📌Definition Table

Term	Definition
Organelle	A membrane-bound compartment within eukaryotic cells, specialized for particular biochemical processes.
Compartmentalisation	The separation of cellular activities into distinct organelles or regions within a cell, increasing efficiency and protection.
Lysosome	Organelle containing hydrolytic enzymes for intracellular digestion and breakdown of waste.
Nucleus	Organelle containing the cell’s genetic material (DNA), controlling gene expression and cell function.
Endocytosis	The process by which cells engulf external substances by forming vesicles from the plasma membrane.
Cell fractionation	A laboratory technique for separating and studying different organelles by centrifugation.

📌Introduction

Eukaryotic cells differ from prokaryotic cells by their compartmentalised internal structure. Organelles such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes are enclosed by membranes, which create specialized environments. This organization enables cells to perform complex and often conflicting biochemical reactions simultaneously. Compartmentalisation provides both efficiency and protection, allowing enzymes and substrates to be localized, harmful by-products to be contained, and optimal conditions (pH, concentration) to be maintained for specific processes.

📌 Advantages of Compartmentalisation

• Separation of incompatible biochemical reactions (e.g., digestive enzymes in lysosomes are kept away from cytoplasm).
• Localisation of substrates and enzymes increases reaction efficiency by maintaining high concentrations in small volumes.
• Maintenance of distinct internal environments, such as low pH in lysosomes or proton gradients in mitochondria.
• Flexibility: the number and size of organelles can change depending on cellular activity (e.g., muscle cells contain more mitochondria).
• Compartmentalisation improves regulation, as processes like transcription in the nucleus are separated from translation in the cytoplasm.

🧠 Examiner Tip: When describing compartmentalisation, always provide examples (e.g., lysosomes preventing self-digestion, or mitochondria isolating respiration steps). Simply stating “it increases efficiency” is not enough for full marks.

📌 The Nucleus and Separation of Processes

• The nucleus is enclosed by a double membrane (nuclear envelope) with pores for exchange of molecules.
• Separates transcription from translation, reducing the likelihood of errors in mRNA before it meets ribosomes.
• mRNA can be modified and processed (e.g., splicing, addition of 5’ cap and poly-A tail) before translation.
• This distinction from prokaryotes highlights the evolutionary advantage of compartmentalisation in maintaining genetic fidelity.

🧬 IA Tips & Guidance: A practical extension is cell fractionation using centrifugation to isolate organelles. Students can design investigations comparing enzyme activity in lysosomal fractions versus cytoplasm, linking structure to compartmentalisation.

📌 Cytoplasmic Compartmentalisation

• Membrane-bound organelles within the cytoplasm (e.g., mitochondria, peroxisomes) keep pathways distinct.
• Harmful substances can be sequestered safely (e.g., oxidative enzymes in peroxisomes, toxins in vacuoles).
• In plants, anaerobic niches in the cytoplasm allow sensitive enzymes like nitrogenase to function away from oxygen.
• Endocytosis and phagocytosis create vacuoles around harmful material, ensuring digestion occurs in controlled compartments.

🌐 EE Focus: An EE could examine how organelle separation contributes to metabolic efficiency, for example by comparing aerobic respiration in isolated mitochondria vs whole cells. Another angle could be the role of compartmentalisation in evolutionary development of eukaryotes.

📌 Microscopy and Study of Organelles

• Early progress in organelle study depended on advances in technology such as electron microscopy and ultracentrifugation.
• Cell fractionation allows isolation of organelles for biochemical study (e.g., separating mitochondria to investigate respiration).
• Staining and fluorescent tagging help localise proteins within organelles, linking structure to function.

❤️ CAS Link: Students could create models or animations of organelles and their compartmentalised functions, then present these in a community school to explain how cells are organized like “factories with departments.”

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📌 Specialisation of Organelles

• Different organelles evolve adaptations for efficiency: mitochondria for ATP production, chloroplasts for photosynthesis, ER for protein and lipid synthesis.
• Organelles work together in coordinated pathways (e.g., protein synthesis: ribosomes → ER → Golgi → vesicles → secretion).
• This cooperation shows that compartmentalisation is not isolation, but controlled interaction.

🔍 TOK Perspective: Compartmentalisation highlights how models simplify complex systems. Scientists study organelles separately to understand function — but does isolating parts risk missing emergent properties of the whole cell? This raises TOK questions about reductionism versus holism in biology.

📝 Paper 2: Expect questions comparing compartmentalisation in prokaryotes vs eukaryotes, describing advantages of organelles, and analyzing diagrams. Data-based questions may involve interpreting centrifugation results or enzyme activity in isolated fractions.