Preview
IB DP Biology — A: Unity and Diversity | SL | A2.2 Cell Structure
📋 What You Will Learn
- Identify the distinguishing ultrastructural features of prokaryotic and eukaryotic cells
- Explain the functional significance of membrane-bound organelles in eukaryotic cells
- Analyse electron micrograph data to identify cell type, organelles, and relative scale
- Apply the magnification formula to calculate actual size and image size from micrographs
- Evaluate the evidence supporting the endosymbiotic theory for the origin of mitochondria and chloroplasts
🔍 Guiding Question How does the internal organisation of cells reflect the functions they carry out, and what does the distinction between prokaryotic and eukaryotic cells reveal about the unity and diversity of life?
🌍 Why This Matters Every antibiotic you have ever taken exploits a structural difference between prokaryotic and eukaryotic cells. Penicillin destroys bacterial cell walls without harming your own cells — because your cells have no cell wall to destroy. Understanding cell structure is not abstract biology; it is the molecular logic behind modern medicine.
📋 Real-Life Context
Look at a drop of pond water under a microscope and you will see two fundamentally different kinds of life sharing the same space. Some organisms are single cells with no visible internal compartments — bacteria, archaea. Others contain elaborate internal machinery: nuclei, mitochondria, chloroplasts, entire cities of membrane-bound structures. This difference — prokaryote versus eukaryote — is the deepest division in all of biology.
It is not merely a difference in complexity. It is a difference in strategy. One cell type solved the problem of life with elegant simplicity. The other solved it by building walls within walls, compartmentalising chemistry so precisely that a single cell can perform thousands of reactions simultaneously without catastrophic interference.
Key Definitions
Before we can compare cell types, we need precise language. The terms below are not arbitrary labels — each one encodes a structural reality that has functional consequences. Understand the definition, and the biology follows naturally.
📖 Key Definition — Prokaryote A prokaryote is an organism whose cells lack a membrane-bound nucleus and membrane-bound organelles. Genetic material is located in a region of the cytoplasm called the nucleoid. All bacteria and archaea are prokaryotes.
📖 Key Definition — Eukaryote A eukaryote is an organism whose cells contain a membrane-bound nucleus enclosing the genetic material, and membrane-bound organelles that compartmentalise cellular functions. Protists, fungi, plants, and animals are all eukaryotes.
📖 Key Definition — Organelle An organelle is a discrete, functionally specialised structure within a cell. In eukaryotes, many organelles are enclosed by one or more phospholipid membranes, creating chemically distinct compartments. Ribosomes are organelles present in all cells but are not membrane-bound.
📖 Key Definition — Ultrastructure Ultrastructure refers to the fine detail of cell structure visible only with an electron microscope, including the internal organisation of organelles, membrane systems, and cytoskeletal elements.