The main difference in Gram (+ve) and Gram (-ve) bacteria resides in their:
(a) Cell wall
(b) Cell membrane
(c) Cytoplasm
(d) Flagella

Options

A. Cell wall
B. Cell membrane
C. Cytoplasm
D. Flagella

Answer

To solve the question regarding the main difference between Gram-positive and Gram-negative bacteria, we will analyze the options provided and focus on the characteristics that distinguish these two groups of bacteria.

### Step-by-Step Solution:

1. **Understanding Gram Staining**:
- Gram staining is a laboratory technique used to differentiate bacterial species into two groups based on the characteristics of their cell walls.

2. **Cell Wall Composition**:
- **Gram-positive bacteria**: They have a thick peptidoglycan layer in their cell wall. This thick layer retains the crystal violet stain used in the Gram staining process, resulting in a blue or purple appearance.
- **Gram-negative bacteria**: They possess a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides. The thin peptidoglycan layer does not retain the crystal violet stain, and they take up the counterstain (safranin), appearing pink.

3. **Analyzing the Options**:
- (a) Cell wall: This is where the main difference lies, as explained above.
- (b) Cell membrane: Both types of bacteria have a cell membrane, but the composition does not differ significantly between them.
- (c) Cytoplasm: The cytoplasmic contents are generally similar in both types of bacteria.
- (d) Flagella: While some bacteria have flagella, the presence and structure of flagella do not define the Gram classification.

4. **Conclusion**:
- The main difference between Gram-positive and Gram-negative bacteria resides in their **cell wall** structure. Therefore, the correct answer is **(a) Cell wall**.

### Final Answer:
(a) Cell wall