Chapter 4: A Tour of the Cell — General Biology Study Notes

Chapter 4: A Tour of the Cell

Introduction

This chapter provides an overview of cell structure and function, focusing on the differences between prokaryotic and eukaryotic cells, the organization of cellular components, and the evolutionary origins of key organelles. Understanding cell structure is fundamental to all areas of biology, as the cell is the basic unit of life.

Cell Theory

• Definition: The cell theory is a foundational concept in biology stating that:

  • All living organisms are composed of one or more cells.

  • The cell is the basic structural and functional unit of life.

  • All cells arise from pre-existing cells.

  • Cells evolved from a common ancestor.

• Implication: No life exists without cells; all biological processes are rooted in cellular activity.

Basic Cellular Organization

• All cells share certain features:

  • Plasma membrane: A selective barrier that surrounds the cell and regulates the passage of substances.

  • Cytosol: Jelly-like fluid inside the cell where organelles are suspended.

  • Chromosomes: Structures that carry genetic information (DNA).

  • Ribosomes: Complexes that synthesize proteins.

Types of Cells

• There are two main types of cells:

  • Prokaryotic cells: Found in Domains Bacteria and Archaea.

  • Eukaryotic cells: Found in Domain Eukarya (includes animals, plants, fungi, and protists).

Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No true nucleus; DNA in nucleoid region	Membrane-bound nucleus
Membrane-bound organelles	Absent	Present (e.g., mitochondria, ER, Golgi)
Cell size	Generally smaller (1–10 μm)	Generally larger (10–100 μm)
Domains	Bacteria, Archaea	Eukarya
Examples	Escherichia coli, Staphylococcus	Animal, plant, fungal, and protist cells

Prokaryotic Cells

• Belong to Domains Bacteria and Archaea.

• Lack a membrane-bound nucleus and organelles.

• DNA is located in an unbound region called the nucleoid.

• Generally smaller and simpler than eukaryotic cells.

Eukaryotic Cells

• Belong to Domain Eukarya (includes animals, plants, fungi, protists).

• Have a membrane-bound nucleus containing DNA.

• Contain numerous membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

• Generally larger and more complex than prokaryotic cells.

• Some have cell walls (plants, fungi), others do not (animals).

Eukaryotic Cell Organization

• Three main regions:

  • Plasma membrane

  • Cytoplasm (includes cytosol and organelles)

  • Nucleus

Plasma Membrane

• Acts as a selective barrier for the cell.

• Composed of a phospholipid bilayer with embedded proteins.

• Regulates the movement of substances in and out of the cell.

Cytoplasm

• Region between the plasma membrane and the nucleus.

• Consists of cytosol (fluid) and organelles.

• Site of many metabolic reactions.

The Nucleus — Information Center

• Contains most of the cell's DNA.

• Enclosed by a double membrane called the nuclear envelope.

• Nuclear pores regulate entry and exit of molecules.

• Contains the nucleolus, where ribosomal RNA (rRNA) is synthesized.

• DNA is organized into chromosomes (chromatin = DNA + histone proteins).

Ribosomes — Protein Factories

• Complexes of rRNA and proteins.

• Carry out protein synthesis in two locations:

  • Free ribosomes (in cytosol)

  • Bound ribosomes (on rough endoplasmic reticulum)

• Consist of two subunits:

  • Small subunit: reads mRNA

  • Large subunit: joins amino acids to form polypeptide chain

Endomembrane System

The endomembrane system regulates protein traffic and performs metabolic functions in the cell. It includes:

• Nuclear envelope

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Lysosomes

• Vacuoles and vesicles

• Plasma membrane

Endoplasmic Reticulum (ER) — Biosynthetic Factory

• Extensive network of membranes continuous with the nuclear envelope.

• Two types:

  • Rough ER: Studded with ribosomes; synthesizes proteins and distributes transport vesicles.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons.

Golgi Apparatus — Shipping & Receiving Center

• Consists of flattened membranous sacs (cisternae).

• Modifies, sorts, and packages products of the ER into transport vesicles.

• Manufactures some macromolecules.

• In animal cells, produces lysosomes; in plant cells, produces some cell wall components.

Lysosomes — Digestive Compartments

• Membranous sacs of hydrolytic enzymes (found mainly in animal cells).

• Digest macromolecules, destroy bacteria, and break down damaged organelles.

Peroxisomes — Oxidation

• Oxidative organelles that remove hydrogen from molecules and transfer it to oxygen, producing hydrogen peroxide ($H_2O_2$).

• Detoxify alcohol and other harmful compounds (especially in liver cells).

Vacuoles — Diverse Compartments

• Large vesicles derived from the ER and Golgi apparatus.

• Types:

  • Food vacuoles: Formed by phagocytosis to bring in food materials.

  • Contractile vacuoles: Pump excess water out of cells (e.g., in protists).

  • Central vacuole: Stores inorganic ions and water (plant cells only).

Vesicles

• Small membrane-bound sacs that transport molecules within the cell.

• Secretory vesicles move materials to the plasma membrane for exocytosis.

Energy-Converting Organelles

• Mitochondria: Sites of cellular respiration; convert chemical energy from food into ATP.

• Chloroplasts: Sites of photosynthesis; convert solar energy into chemical energy (sugars).

• Both are enclosed by a double membrane, contain their own DNA, and can grow and reproduce independently.

Cytoskeleton

• Internal framework of protein fibers extending throughout the cytoplasm.

• Functions:

  • Maintains cell shape and provides structural support.

  • Enables cell movement and division.

  • Organizes cell's structures and activities.

Cytoskeleton Components

Component	Structure	Function
Microtubules	Hollow rods made of tubulin	Shape/support cell, organelle movement, chromosome separation
Microfilaments	Solid rods of actin	Cell shape, muscle contraction, cell movement
Intermediate filaments	Fibrous proteins coiled into cables	Cell shape, anchoring organelles

Extracellular Structures

Extracellular Matrix (ECM) of Animal Cells

• External material surrounding animal cells outside the plasma membrane.

• Composed mainly of glycoproteins (e.g., collagen, fibronectin).

• Functions in cell communication and regulation of cell behavior.

Cell Wall of Plant Cells

• Rigid external structure surrounding the plasma membrane.

• Found in plants, most bacteria, archaea, and fungi.

• Provides protection and structural support.

• Plant cell walls contain cellulose (a structural polysaccharide).

Cell Junctions

• Structures that connect neighboring cells, allowing them to adhere, interact, and communicate.

• Plasmodesmata: Channels in plant cell walls for passage of water and small solutes.

• Animal cells have three main types of junctions:

  • Tight junctions: Watertight seals between adjacent cells.

  • Desmosomes: Anchoring junctions that fasten cells together.

  • Gap junctions: Communicating junctions that allow passage of ions and small molecules.

Animal Cells vs. Plant Cells

Feature	Animal Cells	Plant Cells
Cell wall	Absent	Present (cellulose)
Chloroplasts	Absent	Present
Central vacuole	Absent	Present
Lysosomes	Present	Rare
Shape	Round/irregular	Rectangular/fixed

Endosymbiotic Theory

• Proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Engulfed cells formed a mutualistic relationship with the host cell, eventually becoming permanent organelles.

• Evidence:

  • Both organelles have double membranes.

  • Contain their own ribosomes and circular DNA.

  • Grow and reproduce independently within the cell.

• Example: Lynn Margulis (1960s) developed the modern endosymbiotic theory.

Emergent Properties of Cells

• The cell is more than the sum of its parts; cellular functions arise from the complex organization and interaction of cellular components.

• No single component works alone; emergent properties result from cellular order.