In-text — Parts of the Cell and Why Shape Matters — "The Invisible Living World: Beyond Our Naked Eye"

1. 13 marksCuriosity Grade 8, Chapter 2, Section 2.1, page 12

   What is the cell membrane and what does it do? What does 'porous' mean here?

   Hint. A boundary that let nothing through would kill the cell.

   What it is. The cell membrane is the outer layer of the cell. It encloses the cytoplasm and the nucleus, and it separates one cell from another.

   What 'porous' means. It has fine openings, so it is not a sealed bag. The chapter gives it two jobs, one inward and one outward:

   • it allows the entry of materials essential for life processes
   • it allows the exit of waste material

   Why both matter. A completely sealed boundary would keep the cell distinct but starve it and poison it with its own waste. A boundary with no control would let the cell's contents leak away. Being porous is the compromise that makes a cell both separate and alive.

   Where you see it. It is the outermost layer in the cheek cell of Activity 2.3, and it is present in every cell — plant, animal, fungal and bacterial — which is why it belongs in the 'common to all' region of the exercise diagram.

   ✦ Answer: the outer layer that encloses the cytoplasm and nucleus and separates one cell from another; being porous, it lets essential materials in and waste out.

2. 22 marksCuriosity Grade 8, Chapter 2, Section 2.1, page 12

   What is cytoplasm and why is it described as the place where most life processes happen?

   Hint. It is not just filler between the membrane and the nucleus.

   Where it is. Cytoplasm fills the space between the cell membrane and the nucleus.

   What is in it. It contains the other components of the cell, along with compounds the cell needs — carbohydrates, proteins, fats and mineral salts.

   Why it matters. The chapter states that most of the life processes take place within the cytoplasm. It is therefore not a passive filling but the working space of the cell: the materials that come in through the membrane are used here, and the structures that do the cell's jobs sit here.

   A caution the chapter adds later. "A cell is not just a simple bag of liquid — it is a complex structure made up of many different parts, each with its own special function." The cytoplasm looks featureless at school magnification only because the structures within it are too small to resolve.

   ✦ Answer: the material between the cell membrane and the nucleus, containing the cell's other components together with carbohydrates, proteins, fats and mineral salts — and it is where most life processes take place.

3. 32 marksCuriosity Grade 8, Chapter 2, Section 2.1, page 13

   What does the nucleus do?

   Hint. Two jobs, both mentioned in a single short line of the chapter.

   The nucleus is the round structure in the middle of the cell, itself covered by a thin membrane. The chapter gives it two functions:

   • it regulates all activities that occur within the cell
   • it regulates growth

   How to think about it. If the membrane is the boundary and the cytoplasm is the workspace, the nucleus is the control point — nothing in the cell operates independently of it.

   Where it is absent. Bacteria do not have a well-defined nucleus or a nuclear membrane; they have a nucleoid instead. That single difference is what distinguishes bacterial cells from those of yeast, protozoa, algae, fungi, plants and animals.

   ✦ Answer: it regulates all the activities that occur within the cell, and it regulates growth — and it is present in plant and animal cells but replaced by a nucleoid in bacteria.

4. 43 marksCuriosity Grade 8, Chapter 2, Section 2.1, page 13

   Which cells have a cell wall, what does it do, and how does this explain what you saw in the onion peel?

   Hint. Check the chapter's summary — it is not only plants.

   Who has one. The chapter's summary is explicit: plant, fungal and bacterial cells have an extra covering called a cell wall, outside the cell membrane. Animal cells do not.

   What it does. In plants it provides rigidity and strength.

   How that explains the onion peel. Because each cell is held in a rigid casing, the cells keep a definite, nearly rectangular shape and sit compactly against each other with no gaps, looking firm — exactly the brick-wall appearance of Fig. 2.3c. Cheek cells, having no wall, are soft, irregular and do not hold a fixed outline.

   A point students often get wrong. 'Cell wall = plant' is a half-truth. Since bacteria and fungi have walls too, the cell wall does not belong in the 'only in plant cell' region of the exercise diagram — it belongs where the plant and bacterial circles overlap.

   ✦ Answer: plant, fungal and bacterial cells have a cell wall (animal cells do not); it provides rigidity and strength, which is why onion peel cells hold a regular shape and pack together compactly like bricks.

5. 54 marksCuriosity Grade 8, Chapter 2, A step further, page 13

   What are plastids and vacuoles? How do vacuoles differ between plant and animal cells?

   Hint. One kind of plastid is the reason leaves are green.

   Plastids. Cells in all parts of a plant contain tiny rod-shaped structures called plastids. They are not all alike:

   • Some, called chloroplasts, contain chlorophyll, which makes them green and helps in photosynthesis
   • In non-green parts of the plant, plastids store substances instead

   Vacuoles. Plant cells have a large, empty-looking space called a vacuole. It does three jobs: it stores important substances, gets rid of waste, and maintains the shape of the cell, which gives the plant strength and support.

   The animal comparison. In animal cells vacuoles are usually not present; if present they are usually small, and they store certain substances dissolved in water.

   	Plant cell	Animal cell
   Plastids	Present	Absent
   Vacuole	Large, single, prominent	Usually absent; small if present

   The wider point the chapter draws. These structures are why a cell is "not just a simple bag of liquid" but a complex structure of many parts, each with its own function, allowing the cell — and so the whole organism — to work.

   ✦ Answer: plastids are rod-shaped structures in plant cells, of which chloroplasts contain chlorophyll and carry out photosynthesis while others store substances; the vacuole is a large space in plant cells that stores materials, removes waste and maintains cell shape, whereas animal cells usually lack vacuoles or have only small ones.

6. 64 marksCuriosity Grade 8, Chapter 2, Section 2.1.1, pages 13-14

   Do all cells look alike? Using the examples in the chapter, show how the shape and structure of a cell is related to the function it performs.

   Hint. Compare a muscle cell with a nerve cell, then look at what happens in the food pipe.

   No — cells vary a great deal. The chapter's own comparison is between two human cells:

   Cell	Shape	Why that shape suits the job
   Muscle cell	Spindle-shaped — thick in the middle, tapering at both ends	Thin and flexible, so it can contract and relax
   Nerve cell (neuron)	Very long, with branches	Carries messages; the length and branching let it reach different parts of the body and pass messages quickly
   Cheek cell	Thin and flat	Forms a protective lining over the inner surface of the cheek

   Plants show the same principle. Plant cells may be rectangular, elongated, oval or tube-like, and some form long tubes that carry water throughout the plant — a shape that follows directly from the job of transport.

   A worked example: swallowing. The food pipe contains a group of muscle cells which contract and relax in a wave-like manner, pushing food down to the stomach. This is possible precisely because muscle cells are thin, flexible and spindle-shaped. In the stomach, muscle cells in the wall churn the food, while different cells in the inner lining produce digestive juices and acid that break it down. Several cell types, several shapes, one process.

   ✦ Answer: no — cells differ widely in shape, size and structure, and the shape follows the function. Spindle-shaped muscle cells contract; long branched nerve cells carry messages quickly over distance; flat cheek cells form a lining; tube-like plant cells carry water.

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.