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

Chapter 6: A Tour of the Cell

Introduction

This chapter explores the fundamental unit of life—the cell—by examining its structure, function, and diversity. It distinguishes between prokaryotic and eukaryotic cells, explains the importance of compartmentalization, and describes the major organelles and their roles in cellular processes.

Prokaryotic vs. Eukaryotic Cells

Cell Types and Their Characteristics

• Prokaryotic Cells: Simpler cells lacking a nucleus and membrane-bound organelles. DNA is located in an unbound region called the nucleoid. Examples: Bacteria and Archaea.

• Eukaryotic Cells: More complex cells with a true nucleus (enclosed by a double membrane) and numerous membrane-bound organelles. Examples: Protists, Fungi, Plants, and Animals.

All cells share certain features:

• Plasma membrane

• Cytosol (semifluid substance)

• Chromosomes (carry genes)

• Ribosomes (synthesize proteins)

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (DNA in nucleoid)	Present (DNA in nucleus)
Membrane-bound Organelles	Absent	Present
Cell Size	1–5 μm	10–100 μm
Examples	Bacteria, Archaea	Protists, Plants, Animals, Fungi

Cellular Compartmentalization

Advantages of Compartmentalization

• Internal membranes divide the cell into specialized compartments (organelles), allowing distinct chemical reactions to occur simultaneously.

• Compartmentalization increases efficiency and specialization within the cell.

• Organelles such as the nucleus, mitochondria, and chloroplasts have unique environments suited to their functions.

Example: Lysosomes maintain an acidic environment for digestion, while mitochondria provide a site for ATP production.

Structure and Function of Major Organelles

Nucleus

• Contains most of the cell's genetic material (DNA).

• Enclosed by a double membrane called the nuclear envelope, which contains nuclear pores for molecular exchange.

• DNA is organized into chromosomes (chromatin when not dividing).

• The nucleolus is the site of ribosomal RNA (rRNA) synthesis.

Ribosomes

• Complexes of rRNA and protein; sites of protein synthesis.

• Can be free in the cytosol or bound to the rough endoplasmic reticulum (ER).

Endomembrane System

• Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Components are either continuous or connected via vesicles.

Endoplasmic Reticulum (ER)

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

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs, and stores calcium ions.

Golgi Apparatus

• Consists of flattened membranous sacs (cisternae).

• Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Lysosomes

• Membranous sacs containing hydrolytic enzymes for digesting macromolecules.

• Involved in phagocytosis (engulfing food particles) and autophagy (recycling cellular components).

Vacuoles

• Large vesicles with diverse functions (e.g., storage, waste disposal, water balance).

• Central vacuole in plant cells stores ions and contributes to cell growth.

• Contractile vacuole in protists expels excess water.

Mitochondria and Chloroplasts

• Mitochondria: Sites of cellular respiration; convert chemical energy in food to ATP.

• Chloroplasts: Found in plants and algae; sites of photosynthesis.

• Both have double membranes, their own DNA, and ribosomes; support the endosymbiont theory of origin.

Peroxisomes

• Oxidative organelles that break down fatty acids and detoxify harmful substances.

Plasma Membrane and Surface Area-to-Volume Ratio

Structure and Function

• The plasma membrane is a selective barrier composed of a phospholipid bilayer with embedded proteins.

• Regulates the passage of oxygen, nutrients, and waste.

• Surface area-to-volume ratio limits cell size; as a cell grows, its volume increases faster than its surface area, reducing efficiency of exchange.

Equation:

$\text{Surface Area of a Sphere} = 4\pi r^2$ $\text{Volume of a Sphere} = \frac{4}{3}\pi r^3$

As radius increases, volume grows faster than surface area.

Cytoskeleton

Structure and Functions

• Network of protein fibers that provides structural support, maintains cell shape, and enables movement.

• Microtubules: Hollow rods; involved in cell shape, chromosome movement, and organelle transport.

• Microfilaments: Thin filaments; involved in cell motility, muscle contraction, and cell division.

• Intermediate filaments: Provide mechanical support and anchor organelles.

Plant vs. Animal Cells

Key Differences

Feature	Plant Cells	Animal Cells
Cell Wall	Present (cellulose)	Absent
Chloroplasts	Present	Absent
Central Vacuole	Large, present	Small or absent
Centrosomes with Centrioles	Absent	Present
Lysosomes	Rare	Common
Plasmodesmata	Present	Absent
Extracellular Matrix (ECM)	Absent	Present

Microscopy Techniques

Types and Applications

• Light Microscopy: Used to observe living cells; magnification up to ~1000x.

• Electron Microscopy: Provides higher resolution images of cell ultrastructure; used for detailed study of organelles.

Review & Key Takeaways

• Cells are the fundamental unit of life.

• Compartmentalization and organelles enable efficiency and specialization.

• Understanding cell structure and function is essential for all areas of biology.