A researcher investigating membrane permeability treated red blood cells with a detergent that dissolves the phospholipid bilayer. Explain why this treatment caused haemoglobin to leave the cells but did not cause the loss of glucose-transport proteins from the cell contents.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Red blood cells are specialised cells that lack a nucleus and most membrane-bound organelles. They contain the oxygen-carrying protein haemoglobin dissolved in the cytoplasm and possess integral glucose-transport proteins embedded within their cell-surface membrane.
Model answer (4 marks)
The detergent dissolves the phospholipid bilayer, destroying the cell‑surface membrane.
1. Haemoglobin is a soluble protein in the cytoplasm; once the membrane is gone it can diffuse out of the cell.
2. Glucose‑transport proteins are integral transmembrane proteins that are embedded in the bilayer.
3. When the bilayer is dissolved, these proteins are removed with the membrane material and are not left in the cell contents.
1. Haemoglobin is a soluble protein in the cytoplasm; once the membrane is gone it can diffuse out of the cell.
2. Glucose‑transport proteins are integral transmembrane proteins that are embedded in the bilayer.
3. When the bilayer is dissolved, these proteins are removed with the membrane material and are not left in the cell contents.
Examiner tips
- Use the word ‘dissolve’ for the detergent action. Mention haemoglobin is soluble and can leave. State that transport proteins are integral and are removed with the membrane.
- common_mistakes
- :
- Saying the proteins leak out of the cell. Confusing the proteins with soluble cytoplasmic proteins. Forgetting to link the loss of the bilayer to the removal of the proteins.
Mark scheme (4 marks)
- The phospholipid bilayer is dissolved/disrupted by the detergent, destroying the integrity of the cell-surface membrane.
- Haemoglobin is a soluble protein dissolved in the cytoplasm, so once the membrane is destroyed it is free to leave the cell.
- Glucose-transport proteins are integral/transmembrane proteins embedded within the phospholipid bilayer.
- When the bilayer is dissolved by the detergent, the glucose-transport proteins are also removed/dispersed with the membrane material and are therefore not left behind in the cell contents.
Key terms in this question
phospholipid bilayer · haemoglobin
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →