Bacterial Cell Structure

Cell Shape and Arrangement

Bacteria exhibit a variety of shapes and arrangements, which are important for identification and classification.

• Coccus (plural: cocci): Spherical-shaped bacteria. Arrangements include single, diplococci (pairs), streptococci (chains), staphylococci (clusters), tetrads (groups of four), and sarcinae (cubical packets).

• Bacillus (plural: bacilli): Rod-shaped bacteria. Arrangements include single, diplobacilli (pairs), streptobacilli (chains), and palisades (side-by-side).

• Spirillum: Rigid, spiral-shaped bacteria.

• Spirochete: Flexible, spiral-shaped bacteria.

• Vibrio: Comma-shaped bacteria.

Example: Streptococcus pyogenes forms chains of cocci; Escherichia coli is a single bacillus.

Glycocalyces: Capsule vs. Slime Layer

The glycocalyx is a gelatinous, sticky substance surrounding the outside of some bacterial cells, composed of polysaccharides, polypeptides, or both.

• Capsule: Organized, firmly attached to the cell wall; protects against phagocytosis and desiccation.

• Slime Layer: Loosely attached, unorganized; aids in adherence to surfaces and biofilm formation.

Example: Streptococcus pneumoniae has a capsule that enhances its virulence.

Pili: Attachment Pili (Fimbriae) vs. Sex Pili

Pili are hair-like appendages found on Gram-negative bacteria.

• Attachment Pili (Fimbriae): Short, numerous; help bacteria adhere to surfaces and each other.

• Sex Pili: Longer, fewer; involved in conjugation (transfer of DNA between bacteria).

Gram-Positive vs. Gram-Negative Cell Wall Structures

Bacterial cell walls differ in structure, affecting staining and antibiotic susceptibility.

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

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

Example: Staphylococcus aureus is Gram-positive; Escherichia coli is Gram-negative.

Gram Stain Mechanism

The Gram stain differentiates bacteria based on cell wall structure.

• Crystal violet stains all cells.

• Iodine forms a complex with crystal violet.

• Alcohol decolorizes Gram-negative cells (thin peptidoglycan), but not Gram-positive (thick peptidoglycan retains dye).

• Safranin counterstains Gram-negative cells pink/red.

Unique Cell Walls: Mycoplasma and Mycobacterium

• Mycoplasma: Lack a cell wall; have sterols in the plasma membrane for stability.

• Mycobacterium: Cell wall contains mycolic acid (waxy lipid), making them acid-fast and resistant to desiccation and chemicals.

Key Terms and Definitions

• Peripheral protein: Protein attached to the membrane surface.

• Integral protein: Protein embedded within the membrane, often spanning it.

• Selective permeability: Property of membranes allowing certain substances to pass while restricting others.

• Hypotonic: Solution with lower solute concentration than the cell; water enters the cell.

• Hypertonic: Solution with higher solute concentration than the cell; water leaves the cell.

• Isotonic: Solution with equal solute concentration as the cell; no net water movement.

• Osmotic pressure: Pressure required to prevent water movement across a semipermeable membrane.

Types of Membrane Transport

• Simple diffusion: Movement of small, nonpolar molecules from high to low concentration without energy or proteins.

• Facilitated diffusion: Movement of larger or polar molecules via membrane proteins (channels/carriers), no energy required.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Active transport: Movement of molecules against the concentration gradient using energy (ATP) and transport proteins.

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

Importance of the Cell Wall in Osmotic Pressure

The cell wall prevents bacterial cells from bursting in hypotonic environments by resisting osmotic pressure.

Internal Structures: Eukaryotes vs. Bacteria

• Bacteria: No membrane-bound organelles; have nucleoid, plasmids, ribosomes (70S), inclusions, endospores.

• Eukaryotes: Membrane-bound organelles (nucleus, mitochondria, ER, etc.), ribosomes (80S), cytoskeleton.

Plasmids

Plasmids are small, circular, double-stranded DNA molecules independent of the bacterial chromosome. They often carry genes for antibiotic resistance, virulence, or metabolism.

Endospores

Endospores are dormant, tough, non-reproductive structures formed by some bacteria (e.g., Bacillus, Clostridium) in response to harsh conditions. Sporulation is triggered by nutrient depletion; germination occurs when conditions improve. Endospores ensure survival under extreme conditions.

Comparison: Eukaryote vs. Bacterial Cell Structures

Microbial Metabolism

Key Terms and Definitions

• Metabolism: All chemical reactions in a cell.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.

• Catalyst: Substance that speeds up a chemical reaction without being consumed.

• Activation energy: Minimum energy required to start a reaction.

• Denaturation: Loss of protein structure and function due to environmental stress (e.g., heat, pH).

• Redox (reduction/oxidation) reaction: Transfer of electrons between molecules.

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• Substrate level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP.

• Oxidative phosphorylation: ATP synthesis using energy from electron transport chain and chemiosmosis.

• Electron transport chain (ETC): Series of membrane proteins transferring electrons to generate a proton gradient.

• Proton motive force: Electrochemical gradient of protons across a membrane, driving ATP synthesis.

Endergonic vs. Exergonic Reactions

• Endergonic: Require energy input (e.g., anabolism).

• Exergonic: Release energy (e.g., catabolism).

Cofactor vs. Coenzyme

• Cofactor: Non-protein component required for enzyme activity (e.g., metal ions).

• Coenzyme: Organic cofactor (e.g., NAD+, FAD, vitamins).

Enzyme-Substrate Complex: "Lock and Key" Model

Enzymes have a specific active site where the substrate binds, fitting precisely like a key in a lock, ensuring specificity.

Factors Affecting Enzyme Activity

• Temperature: High temperatures can denature enzymes; low temperatures slow activity.

• pH: Extreme pH can denature enzymes; each enzyme has an optimal pH.

• Saturation: Enzyme activity increases with substrate concentration until all active sites are occupied.

• Competitive inhibitor: Binds to active site, blocking substrate.

• Noncompetitive inhibitor: Binds to allosteric site, changing enzyme shape and reducing activity.

Types of Glycolysis in Prokaryotes

Additional info: EMP is the classic glycolysis pathway; ED and PPP are alternatives found in some bacteria.

Pathways of Aerobic Respiration

• Glycolysis: Occurs in cytoplasm (both cell types); produces 2 ATP, 2 NADH; does not require oxygen; end product is pyruvate.

• Krebs Cycle (Citric Acid Cycle): Occurs in cytoplasm (prokaryotes), mitochondria (eukaryotes); produces 2 ATP, 6 NADH, 2 FADH2 per glucose; requires oxygen indirectly; end products are CO2, NADH, FADH2.

• Electron Transport Chain (ETC): Occurs in plasma membrane (prokaryotes), inner mitochondrial membrane (eukaryotes); produces ~34 ATP; requires oxygen as final electron acceptor; end products are H2O and ATP.

Overall Equation for Aerobic Respiration

The complete oxidation of glucose is represented by:

Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Uses oxygen as final electron acceptor; produces more ATP.

• Anaerobic respiration: Uses inorganic molecules other than oxygen (e.g., nitrate, sulfate) as final electron acceptor; produces less ATP.

Fermentation

• Pathway: Glycolysis followed by conversion of pyruvate to organic end products.

• Types: Lactic acid fermentation (e.g., Lactobacillus), alcoholic fermentation (e.g., yeast).

• Fermentation vs. Anaerobic Respiration: Fermentation does not use an electron transport chain; final electron acceptor is organic.

Final Electron Acceptors

• Aerobic respiration: Oxygen (O2).

• Anaerobic respiration: Inorganic molecules (e.g., NO3-, SO42-).

• Fermentation: Organic molecules (e.g., pyruvate, acetaldehyde).

Catabolism vs. Anabolism

• Catabolism: Degradative, energy-releasing reactions.

• Anabolism: Biosynthetic, energy-consuming reactions.

Classification of Organisms by Metabolism