AQA A-Level Biology 2025/6

Biology • A-Level

Question 1 of 41

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[DIAGRAM: Diagram of a typical animal cell showing organelles]

With reference to the diagram, describe the intracellular pathway that a newly synthesised secretory protein follows from its point of synthesis on the rough endoplasmic reticulum to its final delivery at the plasma membrane. In your answer, explain the specific roles of the rough ER, Golgi apparatus, transport vesicles and the cytoskeletal elements that facilitate vesicle movement.

Hint: Identify the organelles involved in protein synthesis, modification and transport, and consider how vesicles are moved within the cell.

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Correct answer: The diagram shows a typical animal cell with the rough endoplasmic reticulum (RER) studded with ribosomes, a stacked Golgi apparatus, transport vesicles and a network of cytoskeletal filaments. A secretory protein is first synthesised by ribosomes attached to the RER, where it enters the lumen of the RER and undergoes initial folding and N‑linked glycosylation. From the RER, the protein is packaged into coat‑protein complex II (COPII) coated vesicles that bud from the RER membrane and are transported along microtubules toward the Golgi apparatus. Within the Golgi, the protein moves through the cis‑, medial‑ and trans‑cisternae where further processing (e.g., O‑glycosylation, sulfation) and sorting occurs. The mature protein is then packaged into secretory vesicles that bud from the trans‑Golgi network. These vesicles are directed to the plasma membrane by motor proteins (kinesin/dynein) moving along microtubules, and they fuse with the plasma membrane via SNARE protein interactions, releasing the protein to the extracellular space or inserting membrane proteins into the lipid bilayer. Thus, the RER, Golgi apparatus, transport vesicles and the cytoskeleton work together to ensure accurate synthesis, modification, sorting and delivery of secretory proteins.

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The diagram shows a typical animal cell with the rough endoplasmic reticulum (RER) studded with ribosomes, a stacked Golgi apparatus, transport vesicles and a network of cytoskeletal filaments. A secretory protein is first synthesised by ribosomes attached to the RER, where it enters the lumen of the RER and undergoes initial folding and N‑linked glycosylation. From the RER, the protein is packaged into coat‑protein complex II (COPII) coated vesicles that bud from the RER membrane and are transported along microtubules toward the Golgi apparatus. Within the Golgi, the protein moves through the cis‑, medial‑ and trans‑cisternae where further processing (e.g., O‑glycosylation, sulfation) and sorting occurs. The mature protein is then packaged into secretory vesicles that bud from the trans‑Golgi network. These vesicles are directed to the plasma membrane by motor proteins (kinesin/dynein) moving along microtubules, and they fuse with the plasma membrane via SNARE protein interactions, releasing the protein to the extracellular space or inserting membrane proteins into the lipid bilayer. Thus, the RER, Golgi apparatus, transport vesicles and the cytoskeleton work together to ensure accurate synthesis, modification, sorting and delivery of secretory proteins.

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