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

Cells: An Overview

Introduction to Cell Biology

Cells are the fundamental units of life, forming the basis of all living organisms. They exhibit remarkable diversity in structure and function, but share several universal features. Understanding the differences between prokaryotic and eukaryotic cells, as well as the organization of cellular components, is essential for studying biology.

Prokaryotes vs. Eukaryotes

Key Distinctions

• Prokaryotic cells lack a nucleus and membrane-bound organelles. Their DNA is located in a region called the nucleoid.

• Eukaryotic cells possess a true nucleus surrounded by a nuclear envelope and contain various membrane-bound organelles.

• Typical size: Prokaryotes (~1–5 μm), Eukaryotes (~10–100 μm).

• All cells have a plasma membrane, cytosol, ribosomes, and genetic material.

Universal Features of Cells

• Plasma membrane: Selective barrier that maintains internal environment.

• Cytosol: Semi-fluid substance where cellular components are suspended.

• Ribosomes: Sites of protein synthesis.

• Genetic material: DNA stores hereditary information.

Biological Membranes

Structure and Function

Biological membranes are primarily composed of a phospholipid bilayer with embedded proteins and other lipids. They serve as barriers, compartmentalize cellular processes, and facilitate communication and transport.

• Phospholipids: Form the basic structure of the membrane, with hydrophilic heads and hydrophobic tails.

• Proteins: Serve as receptors, transporters, and enzymes.

• Other lipids: Cholesterol and glycolipids contribute to membrane fluidity and function.

Functional Aspects

• Membranes are dynamic and involved in signaling, transport, and energy transduction.

• Compartmentalization allows for specialized environments and efficient metabolic processes.

The Endomembrane System

Components and Functions

The endomembrane system is a network of membranes within eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Studded with ribosomes; synthesizes proteins.

  • Smooth ER (SER): Lacks ribosomes; synthesizes lipids, detoxifies drugs, stores glycogen.

• Golgi Apparatus: Modifies, sorts, and ships proteins and lipids received from the ER.

• Lysosomes: Contain hydrolytic enzymes for digestion and recycling of cellular materials.

• Vacuoles: Membrane-bound sacs with diverse functions (e.g., central vacuole in plants, food vacuoles in protists).

• Transport Vesicles: Shuttle materials between organelles.

Protein Synthesis and Trafficking

• Proteins synthesized by ribosomes on the RER are threaded into the ER lumen, modified, and transported to the Golgi apparatus for further processing.

• From the Golgi, proteins are sorted and sent to their final destinations: secretion, lysosomes, or incorporation into the plasma membrane.

Lysosomes and Vacuoles

• Lysosomes digest macromolecules, old organelles (autophagy), and materials taken up by endocytosis.

• Vacuoles serve as storage, waste disposal, and structural support (especially in plants).

Mitochondria and Chloroplasts

Energy Conversion Organelles

Mitochondria and chloroplasts are specialized organelles responsible for energy transformation. They are not part of the endomembrane system and contain their own DNA and ribosomes, supporting the endosymbiont theory of their origin.

• Mitochondria: Convert chemical energy from food into ATP via cellular respiration.

• Chloroplasts: Convert solar energy into chemical energy (sugars) via photosynthesis (in plants and algae).

Endosymbiont Theory

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

• Evidence: Double membranes, own DNA, ribosomes, and similarities to certain bacteria.

Cytoskeleton

Structure and Functions

The cytoskeleton is a dynamic network of protein fibers that provides structural support, maintains cell shape, and facilitates movement and intracellular transport.

• Microfilaments (actin): Involved in cell movement, muscle contraction, and cytokinesis.

• Intermediate filaments (keratin, etc.): Provide mechanical support and maintain cell integrity.

• Microtubules (tubulin): Guide vesicle movement, form the mitotic spindle, and are components of cilia and flagella.

Cell Walls

Structure and Function

Cell walls are rigid structures found in plants, fungi, prokaryotes, and some protists. They provide support, shape, and protection, and prevent excessive water uptake.

• Plant cell walls: Composed mainly of cellulose.

• Fungal cell walls: Composed mainly of chitin.

• Bacterial cell walls: Composed mainly of peptidoglycan.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid region)	Present
Membrane-bound organelles	Absent	Present
Cell size	1–5 μm	10–100 μm
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Key Terms

• Organelle: Specialized structure within a cell that performs a specific function.

• Endomembrane system: Group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Autophagy: Process by which cells recycle their own components using lysosomes.

• Endosymbiont theory: Hypothesis that mitochondria and chloroplasts originated from engulfed prokaryotic cells.