A Tour of the Cell: Structure and Diversity of Prokaryotic and Eukaryotic Cells

Chapter 6: A Tour of the Cell

Introduction to Cell Structure

Cells are the fundamental units of life, and their internal organization is essential for carrying out the processes necessary for survival. This chapter explores the differences and similarities between prokaryotic and eukaryotic cells, as well as the specialized structures found within each type.

Major Categories of Cells

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells lack a nucleus and membrane-bound organelles.

• Eukaryotic cells contain a membrane-bound nucleus and various organelles.

All living organisms are composed of one of these two cell types, which differ in complexity, size, and internal organization.

Prokaryotic Cells

• Domains: Archaea and Bacteria

• Always single-celled

• Generally smaller and simpler than eukaryotic cells (about 10 times smaller)

• Key structures: plasma membrane, nucleoid region (where DNA is concentrated), ribosomes, cell wall, sometimes capsule, fimbriae, and flagella

Eukaryotic Cells

• Domain: Eukarya (includes animals, plants, fungi, and protists)

• Can be single-celled or multicellular

• Larger and more complex (10–100 μm in size)

• Key structures: plasma membrane, nucleus (membrane-bound DNA), ribosomes, membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus)

Comparisons Between Cell Types

Similarities	Differences
Both have a plasma membrane Both contain ribosomes (protein synthesis) Both contain DNA (genetic instructions) Both have cytoplasm (fluid-filled interior) Genetic code is universal	Eukaryotes: Have organelles (including nucleus) Prokaryotes: DNA in nucleoid region, lack membrane-bound organelles Cell wall: Present in all prokaryotes, only some eukaryotes (e.g., plants, fungi)

Structural Features of Prokaryotic Cells

• Plasma membrane: Encloses the cytoplasm

• Cell wall: Provides structural support and protection

• Nucleoid: Region where DNA is concentrated (not membrane-bound)

• Ribosomes: Sites of protein synthesis

• Capsule: Sticky outer layer (in some bacteria)

• Fimbriae: Attachment structures on the surface

• Flagella: Used for movement

Common shapes: Bacilli (rod-shaped), Cocci (spherical), Spirilli (spiral-shaped)

Structural Features of Eukaryotic Cells

• Nucleus: Contains genetic material (DNA) enclosed by a nuclear envelope

• Membrane-bound organelles: Specialized compartments for specific functions (e.g., mitochondria for energy production, endoplasmic reticulum for protein and lipid synthesis, Golgi apparatus for modification and transport of molecules)

• Cytoskeleton: Network of fibers for structural support and movement

Differences Between Plant and Animal Cells

• Plant cells have three unique structures not found in animal cells:

  • Cell wall: Rigid layer outside the plasma membrane, composed of cellulose, provides support and protection

  • Chloroplasts: Organelles that carry out photosynthesis, converting light energy, CO2, and H2O into glucose and O2

  • Central vacuole: Large storage compartment for water, nutrients, and waste products

• Animal cells lack these structures but may have other specialized organelles (e.g., lysosomes, centrioles)

Summary Table: Key Features of Cell Types

Feature	Prokaryotic Cells	Eukaryotic Cells	Plant Cells	Animal Cells
Nucleus	No	Yes	Yes	Yes
Membrane-bound organelles	No	Yes	Yes	Yes
Cell wall	Yes	Some	Yes (cellulose)	No
Chloroplasts	No	No	Yes	No
Central vacuole	No	No	Yes	No

Key Terms and Concepts

• Organelle: Specialized structure within a cell, surrounded by a membrane, with a specific function

• Plasma membrane: Phospholipid bilayer that encloses the cell, controlling movement of substances in and out

• Ribosome: Molecular machine for protein synthesis

• Cytoplasm: Gel-like substance inside the cell, excluding the nucleus

• Genetic code: Universal set of instructions for building proteins, encoded in DNA

Example: Photosynthesis in Plant Cells

• Chloroplasts in plant cells capture light energy and convert it into chemical energy (glucose) through the process of photosynthesis:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Additional info: The compartmentalization of eukaryotic cells allows for specialized environments that optimize biochemical reactions, increasing cellular efficiency and complexity.