Explain how the structure of the lysosome is related to its function in intracellular digestion.

IB DP Biology Higher Level (2023 syllabus) — B2.2 Organelles and compartmentalisation · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Lysosomes are surrounded by a single phospholipid bilayer, which keeps the hydrolytic enzymes inside and protects the rest of the cell from self‑digestion.

The lumen of a lysosome is kept at an acidic pH (≈4.5–5), the optimum for the acid hydrolases it contains.

These enzymes – proteases, lipases, nucleases and glycosidases – can degrade proteins, lipids, nucleic acids and carbohydrates.

When a lysosome fuses with a phagosome or an autophagosome, the enzymes are delivered directly to the engulfed material, allowing intracellular digestion and recycling of cellular components.

Examiner tips

Mention the single‑membrane isolation, acidic pH, range of enzymes, and fusion with phagosomes/autophagosomes to show functional link.
Use the exact terms ‘phospholipid bilayer’, ‘acidic pH’, ‘acid hydrolases’, and ‘fusion’ – these are the key phrases examiners look for.
Show the sequence: membrane isolation → acidic environment → enzyme activity → fusion for digestion.
Keep the answer concise – 4 points, so one sentence per point is ideal.

Common mistakes

Confusing lysosomes with other organelles (e.g., mitochondria) or omitting the acidic pH detail.
Using vague terms like ‘digestive organelle’ instead of ‘acid hydrolases’.

Mark scheme (4 marks)

Lysosomes are bounded by a single phospholipid bilayer membrane that isolates hydrolytic enzymes from the rest of the cytoplasm, preventing self-digestion of the cell.
The interior (lumen) of the lysosome is maintained at an acidic pH (approximately 4.5–5), which is the optimum pH for the hydrolytic enzymes contained within.
Lysosomes contain acid hydrolases (e.g. proteases, lipases, nucleases, glycosidases) that can break down all major classes of biological macromolecules.
Lysosomes fuse with phagosomes (containing engulfed material) or autophagosomes (containing worn-out organelles), delivering enzymes directly to their substrates to enable digestion and recycling of components.

Key terms in this question

lysosome · intracellular digestion