Inside the Cell: Structure and Function of Prokaryotic and Eukaryotic Cells

Chapter 7: Inside the Cell

Overview

This chapter explores how the properties of life emerge from the collaboration of internal structures within cells. It examines the fundamental differences and similarities between prokaryotic and eukaryotic cells, and how their components contribute to cellular function.

Cell Theory and Basic Cell Components

Cell Theory

• All organisms are composed of one or more cells.

• The cell is the basic unit of structure and function in living things.

Universal Features of Cells

• Proteins: Perform most of the cell’s functions, including catalyzing metabolic reactions, DNA replication, and responding to stimuli.

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

• Carbohydrates: Provide chemical energy, carbon support, and cellular identity.

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

Types of Cells: Prokaryotes vs. Eukaryotes

Classification by Morphology

• Eukaryotes: Have a membrane-bound nucleus.

• Prokaryotes: Lack a membrane-bound nucleus.

Classification by Phylogeny (Evolutionary History)

• Bacteria: Prokaryotic domain.

• Archaea: Prokaryotic domain, distinct from bacteria.

• Eukarya: Eukaryotic domain, includes protists, fungi, plants, and animals.

Prokaryotic Cell Structure

Recent Advances in Understanding

• Prokaryotic cells are more complex than previously thought, as revealed by transmission electron microscopy (TEM).

Key Structural Components

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

• Ribosomes: Macromolecular machines responsible for protein synthesis, composed of RNA and proteins.

• Phospholipid Membranes:

  • Bacterial membranes: Fatty acids bound to glycerol.

  • Archaeal membranes: Branched isoprenoid chains bound to glycerol.

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

Additional Structures

• Cell Wall: Provides structural support and shape; in bacteria, primarily composed of peptidoglycan.

• Flagella: Long, whip-like structures used for movement.

• Fimbriae: Needle-like projections that help cells adhere to surfaces or other cells.

• Cytoskeleton: Long, thin protein filaments that help maintain cell shape and are essential for cell division.

• Internal Membranes: Some prokaryotes, especially photosynthetic species, have internal membranes for specialized functions such as photosynthesis.

• Organelles: Some bacteria have internal compartments for specialized tasks (e.g., storing calcium ions, holding magnetite crystals, concentrating enzymes).

Summary Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present (membrane-bound)
Cell Size	1–10 μm	5–100 μm
Internal Membranes	Few, if any (except in some photosynthetic species)	Extensive, forming organelles
DNA Structure	Single, circular chromosome	Multiple, linear chromosomes
Cell Wall	Usually present (peptidoglycan in bacteria)	Present in plants, fungi, and some protists (cellulose or chitin); absent in animals
Examples	Bacteria, Archaea	Protists, fungi, plants, animals

Key Terms and Definitions

• Transmission Electron Microscopy (TEM): A microscopy technique that uses electrons to visualize cell structures at very high resolution.

• Peptidoglycan: A polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria, providing structural strength.

• Isoprenoid Chains: Branched hydrocarbon chains found in archaeal membrane lipids, contributing to membrane stability under extreme conditions.

• Fimbriae: Short, hair-like appendages on the surface of many bacteria that enable attachment to surfaces.

• Flagella: Long, whip-like appendages that rotate to propel prokaryotic cells through their environment.

Examples and Applications

• Example: Escherichia coli (E. coli) is a model prokaryotic organism with a single circular chromosome, ribosomes, a plasma membrane, and a cell wall composed of peptidoglycan.

• Application: Understanding the differences between prokaryotic and eukaryotic cells is fundamental for fields such as microbiology, medicine, and biotechnology.

Additional info: The notes above are expanded and clarified for academic completeness, based on standard introductory biology content.