Cell Structure and Function: Microbiology Study Notes

Cell Structure and Function

Characteristics of Life in Microbes

Microbes exhibit fundamental processes of life, which distinguish living organisms from non-living matter. These processes include growth, reproduction, responsiveness, and metabolism.

• Growth: Increase in size or number of cells.

• Reproduction: Ability to produce new cells or organisms.

• Responsiveness: Reacting to environmental stimuli.

• Metabolism: Chemical reactions that provide energy and build cellular structures.

• Example: Mycoplasma, the smallest free-living microbe, is alive despite being nonmotile because it performs all other life processes.

Types of Cells: Prokaryotic vs. Eukaryotic

Cells are classified as prokaryotic or eukaryotic based on structural features. This distinction is fundamental in microbiology.

• Prokaryotes:

  • Lack a nucleus

  • Simultaneously read DNA and synthesize proteins

  • Lack internal membrane-bound organelles

  • Typically 1.0 µm or smaller

  • Includes bacteria and archaea

• Eukaryotes:

  • Have a nucleus

  • Contain internal membrane-bound organelles

  • Larger: 10–100 µm

  • More complex structure

  • Includes algae, protozoa, fungi, animals, and plants

Cell Size Comparison

Microbial cells vary greatly in size, from viruses to protozoa and even multicellular organisms.

• Viruses: ~0.3 µm diameter

• Bacteria: ~1 µm diameter

• Protozoa: ~14 µm length

• Chicken egg: 47,000 µm diameter (for scale)

External Structures of Bacterial Cells

Glycocalyces

Bacterial cells may be surrounded by a glycocalyx, a gelatinous, sticky substance composed of polysaccharides, polypeptides, or both.

• Capsule: Organized, firmly attached; protects against host recognition.

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

Flagella

Flagella are long, whip-like structures responsible for bacterial motility. Not all bacteria possess flagella.

• Structure: Composed of filament, hook, and basal body. The basal body anchors the flagellum to the cell wall.

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

Fimbriae and Pili

Fimbriae and pili are surface structures that facilitate attachment and genetic exchange.

• Fimbriae: Sticky, bristlelike projections; important in biofilm formation and adherence.

• Pili: Longer than fimbriae, fewer per cell; specialized for DNA transfer (conjugation).

Bacterial Cell Walls

Structure and Function

Bacterial cell walls provide structural support, shape, and protection from osmotic forces. They are a target for antibiotics and contribute to characteristic cell shapes.

Peptidoglycan Composition

The main component of bacterial cell walls is peptidoglycan, a polymer of sugars and amino acids.

• N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) are the primary sugars.

• Peptidoglycan forms a mesh-like structure with cross-linked chains.

Gram-Positive vs. Gram-Negative Cell Walls

Bacteria are classified based on cell wall structure, which affects staining and susceptibility to antibiotics.

• Gram-Positive:

  • Thick peptidoglycan layer

  • Contains teichoic acids and lipoteichoic acids

  • Stains purple in Gram stain

  • Acid-fast bacteria have mycolic acid for desiccation resistance

• Gram-Negative:

  • Thin peptidoglycan layer

  • Outer membrane with phospholipids, proteins, and lipopolysaccharide (LPS)

  • Lipid A in LPS can cause fever and shock

  • Stains pink in Gram stain

Bacteria Without Cell Walls

Some bacteria lack cell walls and are often mistaken for viruses due to their small size. However, they possess other prokaryotic features such as ribosomes.

Bacterial Cytoplasmic Membranes

Structure

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

• Integral proteins: Span the membrane

• Peripheral proteins: Attached to the membrane surface

Function

The membrane controls passage of substances, maintains gradients, and in photosynthetic bacteria, harvests light energy. It is selectively permeable and proteins facilitate transport.

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 unique, highly resistant structures produced by some bacteria as a defense against unfavorable conditions. They can survive extreme heat, radiation, and chemicals.

Cytoplasm of Eukaryotes

Nonmembranous Organelles: Ribosomes

Ribosomes are the sites of protein synthesis. Prokaryotic ribosomes are 70S, while eukaryotic ribosomes are 80S, reflecting structural differences.

Nonmembranous Organelles: Cytoskeleton

The cytoskeleton is composed of protein fibers and plays roles in cell division, shape, DNA segregation, and movement.

Summary Table: Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
LPS (Lipid A)	Absent	Present
Gram Stain Color	Purple	Pink
Resistance to Desiccation	High (acid-fast)	Lower

Key Equations

Osmosis: Movement of water across a selectively permeable membrane.

$\Delta \Psi = \Psi_{inside} - \Psi_{outside}$

Peptidoglycan Structure: Repeating units of NAG and NAM linked by peptide bridges.

$\text{Peptidoglycan} = (\text{NAG} - \text{NAM})_n$

Conclusion

Understanding cell structure and function is fundamental in microbiology, providing insight into microbial classification, physiology, and the mechanisms underlying their survival and pathogenicity.