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NCERT Solutions for Class 9th Science Chapter 2 Activity 2.3: Let us investigate — Cell wall

Book page 13 Updated on2026-09-08

Q1.
Onion peel cells or Rhoeo leaf peel cells are box-shaped and regularly arranged, whereas cheek cells are irregularly arranged. Why do you think this difference exists?
Answer

Because plant cells have a rigid cell wall and animal cells do not.

Onion peel / Rhoeo leaf peelHuman cheek
Outermost coveringCell wall of cellulose, then cell membraneCell membrane only
ShapeBox-like, straight-edged, definiteRounded, irregular, no fixed outline
ArrangementRegular, packed edge to edge like bricksScattered, often overlapping
Stain usedSafraninMethylene blue
Why it happens: the wall is laid down while the cell is still attached to its neighbours in the tissue, so adjoining walls are shared and the cells are cemented into a fixed brick-like pattern which the peel keeps even after it is torn off. Cheek cells are scraped loose from a moist surface layer, they have no wall to hold a shape, and surface tension pulls each free cell towards a rounded outline. Their irregular arrangement on the slide is partly the shape of the cells and partly the fact that they are simply spread out in a drop of water rather than held in a tissue.
Tip: stain the two slides differently for a reason — safranin binds well to the cellulose wall and the nucleus of plant cells, while methylene blue makes the nucleus of the almost transparent cheek cell visible.
Q2.
Prepare two slides of a Rhoeo leaf peel and human cheek cells again, and put 20 per cent sugar solution on them. Observe them under a microscope after half an hour. What do you observe?
Answer

Both cells lose water, but they look completely different afterwards.

  • Rhoeo (plant) cells: the outer boundary stays exactly where it was, but the inner contents shrink away from it. A clear gap appears between the inner and the outer boundary. This pulling away of the membrane and contents from the wall is called plasmolysis.
  • Cheek (animal) cells: the whole cell shrinks considerably and looks shrivelled, because there is no outer wall to hold the shape.
plant cell in waterwall and contentsboth full plant cell in 20% sugarwall unchanged,contents pulled in cheek cell in waterround and full cheek cell in sugarwhole cell shrunk
In a hypertonic solution both cells lose water; only the plant cell keeps its outline, because the wall does not shrink.
Why it happens: the 20 per cent sugar solution is hypertonic to the cell sap, so water leaves both cells by osmosis. In the plant cell the cellulose wall is rigid and freely permeable — sugar solution seeps into the space between wall and membrane, so while the protoplasm contracts the wall holds its original box shape. In the cheek cell the membrane is the boundary, so when the contents lose water the boundary itself caves in. This single observation is the cleanest proof in the chapter that plant cells have something outside the membrane that animal cells lack.
Check it yourself: flood the plasmolysed Rhoeo slide with plain water. The contents swell back out to meet the wall — the process reverses, showing that plasmolysis has not killed the cell.
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