Ch. 4

Domains of Life and Cell Types

Overview of Prokaryotic and Eukaryotic Cells

The classification of all living organisms is based on cell structure, dividing life into two broad categories: Prokaryotic and Eukaryotic cells. This distinction is fundamental in microbiology and underpins the study of bacteria, archaea, and eukaryotes.

• Prokaryotic Cells: Lack a nucleus and other membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotic Cells: Possess a nucleus and membrane-bound organelles. Includes Protists, Fungi, Plants, and Animals.

Example Table: Domains of Life

Domain	Cell Type	Nucleus	Organelles	Cell Size	Cellularity
Bacteria	Prokaryotic	Absent	Absent	Small (1-10 μm)	Unicellular
Archaea	Prokaryotic	Absent	Absent	Small (1-10 μm)	Unicellular
Eukarya	Eukaryotic	Present	Present	Large (10-100 μm)	Unicellular/Multicellular

Features of Bacterial (Prokaryotic) Cells

General Characteristics

Bacteria are among the most abundant and diverse organisms on Earth. Their cells have unique structural features:

• DNA: Typically circular and located in a region called the nucleoid.

• Cell Division: Occurs by binary fission, a simple splitting process.

• Ribosomes: Prokaryotes have 70S ribosomes.

Diagram: Bacterial cell showing circular DNA in the nucleoid and 70S ribosomes.

Features of Eukaryotic Cells

General Characteristics

Eukaryotic cells are defined by the presence of several membrane-bound organelles, most notably the nucleus:

• DNA: Linear in shape and found inside the nucleus.

• Ribosomes: Larger 80S ribosomes.

• Cell Division: Occurs by mitosis and cytokinesis.

Diagram: Eukaryotic cell showing linear DNA in the nucleus and 80S ribosomes.

Comparing Prokaryotic and Eukaryotic Cells

Key Differences and Similarities

Prokaryotic and eukaryotic cells differ in several fundamental ways, but also share some common features:

Prokaryotic Cells	BOTH	Eukaryotic Cells
No Nucleus Smaller (1-10 μm) Less Complex Circular DNA No membrane-bound organelles Cell Division: Binary Fission	Have a Cell Membrane Contain major biomolecules: - Carbohydrates - Proteins - Nucleic Acids - Lipids	Has a Nucleus Larger (10-100 μm) More Complex Linear DNA Has membrane-bound organelles Cell Division: Mitosis/Cytokinesis

Cell Morphology and Arrangement

Definitions

• Cell Morphology: Refers to the overall shape of individual cells.

• Cell Arrangement: Refers to the organization or alignment of groups of multiple cells.

Types of Bacterial Cell Morphology

Bacterial cells typically range between 0.2–2.0 μm in diameter and 2–8 μm in length. There are three main bacterial cell morphologies:

Type	Shape	Example
Coccus	Circular	Staphylococcus aureus
Bacillus	Rod-shaped	Escherichia coli
Spirillum	Corkscrew-shaped	Spirillum minus

Cocci Bacterial Cell Arrangements

• Diplococci: Pairs of cocci.

• Streptococci: Chains of cocci.

• Tetrad: Groups of four cocci arranged in a square.

• Sarcinae: Cubes of eight cocci.

• Staphylococci: Clusters of cocci in irregular patterns.

Bacilli Bacterial Cell Arrangements

• Single Bacillus: Individual rod-shaped cells.

• Diplobacilli: Pairs of rod-shaped cells.

• Streptobacilli: Chains of rod-shaped cells.

• Coccobacilli: Short rods or ovals, intermediate between cocci and bacilli.

Key Terms and Definitions

• Nucleus: Membrane-bound organelle containing genetic material in eukaryotes.

• Nucleoid: Region in prokaryotes where circular DNA is located.

• Binary Fission: Asexual cell division in prokaryotes.

• Mitosis: Eukaryotic cell division process.

• Ribosome: Organelle responsible for protein synthesis; 70S in prokaryotes, 80S in eukaryotes.

Examples and Applications

• Example: Escherichia coli is a rod-shaped (bacillus) bacterium commonly found in the human gut.

• Application: Understanding cell morphology aids in bacterial identification and classification in clinical microbiology.

Additional info: The study notes expand on brief points from the original slides, providing definitions, examples, and tables for clarity and completeness.