Cell: The Building Block of Life is an important chapter in Class 9 Science that introduces students to the basic structural and functional unit of life—the cell. This chapter explains the discovery of cells, cell theory, cell structure, and the functions of various cell organelles. These NCERT Solutions provide accurate answers in simple language, helping students strengthen their concepts, complete assignments, and prepare effectively for examinations.
Cell the Building Block of Life Class 9 NCERT Solutions
1. Differentiate between the following pairs of terms based on the clues given in parentheses:
(i) Cell membrane and cell wall (permeability)
(ii) RER and SER (structure)
(iii) Chloroplasts and chromoplasts (pigments)
Answer:
(i) Cell membrane and cell wall (permeability)
- Cell membrane: The cell membrane has a thin and flexible covering. It is selectively permeable and allows only some substances like water, nutrients and oxygen to pass.
- Cell wall: Cell wall is a thick, rigid covering outside the membrane in plants, bacteria, and fungi. The cell wall is fully permeable, and water and minerals can dissolve and pass freely. The cell wall helps to give support and shape to plants.
(ii) RER and SER (structure)
- RER: RER is a protein factory. It has ribosomes attached and looks rough; it helps to transport proteins.
- SER: It is a fat/hormone factory. It does not have ribosomes and looks smooth; it stores fats and hormones.
(iii) Chloroplasts and chromoplasts (pigments)
- Chloroplasts: The chloroplasts contain green pigment which is know as chlorophyll. It helps to do photosynthesis and make food for plants.
- Chromoplasts: They contain red, yellow, and orange pigments and give bright colours to flowers and fruits. It attracts pollinators and animals.
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?
- (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.
Answer: (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
Explanation:
(i)This statement is incorrect. The salt molecules cannot move freely into the cell; only water moves.
(ii) This statement is partly correct but it give confusion
(iv) This statement is incorrect. Osmosis is only about water movement, not solute movement.
3. Look at the diagram of a cell in Fig. Identify the parts labelled from (a) to (g) and correctly match them with their functions given below:
- (i) Controlling all the activities of a cell.
- (ii) Site of cellular respiration.
- (iii) Storage organelle that also provides rigidity to the cell.
- (iv) Separates the cell contents from surroundings.
- (v) Provides structural rigidity to the cell.
- (vi) Packs and stores materials received from ER.
- (vii) Helps in manufacturing food.
Answer:
Label Organelle Function
| Nucleus | (i) Controlling all the activities of a cell. |
| Mitochondria | (ii) Site of cellular respiration. |
| Vacuole | (iii) Storage organelle that also provides rigidity to the cell. |
| Cell membrane | (iv) Separates the cell contents from surroundings. |
| Cell wall | (v) Provides structural rigidity to the cell. |
| Golgi apparatus | (vi) Packs and stores materials received from ER. |
| Chloroplast | (vii) Helps in manufacturing food. |
4. Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories?
Answer: (i) Leucoplast (present in plant cells); Cell wall (absent in animal cells)
Explanation: The plant cells have leucoplasts means storage plastids and a cell wall. Animall cells do not have a cell wall.
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.
Answer: The plastids are present in all plant cells, not only in leaves. The roots also contain plastids, but they are not chloroplasts, means roots do not have photosynthesis.
The root has leucoplasts, which are colourless plastids that store food like starch, proteins and oils. Example: Potato and colocasia roots store starch in leucoplasts.
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.
Answer: The mitochondria and chloroplasts are both double-membrane organelles. Both have their own DNA and ribosomes, so they can make some proteins themselves. Both are involved in energy conversion, and both show evidence of bacterial origin.
| Feature | Mitochondria | Chloroplasts |
|---|---|---|
| Function | It break down the food (glucose) and release energy (ATP) | make food (sugar) using sunlight, which is known as photosynthesis |
| Pigment | No pigments Contains | Contains chlorophyll which is green pigment |
| Inner structure | Inner membrane folded into cristae | Inner part has stroma and thylakoid discs with chlorophyll |
| Presence | Found in all plant and animal cells | Found only in plant cells and algae |
| Energy source | Uses glucose and oxygen | Uses sunlight, water and carbon dioxide |
7. Which of the following pairs of cell organelles contains DNA?
(i) Chloroplasts, Ribosomes
(ii) Mitochondria, Nucleus
(iii) Golgi bodies, Ribosomes
(iv) Nucleus, Lysosomes
Answer: (ii) Mitochondria, Nucleus
Explanation: Nucleus contain DNA in the form of chromosomes. Mitochondria have their own DNA and ribosomes, so they can make some proteins independently.
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 (Fig. 2.21). After 24 hours she recorded her observations.
- (i) What hypothesis does she want to test through this experiment?
- (ii) What would you suggest for the improvement of this experiment?
- (iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?
Answer:
(i) Hypothesis being tested
- The researcher wants to test the effect of osmosis on plant cells.
- Hypothesis: The water moves in or out of carrot cells depending on the solutions they are placed in.
(ii) Improvement of the experiment
Cut the carrot pieces of equal size and record their initial weights before placing them in solutions. Keep both carrots under the same conditions, like temperature, time and container size. This will ensure the results are fair and comparable.
(iii) Reason for observations
Carrot in plain water (hypotonic solution): The water outside the carrot has more water and less salt. So, the water moves into the cells through the cell membrane. The vacuole fills up with water and pushes against the cell wall, which helps the cells to become turgid means swollen and firm. That’s why the carrot feels stiff and crunchy.
Carrot in concentrated salt solution (hypertonic solution): The water outside the carrot cells has less water and more salt than inside the cells. So, the water moves out of the cells, the vacuole shrinks, and the cell becomes flaccid (soft and limp). Because of this, the carrot becomes rubbery and limp.
9. Indicate the presence or absence of the following structures in bacterial and animal cells:
| Structures in a cell | Bacterial cell | Animal cell |
|---|---|---|
| Chromosome | ||
| Nucleus | ||
| Mitochondria | ||
| Golgi complex | ||
| Chromoplasts |
Answer:
| Structures in a cell | Bacterial cell | Animal cell |
|---|---|---|
| Chromosome | Present (single circular DNA, not inside nucleus) | Present (many linear chromosomes inside nucleus) |
| Nucleus | Absent (DNA lies in nucleoid region) | Present (well‑defined membrane‑bound nucleus) |
| Mitochondria | Absent | Present |
| Golgi complex | Absent | Present |
| Chromoplasts | Absent | Absent (found only in plant cells) |
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) Explain why water gathers in the hollowed portion of Cup B and Cup C.
- (ii) Why is Cup A necessary for this experiment?
- (iii) Explain why water does not gather in the hollowed portions of Cups A and D.
Answer:
(i) Why water gathers in Cup B and Cup C
Cup B has sugar, and Cup C has salt. When both sugar and salt make the solution inside the potato cup, it makes it more concentrated than the water outside. Water moves into the cup by osmosis. As a result, water collects in the hollow of Cup B and Cup C.
(ii) Why Cup A is needed
Cup A is kept empty as a control experiment. It shows that if no solute means sugar or salt is present, water does not move into the hollow. This proves that the water movement happens because of osmosis. This proves that the water movement happens because of osmosis.
(iii) Why water does not gather in Cup A and Cup D
- Cup A (empty): No solute, no concentration difference, no osmosis and no water.
- Cup D (boiled potato): Boiling destroys the living cell membranes; osmosis cannot happen – no water.
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
Answer: The correct pair matches the cell organelle with its function are –
- (i) Ribosome — Protein synthesis
- (iii) Lysosome — Digestion of foreign agents
12. What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?
Answer: If all mitochondria are removed from a eukaryotic cell, then
- No ATP production: Mitochondria are a “powerhouse” of the cell. Without mitochondria, the cell cannot release energy from food.
- Cell activities stop: Without mitochondria, growth, division, movement, and transport will stop; they need ATP.
- Cell death: The cell will not survive without energy.
13. Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.
Answer: The human body has a safety process called apoptosis. If the cell becomes abnormal, then apoptosis destroys it before it grows into a tumour. The immune system also finds and kills the abnormal cells. If this protection fails, then the tumors can form.
Plants can also develop tumor-like swellings. For example, Agrobacterium tumefaciens bacteria cause crown gall disease in plants. Plant tumors do not spread through the body because plant cells have rigid cell walls.
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.
Answer: The cell membrane is made of lipids (fats) and proteins.
- Lipids (fats) are made in the SER (Smooth Endoplasmic Reticulum).
- Proteins made in ribosomes (on RER).
The pathway steps are
- SER makes lipids.
- Ribosomes on RER make proteins.
- RER carries proteins.
- The Golgi apparatus modifies and packs proteins and lipids.
- Transport vesicles carry them.
- They reach the cell membrane and become part of it.
15. What would happen if gametes are formed by mitotic divisions?
Answer: Normally meiosis makes gametes (sperm and egg). It helps to reduce the chromosome number to half. So, when the sperm and egg join, the new cell (zygote) gets the normal full number of chromosomes.
If the gametes are formed by mitosis, then mitosis does not reduce chromosome number. Gametes would have the same number of chromosomes as the parent cell. When these two diploid gametes fuse, then the zygote would have double chromosomes.
As a result, the zygote would have too many chromosomes. It would not develop normally, and the organism might die or show abnormal growth.
16. A farmer, Deepa, was very happy with the harvest of amla (Indian Gooseberry) and lemons on her farm. However, she could sell only onefourth of the produce in the local market. Recognising that a significant amount of produce may be lost post-harvest, she employed a traditional yet scientifically sound method to extend the shelf life of amla and lemons.
She turned perishable produce into profitable products, such as pickles and sharbat. She used the excess produce to prepare pickles, murabbas, and sharbat by adding appropriate amounts of salt, sugar, or jaggery to small pieces of fruit and their juices. These were then stored in small glass bottles for sale, helping her prevent the wastage of post-harvest produce.
This shift from farming to agro-processing would strengthen food security and boost the local economy, creating a sustainable model that cuts waste while increasing her income. Based on the above passage answer the following questions:
- (i) Which scientific concept has the farmer applied in the preservation of the farm produce?
- (ii) How does the addition of high concentrations of salt and sugar create an environment that prevents the growth of spoilage-causing bacteria and fungi?
- (iii) Suggest a healthy recipe of this kind for food preservation.
- (iv) What are the scientific values addressed in this case?
Answer:
(i)Scientific concept used
The farmer, Deepa, applied food preservative using a high concentration of salt, sugar, and jaggery. This is based on the principle of osmosis, where the water moves out of microbes and helps to prevent their growth.
(ii) Why salt and sugar prevent spoilage
When high salt or sugar is used, it makes the environment hypertonic. Microbes like bacteria or fungi lose water by osmosis, and they shrink and cannot grow. As a result, food stays safe for longer.
(iii) Healthy recipe example
- We can make amla candy using sweet preservation—
- Cut amala into small pieces.
- Boil lightly to soften.
- Add jaggery or honey instead of refined sugar.
- Dry in sunlight and store in airtight jars.
- This helps the amla keep fresh, tasty, and healthy.
(iv) Scientific values shown
- Food security: less wastage, more food available.
- Sustainability: turning farm produce into long‑lasting products.
- Economic empowerment: a farmer earns more income.
- Scientific thinking: using traditional methods explained by science.
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