Cell Structure and Function: Prokaryotic and Eukaryotic Cells

Chapter 7: Cell Structure and Function

Introduction

This chapter explores the fundamental structures and functions of cells, the basic units of life. It examines both prokaryotic and eukaryotic cell types, their internal organization, and how cellular components collaborate to sustain life.

Learning Objectives

• Delineate the structure and function of prokaryotic and eukaryotic cell components.

• Explain how diverse cellular activities are correlated with the function of organelles.

• Summarize the process of transporting molecules into and through the endomembrane system.

• Delineate the structure and function of the three major components of the eukaryotic cytoskeleton.

Inside the Cell

Cell Theory and Universal Cell Components

The cell theory states that all organisms consist of cells. Despite their diversity, all cells share four major types of macromolecules, each with distinct roles:

• Proteins: Perform most of the cell’s functions, including catalyzing metabolic reactions, providing structural support, and facilitating communication.

• Nucleic acids: Store, transmit, and process genetic information (e.g., DNA and RNA).

• Carbohydrates: Provide chemical energy, carbon, structural support, and cellular identity (e.g., cellulose in plant cell walls, glycoproteins on cell surfaces).

• Lipids: Serve as selectively permeable membrane barriers (e.g., plasma membrane), and function in energy storage and signaling.

Prokaryotic Cell Structures

Main Features of Prokaryotic Cells

Prokaryotic cells (Bacteria and Archaea) are characterized by a relatively simple internal structure but possess specialized components:

• Chromosome: Usually a single, circular DNA molecule containing genetic information, often located in a region called the nucleoid.

• Ribosomes: Macromolecular complexes of RNA and protein that synthesize proteins. Prokaryotic ribosomes are similar in size and function across Bacteria and Archaea, but differ in primary structure.

• Phospholipid components:

  • Bacterial phospholipids: Fatty acids bound to glycerol.

  • Archaeal phospholipids: Branched isoprenoid chains bound to glycerol, providing stability in extreme environments.

• Plasmids: Small, circular, supercoiled DNA molecules that carry genes beneficial for survival (e.g., antibiotic resistance).

• Cytoplasm: All contents of the cell inside the plasma membrane, including the nucleoid, ribosomes, and various enzymes.

Cell Envelope and External Structures

• Plasma Membrane: A phospholipid bilayer that acts as a selective barrier for the cell.

• Cell Wall: Most prokaryotes have a rigid cell wall composed primarily of peptidoglycan (in Bacteria) or other polysaccharides (in Archaea), providing structural support and shape.

• External Structures:

  • Flagella: Long, whip-like appendages that rotate to propel the cell.

  • Pili (Fimbriae): Short, needle-like projections that facilitate attachment to surfaces and other cells.

Summary Table: Key Prokaryotic Cell Structures

Structure	Main Function	Present in
Chromosome	Genetic information storage	Bacteria, Archaea
Ribosome	Protein synthesis	Bacteria, Archaea
Plasmid	Accessory genes (e.g., antibiotic resistance)	Bacteria, Archaea
Cell Wall	Structural support, shape	Bacteria (peptidoglycan), Archaea (varied)
Flagellum	Motility	Some Bacteria, Archaea
Pilus/Fimbria	Attachment	Many Bacteria, Archaea

Example: Escherichia coli (E. coli)

E. coli is a model prokaryotic organism with a single circular chromosome, numerous ribosomes, a peptidoglycan cell wall, and flagella for movement.

Additional info:

• Prokaryotic cells lack membrane-bound organelles such as nuclei, mitochondria, or endoplasmic reticulum.

• Some prokaryotes form biofilms, complex communities attached to surfaces.