Which of the following is correct ?

Options

A. In ribosomes, ‘S’ is directly a measure of density and size.
B. Both animal and plant cells have a well defined cell wall.
C. The content of nucleolus is continuous with the rest of the nucleoplasm as it is a membrane bound structure.
D. Nucleus as a cell organelle was first described by Robert Brown as early as 1831.

Answer

To determine which of the statements is correct, we will evaluate each statement one by one.

### Step-by-Step Solution:

1. **Evaluate the First Statement:**
- The first statement discusses the ribosome and its sedimentation coefficient, denoted as S (Swedberg's unit). It states that a higher S value indicates a larger sedimentation coefficient, which is true. However, it claims that S is a direct measure of density and size, which is incorrect. The S value is an indirect measure of these properties.
- **Conclusion:** This statement is **false**.

2. **Evaluate the Second Statement:**
- The second statement claims that both animal and plant cells have a well-defined cell wall. This is incorrect because only plant cells have a cell wall; animal cells do not possess a cell wall.
- **Conclusion:** This statement is **false**.

3. **Evaluate the Third Statement:**
- The third statement mentions that the content of the nucleolus is organized continuously with the rest of the nucleoplasm and states that it is a membrane-bound organelle. This is incorrect because the nucleolus is not membrane-bound; it is a non-membrane-bound structure within the nucleus.
- **Conclusion:** This statement is **false**.

4. **Evaluate the Fourth Statement:**
- The fourth statement asserts that the nucleus as a cell organelle was first described by Robert Brown in 1831. This is a true statement, as Robert Brown indeed described the nucleus during that time.
- **Conclusion:** This statement is **true**.

### Final Answer:
- The correct statement is **option number 4**: "The nucleus as a cell organelle was first described by Robert Brown in the year 1831."

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