Welcome to the NCERT Solutions for Class 9 Science Chapter 2: Cell – The Building Block of Life . Here, you will find complete and accurate answers to all the exercise questions from the NCERT textbook. These solutions explain important concepts such as cell structure, cell organelles, and their functions in a simple and easy-to-understand manner. Prepared according to the latest CBSE syllabus, these answers will help students strengthen their understanding of the chapter and perform well in examinations.
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CH :- 02 - Cell: The Building Block of Life
Page No. :- 24 - Revise, Reflect, Refine
Ques 1 :- Differentiate between the following pairs of terms based on the clues given in parentheses:
- Cell membrane and cell wall (permeability)
- RER and SER (structure)
- Chloroplasts and chromoplasts (pigments)
Ans:-
(i) Cell membrane vs Cell wall (permeability)
- Cell membrane : Selectively permeable (allows some substances to pass).
- Cell wall : Freely permeable (allows most substances to pass).
(ii) RER vs SER (structure)
- RER (Rough ER) : Has ribosomes on surface; appears rough.
- SER (Smooth ER) : Lacks ribosomes; appears smooth.
(iii) Chloroplasts vs Chromoplasts (pigments)
- Chloroplasts : Contain chlorophyll (green pigment).
- Chromoplasts : Contain carotenoids (yellow, orange, red pigments).
Ques 2 :- Two similar animal cells are placed in two different solutions :
• Cell X is placed in pure water.
• Cell Y is placed in a concentrated salt solution.
• Cells are observed after some time. Cell X swells, and Cell Y shrinks. Which statement provides the correct explanation for the above observations?
Options:
(i) Salt molecules moved into Cell Y, causing it to shrink.
(ii) Water moved into Cell X and more water moved out of Cell Y than the salt solution entered in it.
(iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
(iv) Solute movement caused osmosis in both cells.
Ans:- (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
Explanation :- Osmosis = movement of water across a semipermeable membrane from a dilute solution to a concentrated solution. Therefore, Cell X swells and Cell Y shrinks.
Ques 3 :- Look at the diagram of a cell in figure. Identify the parts labelled from (a) to (g) and correctly match them with their functions given below:
Ans:-
| Function | Cell Organelle |
|---|---|
| (i) Controlling all the activities of a cell | Nucleus |
| (ii) Site of cellular respiration | Mitochondrion |
| (iii) Storage organelle that also provides rigidity to the cell | Vacuole |
| (iv) Separates the cell contents from surroundings | Cell membrane |
| (v) Provides structural rigidity to the cell | Cell wall |
| (vi) Packs and stores materials received from ER | Golgi apparatus |
| (vii) Helps in manufacturing food | Chloroplast |
Ques 4:- Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories?
| Option | Present in the plant cells | Absent in the animal cells |
|---|---|---|
| (i) | • Leucoplast. | • Cell wall. |
| (ii) | • Mitochondria. | • Ribosome. |
| (iii) | • Cell wall. | • Golgi apparatus. |
| (iv) | • Lysosome. | • Endoplasmic reticulum. |
Ans:- (i) Leucoplast
Leucoplasts are present in plant cells and absent in animal cells. They are colourless plastids that store food materials such as starch, oils and proteins.
Ques 5:- Two students, Renu and Rohit, were having a discussion on the plastids. Renu emphasised that all parts of the plants, even roots, contain plastids. However, Rohit did not agree with the statement and told her that plastids are absent in plant roots since the roots are underground and do not need to perform photosynthesis. Who is correct? Justify your answer.
Ans :- Renu is correct.
Justification:- Plastids are present in all plant cells, including roots. In roots, plastids occur as leucoplasts (colourless plastids) which store food like starch. Although roots do not perform photosynthesis, they still contain plastids for storage and other functions.
Ques 6:- Mitochondria and chloroplasts are two important organelles in a plant cell. Discuss how these two organelles are structurally and functionally similar to each other, and different from each other.
Ans :-
Similarities (Structure + Function) :-
- Both are double-membrane-bound organelles.
- Both have their own DNA and ribosomes (semi-autonomous organelles).
- Both are involved in energy transformations in the cell.
Differences:
| Mitochondria | Chloroplasts |
|---|---|
| • Present in both plant and animal cells. | • Present only in plant cells and some algae. |
| • Site of cellular respiration. | • Site of photosynthesis. |
| • Convert food energy into ATP. | • Convert solar energy into chemical energy (food). |
| • Do not contain chlorophyll. | • Contain chlorophyll (green pigment). |
Ques 7 :- Which of the following pairs of cell organelles contains DNA?
(i) Chloroplasts, Ribosomes
(ii) Mitochondria, Nucleus
(iii) Golgi bodies, Ribosomes
(iv) Nucleus, Lysosomes
Ans:- Explanation: Mitochondria and nucleus both contain DNA.
Ques 8 :- A researcher carried out an experiment in which she took two carrots of similar size. She placed one carrot in plain water and the other carrot in concentrated salt solution. After 24 hours she recorded her observations.
(i) What hypothesis does she want to test through this experiment?
Ans:- Hypothesis: Osmosis causes water movement across cell membranes – cells gain water in a dilute solution and lose water in a concentrated solution.
(ii) What would you suggest for the improvement of this experiment?
Ans:- Improvement:
• Use carrot pieces of equal size and weight
• Measure initial and final mass/length
• Keep time and temperature constant
• Repeat for accuracy
(iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp ?
Ans:- Reason for observations:
• In plain water : water enters cells → cells become turgid → carrot stays stiff
• In salt solution : water leaves cells → cells become flaccid/plasmolysed → carrot becomes limp.
Ques 9 :- Indicate the presence or absence of following structures in bacterial and animal cells:
| Structures in a Cell | Bacterial Cell | Animal Cell |
|---|---|---|
| Chromosome | Present (Single, circular) | Present (Multiple, linear) |
| Nucleus | Absent (undefined nucleoid) | Present (Well-defined with nuclear membrane) |
| Mitochondria | Absent | Present |
| Golgi Complex | Absent | Present |
| Chromoplasts | Absent | Absent (Found only in plant cells) |
Ques 10 :- Carry out the following experiment: Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato.
Place each of the potato cups in a beaker containing water (Fig. 2.22).
Now, set up the experiment as follows:
(a) Keep Cup A empty.
(b) Add one teaspoon sugar in Cup B.
(c) Add one teaspoon salt in Cup C.
(d) Add one teaspoon sugar in the boiled potato in Cup D
Observe the four potato cups at least two hours and answer the following
questions:
(i) Why Does Water Gather in Cup B and Cup C?
Ans :- Cup B has sugar, and Cup C has salt inside. These make the solution
inside the cup concentrated.
The potato cells around the cup have more water. So water moves
from the potato cells into the hollow cup through the selectively
permeable cell membrane. This is called osmosis.
That is why water gathers in both cups.
(ii) Why is Cup A necessary for this experiment?
Ans:- Cup A is the control of the experiment. It has no solute inside, so no osmosis takes place and no water gathers. It helps us compare results and confirms that water gathered in other cups only because of osmosis — and not for any other reason.
(iii) Why Does Water NOT Gather in Cup A and Cup D?
Ans:- Cup A — There is no solute inside. So there is no concentration
difference, and no osmosis takes place.
Cup D — The potato is boiled, so its cells are dead. Boiling destroys
the cell membrane. Since osmosis requires a selectively permeable
membrane, it cannot occur in Cup D even though sugar IS present.
Ques 11:- Identify the pair that incorrectly matches the cell organelle with
its function.
(i) Ribosome — Protein synthesis
(ii) SER — Lipid and cellulose synthesis
(iii) Lysosome — Digestion of foreign agents
Ans:- The incorrect pair is: (ii) SER — Lipid and cellulose synthesis.
Ques 12 :- What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?
Ans :- If all the mitochondria are removed from a eukaryotic cell,
• the cell will not be able to produce enough ATP (energy) through cellular respiration.
• As a result, all energy-dependent activities of the cell will stop, and the cell will eventually die.
Ques 13 :- Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.
Ans :- The phenomenon that inhibits the formation of tumors in the human body is
contact inhibition.
• In normal cells, when cells come into contact with each other, they stop dividing. This prevents uncontrolled cell growth and helps maintain the proper size and shape of tissues.
• Yes, plants can also develop tumors. In plants, tumors are commonly caused by the bacterium Agrobacterium tumefaciens.
• This bacterium transfers part of its DNA into plant cells, causing them to divide uncontrollably and form tumors known as crown galls.
• Thus, while contact inhibition helps prevent tumor formation in humans, plants can develop tumors due to infection by certain bacteria.
Ques 14 :- The cell membrane of a cell is made up of proteins and lipids. Which cell organelles help in the synthesis of cell membrane? Write the path of these compounds from their site of synthesis to the cell membrane and show this through a labelled diagram.
Ans :- The cell membrane is made up of proteins and lipids.
• Lipids are synthesized in the Smooth Endoplasmic Reticulum (SER).
• Proteins are synthesized by ribosomes present on the Rough Endoplasmic Reticulum (RER).
After synthesis, these substances are transported to the Golgi apparatus, where they are modified, packaged, and sent in vesicles to the cell membrane. These vesicles fuse with the cell membrane, helping in its growth and repair.
Path of Proteins and Lipids
Proteins:-
Ribosomes on RER → RER → Golgi Apparatus → Vesicles → Cell Membrane
Lipids:-
SER → Golgi Apparatus → Vesicles → Cell Membrane
The endoplasmic reticulum (RER and SER) and the Golgi apparatus together help in the synthesis, processing, and transport of proteins and lipids required for the formation of the cell membrane.
Ques 15 :- What would happen if gametes are formed by mitotic divisions?
Ans :- If gametes are formed by mitosis, they will contain the full set of chromosomes instead of half.
• During fertilisation, the fusion of two such gametes would produce a zygote with double the normal chromosome number. This would disrupt the genetic balance, cause abnormalities in offspring, and the chromosome number would keep increasing in successive generations.
• Therefore, meiosis is essential to maintain a constant chromosome number in sexually reproducing organisms.
We hope these NCERT Solutions for Class 9 Science Chapter 2: Cell – The Building Block of Life have helped you understand the chapter better. Regular practice of these exercise questions will strengthen your concepts and improve your confidence in exams. Keep revising the key topics and use these solutions as a quick reference for effective preparation.
Frequently Asked Questions (FAQs)
Q1. Why is the cell called the basic structural and functional unit of life?
Cells are called the basic structural and functional units of life because all living organisms are made up of cells, and all life processes take place within cells.
Q2. Who discovered the cell?
The cell was first discovered by Robert Hooke in 1665 while observing a thin slice of cork under a microscope.
Q3. What is the function of the cell membrane?
The cell membrane surrounds the cell and controls the movement of substances into and out of the cell. It is selectively permeable in nature.
Q4. What are cell organelles?
Cell organelles are specialized structures present inside a cell that perform specific functions. Examples include the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus.
Q5. What is the difference between plant cells and animal cells?
Plant cells have a cell wall, chloroplasts, and a large central vacuole, whereas animal cells do not have a cell wall or chloroplasts and usually contain smaller vacuoles.
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