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

IB DP Biology Standard 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 single‑membrane organelles; the phospholipid bilayer isolates the hydrolytic enzymes from the cytoplasm, preventing self‑digestion.
They contain hydrolases that catalyse the breakdown of proteins, lipids, carbohydrates and nucleic acids.
The lumen is kept acidic (pH 4.5–5) by proton pumps in the membrane, which is the optimum pH for the enzymes.
When a lysosome fuses with a phagosome, endosome or damaged organelle, the enzymes are delivered directly to the substrate, allowing intracellular digestion and recycling.

Examiner tips

Mention the single‑membrane isolation first, then the enzymes, acidic pH, and fusion with vesicles – this order matches the mark scheme.
Use the exact terms ‘hydrolases’, ‘phospholipid bilayer’, ‘acidic pH’, and ‘fusion’ to gain full marks.

Common mistakes

Confusing lysosomes with other vesicles (e.g. endosomes) or omitting the membrane isolation.
Failing to state the acidic pH or the role of proton pumps.
Using vague language such as ‘digestive enzymes’ without specifying hydrolases.

Mark scheme (4 marks)

Lysosomes are surrounded by a single phospholipid bilayer membrane, which isolates hydrolytic enzymes from the rest of the cytoplasm / prevents self-digestion of the cell.
Lysosomes contain hydrolytic (digestive) enzymes / hydrolases, which catalyse the breakdown of macromolecules such as proteins, lipids, carbohydrates and nucleic acids.
The interior of the lysosome is maintained at an acidic pH (approximately pH 4.5–5), which is the optimum pH for lysosomal hydrolases to function / proton pumps in the membrane actively transport H⁺ ions into the lysosome to maintain this low pH.
Lysosomes can fuse with vesicles containing engulfed material (phagosomes / endosomes) or with damaged organelles, delivering enzymes directly to the substrate for digestion / allowing recycling of cellular components.

Key terms in this question

lysosome