Prokaryotic and Eukaryotic Cells: Structure and Classification

Domains of Life and Cell Types

Classification of Cells

Cells are classified into two major types: prokaryotic and eukaryotic. These types are distinguished by the presence or absence of a nucleus and other membrane-bound organelles.

• Prokaryotic Cells: Do not have a nucleus (includes Bacteria and Archaea).

• Eukaryotic Cells: Have a nucleus and other membrane-bound organelles.

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

Example: Bacteria and Archaea are classified as prokaryotes, while Eukarya includes protists, fungi, plants, and animals.

Features of Bacterial (Prokaryotic) Cells

Structure and Function

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

• Bacterial DNA: Is circular in shape and found in a region called the nucleoid.

• Ribosomes: Small (70S), responsible for protein synthesis.

• Cell Division: Occurs by binary fission, a simple process of splitting into two cells.

Example: Escherichia coli (E. coli) is a common bacterium with these features.

Feature	Prokaryotic Cell
DNA Shape	Circular
Ribosome Size	70S
Cell Division	Binary Fission

Features of Eukaryotic Cells

Structure and Function

Eukaryotic cells are more complex and contain several membrane-bound organelles, including a nucleus.

• Eukaryotic DNA: Is linear in shape and found inside the nucleus.

• Ribosomes: Large (80S), responsible for protein synthesis.

• Cell Division: Occurs by mitosis and cytokinesis.

Example: Human cells are eukaryotic, containing a nucleus and organelles such as mitochondria and endoplasmic reticulum.

Feature	Eukaryotic Cell
DNA Shape	Linear
Ribosome Size	80S
Cell Division	Mitosis & Cytokinesis

Comparing Prokaryotic and Eukaryotic Cells

Key Differences and Similarities

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

Prokaryotic Cells	BOTH	Eukaryotic Cells
No nucleus	Cell membrane	Has nucleus
Smaller (1-10 μm)	Contain macromolecules: carbohydrates, proteins, nucleic acids, lipids	Larger (10-100 μm)
Less complex		More complex
Circular DNA		Linear DNA
Binary fission		Mitosis & cytokinesis
No membrane-bound organelles		Membrane-bound organelles
Unicellular		Unicellular or multicellular

Practice Questions and Applications

Understanding Cell Types

• Which domains of life are classified as prokaryotes? Answer: Bacteria & Archaea.

• Which organelle packages the genetic/hereditary material in eukaryotes but not in prokaryotes? Answer: Nucleus.

• Primary differences between prokaryotes and eukaryotes:

  • Prokaryotes have DNA that floats freely in the cytoplasm.

  • Prokaryotes are significantly smaller than eukaryotic cells.

  • Prokaryotes do not have ribosomes like eukaryotic cells.

• Common feature shared by all types of cells: Contains DNA as the genetic material.

• Cell identification: A cell with no nucleus, a cell wall, and a size of 2 μm is likely a bacterium.

Key Terms and Definitions

• Prokaryote: An organism whose cells lack a nucleus and membrane-bound organelles.

• Eukaryote: An organism whose cells contain a nucleus and membrane-bound organelles.

• Nucleus: Membrane-bound organelle that contains genetic material in eukaryotic cells.

• Nucleoid: Region in a prokaryotic cell where circular DNA is located.

• Binary Fission: A method of asexual reproduction in prokaryotes.

• Mitosis: Process of cell division in eukaryotes resulting in two identical daughter cells.

• Ribosome: Organelle responsible for protein synthesis; size differs between prokaryotes (70S) and eukaryotes (80S).

Formulas and Equations

• Cell Size Comparison:

$\text{Prokaryotic Cell Size} \approx 1-10\ \mu m$ $\text{Eukaryotic Cell Size} \approx 10-100\ \mu m$

• DNA Structure:

$\text{Prokaryotic DNA: Circular}$ $\text{Eukaryotic DNA: Linear}$

Additional info: Eukaryotic cells may be unicellular (e.g., protists) or multicellular (e.g., plants, animals, fungi). Prokaryotic cells are always unicellular.