Preview
IB DP Biology — Unity and Diversity | SL | A2.2 Cell Structure
📋 What You Will Learn
- Identify the key membrane-bound organelles of eukaryotic cells and their structural features
- Explain how compartmentalization allows different biochemical processes to occur simultaneously within a single cell
- Describe the structure and function of the nucleus, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles
- Analyse electron micrographs to identify organelles and deduce the metabolic activity of a cell from its organelle composition
- Apply understanding of organelle function to explain why different cell types contain different proportions of organelles
🔍 Guiding Question How does the compartmentalization of cellular functions in eukaryotes allow for greater complexity than is possible in prokaryotes?
🌍 Why This Matters Mitochondrial dysfunction underlies dozens of human diseases — from Parkinson's to rare inherited metabolic disorders — because when the cell's power stations fail, energy-hungry tissues like the brain and heart are the first to suffer. Understanding organelle structure is not abstract biology; it is the foundation of modern medicine.
The Problem the Eukaryotic Cell Had to Solve
Here is the problem: chemistry is messy. If you run every reaction in the same open space, products interfere with substrates, acids destroy enzymes, and competing pathways cancel each other out. A prokaryotic cell manages — just barely — by keeping things simple. But complexity demands separation.
The eukaryotic cell's answer was membranes. Not just one outer boundary, but an entire internal architecture of lipid bilayers dividing the cytoplasm into discrete compartments. Each compartment maintains its own chemical environment — its own pH, its own enzyme cocktail, its own concentration of ions. The result is a cell that can simultaneously synthesise proteins, digest old organelles, generate ATP, and replicate its DNA without any of these processes sabotaging the others.
📖 Key Definition — Compartmentalization The division of the interior of a eukaryotic cell into membrane-bound regions (organelles), each maintaining a distinct chemical environment that allows specific biochemical reactions to proceed without interference from other cellular processes.
📖 Key Definition — Organelle A discrete, membrane-bound structure within a eukaryotic cell that performs a specific function. The term literally means "little organ" — each organelle is to the cell what an organ is to the body.
The Nucleus: Command and Control
Every decision the cell makes traces back here. The nucleus is the only organelle that holds the master copy of the genome — the complete set of instructions for building and running the organism. Protecting that information is not optional; a single unrepaired error in DNA can cascade into cancer or cell death.
Nuclear Envelope and Pores
The nucleus is bounded by a double membrane called the nuclear envelope. This is not simply two lipid bilayers stacked together — the space between them (the perinuclear space) is continuous with the lumen of the endoplasmic reticulum, connecting the nucleus to the cell's protein-trafficking network. Punctuating the nuclear envelope are nuclear pore complexes: large protein channels roughly 120 nm in diameter that regulate the passage of molecules between nucleus and cytoplasm.
mRNA molecules transcribed from DNA must exit through these pores to reach ribosomes in the cytoplasm. Proteins destined