Explain how the structural features of the smooth endoplasmic reticulum (SER) are related to its functions in a hepatocyte (liver cell).

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)

The smooth endoplasmic reticulum (SER) in a hepatocyte is a vast network of membranous tubules, giving it a large surface area for enzyme‑mediated reactions. Cytochrome P450 and other detoxification enzymes are embedded in the SER membrane, allowing lipid‑soluble drugs and toxins to be oxidised into water‑soluble metabolites that can be excreted. Because the SER is a closed, continuous compartment, the toxic intermediates produced during detoxification are sequestered away from the rest of the cytoplasm, protecting cellular components. In addition, the SER membrane contains Ca²⁺‑ATPase pumps and Ca²⁺ channels that store and release calcium ions, thereby regulating cellular signalling and metabolic pathways in the hepatocyte.

Examiner tips

  • Use the word ‘smooth’ to distinguish from rough ER; link surface area to enzyme activity; mention cytochrome P450 explicitly; explain the closed compartment protects the cell; include Ca²⁺ storage and signalling.
  • Structure‑function links should be clear: network → surface area → detox enzymes; membrane localisation → lipid‑soluble substrates; closed system → protection; Ca²⁺ pumps → signalling.

Common mistakes

  • Confusing rough ER functions (protein synthesis) with SER; not naming cytochrome P450; omitting the protective aspect of the closed compartment; failing to mention Ca²⁺ pumps or channels.

Mark scheme (4 marks)

  1. The SER is an extensive network of membranous tubules, providing a large surface area for enzyme-mediated reactions such as detoxification of lipid-soluble drugs and toxins.
  2. Detoxification enzymes (e.g. cytochrome P450) are embedded within the SER membrane, allowing lipid-soluble compounds to be converted into water-soluble forms for excretion.
  3. The SER forms a closed, continuous membrane compartment that sequesters toxic intermediates, protecting the rest of the cytoplasm from damage during detoxification.
  4. The SER also stores and releases calcium ions (Ca²⁺) by means of membrane-embedded Ca²⁺-ATPase pumps and Ca²⁺ channels, enabling regulation of cellular signalling and metabolic processes in the hepatocyte.

Key terms in this question

smooth endoplasmic reticulum (SER) · hepatocyte

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