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, let's analyze each one step by step.

### Step 1: Analyze Statement 1
**Statement:** In ribosome, S is directly a measure of density and size.

- **Explanation:** The "S" in ribosomes refers to the Svedberg unit, which is a measure of sedimentation rate during centrifugation. It does not directly measure density and size but rather how quickly particles settle in a centrifugal field. Ribosomes are classified into two types based on their Svedberg values: 70S (prokaryotic) and 80S (eukaryotic). Therefore, this statement is **incorrect**.

### Step 2: Analyze Statement 2
**Statement:** Both plant and animal cells have a defined cell wall.

- **Explanation:** Plant cells have a rigid cell wall made of cellulose, while animal cells do not have a cell wall; they only have a flexible cell membrane. Thus, this statement is also **incorrect**.

### Step 3: Analyze Statement 3
**Statement:** The content of the nucleolus is continuous with the rest of the nucleoplasm as it is a membrane-bound structure.

- **Explanation:** The nucleolus is not a membrane-bound structure; it is a dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins. The nucleolus is continuous with the nucleoplasm, but it does not have a membrane separating it from the rest of the nucleoplasm. Therefore, this statement is **incorrect**.

### Step 4: Analyze Statement 4
**Statement:** S as the cell organ was first described by Robert Brown as early 1831.

- **Explanation:** Robert Brown did describe the nucleus in 1831, but the term "S" (Svedberg unit) is not related to his work. The Svedberg unit was named after Theodor Svedberg, who worked in the early 20th century. Therefore, this statement is **incorrect**.

### Conclusion
After analyzing all four statements, we find that **none of the statements are correct**.