The Fundamental Unit of Life

Why This Chapter Matters

A human body contains trillions of cells, yet many organisms consist of one cell. The cell is therefore not merely a building block like a brick; it is the smallest unit capable of carrying out the processes of life. A cell must maintain a boundary without becoming sealed, store instructions, obtain energy, build molecules, transport materials and remove waste. Organelles make sense when viewed as solutions to these problems.

Bridge from Earlier Learning

• Basic plant and animal cell diagrams
• Diffusion
• Living and non-living characteristics
• Microscope idea

These ideas are reused and deepened in this chapter.

1. Discovery and Cell Theory

2. Plasma Membrane — Controlled Boundary

The plasma membrane separates cell contents from the external environment but is selectively permeable. Small molecules such as oxygen and carbon dioxide can diffuse according to concentration differences. Water movement across a selectively permeable membrane is osmosis. Selective exchange allows the cell to maintain internal conditions instead of simply matching the surroundings.

3. Osmosis and Tonic Effects

In a dilute external solution, water may enter a cell; in a concentrated external solution, water may leave. Plant cells resist excessive swelling because the rigid wall supports the membrane. Loss of water can cause the membrane and contents to pull away from the wall, called plasmolysis in the school context. Direction depends on relative water potential, not on vague ideas of 'more water' alone.

4. Cell Wall

Plant cells have a cellulose-rich wall outside the plasma membrane. It is more freely permeable than the plasma membrane and provides support, protection and resistance to bursting. The wall allows plant cells to become turgid when water enters; turgor helps non-woody parts stay firm.

5. Nucleus and Genetic Control

The nucleus is surrounded by a nuclear envelope and contains chromatin, which condenses into chromosomes during division. Chromosomes carry DNA and genes. Genes contain inherited information that influences cell structure and function. Prokaryotic cells lack a membrane-bound nucleus; eukaryotic cells have one and possess more extensive compartmentalisation.

6. Endoplasmic Reticulum and Golgi Apparatus

The endoplasmic reticulum forms an internal membrane network. Rough ER has ribosomes and is closely involved in protein production; smooth ER participates in lipid synthesis and other specialised functions. Golgi stacks modify, sort and package products for transport inside or outside the cell.

7. Lysosomes, Mitochondria and Plastids

Lysosomes contain digestive enzymes used to break down materials. Mitochondria release usable energy through respiration and produce ATP; they contain their own DNA and ribosomes. Plastids occur in plant cells. Chloroplasts contain chlorophyll and perform photosynthesis; other plastids store pigments or food materials.

8. Vacuoles

Vacuoles are membrane-bound storage compartments. Plant cells often have one large central vacuole containing cell sap, contributing to storage and turgor. Animal cells may have smaller, temporary vacuoles depending on cell type.

9. Prokaryotic and Eukaryotic Organisation

Prokaryotic cells are generally smaller and lack membrane-bound organelles such as nucleus, mitochondria and Golgi apparatus. Their DNA lies in a nucleoid region. Eukaryotic cells compartmentalise functions with membrane-bound organelles. Both cell types still possess plasma membrane, cytoplasm, genetic material and ribosomes.

Compare the Ideas

Visualise the Concept

Organelles make more sense when linked to cellular needs such as energy, transport, storage and control.

Worked Examples & Scientific Reasoning

Observe Like a Scientist

Exam Thinking Strategy

Common Mistakes

Quick Revision

Focused Practice

Final Chapter Summary