 60S and 40S subunits.
Cytoskeleton: Tubulin microtubules, actin microfilaments, intermediate filaments; anchors organelles and shapes cell.
Centrioles and Centrosome: Involved in mitosis, cytokinesis, flagella/cilia formation; not present in all eukaryotes.

Eukaryotic cytoskeleton Centrosome structure

Membranous Organelles

Nucleus: Contains DNA, nucleoplasm, chromatin, nucleoli, nuclear envelope with pores.
Endoplasmic Reticulum (ER): Transport system; smooth (SER) and rough (RER).
Golgi Body: Processes and packages molecules for export.
Lysosomes, Peroxisomes, Vacuoles, Vesicles: Storage, digestion, detoxification.
Mitochondria: ATP production; two membranes, matrix with 70S ribosomes and circular DNA.
Chloroplasts: Photosynthesis; two membranes, DNA, 70S ribosomes.

Eukaryotic nucleus structure Endoplasmic reticulum structure Golgi body structure

Comparison Table: Organelles in Prokaryotes and Eukaryotes

Organelle	Prokaryotes	Eukaryotes
Ribosomes	Present (70S)	Present (80S in cytosol/ER, 70S in mitochondria/chloroplasts)
Cytoskeleton	Present in some	Present in all
Centrosome	Absent	Present in animals
Nucleus	Absent	Present in all
ER	Absent	Present in all
Golgi bodies	Absent	Present in some
Lysosomes	Absent	Present in some
Peroxisomes	Absent	Present in some
Vacuoles/Vesicles	Absent	Present in some/all
Mitochondria	Absent	Present in most
Chloroplasts	Absent	Present in plants/algae

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.

Comparison Table: Archaeal, Bacterial, and Eukaryotic Cells

Characteristic	Archaea	Bacteria	Eukaryotes
Nucleus	Absent	Absent	Present
Membrane-bound organelles	Absent	Present in few	Various types present in all
Glycocalyx	Present	Present	Present in some
Motility	Present in some	Present in some	Present in some
Flagella	Some have flagella	Some have flagella	Some have flagella/cilia
Cilia	Absent	Absent	Present in some
Fimbriae/Pili	Present in some	Present in some	Absent
Hami	Present in some	Absent	Absent
Cell wall	Most lack peptidoglycan	Most have peptidoglycan	Present in plants, algae, fungi
Cytoplasmic membrane	Present in all	Present in all	Present in all
Cytosol	Present in all	Present in all	Present in all
Inclusions	Present in most	Present in most	Present in some
Endospores	Absent	Present in some	Absent
Ribosomes	Small (70S)	Small (70S)	Large (80S), small (70S in mitochondria/chloroplasts)
Chromosomes	Single, circular	Single, circular	Linear, multiple