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IB DP Biology B2.2 Organelles and compartmentalization -FA 2025-IB Style questions SL Paper 2

IBDP Biology Paper 2 - All Chapters

Question

Cystic fibrosis is a genetic condition that is associated with defects in the CFTR protein. The CFTR protein is a gated ion channel that requires ATP binding in order to allow chloride ions (Cl−) to diffuse across the membrane.

(a) In the provided model of a cell, draw arrows to describe the pathway for production of a normal CFTR protein from gene expression to final cellular location.

(b) Identify the most likely cellular location of the ribosomes that synthesize CFTR protein.

(c) Identify the most likely cellular location of a mutant CFTR protein that has an amino acid substitution in the ATP-binding site.

▶️ Answer/Explanation

 

(a) 

(b) 

The Endoplasmic Reticulum (specifically the rough ER).
Because CFTR is a membrane protein, it is synthesized on ribosomes bound to the rough ER.

(c)

The Plasma Membrane.

Even though the CFTR protein is mutated, it can still be processed and embedded in the plasma membrane. However, the faulty ATP-binding site may prevent it from functioning properly as a chloride channel.

Markscheme:

(a) The pathway should show:
1. Transcription in nucleus (DNA → mRNA)
2. mRNA export to cytoplasm
3. Translation on rough ER ribosomes
4. Processing in Golgi apparatus
5. Final insertion in plasma membrane
Note: For the image, arrows should connect these locations in sequence.

(b) The ribosomes that synthesize the CFTR protein would probably be found on the rough ER.

(c) The mutant CFTR protein is likely embedded in the plasma membrane.

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