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Cell Structure: Question 5

Syllabus 1.2

Multiple choice AS 1 mark

A gland cell lining the small intestine synthesises a digestive enzyme, which is a protein, and then secretes it out of the cell.

Which sequence correctly shows the structures this enzyme passes through, in order, from its synthesis to its release from the cell?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Where the protein is made

The enzyme is a protein secreted out of the cell, so it is synthesised by ribosomes attached to the rough endoplasmic reticulum (rough ER). As it is made, the growing polypeptide chain enters the space inside the rough ER, where it begins to fold.

Step 2: From the rough ER to the Golgi body

The protein does not move directly from the rough ER to the Golgi body. Instead, a small piece of the rough ER membrane pinches off, enclosing the protein in a vesicle, which then travels through the cytoplasm and fuses with the Golgi body.

Step 3: Processing and packaging at the Golgi body

Inside the Golgi body, the protein is further modified and processed (for example, by the addition of carbohydrate groups). The Golgi body then packages the finished protein into a second vesicle, which buds off and moves towards the edge of the cell.

Step 4: Release from the cell

This second vesicle travels to and fuses with the cell surface membrane, releasing the protein outside the cell by exocytosis. This completes the pathway: ribosome on the rough ER → vesicle → Golgi body → vesicle → cell surface membrane.

Why the other options are wrong

  • B: places the Golgi body before the first vesicle and wrongly routes the protein through the smooth ER, which is not part of the protein secretion pathway.
  • C: starts from the nucleus and the smooth ER rather than ribosomes on the rough ER, and puts the cell surface membrane before the Golgi body, which is out of order.
  • D: replaces the first transport vesicle with a lysosome; a lysosome contains digestive enzymes for breaking down material inside the cell and plays no part in exporting a secreted protein.

Final answer

  • The correct order is: ribosome on the rough ER → vesicle → Golgi body → vesicle → cell surface membrane, option A.