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 solve the question "Which of the following is closest to the plasma membrane?" we need to analyze the options provided and understand the structure of plant cells.

### Step-by-step Solution:

1. **Identify the Options**: The options given are:
- a) Tonoplast
- b) Middle lamella
- c) Primary cell wall
- d) Secondary cell wall

2. **Understanding Tonoplast**:
- The tonoplast is the membrane that surrounds the vacuole in plant cells. It is not related to the plasma membrane of the cell itself. Therefore, this option is incorrect.

3. **Understanding Middle Lamella**:
- The middle lamella is a layer that is rich in pectin and is formed between the cell walls of adjacent plant cells. It is not directly adjacent to the plasma membrane of a single cell but rather acts as a glue between cells. Thus, this option is also incorrect.

4. **Understanding Primary Cell Wall**:
- The primary cell wall is the outermost layer of a plant cell, which is flexible and allows for cell growth. However, it is not the closest structure to the plasma membrane since it is located outside of it. Therefore, this option is incorrect.

5. **Understanding Secondary Cell Wall**:
- The secondary cell wall is formed after the primary cell wall and is located inside the primary cell wall. It is situated directly adjacent to the plasma membrane. Hence, this is the closest structure to the plasma membrane.

6. **Conclusion**:
- Based on the analysis, the correct answer to the question "Which of the following is closest to the plasma membrane?" is **d) Secondary cell wall**.

### Summary:
- The closest structure to the plasma membrane in a plant cell is the **secondary cell wall**.