Activities 2.2 and 2.3 — Observing Plant and Animal Cells — "The Invisible Living World: Beyond Our Naked Eye"

1. 14 marksCuriosity Grade 8, Chapter 2, Activity 2.2, page 11

   Describe how an onion peel slide is prepared. Explain the purpose of the safranin, the rinse, the glycerin, and the careful placing of the coverslip.

   Hint. Each step exists to solve one specific problem with looking at a transparent, drying-out tissue.

   The steps. Wash an onion bulb, cut it vertically, and pull the thin transparent layer from the inner surface of a piece with forceps — this is the onion peel. Place it in a petri dish with a few drops of safranin for 30 seconds, transfer it with a thin brush to a dish of water to rinse, lay it flat on a glass slide without breaking or folding it, add a drop of glycerin, lower a coverslip with a needle so no air bubbles are trapped, wipe off excess glycerin with blotting paper, and observe.

   Why each step is there:

   Step	The problem it solves
   Safranin (red stain), 30 s	The peel is nearly transparent, so its parts have almost no contrast; the stain gives the cells a pinkish colour and makes them visible
   Rinse in water	Removes excess stain, which would otherwise darken everything and hide the detail the stain was meant to reveal
   Glycerin	Prevents the cells drying out, and improves clarity
   Coverslip lowered slowly at an angle	Trapped air bubbles look like dark-edged circles under the microscope and are easily mistaken for cells
   Blotting paper	Excess glycerin would smear the lens and let the coverslip slide

   Do not break or fold the peel. A folded peel gives two overlapping layers, so you see a confusing double image instead of one clean sheet of cells.

   ✦ Answer: safranin stains the near-transparent cells so they can be seen, the rinse removes excess stain, glycerin stops the cells drying and improves clarity, and the coverslip is lowered carefully so that no air bubbles are trapped and mistaken for cells.

2. 23 marksCuriosity Grade 8, Chapter 2, Activity 2.2, page 11

   What do you see when you observe the onion peel under the microscope? What is it compared with in Fig. 2.3d, and why is the comparison apt?

   Hint. Fig. 2.3d is not a biological picture at all.

   What you see. Nearly rectangular structures, arranged closely together with no space between them. These are the cells of the onion peel.

   The comparison. Fig. 2.3d is a photograph of a wall made of bricks. The similarity is immediate: both are made of many small units of roughly the same shape, packed tightly in rows with no gaps.

   Why the comparison is apt. It carries the chapter's central idea. A wall is not a single object that happens to have lines on it — it is built from bricks, and the brick is the unit you would have to understand to understand walls. In the same way, the onion peel is built from cells, and the chapter returns to this analogy later when it says the cell is the basic unit of life just as a brick is the basic unit of a wall.

   Where the comparison stops. Bricks are identical, inert and made by us; cells differ in shape according to their job, are alive, and build themselves. So the picture is a way in, not the whole story.

   Try it more widely. Peeling the leaves of different plants shows the same thing every time, which is how you convince yourself that all plants are made of cells.

   ✦ Answer: nearly rectangular cells packed closely with no gaps, compared with a brick wall — apt because it shows that a living structure is built from repeated units, the same idea as the cell being the basic unit of life.

3. 33 marksCuriosity Grade 8, Chapter 2, Activity 2.3, page 12

   Describe how a slide of human cheek cells is prepared, and explain why the mouth is rinsed first and why methylene blue is added.

   Hint. The blunt end of the toothpick, and the rinse, are both there for reasons.

   The steps. Rinse your mouth with clean water. Using the blunt end of a clean toothpick, gently scrape the inside of your cheek. Place the scraped material in a drop of water on a clean slide and spread it evenly. Add a drop of methylene blue, wait one minute, add a drop of glycerin, lower a clean coverslip, remove excess glycerin with blotting paper, and observe — then draw what you see.

   Why rinse the mouth first. To wash away food particles and loose debris. Without it the slide is cluttered with fragments that are not cells at all, and a beginner will happily identify a starch grain as a cell.

   Why the blunt end, gently. The point is to collect the loose cells of the surface lining, not to injure the cheek. These cells are shed and replaced constantly, so a gentle scrape is enough.

   Why methylene blue. Exactly as with safranin: the material is nearly colourless, and the stain improves visibility by increasing contrast. Different stains suit different material — safranin (red) for the plant peel, methylene blue for animal cells.

   Why glycerin again. To keep the cells from drying out while you observe them.

   ✦ Answer: rinse, scrape gently with the blunt end of a toothpick, spread in water on a slide, stain with methylene blue, add glycerin, cover and observe — the rinse removes food debris that would be mistaken for cells, and the stain increases contrast so the near-colourless cells become visible.

4. 42 marksCuriosity Grade 8, Chapter 2, Activity 2.3, page 12

   What do cheek cells look like under the microscope, and what are they in the body?

   Hint. Compare their outline with the onion peel's.

   What you see. Polygon-shaped structures — many-sided, irregular outlines, quite unlike the neat rectangles of the onion peel.

   What they are. These are cheek cells, and they form the inner lining of your mouth.

   The three parts visible. The outer boundary is the cell membrane, the round structure in the middle is the nucleus, and the space between them is filled with cytoplasm.

   Why they are shaped as they are. The chapter returns to this later: cheek cells are thin and flat, which suits their job of forming a protective lining over the inner surface of the cheek. A flat tile-like shape covers a surface efficiently, which is why lining cells throughout the body tend to look like this.

   ✦ Answer: polygon-shaped cells showing a cell membrane, cytoplasm and nucleus — they form the inner lining of the mouth, and their thin flat shape suits that protective covering role.

5. 54 marksCuriosity Grade 8, Chapter 2, Activities 2.2 and 2.3, pages 11-12

   What similarities and differences did you observe between onion peel cells and human cheek cells?

   Hint. Three parts are shared; the differences are in what surrounds them and how they are arranged.

   Similarities. Both are made of cells, and every cell of both kinds has the same three basic parts:

   • a cell membrane as the outer layer
   • cytoplasm filling the space
   • a nucleus in the middle, itself covered by a thin membrane

   Differences:

   	Onion peel (plant)	Cheek cell (animal)
   Extra outer layer	Has a cell wall outside the membrane	No cell wall
   Shape	Nearly rectangular, regular	Polygonal, irregular
   Arrangement	Closely packed with no gaps, like bricks	Loose, scattered on the slide
   Firmness	Rigid and firm	Soft and flexible

   The connection between the last three rows. They are not three separate facts. The cell wall is what provides rigidity and strength, and that is why plant cells hold a definite shape and sit compactly against one another, while animal cells without walls are flexible and irregular.

   One thing you cannot see here. Onion peel cells have no chloroplasts, because the inner peel of a bulb grows underground and is not green. So this particular plant cell does not show every plant feature — a useful reminder that one specimen is not the whole category.

   ✦ Answer: both have a cell membrane, cytoplasm and nucleus; the onion cell additionally has a cell wall, which makes it rigid, regular in shape and packed closely with its neighbours, whereas the cheek cell has no wall and so is soft, polygonal and irregular.

6. 63 marksCuriosity Grade 8, Chapter 2, Activities 2.2 and 2.3, pages 11-12

   Both activities use a stain. Why can cells not simply be viewed without one, and what would happen if you used far too much stain?

   Hint. Magnification and visibility are not the same problem.

   Magnifying is not enough. A microscope makes things bigger, but it cannot create contrast. Cell membranes, cytoplasm and nuclei are all nearly transparent and nearly the same shade, so an unstained cell magnified 400 times is simply a larger patch of almost nothing. You might make out the outlines of thick plant cell walls, but the nucleus would be very hard to find.

   What a stain does. It is taken up unevenly by different parts of the cell, so some structures become darker than others. That difference is what the eye actually detects. The chapter puts it directly: adding stain improves the visibility of the material under the microscope by increasing contrast.

   Too much stain. Everything darkens together, so the contrast you were trying to create disappears again and the slide goes uniformly dark. That is precisely why Activity 2.2 rinses the peel in water after 30 seconds in safranin — the rinse removes the excess.

   Which stain for which. Safranin for the plant peel, methylene blue for animal cells; different materials take up different stains well.

   ✦ Answer: because a microscope supplies magnification but not contrast, and unstained cells are nearly transparent — the stain is taken up unevenly and makes structures stand out; too much stain darkens everything equally and destroys the contrast, which is why excess is rinsed off.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity - Textbook of Science for Grade 8 (hecu102.pdf), Chapter 2 'The Invisible Living World: Beyond Our Naked Eye', pages 8-27, Reprint 2026-27. HAND-WRITTEN throughout. Page count verified against the printed contents page (hecu1ps.pdf p.18): 20 pages, 8-27, matching exactly. Covers the 3 'Probe and ponder' prompts (p.8), all 9 activities (2.1 to 2.9), every in-text section (2.1 to 2.5), the 9 numbered 'Keep the curiosity alive' exercises (pp.25-26) and the 4 'Discover, design, and debate' projects (p.27). EXERCISE Q1 VENN DIAGRAM READ OFF A 500-DPI RENDER of p.25: three circles labelled 'Only in Animal Cell' (top), 'Only in Bacterial Cell' (lower left) and 'Only in Plant Cell' (lower right), with an arrow labelling the central three-way overlap 'Common to all three cells'. Each of those four labelled regions carries two ruled lines; the three PAIRWISE overlap regions carry none. Working from the book's own statements (p.13 'the cell wall in the plant cell provides rigidity'; p.24 'bacteria do not have a well-defined nucleus... instead they have a nucleoid'; and the Snapshots line 'Plant, fungal, and bacterial cells have an extra covering, called a cell wall'), the correct placement is: cytoplasm and cell membrane in the three-way overlap (exactly two items, matching the two ruled lines); chloroplast in plant-only; nucleoid in bacterial-only; NUCLEUS in the animal-plant pairwise overlap (plant cells have nuclei, bacteria do not); CELL WALL in the plant-bacterial pairwise overlap (not plant-only, since the book says bacterial and fungal cells have walls too); and NO listed part is unique to the animal cell. The solution states both traps explicitly. FACTUAL FIGURES TAKEN VERBATIM FROM THE BOOK: Hooke's Micrographia 1665, microscope 200-300x, cork, honeycomb, first use of 'cell'; Leeuwenhoek 1660s, Father of Microbiology; ordinary microscope 100-400x; electron microscope about 10,00,000x; ostrich egg yolk 130-170 mm as the largest known cell; Ananda Mohan Chakrabarty 1938-2020, oil-degrading bacterium 1971, patent 1980; biogas mainly carbon dioxide with a high proportion of methane; one of India's oldest biogas plants late 1850s; microalgae produce more than half of Earth's oxygen; Spirulina over 60 per cent protein by body weight and a source of vitamin B12, harvested after 3-6 weeks. The MNRE biogas-programme project answer deliberately does NOT assert current programme details - it names the programmes to search for and tells the student to verify against the current official MNRE page, since names and subsidy rules change.. Questions are referenced from the NCERT textbook for identification.