2.4.2Additional lessons
Cell Compartmentalisation — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 25 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Compartments in Action
Nucleus and cytoplasm
- Nucleus and cytoplasm Separation of DNA from the cytoplasm allows mRNA to be processed before it reaches ribosomes, which reduces errors.
Mitochondria
- Mitochondria The space between the inner and outer membranes can build up a high concentration of hydrogen ions for ATP synthesis.
Chloroplasts
- Chloroplasts The interior of thylakoids accumulates hydrogen ions, while the stroma holds the enzymes of the Calvin cycle.
Lysosomes
- Lysosomes An acid interior around pH 5 suits the hydrolytic enzymes, which work poorly at the pH of the cytosol.
Vesicles
- Vesicles Vesicles move materials between compartments without letting their contents mix with the cytosol.
Chloroplast stroma
- Chloroplast stroma Enzymes of the Calvin cycle are concentrated in the stroma, close to the thylakoids that supply ATP and NADPH.
Section 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.
Section 3 of 4Why Use This Technique
Differential centrifugation separates organelles by density and size.
Why Use This Technique
Isotonic solution
- Isotonic solution Prevents osmosis, which could burst or shrink organelles during the procedure.
Cold temperature
- Cold temperature Slows the activity of enzymes that could damage the organelles.
Buffer
- Buffer Maintains a stable pH so that enzymes and membranes are not denatured.
Nature of science
- Nature of science Advances in techniques such as centrifugation allowed scientists to study the functions of individual organelles.
Differential centrifugation
- Differential centrifugation Increasing the spin speed in steps separates organelles in order of size and density, from nuclei to ribosomes.
Section 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.