Chapter 6: A Tour of the Cell – Structure and Function of Cells

Chapter 6: A Tour of the Cell

Introduction

Cells are the fundamental units of life, exhibiting remarkable diversity due to the presence of various macromolecules, structures, and processes. Understanding cell structure and function is essential for all biological sciences.

History of Cell Discovery

Development of Microscopy

• Robert Hooke (1665): Used the first simple microscope to observe cork, coining the term cellulae (cells).

• Antonie van Leeuwenhoek (1680s): Improved the microscope and was the first to observe and describe microorganisms (“animalcules”) in water.

• Microscopes: Revealed the existence of cells and allowed scientists to study their detailed structures.

Key Concepts in Microscopy

• Magnification: The ratio of an object’s image size to its real size.

• Resolution: The measure of clarity or the ability to distinguish two close objects as separate.

• Contrast: The difference in brightness between light and dark areas, enhancing visibility of structures.

Types of Microscopy

Light Microscopy

• Uses visible light to illuminate specimens.

• Maximum useful magnification is about x1000.

• Commonly used for viewing live cells and tissues.

Electron Microscopy

• Transmission Electron Microscope (TEM): Uses electrons to visualize the interior of cells at very high magnification (up to x500,000).

• Scanning Electron Microscope (SEM): Provides detailed 3D images of cell surfaces.

• Cryo-electron Microscopy: Allows specimens to be preserved in their native state for imaging.

Cell Fractionation

• A laboratory technique used to separate different cell components for individual study.

Prokaryotic vs. Eukaryotic Cells

Overview

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No true nucleus; DNA in nucleoid region	True nucleus with nuclear envelope
Membrane-bound Organelles	Absent	Present (e.g., mitochondria, ER, Golgi)
Cell Wall	Usually present (e.g., bacteria)	Present in plants, fungi; absent in animals
Size	Generally smaller (1–10 μm)	Generally larger (10–100 μm)
Examples	Bacteria, Archaea	Animals, plants, fungi, protists

Prokaryotic Cells – Bacteria

Key Features

• No nucleus: Genetic material is located in a region called the nucleoid.

• Cell wall: Provides structural support and protection.

• Capsule or slime layer: An additional protective outer layer.

• Lack membrane-bound organelles: No mitochondria, ER, or Golgi apparatus.

• Ribosomes: Present for protein synthesis, but structurally different from eukaryotic ribosomes.

Example: Escherichia coli (E. coli)

• A common bacterium with a nucleoid, cell wall, capsule, and ribosomes.

Eukaryotic Cells

Key Features

• Nucleus: Contains the cell’s DNA and is surrounded by a double membrane (nuclear envelope).

• Membrane-bound organelles: Specialized structures such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Complex internal structure: Includes cytoskeleton and various organelles for compartmentalized functions.

• Examples: Animals, plants, fungi, and protists.

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Characteristic	Prokaryotic	Eukaryotic
DNA Location	Nucleoid (no membrane)	Nucleus (membrane-bound)
Organelles	Absent	Present
Cell Size	Small	Larger
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Key Terms and Definitions

• Cell: The basic structural and functional unit of all living organisms.

• Organelle: A specialized subunit within a cell that has a specific function, usually membrane-bound in eukaryotes.

• Nucleus: The organelle that contains genetic material in eukaryotic cells.

• Nucleoid: The region in prokaryotic cells where DNA is located.

• Ribosome: A molecular machine for protein synthesis, found in all cells.

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

Additional info:

• Further details on eukaryotic organelles, cytoskeleton, and cell junctions are typically covered in subsequent sections of this chapter.