Cell Structure and Function: Prokaryotic and Eukaryotic Cells

Chapter 7: Cell Structure and Function

Overview

This chapter explores how the properties of life emerge from the collaboration of internal structures within cells. It focuses on the organization, function, and diversity of cell components in both prokaryotic and eukaryotic cells.

Inside the Cell

Cell Theory and Universal Cell Components

Cell theory states that all organisms are composed of cells, which are the fundamental units of life. All cells share several key components that enable them to function and maintain life.

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

• Nucleic acids: Store, transmit, and process genetic information. Examples include DNA and RNA.

• Carbohydrates: Provide chemical energy, carbon support, and cellular identity. Examples include glucose and glycoproteins.

• Plasma membrane: Serves as a selectively permeable barrier, controlling the movement of substances into and out of the cell.

Types of Cells: Morphology and Phylogeny

Cells are classified based on their structure (morphology) and evolutionary history (phylogeny).

• Eukaryotes: Have a membrane-bound nucleus.

• Prokaryotes: Lack a membrane-bound nucleus.

Based on phylogeny, organisms are divided into three domains:

• Bacteria – prokaryotic

• Archaea – prokaryotic

• Eukarya – eukaryotic

7.1 Bacterial and Archaeal Cell Structures and Their Functions

Complexity of Prokaryotic Cells

Recent advances, especially the invention of transmission electron microscopy (TEM), have revealed that prokaryotic cells are more complex than previously thought.

• Prokaryotic cells contain various internal structures, including chromosomes, ribosomes, and specialized membranes.

Prokaryotic Cell Structures: A Parts List

Prokaryotic cells possess a variety of structures, each with specific functions.

• Chromosome: Most prominent structure inside prokaryotic cells; usually a single, circular DNA molecule associated with proteins.

• Ribosomes: Macromolecular machines responsible for protein synthesis; consist of large and small subunits made of RNA and proteins.

• Phospholipids: The plasma membrane is composed of phospholipids, which differ between Bacteria and Archaea:

  • Bacterial phospholipids: Fatty acids bound to glycerol.

  • Archaeal phospholipids: Branched isoprenoid chains bound to glycerol.

• Cytoplasm: All contents of the cell inside the plasma membrane.

Organization of Genetic Material

The chromosome is organized into a region called the nucleoid. Some prokaryotes also contain small, circular DNA molecules called plasmids, which carry additional genes.

Protein Synthesis: Ribosomes

Ribosomes are essential for translating genetic information into proteins. While ribosomes in Bacteria and Archaea are similar in size and function, their RNA and protein components differ.

Cytoskeleton in Prokaryotes

Prokaryotic cells contain thin protein filaments that form the basis of the cytoskeleton. The cytoskeleton is crucial for cell division and maintaining cell shape.

Internal Membrane Systems

Some prokaryotes, especially photosynthetic species, have internal membranes that convert energy from sunlight into chemical energy. These membranes often develop as infoldings of the plasma membrane and contain enzymes and pigments required for photosynthesis.

Specialized Organelles in Prokaryotes

Certain bacteria possess internal compartments called organelles that perform specialized tasks, such as:

• Storing calcium ions

• Holding magnetite crystals for navigation

• Concentrating enzymes for building organic compounds

Cell Wall: Protective Exoskeleton

Most prokaryotes have a cell wall composed of a tough, fibrous layer that surrounds the plasma membrane, providing shape and rigidity.

• Bacterial cell wall: Primary structural component is peptidoglycan.

• Some bacteria have an additional outer membrane made of glycolipids.

External Structures: Flagella and Fimbriae

Prokaryotes interact with their environment using structures that extend from the plasma membrane:

• Flagella: Long filaments that propel the cell.

• Fimbriae: Needlelike projections that promote attachment to other cells or surfaces.

Comparative Table: Bacterial vs. Archaeal Phospholipids

Feature	Bacteria	Archaea
Phospholipid Structure	Fatty acids bound to glycerol	Branched isoprenoid chains bound to glycerol
Cell Wall Component	Peptidoglycan	Varies; lacks peptidoglycan
Genetic Material	Circular chromosome, plasmids	Circular chromosome, plasmids

Additional info:

• Transmission electron microscopy (TEM) allows visualization of cell ultrastructure at high resolution.

• Plasmids often carry genes for antibiotic resistance or other specialized functions.

• Peptidoglycan is a polymer consisting of sugars and amino acids, forming a mesh-like layer outside the plasma membrane.