Which of the following statements is false regarding protein transport in the endomembrane system?

• A. Proteins destined for secretion are synthesized on ribosomes bound to the endoplasmic reticulum.
• B. Proteins are directly transported from the cytosol to the plasma membrane without any vesicular transport.
• C. Proteins are transported from the Golgi apparatus to the plasma membrane via vesicles.
• D. The Golgi apparatus modifies proteins before they are secreted from the cell.

Answer: (B)

Verified step by step guidance

1. Understand the endomembrane system: The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the endoplasmic reticulum (ER), Golgi apparatus, vesicles, and the plasma membrane.
2. Identify the role of ribosomes and the ER: Proteins destined for secretion are synthesized on ribosomes that are bound to the rough endoplasmic reticulum (ER). This is because the ER is involved in the initial stages of protein synthesis and folding.
3. Examine the role of the Golgi apparatus: After synthesis in the ER, proteins are transported to the Golgi apparatus. The Golgi apparatus modifies proteins, which may include glycosylation, phosphorylation, and other modifications, before they are sent to their final destinations.
4. Understand vesicular transport: Proteins are transported from the Golgi apparatus to the plasma membrane via vesicles. Vesicles are small membrane-bound sacs that transport substances within the cell. This is a key step in the secretion pathway.
5. Identify the false statement: The statement 'Proteins are directly transported from the cytosol to the plasma membrane without any vesicular transport' is false. Proteins are not directly transported from the cytosol to the plasma membrane; they require vesicular transport to move between organelles and to the plasma membrane.