Cell Structure and Function

Introduction

This study guide covers the fundamental aspects of microbial cell structure and function, focusing on microscopy, staining techniques, cell morphology, membrane organization, and mechanisms of nutrient transport. These topics are essential for understanding how microorganisms are visualized, classified, and how they maintain cellular integrity and interact with their environment.

Microscopy in Microbiology

Principles of Light Microscopy

• Light Microscopy uses visible light to magnify specimens, allowing visualization of cells and some subcellular structures.

• Magnification is achieved through a series of lenses (objective and ocular).

• Resolving Power refers to the ability to distinguish two close objects as separate entities. Light microscopes typically resolve down to ~0.2 μm.

• Key Components: ocular lens, objective lens, stage, condenser, focusing knobs.

• Visualization Path: Light passes through the specimen, is focused by lenses, and forms a magnified image.

Electron Microscopy

• Transmission Electron Microscope (TEM): Provides detailed internal cell structure by passing electrons through thin sections.

• Scanning Electron Microscope (SEM): Visualizes surface structures by scanning specimens with electrons.

• Electron microscopes have much higher resolving power than light microscopes, allowing visualization of viruses and subcellular details.

Microscope Resolving Power Table

Staining Techniques

Why Stain Bacteria?

• Most cells lack natural pigment and are difficult to see under a light microscope unless stained.

• Staining reveals biochemical or structural properties, such as cell wall type and presence of endospores.

• Helps in identification, classification, and diagnosis of bacterial infections.

Types of Staining

• Simple Staining: Uses a single dye to color cells, enhancing visibility.

• Differential Staining: Distinguishes between cell types or structures. Includes:

  • Gram Staining

  • Endospore Staining

  • Capsule Staining

• Negative Staining: Stains the background, leaving cells unstained for contrast.

Staining Procedure Table

Gram Staining

• Gram-Positive Bacteria: Thick peptidoglycan layer, retains crystal violet stain (purple).

• Gram-Negative Bacteria: Thin peptidoglycan layer, outer membrane, does not retain crystal violet, counterstained (pink/red).

• Steps: Crystal violet → Iodine → Alcohol (decolorizer) → Safranin (counterstain).

Negative Staining

• Uses acidic dyes (e.g., nigrosin) that do not penetrate cells, resulting in a colored background and clear cells.

• Useful for visualizing capsules and cell morphology.

Cell Morphology and Size

Cell Shape and Size

• Cell Morphology: Refers to the shape and arrangement of bacterial cells.

• Common shapes: Cocci (spherical), Bacilli (rod-shaped), Spirilla (spiral), Vibrio (comma-shaped), Filamentous.

• Arrangements: chains, clusters, pairs, etc.

• Cell Size: Bacteria typically range from 0.5 to 5 μm; viruses are much smaller (20–300 nm).

Cell Morphology Table

Cell Organization

Prokaryotic vs. Eukaryotic Cells

• Prokaryotes: Lack a nucleus and membrane-bound organelles; include Bacteria and Archaea.

• Eukaryotes: Have a nucleus and organelles; include fungi, protozoa, algae.

• Viruses are not cells but are major microorganisms.

Common Bacterial Structures and Functions Table

Membrane Structure and Function

Fluid Mosaic Model

• Fluid Mosaic Model: Proposes that membranes are lipid bilayers with proteins floating within.

• Cell membranes are thin, typically 2–3 nm, and appear as two dark lines under TEM.

• Membranes are dynamic and flexible, with domains of concentrated lipids.

Bacterial Lipids

• Bacterial membranes contain phospholipids and often hopanoids (stabilizing molecules).

• Hopanoids are similar to sterols in eukaryotes but are unique to bacteria.

Phospholipid Structure

• Phospholipids are amphipathic: polar (hydrophilic) heads and nonpolar (hydrophobic) tails.

• In water, they form bilayers with hydrophobic tails inside and hydrophilic heads outside.

Membrane Proteins

• Peripheral Proteins: Loosely attached, easily removed, soluble.

• Integral Proteins: Embedded in membrane, insoluble, often span the bilayer.

Archaeal Membranes

• Archaea have unique membrane lipids: branched hydrocarbons attached to glycerol by ether links (not ester links).

• Some archaeal membranes are monolayers (C40 tetraethers) for stability in extreme environments.

Transport Mechanisms in Bacteria

Overview

• Plasma membranes act as barriers but must allow nutrient uptake.

• Transport mechanisms include:

  • Passive diffusion

  • Facilitated diffusion

  • Primary and secondary active transport

  • Group translocation

Passive Diffusion

• Molecules move from high to low concentration without energy input.

• Requires a large concentration gradient for effective nutrient uptake.

• Rate decreases as internal concentration increases.

Facilitated Diffusion

• Transport proteins (channels/carriers) assist movement across the membrane.

• Channels form pores; carriers bind and transport specific substances.

Primary Active Transport

• Uses energy from ATP hydrolysis to move substances against a concentration gradient.

• ABC transporters are important examples; they are uniporters (move one molecule at a time).

Secondary Active Transport

• Uses ion gradients to drive transport without direct energy input.

• Cotransporters move two substances simultaneously:

  • Symport: Both substances move in the same direction.

  • Antiport: Substances move in opposite directions.

Group Translocation

• Molecule is chemically modified as it is transported into the cell.

• Best-known system: phosphoenolpyruvate:sugar phosphotransferase system (PTS).

• PEP provides energy for sugar uptake and phosphorylation.

Bacterial Cell Walls

Types of Bacterial Cell Walls

• Gram-Positive: Thick (20–80 nm) peptidoglycan layer outside plasma membrane.

• Gram-Negative: Thin (2–7 nm) peptidoglycan layer plus 7–8 nm outer membrane.

Peptidoglycan Structure

• Composed of repeating subunits: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked by peptide chains.

• Provides rigidity and shape to the cell wall.

Cell Wall Structure Table

Summary

Understanding cell structure and function is foundational in microbiology. Microscopy and staining techniques are essential for visualizing and classifying microorganisms. The organization of cell membranes and walls, as well as mechanisms for nutrient transport, are critical for microbial survival and adaptation.