Cell Structure and Function

Processes of Life

All living cells exhibit fundamental processes that define life. These include growth, reproduction, responsiveness, and metabolism. Understanding these processes is essential for studying microbiology.

• Growth: Increase in size and complexity.

• Reproduction: Ability to produce new cells or organisms.

• Responsiveness: Reacting to environmental stimuli.

• Metabolism: Chemical reactions that sustain life.

Types of Cells: Prokaryotes vs. Eukaryotes

Cells are classified into two main types: prokaryotic and eukaryotic. Each type has distinct structural and functional characteristics.

• Prokaryotes: Lack a nucleus and membrane-bound organelles; include bacteria and archaea; typically small (~1.0 µm).

• Eukaryotes: Possess a nucleus and internal membrane-bound organelles; include algae, protozoa, fungi, animals, and plants; larger (10–100 µm).

Cell Size Comparison

Microbial cells vary greatly in size, from viruses to protozoa and multicellular organisms. Understanding scale is crucial for microscopy and classification.

• Viruses: ~0.3 µm diameter

• Bacteria: ~1 µm diameter

• Protozoa: ~14 µm length

• Chicken egg: ~47,000 µm diameter

Prokaryotic Cell Structure

Overview of Prokaryotic Cell

Prokaryotic cells have a simple structure, lacking a nucleus and most organelles. Key components include the cytoplasmic membrane, cell wall, ribosomes, nucleoid, and external structures such as flagella and glycocalyx.

External Structures of Bacterial Cells

Glycocalyces

The glycocalyx is a gelatinous, sticky substance surrounding the cell, composed of polysaccharides, polypeptides, or both. It plays a role in protection and adherence.

• Capsule: Organized, firmly attached, may prevent recognition by host.

• Slime layer: Loosely attached, water soluble, aids in surface attachment.

Flagella

Flagella are long, whip-like structures responsible for bacterial motility. They consist of a filament, hook, and basal body, which anchors the flagellum to the cell wall and membrane.

• Structure: Filament, hook, basal body (with rings and rods).

• Function: Rotation propels bacteria; movement can be toward (runs) or away (tumbles) from stimuli (taxis).

Fimbriae and Pili

Fimbriae are short, bristle-like projections used for adherence, while pili (conjugation pili) are longer and involved in DNA transfer between cells.

• Fimbriae: Important in biofilm formation and attachment.

• Pili: Mediate conjugation, typically one or two per cell.

Bacterial Cell Walls

Bacterial cell walls provide structure, shape, and protection from osmotic forces. They are primarily composed of peptidoglycan and are classified as Gram-positive or Gram-negative based on their structure and staining properties.

• Gram-positive: Thick peptidoglycan, teichoic acids, purple after Gram stain.

• Gram-negative: Thin peptidoglycan, outer membrane with LPS, pink after Gram stain.

Bacterial Cytoplasmic Membranes

Structure

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

Function

• Energy storage

• Selective permeability

• Maintains concentration and electrical gradients

Transport Processes

• Passive: Diffusion, facilitated diffusion, osmosis

• Active: Active transport, group translocation (substance chemically modified during transport)

Cytoplasm of Bacteria

• Cytosol: Liquid portion of cytoplasm

• Inclusions: Reserve deposits of chemicals

• Endospores: Defensive structures produced by some bacteria

Nonmembranous Organelles

• Ribosomes: Sites of protein synthesis

• Cytoskeleton: Maintains cell shape

Archaeal Cell Structure

External Structures

• Glycocalyces: Biofilm formation, adherence

• Flagella: Basal body, hook, filament; differences from bacterial flagella

• Fimbriae and Hami: Attachment structures

Cell Walls and Membranes

• Cell walls: No peptidoglycan, specialized polysaccharides and proteins

• Cytoplasmic membranes: Maintain gradients, control import/export

Cytoplasm

• Similarities: 70S ribosomes, fibrous cytoskeleton, circular DNA

• Differences: Ribosomal proteins, metabolic enzymes, genetic code

Eukaryotic Cell Structure

External Structures

• Glycocalyces: Less organized, anchor cells, strengthen surface, protection, recognition

Cell Walls and Cytoplasmic Membranes

• Cell walls: Found in fungi, algae, plants, some protozoa; composed of cellulose, chitin, glucomannan, or other polysaccharides

• Cytoplasmic membrane: Fluid mosaic, phospholipids, proteins, steroids, membrane rafts

Cytoplasm of Eukaryotes

Flagella and Cilia

• Flagella: Tubulin microtubules, undulate rhythmically, anchored by basal body

• Cilia: Shorter, numerous, coordinated beating for movement and substance transport

Other Nonmembranous Organelles

• Ribosomes: Larger (80S), composed of 60S and 40S subunits

• Cytoskeleton: Tubulin microtubules, actin microfilaments, intermediate filaments

• Centrioles and centrosome: Mitosis, cytokinesis, flagella/cilia formation

Membranous Organelles

• Nucleus: Largest organelle, contains DNA, nucleoplasm, chromatin, nucleoli, nuclear envelope, nuclear pores

• Endoplasmic reticulum: Transport system; smooth (SER) and rough (RER)

• Golgi body: Processes and packages molecules for export

• Lysosomes, peroxisomes, vacuoles, vesicles: Storage, transfer, catabolic enzymes, degradation of wastes

• Mitochondria: ATP production, two membranes, matrix with 70S ribosomes and DNA

• Chloroplasts: Light-harvesting, two membranes, DNA, 70S ribosomes

Endosymbiotic Theory

The endosymbiotic theory proposes that eukaryotes originated from a symbiotic relationship between small aerobic prokaryotes and larger anaerobic prokaryotes. The smaller prokaryotes became mitochondria and chloroplasts.

Example: Bacterial cell walls can be targeted by antibiotics, such as penicillin, which inhibits peptidoglycan synthesis.

Additional info: The notes expand on brief points to provide context for cell structure and function, including definitions, examples, and comparisons relevant to microbiology students.