Question Think It Over 1
Where does a cell come from?
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New cells arise from pre-existing cells.
Answer
New cells arise from pre-existing cells.
CLASS 9 · SCIENCE · CHAPTER 2
Cell: The Building Block of Life
Simple answers to the chapter questions, investigations, all sixteen end questions, and all four projects.
41 worked answersComplete chapter answers
41 worked answers · short steps first, explanations when you need them
Chapter Investigations
Textbook pages 9–35
Question Think It Over 2
How have tools helped us learn about things too small to see with our eyes?
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Microscopes let us see cells and their smaller structures.
Answer
Microscopes let us see cells and their smaller structures.
Question Think It Over 3
Why is a cell called the structural and functional unit of life?
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Cells build organisms and carry out life processes.
Answer
Cells build organisms and carry out life processes.
Question Think It Over 4
How does a cell multiply?
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Cells multiply by division; mitosis and meiosis have different roles.
Answer
Cells multiply by division; mitosis and meiosis have different roles.
Question Activity 2.1
A microscope’s field of view is 5 mm wide and 25 onion cells fit across it. Estimate one cell’s size.
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About 200 µm.
Answer
About 200 µm.
Question Ready to Go Beyond
If a cell is 200 µm long and the microscope magnifies it 100 times, how long does it appear?
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20 mm (2 cm).
Answer
20 mm (2 cm).
Question Activity 2.2
Why does a potato piece swell in plain water and shrink in concentrated salt solution?
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Water enters in plain water and leaves in concentrated salt solution.
Answer
Water enters in plain water and leaves in concentrated salt solution.
Question What if 1
What may happen to soaked mung beans placed in a concentrated salt solution?
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They may lose water and shrink.
Answer
They may lose water and shrink.
Question What if 2
What happens to a cell in hypotonic, isotonic, and hypertonic solutions?
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Hypotonic: water enters; isotonic: no net movement; hypertonic: water leaves.
Answer
Hypotonic: water enters; isotonic: no net movement; hypertonic: water leaves.
Question Activity 2.3
Why do onion peel cells look regular and box-shaped while cheek cells look irregular?
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The plant cell wall gives onion cells a firm, regular shape.
Answer
The plant cell wall gives onion cells a firm, regular shape.
Question Activity 2.3
What happens to plant and cheek cells after being placed in concentrated sugar solution?
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Both lose water; cheek cells shrink, while plant cell contents pull away from the wall.
Answer
Both lose water; cheek cells shrink, while plant cell contents pull away from the wall.
Question Cell wall
The cell wall gives support and protection and helps the cell keep its shape. In plants, it helps stems and leaves stay firm. The wall is porous, so water and some dissolved substances can pass through it; the cell membrane controls entry more selectively.
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It supports and protects cells and helps maintain shape.
Answer
It supports and protects cells and helps maintain shape.
Question Activity 2.4
Which pictured cells are prokaryotic and which are eukaryotic?
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Bacterial: prokaryotic; plant and animal: eukaryotic.
Answer
Bacterial: prokaryotic; plant and animal: eukaryotic.
Question Nucleus
The nucleus stores most of the cell’s DNA. Genes in DNA carry instructions that help control cell activities and pass inherited features to new cells or offspring. The nucleolus inside the nucleus helps make ribosome parts.
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It stores DNA and helps direct cell activities.
Answer
It stores DNA and helps direct cell activities.
Question Activity 2.5
Why do onion root-tip cells look different from one another under a microscope?
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They are at different stages of cell division and growth.
Answer
They are at different stages of cell division and growth.
Question Plastids
Yes. Root cells can contain colourless plastids called leucoplasts, which store food such as starch. Roots usually do not photosynthesise, so they do not need green chloroplasts in the same way as leaves.
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Yes. Roots can have leucoplasts for food storage.
Answer
Yes. Roots can have leucoplasts for food storage.
Question White flowers
Often, white flowers look white because they reflect or scatter most visible wavelengths rather than having a strong coloured pigment. Some white petals may contain small amounts of pigments that are not visible as a colour. So a white appearance does not always mean there are no pigment molecules at all.
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White petals often lack strong visible pigment; whiteness can come from light scattering.
Answer
White petals often lack strong visible pigment; whiteness can come from light scattering.
Question Pause and Ponder 5
What should be included in a labelled plant-cell sketch using the given clues?
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Label cell wall, membrane, cytoplasm, nucleus, ER, vacuole, mitochondria, and chloroplasts.
Answer
Label cell wall, membrane, cytoplasm, nucleus, ER, vacuole, mitochondria, and chloroplasts.
Question Pause and Ponder 6
Why might cells use many small mitochondria instead of one giant one?
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Many small mitochondria provide more usable membrane area and can be placed where energy is needed.
Answer
Many small mitochondria provide more usable membrane area and can be placed where energy is needed.
Question Pause and Ponder 7
What might happen if skin cells divided by meiosis instead of mitosis?
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Repair would fail because skin needs mitosis, not meiosis.
Answer
Repair would fail because skin needs mitosis, not meiosis.
Question Pause and Ponder 8
Do cells grow and divide forever?
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Normal cells are controlled and have a life span; they do not divide forever.
Answer
Normal cells are controlled and have a life span; they do not divide forever.
Revise, Reflect, Refine
Textbook pages 35–37
Question 1
Differentiate between (i) cell membrane and cell wall, (ii) RER and SER, and (iii) chloroplasts and chromoplasts.
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Membrane controls passage; wall supports. RER has ribosomes; SER does not. Chloroplasts are green; chromoplasts have other colours.
Answer
Membrane controls passage; wall supports. RER has ribosomes; SER does not. Chloroplasts are green; chromoplasts have other colours.
Question 2
Why does animal cell X swell in pure water and cell Y shrink in concentrated salt solution?
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Option 3.
Answer
Option 3.
Question 3
In Fig. 2.20, match labels (a)–(g) to these functions: cell control, cellular respiration, storage and rigidity, boundary around cell contents, structural support, packaging materials from the ER, and food-making.
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(a) mitochondrion, (b) nucleus, (c) Golgi apparatus, (d) chloroplast, (e) cell wall, (f) cell membrane, (g) vacuole.
Answer
(a) mitochondrion, (b) nucleus, (c) Golgi apparatus, (d) chloroplast, (e) cell wall, (f) cell membrane, (g) vacuole.
Question 4
Which listed pair is correctly placed as present in plant cells and absent in animal cells?
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Option (i).
Answer
Option (i).
Question 5
Who is correct: Renu, who says roots have plastids, or Rohit, who says roots do not?
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Renu: roots can have leucoplasts for storage.
Answer
Renu: roots can have leucoplasts for storage.
Question 6
How are mitochondria and chloroplasts alike and different?
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Both have two membranes, DNA, and ribosomes; mitochondria release energy, chloroplasts capture light for photosynthesis.
Answer
Both have two membranes, DNA, and ribosomes; mitochondria release energy, chloroplasts capture light for photosynthesis.
Question 7
Which listed pair of organelles contains DNA?
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Option 2.
Answer
Option 2.
Question 8
(i) What hypothesis is tested by placing similar carrots in plain water and concentrated salt solution? (ii) How can the experiment be improved? (iii) Why does each carrot feel different?
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Water enters in plain water, making it firm; water leaves in salt solution, making it limp.
Answer
Water enters in plain water, making it firm; water leaves in salt solution, making it limp.
Question 9
State whether each structure is present in bacterial and animal cells: chromosome, nucleus, mitochondria, Golgi complex, chromoplasts.
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Bacteria: chromosome yes; nucleus, mitochondria, Golgi, chromoplasts no. Animal: chromosome, nucleus, mitochondria, Golgi yes; chromoplasts no.
Answer
Bacteria: chromosome yes; nucleus, mitochondria, Golgi, chromoplasts no. Animal: chromosome, nucleus, mitochondria, Golgi yes; chromoplasts no.
Question 10
Why does water collect in sugar Cup B and salt Cup C, why is empty Cup A needed, and why does boiled Cup D not collect water?
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B and C collect water by osmosis; A is the control; boiling damages D’s membranes.
Answer
B and C collect water by osmosis; A is the control; boiling damages D’s membranes.
Question 11
Which organelle-function pair is incorrectly matched: ribosome—protein synthesis; SER—lipid and cellulose synthesis; lysosome—digestion of foreign agents?
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SER–cellulose synthesis is the incorrect pair.
Answer
SER–cellulose synthesis is the incorrect pair.
Question 12
What might happen if all mitochondria were removed from a eukaryotic cell?
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ATP supply would fall greatly; essential cell activities would fail.
Answer
ATP supply would fall greatly; essential cell activities would fail.
Question 13
What phenomenon helps stop tumours forming in the human body? Can plants also develop tumours?
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Contact inhibition helps control cell division; plants can also develop abnormal growths.
Answer
Contact inhibition helps control cell division; plants can also develop abnormal growths.
Question 14
Which organelles help make the cell membrane’s lipids and proteins, and how do they reach the membrane?
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SER/RER → vesicles → Golgi → vesicles → cell membrane.
Answer
SER/RER → vesicles → Golgi → vesicles → cell membrane.
Question 15
What would happen if gametes were formed by mitosis instead of meiosis?
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Chromosome numbers would double at fertilisation.
Answer
Chromosome numbers would double at fertilisation.
Question 16
(i) Which scientific concept helps preserve fruit with salt or sugar? (ii) How do concentrated salt or sugar prevent microbial growth? (iii) Suggest a healthy recipe. (iv) What values does the example show?
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Osmosis lowers available water; salt/sugar can slow spoilage, with safe handling.
Answer
Osmosis lowers available water; salt/sugar can slow spoilage, with safe handling.
The Journey Beyond
Textbook pages 37
Question Project 1
What should a clear animation of cell division show?
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Show the parent cell, copied DNA, divisions, and resulting daughter cells.
Answer
Show the parent cell, copied DNA, divisions, and resulting daughter cells.
Question Project 2
What could a safe, low-cost model of a synthetic cell include?
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Represent major cell parts with labelled reused materials; explain the model’s limits.
Answer
Represent major cell parts with labelled reused materials; explain the model’s limits.
Question Project 3
How can teamwork improve a class model of mitosis or meiosis?
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Share roles, check the sequence together, and reflect on what the model clarified.
Answer
Share roles, check the sequence together, and reflect on what the model clarified.
Question Project 4
How could a short play explain the roles of cell organelles?
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Show each organelle’s job and how they cooperate in a cell.
Answer
Show each organelle’s job and how they cooperate in a cell.