Biology · Cell: The Unit of Life · NEET
| Peptidoglycan (murein) layer | Thick | Thin |
| Outer membrane (LPS) | Absent | Present |
| Gram stain colour | Purple / violet | Pink / red |
| Reaction to gram stain | Takes up (retains) the stain | Does not retain the stain |
| Example structure | Simple thick wall | Wall + extra outer membrane |
The main difference is the cell wall. Gram-positive bacteria have a THICK layer of peptidoglycan in the wall. Gram-negative bacteria have a THIN peptidoglycan layer plus an extra outer membrane on top. Because of this wall difference, they react differently to the Gram stain. NCERT groups bacteria into these two types based on their cell envelope and staining response.
Gram-positive bacteria stay PURPLE (violet). Gram-negative bacteria turn PINK or red. In the test, the violet dye is trapped in the thick wall of gram-positive cells, so they stay purple. Gram-negative cells have a thin wall, so the violet washes out and they take the pink counterstain (safranin). Easy hook: Positive = Purple, Negative = piNk.
Yes, they DO have peptidoglycan, but only a thin layer. Many students think gram-negative bacteria have no cell wall. That is wrong. Both types have a peptidoglycan (murein) cell wall. The difference is the thickness: thick in gram-positive, thin in gram-negative. Only Mycoplasma has no cell wall at all.
Because their cell wall has a thick peptidoglycan layer. When the alcohol wash is added, this thick wall shrinks and traps the crystal-violet-iodine dye inside. So the cell stays purple. In gram-negative cells the wall is thin and has an outer membrane that gets damaged by alcohol, so the dye escapes and the cell loses its purple colour.
No. NCERT only says that bacteria are grouped into gram-positive (those that TAKE UP the gram stain) and gram-negative (those that DO NOT), based on cell envelope differences and the staining method developed by Gram. For NEET you should remember this NCERT line plus the basic reason: thick wall keeps the stain, thin wall loses it.
The three-layer plan is the same: glycocalyx, then cell wall, then plasma membrane. But the cell wall part differs. Gram-positive walls are mostly thick peptidoglycan. Gram-negative walls have thin peptidoglycan plus an outer membrane containing lipopolysaccharide. NCERT states bacteria are classified into two groups because of these cell envelope differences.
Select the wrong statement:
Statement I: Mycoplasma can pass through less than 1 micron filter size. Statement II: Mycoplasma are bacteria with a cell wall. Choose the correct option:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Peptidoglycan (also called murein) is the tough material of the bacterial cell wall. It is made of sugar chains linked by short amino acid bridges. It is thick in gram-positive bacteria and thin in gram-negative bacteria.
Almost all bacteria have a peptidoglycan cell wall. The only exception in NCERT is Mycoplasma, which has no cell wall. Both gram-positive and gram-negative bacteria have a wall.
Gram-positive = purple/violet. Gram-negative = pink/red. The thick wall of gram-positive cells traps the violet dye, while the thin wall of gram-negative cells lets it wash out.
It was developed by Hans Christian Gram. NCERT names the method after 'Gram' and uses it to split bacteria into gram-positive and gram-negative types.
The extra outer membrane (with lipopolysaccharide) is found in gram-negative bacteria. Gram-positive bacteria do not have this outer membrane; they rely on their thick peptidoglycan wall.
