Prokaryotic Cells: Structure, Function, and Laboratory Techniques

Prokaryotic and Eukaryotic Cells: An Overview

Major Types of Cells

Cells are the fundamental units of life and can be classified into two major types: prokaryotic cells and eukaryotic cells. Understanding their differences is essential for studying cellular biology.

• Prokaryotic Cells: Generally unicellular, small (0.1–10 μm), and structurally simpler. Examples include bacteria and archaea.

• Eukaryotic Cells: Can be unicellular or multicellular, larger (10–100 μm), and more complex. Examples include plants, animals, fungi, and protists.

Genetic Material Organization

The organization of genetic material is a key distinction between prokaryotes and eukaryotes.

• Prokaryotes: Possess a single, circular DNA molecule located in a region called the nucleoid. DNA is not enclosed by a membrane.

• Eukaryotes: Contain multiple, linear chromosomes housed within a membrane-bound nucleus.

Cellular Organelles

Organelles are specialized structures within cells that perform distinct functions.

• Prokaryotes: Lack membrane-bound organelles but contain ribosomes for protein synthesis.

• Eukaryotes: Possess numerous membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus) and ribosomes.

Storage of Genetic Material

• Prokaryotes: DNA is found in the cytoplasm within the nucleoid region.

• Eukaryotes: DNA is stored as linear chromosomes inside a membrane-bound nucleus, separated from the cytoplasm.

Nutrition in Prokaryotes

Autotrophs vs. Heterotrophs

Prokaryotes can obtain energy and carbon in different ways, classified as autotrophs or heterotrophs.

• Autotrophs: Organisms that produce their own food, typically using sunlight (photosynthesis) or inorganic chemicals (chemosynthesis).

• Heterotrophs: Organisms that obtain food by consuming other organisms or organic matter.

Bacterial Morphology and Identification

Common Shapes of Bacteria

Bacteria are classified by their morphology (shape):

• Coccus (cocci): Spherical-shaped bacteria.

• Bacillus (bacilli): Rod-shaped bacteria.

• Spirillum (spirilli): Spiral or helical-shaped bacteria.

Bacterial Cell Walls and Gram Staining

Structure of Bacterial Cell Walls

Bacterial cell walls are essential for maintaining cell shape and integrity. They are classified as Gram-positive or Gram-negative based on their structure and staining properties.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Lipopolysaccharide Layer	Absent	Present
Outer Membrane	Absent	Present
Stain Color	Purple	Pink

Peptidoglycan is a polymer of sugars and amino acids that forms a mesh-like layer outside the plasma membrane.

Gram Staining Procedure

Gram staining differentiates bacteria based on cell wall structure using a series of dyes and reagents:

1. Crystal Violet: Primary stain; stains all cells purple.

2. Gram's Iodine: Mordant; forms a complex with crystal violet, fixing the dye in Gram-positive cells.

3. Ethanol: Decolorizing agent; dehydrates thick peptidoglycan in Gram-positive cells (retains dye), dissolves outer membrane in Gram-negative cells (dye washes out).

4. Safranin: Counterstain; stains Gram-negative cells pink, Gram-positive cells remain purple.

Gram-positive bacteria appear purple, while Gram-negative bacteria appear pink under the microscope.

Cyanobacteria

Characteristics of Cyanobacteria

Cyanobacteria are photosynthetic prokaryotes, often called "blue-green algae" due to their pigmentation. They are important for oxygen production and nitrogen fixation in aquatic environments.

• Contain chlorophyll a (unlike most other photosynthetic prokaryotes, which have bacteriochlorophyll).

• Lack chloroplasts; photosynthesis occurs in thylakoid membranes within the cytoplasm.

• Often form linear filaments and do not fit standard bacterial morphology terms.

• Phycobilins give them a blue-green hue.

Laboratory Techniques: Culturing and Identifying Bacteria

Isolation Streak Plate (4-Quadrant Streak Plate)

This technique is used to isolate individual bacterial colonies from a mixed sample by streaking bacteria across four quadrants of an agar plate, decreasing the concentration with each quadrant.

Spread Plate (Lawn Plate)

The spread plate technique creates a uniform layer of bacterial growth across the surface of the agar, useful for quantifying bacteria or testing antibiotic sensitivity.

Laboratory Safety and Best Practices

• Always label agar plates with your name, lab section, date, and type of plate. Store agar side up and wrap in parafilm.

• Slides must be air-dried before heat fixing; gently pass glass through the flame to avoid breakage.

• Dispose of used slides and plates in designated discard bins.

Additional info: Proper aseptic technique is essential to prevent contamination and ensure accurate results in microbiology labs.