A protein is synthesized on ribosomes bound to the endoplasmic reticulum and is destined for the inner thylakoid space of a chloroplast. Through how many double layered (lipid bilayer) membranes must this protein pass to reach its final destination inside the thylakoid lumen?

Introduction / Context:
Protein targeting in eukaryotic cells is a complex but important topic in cell biology. Many proteins are synthesized at one location in the cell and then transported to specific organelles where they function. Chloroplasts are double membrane bound organelles with an additional internal membrane system, the thylakoid membranes. When a protein is synthesized on the rough endoplasmic reticulum and is destined for the inner thylakoid space (thylakoid lumen), it must cross several lipid bilayers on its journey. This question tests whether you can carefully count how many double layered membranes are involved in this trafficking pathway.

Given Data / Assumptions:

• The protein is synthesized on ribosomes bound to the endoplasmic reticulum, so it enters the ER lumen initially.
• Its final destination is the inner thylakoid space of a chloroplast.
• Chloroplasts have an outer membrane and an inner membrane, forming a double membrane envelope.
• Thylakoid membranes form closed sacs inside the chloroplast, enclosing the thylakoid lumen.
• We count how many double layered lipid membranes must be crossed or passed through.

Concept / Approach:
The key idea is to track the route of the protein from its site of synthesis to its final location and to identify each double membrane it must cross. Starting in the ER lumen, the protein must move to the cytosol or to transport vesicles that eventually deliver it to the chloroplast. From outside the chloroplast, it must pass first through the outer chloroplast membrane and then through the inner chloroplast membrane to reach the stroma. Finally, to enter the thylakoid lumen, it must cross the thylakoid membrane. The standard reasoning for many exam questions is to count the double layered membranes that lie between the origin and the destination compartments.

Step-by-Step Solution:
Step 1: Recognize that chloroplasts have a double membrane envelope, which consists of an outer membrane and an inner membrane, both being lipid bilayers. Step 2: The protein must first pass through the outer chloroplast membrane to enter the organelle. Step 3: Next, it must traverse the inner chloroplast membrane to reach the stroma, the fluid filled space inside the envelope. Step 4: From the stroma, the protein must then cross the thylakoid membrane to enter the inner thylakoid space or lumen. Step 5: Each of these membranes is a double layered lipid bilayer, so the total count is outer envelope membrane, inner envelope membrane, and thylakoid membrane, giving three double layered membranes in total.

Verification / Alternative check:
You can visualize the membranes by drawing a simple diagram. Start with the cytosol outside the chloroplast. The chloroplast envelope has two membranes (outer and inner), and inside the stroma there are many thylakoid sacs, each with its own membrane surrounding the thylakoid lumen. The protein must end up inside this lumen. Therefore, it must cross the outer membrane (one double layer), the inner membrane (second double layer), and the thylakoid membrane (third double layer). No additional double membranes are crossed on the way if we assume a direct targeting pathway. This diagram based reasoning confirms the total of three double layered membranes.

Why Other Options Are Wrong:
2 double layered membranes: This would only account for the outer and inner chloroplast membranes and ignore the thylakoid membrane, which also encloses the lumen where the protein is going. 4 double layered membranes: This would over count and imply an additional double membrane that does not exist in the described route. 5 double layered membranes: This is an even larger overestimate and does not match the known chloroplast structure. 1 double layered membrane: This would seriously underestimate the number of membranes, failing to recognize the double envelope structure and the internal thylakoid membrane.

Common Pitfalls:
Students often confuse the counting of membranes in organelles with double membranes. Some learners only think about the chloroplast envelope and forget the thylakoid membrane as another barrier. Others count both sides of a double membrane separately instead of as one double layered structure. In exam questions, it is important to read whether the question is asking for the number of double membranes or total lipid layers. Here, we count each distinct bilayer bounded membrane system, leading to the answer of three. Drawing a simple labelled sketch during practice can help fix this concept in memory.

Final Answer:
Therefore, the protein must pass through 3 double layered membranes to reach the inner thylakoid space of the chloroplast.