Which of the following is closest to the plasma membrane ?

Options

A. Tonoplast
B. Middle lamella
C. Primary cell wall
D. Secondary cell wall

Answer

To determine which of the following options is closest to the plasma membrane, we will analyze each option provided.

### Step-by-step Solution:

1. **Understanding Plasma Membrane**: The plasma membrane is the outer boundary of the cell, separating the interior of the cell from the external environment. It is crucial for maintaining the cell's integrity and regulating the movement of substances in and out of the cell.

2. **Analyzing the Options**:
- **Option 1: Tonoplast**: This is the membrane that surrounds the central vacuole in plant cells. It is not directly adjacent to the plasma membrane, as it is located inside the cell, enclosing the vacuole.
- **Option 2: Middle Lamella**: This is a layer that acts as a cementing substance between the cell walls of adjacent plant cells. It is not part of the cell membrane structure and is located outside the plasma membrane.
- **Option 3: Primary Cell Wall**: This is the first layer of the cell wall in plant cells, which is located outside the plasma membrane. Therefore, it is not the closest structure to the plasma membrane.
- **Option 4: Secondary Cell Wall**: This layer is formed inside the primary cell wall and is typically found in some plant cells. It is located just outside the plasma membrane, making it the closest structure to the plasma membrane.

3. **Conclusion**: After analyzing all the options, the closest structure to the plasma membrane is the **secondary cell wall**. Therefore, the correct answer is **Option 4**.