Questions: Organelles and Their Functions

Question 1

A cell is producing a secreted protein (one that will be exported outside the cell). After the ribosome synthesizes it, which sequence of organelles will the protein pass through before leaving the cell?

• A. Nucleus → mitochondria → plasma membrane
• B. Rough ER → Golgi apparatus → secretory vesicle → plasma membrane
• C. Ribosome → lysosome → vacuole → plasma membrane
• D. Smooth ER → nucleus → Golgi apparatus → plasma membrane

Answer: (B)

Secreted proteins are synthesized by ribosomes on the rough ER, threaded into the ER lumen, processed and packaged by the Golgi apparatus, and then shipped to the plasma membrane in secretory vesicles. This secretory pathway is a key illustration of how organelle compartmentalization enables complex multi-step processing of cell products.

Question 2

Ribosomes are not considered true organelles because they lack a surrounding membrane.

• T. True
• F. False

Answer: False

Ribosomes are universally recognized as organelles even though they are not membrane-bound. The term 'organelle' refers to a discrete, functionally specialized structure within the cell — membrane-bound or not. Ribosomes qualify because they are complex, dedicated molecular machines with a clearly defined function (protein synthesis). This is a common misconception because many textbook definitions over-emphasize the membrane criterion.

Question 3

What is the key functional advantage of compartmentalizing biochemical reactions into separate membrane-bound organelles?

Model answer: Compartmentalization allows different chemical environments (pH, ion concentrations, enzyme sets) to be maintained simultaneously in one cell, so reactions that would interfere with each other can proceed in parallel without disruption.

For example, lysosomes maintain an acidic pH (~4.5–5) optimal for hydrolytic enzymes — if those enzymes were released into the cytoplasm (pH ~7.2), they would damage the cell. The Golgi and ER maintain their own distinct environments for protein modification. This is precisely why eukaryotes, with their elaborate organelle system, can run far more complex biochemistry than prokaryotes.