A Tour of the Cell – Structure and Function of Cellular Components

Introduction to the Cell

Cells are the fundamental units of life, and their discovery and study have revolutionized biology. The development of microscopy has enabled scientists to explore the structure and function of cells in detail.

Microscopy and Cell Theory

Types of Microscopes

• Light Microscope: Allows observation of living cells and tissues with moderate magnification and resolution.

• Electron Microscopes: Include scanning (SEM) and transmission (TEM) types, providing much higher magnification and resolution, revealing cell ultrastructure.

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

• Resolution: The ability to distinguish two close objects as separate entities.

Cell Theory

• All living things are composed of cells.

• All cells arise from pre-existing cells.

The Small Size of Cells and the Plasma Membrane

Surface-to-Volume Ratio

The small size of cells ensures a large surface area relative to volume, facilitating efficient exchange of materials across the plasma membrane.

Structure of the Plasma Membrane

• Phospholipid Bilayer: Forms the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Membrane Proteins: Include channels for passive transport and pumps for active transport of molecules.

Prokaryotic vs. Eukaryotic Cells

Common Features of All Cells

• Plasma membrane

• DNA

• Ribosomes

• Cytosol

Prokaryotic Cells

• Domains: Bacteria and Archaea

• Lack a membrane-bound nucleus and organelles

• Generally smaller and structurally simpler

Eukaryotic Cells

• Domain: Eukarya (includes plants, animals, fungi, and protists)

• Have a membrane-bound nucleus and various organelles

Compartmentalization in Eukaryotic Cells

Eukaryotic cells contain membrane-bound organelles that compartmentalize cellular functions into four main groups:

• Genetic control: Nucleus and ribosomes

• Manufacture, distribution, and breakdown: Endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes

• Energy processing: Mitochondria (all cells), chloroplasts (plants and algae)

• Structural support, movement, communication: Cytoskeleton, plasma membrane, cell wall (plants)

The Nucleus and Ribosomes

The Nucleus

• Houses the cell's DNA and directs protein synthesis via messenger RNA (mRNA).

• Contains the nucleolus, where ribosomal subunits are assembled.

Ribosomes

• Composed of ribosomal RNA and proteins.

• Sites of protein synthesis, following instructions from DNA.

• Can be free in the cytosol or bound to the endoplasmic reticulum.

The Endomembrane System

Components and Functions

• Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Responsible for synthesis, distribution, storage, and export of molecules.

Endoplasmic Reticulum (ER)

• Smooth ER: Synthesizes lipids, processes toxins.

• Rough ER: Studded with ribosomes; produces membranes and secretory proteins.

Golgi Apparatus

• Consists of stacked sacs that modify, sort, and ship products from the ER to other destinations.

Lysosomes

• Membranous sacs containing digestive enzymes.

• Break down ingested substances and damaged organelles (cellular recycling).

Vacuoles

• Large vesicles with diverse functions (e.g., storage, waste disposal, growth in plants).

• Contractile vacuoles in protists expel excess water.

Peroxisomes

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

• Not part of the endomembrane system.

Integration of the Endomembrane System

• Transport vesicles shuttle materials between organelles, integrating their functions.

Energy-Converting Organelles

Mitochondria

• Sites of cellular respiration, converting chemical energy from food into ATP.

• Contain an outer membrane, an inner membrane with folds (cristae), and a matrix with DNA and ribosomes.

Chloroplasts

• Found in plants and algae; sites of photosynthesis (conversion of solar energy to chemical energy).

• Contain inner and outer membranes, thylakoids (site of light reactions), and stroma (site of Calvin cycle).

Endosymbiont Theory

• Mitochondria and chloroplasts likely evolved from prokaryotic cells engulfed by ancestral eukaryotes.

The Cytoskeleton and Cell Surfaces

The Cytoskeleton

• Network of protein fibers: microfilaments (actin), intermediate filaments, and microtubules (tubulin).

• Functions: maintain cell shape, anchor/move organelles, cell movement, muscle contraction.

Cilia and Flagella

• Locomotor appendages made of microtubules in a "9+2" arrangement.

• Flagella: longer, undulating motion; Cilia: shorter, coordinated oar-like movement.

Extracellular Matrix (ECM) of Animal Cells

• Composed of glycoproteins (e.g., collagen) and other molecules.

• Provides structural support, binds cells, and communicates with the cytoskeleton via integrins.

Cell Junctions in Animal Tissues

• Tight junctions: Form leakproof sheets.

• Anchoring junctions: Rivet cells into strong tissues.

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

Plant Cell Walls and Plasmodesmata

• Rigid cell wall (mainly cellulose) provides support and protection.

• Plasmodesmata: Channels that connect plant cells, allowing exchange of water, nutrients, and signals.

Summary Table: Major Eukaryotic Cell Structures and Functions

Structure	Main Function(s)
Nucleus	Genetic control, houses DNA
Ribosomes	Protein synthesis
Endoplasmic Reticulum	Lipid and protein synthesis, detoxification
Golgi Apparatus	Modification, sorting, shipping of cell products
Lysosomes	Digestion and recycling
Vacuoles	Storage, waste disposal, growth (plants)
Peroxisomes	Breakdown of fatty acids, detoxification
Mitochondria	ATP production (cellular respiration)
Chloroplasts	Photosynthesis (plants, algae)
Cytoskeleton	Structural support, movement
Plasma Membrane	Selective barrier, communication
Cell Wall (plants)	Support, protection
Extracellular Matrix (animals)	Support, adhesion, signaling