Tour of the Cell: Structure, Function, and Organization

Introduction to the Cell

The Role of Microscopes in Cell Biology

Advancements in microscopy have been fundamental to our understanding of cell structure and function. The invention and refinement of microscopes allowed scientists to observe cells and their components, leading to the development of cell theory and modern cell biology.

• Robert Hooke (1665): Used a crude microscope to examine cork bark, coining the term "cell" after observing small compartments he called cellulae.

• Antonie van Leeuwenhoek: Improved lens technology to describe living cells from blood, sperm, and pond water.

• Modern Microscopy: Enhanced magnification and resolution have vastly expanded our view of cellular structures.

Key Terms:

• Magnification: The increase in an object's image size compared to its actual size.

• Resolution: The clarity of an image; the ability to distinguish two close objects as separate.

Cell Theory and Types of Cells

Cell Theory

Early studies of cells led to the formulation of cell theory, which is foundational to biology.

• All living things are composed of cells.

• All cells arise from pre-existing cells.

Types of Microscopes

• Light Microscope (LM): Can display living cells; suitable for observing cell shape and movement.

• Scanning Electron Microscope (SEM): Reveals surface details of cells and structures.

• Transmission Electron Microscope (TEM): Shows internal ultrastructure of cells.

Example: To study the changes in shape of a living white blood cell, use a light microscope. For surface texture of a hair, use SEM. For organelle details, use TEM.

Cell Size and the Plasma Membrane

Surface Area-to-Volume Ratio

The small size of cells is crucial for efficient exchange of materials across the plasma membrane. A high surface area-to-volume ratio allows for adequate nutrient uptake and waste removal.

• Plasma Membrane: A phospholipid bilayer with embedded proteins that regulates the movement of substances in and out of the cell.

• Some proteins form channels for hydrophilic molecules; others act as pumps for active transport.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells

Prokaryotic cells (Bacteria and Archaea) are structurally simpler than eukaryotic cells.

• Lack a membrane-bound nucleus and organelles.

• Components: plasma membrane, DNA (in nucleoid), ribosomes, cytosol, cell wall, sometimes capsule, fimbriae, and flagella.

Eukaryotic Cells

Eukaryotic cells (plants, animals, fungi, protists) have a membrane-enclosed nucleus and various organelles.

• Organelles compartmentalize cellular functions.

• Four functional groups: genetic control (nucleus, ribosomes), manufacturing/distribution/breakdown (ER, Golgi, lysosomes, vacuoles, peroxisomes), energy processing (mitochondria, chloroplasts), and structural support/movement/communication (cytoskeleton, plasma membrane, cell wall).

Cellular Organelles and Their Functions

The Nucleus and Ribosomes

The nucleus contains the cell’s genetic instructions and directs protein synthesis via messenger RNA (mRNA). Ribosomes, composed of rRNA and proteins, synthesize proteins according to mRNA instructions.

• Nucleolus: Site of ribosome subunit assembly.

• Free Ribosomes: Synthesize proteins for use in the cytosol.

• Bound Ribosomes: Attached to the rough ER; synthesize proteins for membranes or export.

The Endomembrane System

The endomembrane system includes organelles that manufacture, distribute, store, and export molecules.

• Endoplasmic Reticulum (ER): Network of membranes; smooth ER synthesizes lipids and detoxifies, rough ER produces membranes and proteins.

• Golgi Apparatus: Modifies, sorts, and ships products from the ER.

• Lysosomes: Contain digestive enzymes to break down ingested substances and damaged organelles.

• Vacuoles: Large vesicles for storage and maintenance; plant cells have a central vacuole for storage and growth.

• Peroxisomes: Metabolic compartments that break down fatty acids and detoxify harmful substances.

Energy-Converting Organelles

• Mitochondria: Site of cellular respiration, converting chemical energy in food to ATP.

• Chloroplasts: Found in plants and algae; site of photosynthesis, converting solar energy to chemical energy.

Endosymbiont Theory: Mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

The Cytoskeleton and Cell Surfaces

The Cytoskeleton

The cytoskeleton is a network of protein fibers that organizes cell structure and activities.

• Microfilaments: Actin filaments involved in cell shape and movement.

• Intermediate Filaments: Provide structural support and anchor organelles.

• Microtubules: Tubulin polymers that guide organelle movement and form cilia/flagella.

Cilia and Flagella

Cilia and flagella are locomotor appendages made of microtubules in a "9+2" arrangement. Flagella are longer and move with a whiplike motion; cilia move like oars.

Extracellular Matrix (ECM) and Cell Junctions

• ECM: Network of glycoproteins and polysaccharides that supports animal cells and communicates with the cytoskeleton via integrins.

• Cell Junctions:

  • Tight Junctions: Form leakproof sheets.

  • Anchoring Junctions (Desmosomes): Rivet cells into strong tissues.

  • Gap Junctions: Allow ions and small molecules to pass between cells.

Plant Cell Walls and Plasmodesmata

• Cell Wall: Rigid structure made of cellulose that provides support and protection.

• Plasmodesmata: Channels that connect plant cells for sharing water, nutrients, and chemical messages.

Review: Functional Categories of Eukaryotic Organelles

• Genetic Control: Nucleus, ribosomes

• Manufacturing, Distribution, Breakdown: ER, Golgi, lysosomes, vacuoles, peroxisomes

• Energy Processing: Mitochondria, chloroplasts

• Structural Support, Movement, Communication: Cytoskeleton, plasma membrane, ECM, cell wall, cell junctions

Key Metric Equivalents

Unit	Equivalent
1 meter (m)	100 cm = 1,000 mm = 39.4 inches
1 centimeter (cm)	0.01 m = 0.4 inch
1 millimeter (mm)	0.001 m
1 micrometer (μm)	0.000001 m = 0.001 mm
1 nanometer (nm)	0.000000001 m = 0.000001 μm

Summary Table: Eukaryotic Cell Structures and Their Functions

Structure	Function
Nucleus	Genetic control; houses DNA
Ribosomes	Protein synthesis
Endoplasmic Reticulum (ER)	Smooth: lipid synthesis, detoxification; Rough: protein synthesis
Golgi Apparatus	Modification, sorting, shipping of cell products
Lysosomes	Digestion of macromolecules and damaged organelles
Vacuoles	Storage, waste disposal, growth (plants)
Peroxisomes	Breakdown of fatty acids, detoxification
Mitochondria	Cellular respiration; ATP production
Chloroplasts	Photosynthesis (plants, algae)
Cytoskeleton	Cell shape, movement, organelle transport
Plasma Membrane	Regulates entry/exit of substances
Extracellular Matrix (ECM)	Support, regulation, communication (animals)
Cell Wall	Support, protection, binding of cells (plants)
Cell Junctions	Communication and adhesion between cells