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2.4.2Additional lessons

Cell Compartmentalisation — IB Biology SL Revision Notes & Topical Questions

5 min read 4 parts Lesson 25 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 4Compartments in Action

2.4.2 Cell Compartmentalisation
Nucleus and cytoplasm
  • Separation of DNA from the cytoplasm allows mRNA to be processed before it reaches ribosomes, which reduces errors.
Mitochondria
  • The space between the inner and outer membranes can build up a high concentration of hydrogen ions for ATP synthesis.
Chloroplasts
  • The interior of thylakoids accumulates hydrogen ions, while the stroma holds the enzymes of the Calvin cycle.
Lysosomes
  • An acid interior around pH 5 suits the hydrolytic enzymes, which work poorly at the pH of the cytosol.
Vesicles
  • Vesicles move materials between compartments without letting their contents mix with the cytosol.
Chloroplast stroma
  • Enzymes of the Calvin cycle are concentrated in the stroma, close to the thylakoids that supply ATP and NADPH.

Part 2 of 4Cell Fractionation

1
Cells are broken open in a cold, buffered, isotonic solution so the organelles are not damaged and their enzymes remain active.
2
The mixture, called the homogenate, is filtered to remove large debris such as cell wall fragments.
3
The filtrate is spun at a low speed in a centrifuge, and the heaviest organelles, such as nuclei, form a pellet.
4
The supernatant is transferred to another tube and spun faster, so that mitochondria and then smaller structures sediment.
5
Each pellet can be examined with a microscope or tested for enzyme activity to identify its organelles.

Part 3 of 4Why Use This Technique

Differential centrifugation separates organelles by density and size.

Why Use This Technique
Isotonic solution
  • Prevents osmosis, which could burst or shrink organelles during the procedure.
Cold temperature
  • Slows the activity of enzymes that could damage the organelles.
Buffer
  • Maintains a stable pH so that enzymes and membranes are not denatured.
Nature of science
  • Advances in techniques such as centrifugation allowed scientists to study the functions of individual organelles.
Differential centrifugation
  • Increasing the spin speed in steps separates organelles in order of size and density, from nuclei to ribosomes.

Part 4 of 4Exam Checklist & Tips

2.4.2 – Key Points for the Exam

  • Compartmentalisation Separation of cell processes in different membrane-bound regions.
  • Homogenate The mixture of broken cells produced during cell fractionation.
  • Centrifuge A machine that spins samples to separate components by density.
  • Isotonic Having the same solute concentration as the cell or organelle, so there is no net water movement.

Common Mistakes

  • Using a hot solution during homogenisation, which would denature enzymes and damage organelles.
  • Saying the lightest organelles settle first; heavier structures sediment at lower speeds.
  • Using pure water instead of an isotonic solution, which would burst organelles by osmosis.
  • Confusing compartmentalisation with cell division.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.