Microbiology Chapter 3: Cell Structure and Function – Study Guide

Chapter Overview

This chapter explores the fundamental differences and similarities among prokaryotic and eukaryotic cells, their structures, and the mechanisms that support cellular life. Key topics include cell membranes, cell walls, organelles, transport mechanisms, and the endosymbiotic theory.

Characteristics of Life

Defining Features of Living Organisms

• Growth: Increase in size.

• Reproduction: Increase in number of cells or organisms.

• Responsiveness: Ability to react to environmental stimuli.

• Metabolism: Controlled chemical reactions that sustain life.

Viruses vs Living Cells: Viruses are not considered fully living because they do not grow, cannot reproduce independently, rely on host metabolism, and lack cellular structure.

Prokaryotic vs Eukaryotic Cells

Major Differences and Examples

• Prokaryotes: Include Bacteria and Archaea. No nucleus or membrane-bound organelles, smaller size (≤1 µm), DNA in nucleoid, simpler structure, can simultaneously transcribe and translate DNA.

• Eukaryotes: Include Fungi, Protozoa, Algae, Plants, and Animals. Have a nucleus, membrane-bound organelles, larger size (10–100 µm), and more complex structure.

External Structures of Bacterial Cells

Glycocalyx

• Sticky outer coating for protection, adhesion, biofilm formation, and preventing dehydration.

• May help bacteria evade immune detection.

Type	Attachment	Function
Capsule	Firmly attached	Organized, protective, immune evasion
Slime Layer	Loosely attached	Water-soluble, surface attachment

Flagella

• Responsible for motility; composed of filament, hook, and basal body.

• Movement: Rotation propels bacteria (counterclockwise = runs, clockwise = tumbles).

• Taxis: Movement in response to stimuli (chemotaxis, phototaxis, aerotaxis).

Spirochetes and Axial Filaments

• Axial filaments (endoflagella) enable corkscrew motion through fluids/tissues.

• Examples: Treponema pallidum, Borrelia burgdorferi.

Fimbriae and Pili

• Fimbriae: Short, sticky projections for attachment and biofilm formation.

• Pili: Specialized fimbriae for DNA transfer (conjugation) and genetic exchange.

Bacterial Cell Walls

Main Functions

• Maintain shape, prevent osmotic lysis, provide protection, and aid in attachment.

Peptidoglycan

• Major structural component, composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

Gram-Positive vs Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Gram Stain	Purple	Pink

• Acid-Fast Bacteria: Contain mycolic acid (waxy, resistant to drying/chemicals). Example: Mycobacterium tuberculosis.

• Lipopolysaccharide (LPS): Found in Gram-negative outer membrane; Lipid A acts as endotoxin (causes fever, inflammation, shock, blood clotting).

Bacteria Without Cell Walls

• Example: Mycoplasma (lack peptidoglycan, sensitive to osmotic pressure, often mistaken for viruses).

Cytoplasmic Membrane

Structure and Function

• Described by the fluid mosaic model: phospholipid bilayer with integral and peripheral proteins.

• Functions: selective permeability, transport, maintaining concentration gradients, ATP production (in some bacteria).

Transport Across Membranes

Passive Transport

• No energy required.

• Diffusion: Movement from high to low concentration.

• Facilitated Diffusion: Uses carrier/channel proteins.

• Osmosis: Movement of water across membrane.

Solution Type	Solute Concentration	Water Movement	Cell Effect
Isotonic	Equal	None	No change
Hypertonic	Higher outside	Leaves cell	Cell shrinks
Hypotonic	Lower outside	Enters cell	Cell may burst

Active Transport

• Requires ATP; moves substances against concentration gradient.

• Examples: Na+, K+, Ca2+.

• Group Translocation: Substance is chemically modified during transport (e.g., glucose phosphorylation).

Cytoplasm of Bacteria

Components

• Cytosol: Liquid portion containing DNA, ribosomes, and nutrients.

• Inclusions: Storage granules for lipids, nutrients, and energy reserves.

• Endospores: Highly resistant survival structures (not reproductive), produced under nutrient limitation. Resistant to heat, radiation, chemicals, and drying. Examples: Bacillus, Clostridium.

Ribosomes

• Site of protein synthesis.

Cell Type	Ribosome Size
Prokaryotes	70S
Eukaryotes	80S

Cytoskeleton

• Maintains cell shape, assists in DNA segregation, movement, and cell division.

Archaea

Similarities and Differences with Bacteria

• Prokaryotic, no nucleus, 70S ribosomes, circular DNA.

• No peptidoglycan, unique membrane lipids, ribosomal proteins more similar to eukaryotes.

Archaeal Structures

• Hami: Hooked attachment structures for surface attachment.

• Archaeal Flagella: Structurally distinct from bacterial flagella; different proteins, grow at base.

Eukaryotic Cells

Cell Walls

Organism	Main Component
Plants	Cellulose
Fungi	Chitin
Algae	Various polysaccharides

Eukaryotic Membranes

• Composed of phospholipids, proteins, and sterols.

• Functions: transport, communication, fluidity maintenance.

Endocytosis and Exocytosis

• Endocytosis: Brings material into cell.

• Phagocytosis: Engulfs solids.

• Pinocytosis: Engulfs liquids.

• Exocytosis: Releases materials out of cell.

Eukaryotic Flagella and Cilia

• Flagella: Long, few, undulating movement.

• Cilia: Short, numerous, coordinated beating.

Major Eukaryotic Organelles

• Nucleus: Contains DNA, control center.

• Rough ER: Protein synthesis, ribosomes attached.

• Smooth ER: Lipid synthesis, detoxification.

• Golgi Body: Modifies and packages proteins.

• Lysosomes: Digestive enzymes.

• Peroxisomes: Detoxify harmful substances.

• Vacuoles: Storage.

• Mitochondria: ATP production, double membrane, own DNA and 70S ribosomes.

• Chloroplasts: Photosynthesis, found in plants/algae, own DNA and ribosomes.

Endosymbiotic Theory

Origin of Mitochondria and Chloroplasts

• Proposes that mitochondria and chloroplasts originated from engulfed bacteria.

• Evidence: own circular DNA, 70S ribosomes, double membranes, reproduce independently.

Important Comparisons to Memorize

Feature	Bacteria	Archaea	Eukaryotes
Nucleus	No	No	Yes
Organelles	No	No	Yes
Ribosomes	70S	70S	80S
Cell Wall	Peptidoglycan	No peptidoglycan	Variable
DNA	Circular	Circular	Linear
Endospores	Some	No	No

High-Yield Exam Concepts

• Know the structure and function of: capsule, slime layer, flagella, pili, fimbriae, peptidoglycan, Lipid A, endospore.

• Be able to compare: Gram-positive vs Gram-negative, prokaryote vs eukaryote, passive vs active transport, diffusion vs osmosis, flagella vs pili vs fimbriae.

• Know the function of organelles: nucleus, ER, Golgi, lysosome, mitochondria, chloroplast.

Common Exam Questions

• Which structure transfers DNA? Pili

• Which bacteria stain purple? Gram-positive

• Which structure contains endotoxin? Lipid A of LPS

• Which organelle produces ATP? Mitochondria

• Which cells contain peptidoglycan? Bacteria

• Which process requires ATP? Active transport

• Which structures help bacteria attach? Fimbriae and glycocalyx

• Which structures resist harsh environments? Endospores

Key Vocabulary

• Glycocalyx

• Capsule

• Slime layer

• Peptidoglycan

• Teichoic acid

• Lipopolysaccharide

• Endotoxin

• Osmosis

• Diffusion

• Active transport

• Group translocation

• Nucleoid

• Endospore

• Cytoskeleton

• Endocytosis

• Exocytosis

• Endosymbiotic theory