Functional Anatomy of Prokaryotic and Eukaryotic Cells & Microbial Growth Media

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Bacterial Cell Shape and Arrangement

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

• Shapes:

  • Coccus: Spherical shape

  • Bacillus: Rod-shaped

  • Vibrio: Bent rod shape

  • Spirillum: Rigid corkscrew shape

  • Spirochete: Flexible spiral, usually longer

• Arrangements:

  • Single: Individual cells

  • Diplococcus: Pairs of cocci

  • Tetrad: Groups of four

  • Sarcina: Groups of eight (cube shape)

  • Streptococcus: Chains of cocci

  • Staphylococcus: Clusters of cocci

• Micron: A millionth of a meter (μm), typical unit for bacterial size

Plasma Membrane Structure and Function

The plasma membrane is the outermost boundary of the cell, crucial for maintaining cellular integrity and regulating transport.

• Phospholipid Bilayer: Composed of hydrophilic (polar) phosphate heads and hydrophobic (nonpolar) fatty acid tails

• Integral Membrane Proteins: Embedded in the membrane, some act as channels or transporters

• Transport:

  • Passive Transport: Channels allow molecules to move down their concentration gradient without energy

  • Active Transport: Transporters use energy (often ATP) to move molecules against their gradient

• Receptors: Proteins that receive signals from the environment, triggering cellular responses via second messengers (transducers)

• Diffusion: Random movement of molecules, leading to even distribution; described by Brownian motion

Cytoplasmic Components

The cytoplasm is a gel-like substance inside the cell, containing essential structures for cellular function.

• Nucleoid: Region containing the bacterial chromosome (circular DNA)

• Ribosomes: Sites of protein synthesis; bacterial ribosomes are 70S (Svedberg units)

• Cytoskeleton: Protein fibers that maintain cell shape

• Inclusion Bodies: Storage areas for nutrients (e.g., sugars, lipids, iron)

Cell Wall Structure

The cell wall provides structural support and protection. Its composition is key to bacterial classification.

• Peptidoglycan: Polymer of N-acetyl muramic acid (NAM) and N-acetyl glucosamine (NAG) linked in chains

• Tetrapeptides: Short chains of amino acids cross-link peptidoglycan strands, forming a rigid structure

• Gram Positive: Thick peptidoglycan layer

• Gram Negative: Thin peptidoglycan layer plus an outer membrane

Gram Negative Cell Envelope

• Outer Membrane: Contains lipopolysaccharide (LPS), also known as endotoxin

• LPS Structure: Lipid A (nonpolar), core oligosaccharide (polar), O antigen (long sugar chain)

• Periplasmic Space: Area between plasma membrane and cell wall; site of peptidoglycan repair and antibiotic modification

External Structures

Bacteria may possess additional structures outside the cell wall, aiding in protection, attachment, and motility.

• Glycocalyx: Sugar coating; can be a capsule (organized, hard to remove) or slime layer (loose, easy to remove)

• Functions: Adhesion, protection from desiccation, evasion of immune system, biofilm formation

• Biofilm: Community of cells attached to a surface, embedded in glycocalyx

• S Layer: Protein layer outside the cell, self-assembling, aids in surface attachment

Pili and Motility Structures

• Pilus: Protein tube for attachment; types include:

  • Fimbria: Attachment to surfaces

  • F Pilus: DNA transfer (conjugation)

  • Type IV Pilus: Twitching motility

• Flagellum: Structure for swimming; consists of basal body, hook, and tail

• Movement: Taxis (response to stimulus); chemotaxis (chemical), phototaxis (light); positive (toward) or negative (away)

• Biased Random Walk: Alternates between straight swimming and tumbling to change direction

• Axial Filaments (Endoflagella): Found in spirochetes, located in periplasm, enable corkscrew movement

Eukaryotic Cell Structure

Eukaryotic cells are larger and more complex than prokaryotic cells, with compartmentalized organelles.

• Nucleus: Contains linear chromosomes; surrounded by a phospholipid bilayer

• Nucleolus: Area within nucleus, site of RNA synthesis

• Endoplasmic Reticulum (ER): Membranous network; rough ER (with ribosomes, protein synthesis), smooth ER (lipid synthesis)

• Ribosomes: Eukaryotic ribosomes are 80S

• Golgi Body: Processes and folds proteins; forms vesicles for export, vacuoles for storage, lysosomes for degradation

• Vesicles: Membrane-bound compartments for transport and export

• Lysosomes: Acidic organelles for breakdown of cellular waste

Microbial Growth Media and Sterile Technique

Types of Growth Media

Microbial media provide nutrients for growth and can be solid or liquid.

• Liquid Medium: Broth; used for growing bacteria in suspension

• Solid Medium: Agar; firm, high melting point, not degraded by bacteria

• Rich Medium: Contains abundant nutrients; supports rapid growth (often undefined, e.g., beef extract)

• Minimal Medium: Contains only essential nutrients; often defined (synthetic, exact ingredients known)

Preparation and Sterilization

• Agar Forms: Can be poured in tubes (deep, slant) or plates

• Sterility: Absence of living organisms, including microbes, viruses, and endospores

• Autoclave: Common sterilization method; uses steam at 121°C and 15 psi for 15 minutes

• Alternative Methods: Filtration (for heat-sensitive materials), ethylene oxide (for moisture-sensitive items)

Cultures and Contamination

• Pure Culture: Contains only one species of microbe

• Mixed Culture: Contains multiple species; used intentionally in some experiments

• Contaminant: Unwanted organism in a culture; can affect results

Sterile Technique

• Purpose: Prevent contamination by using sterile media and disinfecting work surfaces

• Examples: Sampling air, surfaces, skin, and soil; inoculating plates and incubating at room temperature for 48 hours

• Labeling: Plates labeled with name, date, medium, and sample source

Table: Comparison of Gram Positive and Gram Negative Cell Walls

Feature	Gram Positive	Gram Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present (contains LPS)
Teichoic Acids	Present	Absent
Periplasmic Space	Small or absent	Large
Sensitivity to Penicillin	High	Low
LPS (Endotoxin)	Absent	Present

Table: Types of Microbial Media

Medium Type	Definition	Example
Rich Medium	Contains abundant nutrients, often undefined	Beef extract, soy extract
Minimal Medium	Contains only essential nutrients, defined composition	Synthetic medium
Solid Medium	Contains agar for firmness	Nutrient agar plate
Liquid Medium	No solidifying agent	Broth

Key Equations

• Diffusion: Movement from high to low concentration $\text{Rate of diffusion} = \frac{D \cdot (C_1 - C_2)}{d}$ Where: D = diffusion coefficient, C1 and C2 = concentrations, d = distance

• Autoclave Conditions: $\text{Temperature} = 121^\circ \text{C}$ $\text{Pressure} = 15 \text{ psi}$ $\text{Time} = 15 \text{ min}$

Example: Staphylococcus aureus forms clusters of cocci, is Gram positive, and grows well on rich media such as nutrient agar.

Additional info: Academic context was added to clarify cell wall structure, media types, and eukaryotic cell organelles.