Question

Which of the following are examples of trans-membrane transport between different subcellular compartments?
(A) Transport from the stroma into thylakoid space
(B) Transport from the cytoplasm into the lumen of the endoplasmic reticulum
(C) Transport from the endoplasmic reticulum into the Golgi complex
(D) Transport from mitochondrial intermembrane space into the mitochondrial matrix
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only
• B. (A), (B) and (C) only
• C. (A), (B), (C) and (D)
• D. (B), (C) and (D) only

Hint:

To distinguish these transport types, remember that any transport between organelles of the secretory pathway (like ER to Golgi, or Golgi to Lysosomes) is vesicular transport.
In contrast, importing proteins from the cytosol into the ER, Mitochondria, or Plastids requires crossing a membrane directly via a channel, which is transmembrane transport.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Intracellular protein trafficking occurs via three main mechanisms:
1. Gated transport: Movement of proteins between the cytosol and nucleus through the nuclear pore complexes.
2. Transmembrane transport: Direct translocation of unfolded polypeptide chains across a membrane barrier via specialized protein translocators.
3. Vesicular transport: Movement of proteins encapsulated within membrane-bound transport vesicles that bud from one compartment and fuse with another.

Step 2: Detailed Explanation:
Let us analyze each of the transport pathways listed:
- (A) Transport from the stroma into the thylakoid space: This requires translocation of proteins across the thylakoid membrane using specific translocase pathways (such as Sec or Tat pathways). This is a classic transmembrane transport mechanism.
- (B) Transport from the cytoplasm into the lumen of the ER: This involves the translocation of nascent polypeptide chains across the ER membrane via the Sec61 translocon complex (co-translational or post-translational translocation). This is transmembrane transport.
- (C) Transport from the ER to the Golgi complex: This occurs via membrane-bound COP-II coated vesicles that bud from the ER and fuse with the cis-Golgi network. This is vesicular transport, not transmembrane transport.
- (D) Transport from the mitochondrial intermembrane space into the matrix: This requires crossing the inner mitochondrial membrane via the TIM (Translocase of the Inner Membrane) complex. This is transmembrane transport.
Therefore, (A), (B), and (D) are examples of transmembrane transport, while (C) is vesicular transport.

Step 3: Final Answer:
The correct option is (A), which selects (A), (B), and (D) only.