Keywords: IB Biology Topic A2.2, Cell Structure, Prokaryotic vs Eukaryotic, Organelles, Resolution vs Magnification, Binary Fission, Compartmentalization, Ultrastructure, New IB Biology Syllabus.
Welcome to the microscopic world of Topic A2.2: Cell Structure. In the new IB Biology syllabus, the focus has shifted from a simple 'identify the parts' approach to a functional understanding of compartmentalization. The IBO wants you to understand why eukaryotes bother with membranes around their organelles, while prokaryotes keep everything in one 'open-plan' room. This is the logic of efficiency: by separating chemical reactions into different compartments, a cell can optimize the environment for each specific task.
A major hurdle in this unit is mastering the scale of cells and the mathematics of microscopy. You are expected to move fluidly between millimeters, micrometers, and nanometers. In Paper 1A (MCQs), you will frequently face questions where you must calculate the actual size of a cell or determine the magnification of an electron micrograph. Understanding the difference between resolution (the ability to see detail) and magnification (the ability to see things larger) is critical for scoring a 7.
Before we dive into the organelles, internalize this fundamental truth: structure follows function. If a cell has a massive amount of Rough Endoplasmic Reticulum, it is a protein factory. If it is packed with mitochondria, it is an energy-intensive worker. By looking at the ultrastructure, you can predict exactly what that cell does for a living.
Prokaryotes (Bacteria and Archaea) are the simplest cells. They lack a nucleus and membrane-bound organelles. Their DNA is a single, circular chromosome found in the nucleoid region, and they may have smaller loops of DNA called plasmids.
Take a look at the question below:
The Bio-Logic: Prokaryotes lack membrane-bound organelles (Option A) and a nucleus (Option D). While eukaryotes have 80S ribosomes, prokaryotes have 70S ribosomes (Option B). However, every living cell requires a plasma membrane (Option C) to maintain a distinct internal environment. This is a "universal" feature of life.
Eukaryotic cells are significantly more complex due to compartmentalization. By enclosing enzymes and substrates within organelles, the cell can maintain different pH levels or concentrations in different areas simultaneously.
Take a look at the question below:
The Approach: Option B is the functional "Why." For example, the digestive enzymes inside a lysosome would destroy the rest of the cell if they weren't tucked away behind a membrane. This "division of labor" is what allows eukaryotes to be so much more metabolically diverse and physically larger than prokaryotes.
Resolution is the key difference between light and electron microscopes. Light microscopes are limited by the wavelength of visible light, whereas electron microscopes use beams of electrons, which have a much shorter wavelength.
Take a look at the two questions below:
The Bio-Logic for Question A: Resolution is physically limited by wavelength. Because electrons have a shorter wavelength (Option B), they can distinguish between two points that are much closer together, allowing us to see the ultrastructure of organelles like the Golgi or the double membrane of the nucleus.
The Bio-Logic for Question B: Use the formula: Magnification = Image size / Actual size. First, convert everything to the same units. 40 mm = 40,000 micrometers. 40,000 / 20 = 2000x (Option C). Always double-check your unit conversions!
You must be able to identify organelles from electron micrographs and link them to their primary function.
The Logic: Enzymes are proteins. Therefore, the cell needs Rough ER to build them and the Golgi (Option B) to wrap them up for delivery outside the cell. This is the "Secretory Pathway."
The IBO loves to mix units. Use this checklist to stay accurate:
Final Summary: Topic A2.2 is about the relationship between space and efficiency. Prokaryotes are small and fast; eukaryotes are large and complex. By using compartmentalization, eukaryotes can 'multi-task' chemically. Master the visual identification of organelles and the math of magnification, and you will dominate this section of the Paper 1A.
Click the black box to reveal the answers!