02. Cell: The Building Block of Life – Textbook Solutions

Page 8 – Think It Over

Q1: Where does a cell come from?

Ans: Cells arise from pre-existing cells. This idea comes from the cell theory which states that all cells are formed by the division of already existing cells.

Q2: How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye?

Ans: The invention of microscopes has made it possible to observe structures that are too small to be seen with the naked eye. Light microscopes allow us to see cells, while electron microscopes reveal finer details like organelles. These technologies have helped scientists understand cell structure, functions, and microorganisms.

Q3: How is the cell structural and functional unit of life?

Ans: Cells form the structure of all living organisms, as all organisms are made up of one or more cells. They also perform all vital functions such as respiration, nutrition, excretion, and reproduction. Therefore, the cell is both the structural and functional unit of life.

Q4: How does a cell multiply?

Ans: Cells multiply through the process of cell division, of which there are two main types. In mitosis, one cell divides to produce two daughter cells that are genetically identical to the parent cell – same DNA, same number of chromosomes. This type of division is used for normal growth, repair of damaged tissues, and replacement of old cells. In meiosis, the parent cell divides twice to produce four daughter cells, each with half the number of chromosomes of the parent. This type occurs only in reproductive organs and is used to produce gametes (sperm and egg cells) for sexual reproduction.

Page 12 – What If

Q5: mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them?

Ans: After soaking in water for 12 hours, the seeds absorb water by osmosis and become swollen and plump, since water moves from the dilute surroundings into the more concentrated cell contents.

When transferred to a concentrated salt or sugar solution, the process reverses. The solution outside is now more concentrated (hypertonic) than the cell contents, so water moves out of the seed cells by osmosis. The seeds shrink, become wrinkled and limp, and any germination that had begun will stop due to dehydration of the cells.

Page 14 – Pause and Ponder

Q6: What argument would you give for the necessity of a cell wall in plants usually fixed in one place versus in animals usually moving from one place to the other?

Ans: Plants require a cell wall for rigidity, protection, and maintaining shape because they cannot move. Animals need flexibility for movement, so they do not have a cell wall.

Q7: What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?

Ans: Without a cell wall, plant cells would lose their shape, become weak, and could burst due to osmotic pressure.

Q8: Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?

Ans: To ensure that the results of the experiment are accurate and not affected by differences in size.

Page 16 – Threads of Curiosity

Some cells are specialised to perform specific functions. For example, mature Red Blood Cells (RBCs) in humans do not have a nucleus (enucleate). The absence of a nucleus provides more space for haemoglobin, allowing it to transport a larger amount of oxygen to all cells of the body. Since they lack a nucleus, they cannot repair or divide themselves. As a result, their lifespan is short and they survive approximately for 120 days.

Q9: Do you know any other cells without nucleus?
Ans: Platelets (thrombocytes) are another example of cells in the human body that lack a nucleus. In plants, sieve tube cells (found in the phloem) also lose their nucleus when they mature.

Page 19

Q10: Are there any other plastids in plant cells that contain any pigments other than the green pigments?

Ans: Yes, chromoplasts contain pigments that give colour to fruits and flowers.

Page 19 – Pause and Ponder

Q11: Do white flowers contain any pigment? Give reasons.

Ans: White flowers lack visible pigments like chlorophyll or carotenoids. They may contain colourless plastids or pigments that do not absorb visible light.

Q12: Draw a well-labelled schematic diagram of a plant or an animal cell using these clues –
(i) Nucleus appears as a dark and round body inside the cell.
(ii) ER spreads like a network of extended nuclear envelope.
(iii) Mitochondria and chloroplasts are rod shaped.

Ans:

Page 22 – Pause and Ponder

Q13: Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area?

Ans: Many small mitochondria provide a larger total surface area, allowing efficient energy production and distribution throughout the cell.

Q14: If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to а cut on the skin?

Ans: Cells would have half the chromosome number, leading to improper functioning and abnormal tissue development.

Revise, Reflect, Refine

Q1: Differentiate between the following pairs of terms based on the clues given in parentheses:

(i) Cell membrane and cell wall (permeability)

Ans:

The cell membrane is selectively permeable – it allows only certain substances to pass through while blocking others, thus controlling what enters and exits the cell.
The cell wall, on the other hand, is fully permeable – it allows water and dissolved minerals to pass through freely without any selectivity. It acts as a structural support rather than a control barrier.

(ii) RER and SER (structure)

Ans:

Rough Endoplasmic Reticulum (RER) has ribosomes attached to its surface, which give it a rough, bumpy appearance under an electron microscope. These ribosomes are responsible for protein synthesis.
Smooth Endoplasmic Reticulum (SER)lacks ribosomes on its surface, making it appear smooth. Because of this structural difference, RER is involved in protein synthesis and secretion, while SER is involved in the synthesis and storage of fats and hormones.

(iii) Chloroplasts and Chromoplasts (pigments)

Ans:

Chloroplasts contain the green pigment chlorophyll, which absorbs sunlight for photosynthesis. They are found in the green parts of plants, mainly leaves.
Chromoplasts contain pigments other than chlorophyll, such as yellow, orange, or red pigments called carotenoids. They are found in flowers and fruits and are responsible for their bright colours, which attract pollinators and seed-dispersing animals.

Q2: 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.

Ans: Option (iii) – Water moved into Cell X and moved out of Cell Y through the cell membrane.

Explanation:

Pure water is a hypotonic solution relative to Cell X. The cell has a higher solute concentration inside than outside. So water moves into Cell X by osmosis → Cell X swells.
Concentrated salt solution is a hypertonic solution relative to Cell Y. The outside has more solute than inside the cell. So water moves out of Cell Y by osmosis → Cell Y shrinks.
The cell membrane is selectively permeable – it allows water to move through, but not large solute molecules like salt.
Option (i) is wrong – salt molecules do not move into the cell through the membrane.
Option (iv) is wrong – solute movement does not cause osmosis; water movement does.

Q3: Look at the diagram of a cell in Fig. 2.20. 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.

Based on a typical plant cell diagram, the labels and their matching functions are:Label OrganelleFunction from the list(a)Mitochondria(ii) Site of cellular respiration(b)Nucleus(i) Controlling all the activities of a cell(c)Golgi Apparatus(vi) Packs and stores materials received from ER.(d)Chloroplast(vii) Helps in manufacturing food(e)Cell wall (v) Provides structural rigidity to the cell(f)Cell membrane(iv) Separates the cell contents from surroundings(g)Vacuole(iii) Storage organelle that also provides rigidity to the cell

Q4: Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories?

Ans: Option (i) – Leucoplast (present in plant cells) and Cell wall (absent in animal cells)

Explanation:

Leucoplasts are plastids found only in plant cells → present in plant cells and absent in animal cells
Cell wall is present in plant cells and absent in animal cells
Option (ii) is wrong – mitochondria and ribosomes are present in both plant and animal cells
Option (iii) is wrong – Golgi apparatus is present in animal cells too
Option (iv) is wrong – lysosomes are found in animal cells

Q5: 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: Rohit is partially correct, but Renu is more accurate overall.

Rohit is right that roots do not perform photosynthesis since they are underground and do not receive sunlight, so chloroplasts are indeed absent in root cells.

However, Renu is correct that plastids are still present in roots – just not chloroplasts. Roots contain leucoplasts, which are colourless plastids that store food materials like starch, oils, and proteins. Leucoplasts do not need light to function – their job is storage, not food synthesis. A well-known example is potato and taro roots, where leucoplasts store large amounts of starch.

So the key point is that plastids are not limited to chloroplasts alone. There are three types of plastids – chloroplasts, chromoplasts, and leucoplasts – and not all of them require sunlight or perform photosynthesis. Roots contain leucoplasts, which makes Renu’s broader statement correct: all parts of a plant, including roots, do contain plastids, just not the same type.

Q6: 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.

Similarities:FeatureMitochondriaChloroplastsNumber of membranesDouble membrane boundDouble membrane boundOwn DNAYesYesOwn ribosomesYesYesCan make some own proteinsYesYesSemi-fluid interiorYes (matrix)Yes (stroma)

Both mitochondria and chloroplasts have their own DNA and ribosomes, which suggests they share an evolutionary history with ancient bacteria that were absorbed into early eukaryotic cells.

Differences:FeatureMitochondriaChloroplastsFunctionCellular respiration – breaks down glucose to release energy (ATP)Photosynthesis – uses sunlight to make glucose (food)Energy directionReleases energyStores energyPresent inBoth plant and animal cellsOnly plant cells (and some algae)PigmentNoneContains green chlorophyllInner membraneFolded into cristaeHas disc-shaped structures containing chlorophyll in stromaRaw materials usedGlucose + oxygenCO₂ + water + sunlightProductATP (energy) + CO₂ + waterGlucose + oxygen

In simple words: Mitochondria burn food to release energy; chloroplasts use sunlight to make food.

Q7: Which of the following pairs of cell organelles contains DNA?
(i) Chloroplasts, Ribosomes
(ii) Mitochondria, Nucleus
(iii) Golgi bodies, Ribosomes
(iv) Nucleus, Lysosomes

Ans: Option (ii) – Mitochondria, Nucleus

Explanation:

The nucleus contains chromosomes, which are made of DNA
Mitochondria have their own DNA (circular, like bacterial DNA)
Chloroplasts also have their own DNA – but that option is not in the choices
Ribosomes do not contain DNA – they contain RNA and proteins
Golgi bodies and lysosomes do not contain DNA

Q8 : 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?

Ans: She wants to test whether osmosis occurs across the cell membrane of plant cells when placed in solutions of different concentrations. More specifically, she wants to test whether a difference in solute concentration between the inside of a carrot cell and the surrounding solution causes water to move in or out of the cells, affecting their firmness.

(ii) What would you suggest for the improvement of this experiment?

Ans: Suggestions:

Weigh both carrots before and after the experiment to measure the change in mass – this gives a more precise, measurable result
Use equal-sized, equal-weight pieces of carrots (not whole carrots) for better comparison
Repeat the experiment multiple times (replication) to ensure results are consistent
Keep other variables constant – same temperature, same volume of liquid, same time duration
Consider using intermediate salt concentrations to find the point where no change occurs (isotonic point)

(iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?

Ans: Carrot in plain water (hypotonic solution):Plain water has a lower solute concentration than the inside of carrot cells. By osmosis, water moves into the cells. The cells become turgid (firm and full of water). The carrot stays stiff and crunchy.

Carrot in concentrated salt solution (hypertonic solution):The salt solution has a higher solute concentration than inside the carrot cells. By osmosis, water moves out of the cells. The cells lose water and become flaccid (limp and soft). The carrot becomes rubbery and limp.

Q9: Indicate the presence or absence of following structures in bacterial and animal cells:

Ans: StructuresBacterial CellAnimal CellChromosomeAbsent (has nucleoid with circular DNA, no membrane-enclosed chromosomes)PresentNucleusAbsent (only nucleoid region – no nuclear membrane)PresentMitochondriaAbsentPresentGolgi complexAbsentPresentChromoplastsAbsentAbsent

Note: Chromoplasts are plastids – present only in plant cells, absent in both bacterial and animal cells.

Q10: 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 С.
(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 Qs:

(i) Explain why water gathers in the hollowed portion of Cup B and Cup C.
Ans: The sugar solution in Cup B and salt solution in Cup C create a hypertonic environment inside the potato cup relative to the water in the surrounding beaker. Since the potato walls are made of cells with a selectively permeable membrane, water from outside moves into the hollowed portion by osmosis – from the region of lower solute concentration (surrounding water) to higher solute concentration (inside the cup). This is why water gathers inside Cups B and C.

(ii) Why is Cup A necessary for this experiment?
Ans: Cup A is the control in the experiment. It has no solute added to it, so no osmosis should occur. If water is also observed gathering in Cup A, it means the results in B and C may not be due to osmosis alone (perhaps due to some leakage or other reason). Cup A lets us confirm that the water gathering in Cups B and C is specifically because of the solute concentration difference – not because of some unrelated factor.

(iii) Explain why water does not gather in the hollowed portions Cups A and D.

Ans: Cup A (empty): There is no solute concentration difference between the inside of Cup A and the surrounding water. So no osmosis occurs and no water gathers.

Cup D (sugar in boiled potato): The potato used to make Cup D has been boiled. Boiling kills the cells and destroys the cell membrane. A dead, damaged membrane is no longer selectively permeable – it cannot control the passage of water. Without a functional selectively permeable membrane, osmosis cannot occur. So even though sugar is present, water does not gather because the membrane is non-functional.

Q11: 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: Option (ii) – SER – Lipid and cellulose synthesis is INCORRECT

Explanation:

Option (i): Ribosome – Protein synthesis ✓ Correct
Option (ii): SER – Lipid and cellulose synthesis ✗ Incorrect – SER synthesises lipids and hormones, NOT cellulose. Cellulose is a component of the cell wall, not made by the SER.
Option (iii): Lysosome – Digestion of foreign agents ✓ Correct – Lysosomes break down foreign materials and waste

Q12: What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?

Ans: If all mitochondria are removed from a eukaryotic cell, the cell would lose its primary source of energy (ATP) and would not be able to carry out most of its functions.

Detailed effects:

Cellular respiration cannot occur → glucose cannot be broken down to release energy
ATP production stops → the cell runs out of its energy currency
All energy-dependent activities stop: protein synthesis, movement, transport of materials across membranes, cell division, and communication with other cells
The cell will no longer be able to repair itself or reproduce
Eventually the cell will die because it cannot perform any of its vital functions

In short, removing all mitochondria is fatal for eukaryotic cells because they depend on mitochondria for their energy supply. (Note: Some very limited energy can be produced through anaerobic processes in the cytoplasm – called glycolysis – but this is far too little to sustain normal cell functions for long.)

Q13: Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.

Ans: Phenomenon that inhibits tumours in animals – Contact Inhibition:

In normal animal cells, when a cell comes into contact with neighbouring cells, a signal is sent to stop dividing. This natural “stop signal” is called contact inhibition. It prevents cells from dividing uncontrollably and therefore inhibits the formation of tumours.

When cancer cells lose this control – they no longer respond to the stop signal – they keep dividing uncontrollably, forming masses called tumours.

Can plants develop tumours?

Yes, but in a different way. Plant cells have rigid cell walls and do not show contact inhibition. They follow a different pattern of growth. However, plants can develop abnormal growths or galls – tumour-like masses – often caused by bacterial infections (e.g., Agrobacterium tumefaciens causes crown gall disease in plants) or by abnormal cell division triggered by external factors.

Since plant cells have cell walls and do not move, plant tumours do not spread through the body the way animal cancer does. They stay localised. This is why plant tumours are generally less dangerous than animal tumours.

Q14: 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: Organelles involved: Ribosomes, Rough ER (RER), Smooth ER (SER), and Golgi apparatus

Path of proteins:

Ribosomes (attached to RER) → synthesise proteins
Proteins enter the RER → undergo initial processing
From RER → Golgi apparatus (via transport vesicles) → further modification, sorting, and packaging
Golgi packages proteins into vesicles that travel to and fuse with the cell membrane

Path of lipids:

Smooth ER (SER) → synthesises lipids (fats)
Lipids move to the Golgi apparatus → packaged into vesicles
Vesicles travel to and fuse with the cell membrane → add new lipid molecules

So the full path is:

Ribosomes / SER → RER / SER → Golgi apparatus → Vesicles → Cell membrane

Q15: What would happen if gametes are formed by mitotic divisions?

Ans: If gametes (sperm and egg) were formed by mitosis instead of meiosis, they would have the full number of chromosomes (called the diploid number) instead of half.

What would happen at fertilisation:When sperm and egg join during fertilisation, their chromosomes combine. If both had the full set, the fertilised egg (zygote) would have double the normal number of chromosomes.

Effects over generations:

Each generation would have double the chromosomes of the previous one
After just a few generations, the chromosome number would become so large that cells could not function
The organism would not be able to survive normally
The entire species would eventually become non-viable

Also:

All offspring would be genetically identical (no variation) because mitosis produces genetically identical cells
Meiosis creates genetic variation through shuffling of chromosomes – this drives evolution
Without meiosis and genetic variation, organisms would be unable to adapt to changing environments

Q16: 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 Qs:

(i) Which scientific concept has the farmer applied in the preservation of the farm produce?

Ans: Farmer Deepa has applied the concept of osmosis to preserve the produce. By adding high concentrations of salt, sugar, or jaggery to the fruit, she creates a hypertonic environment around the microorganisms (bacteria and fungi) that cause spoilage. Water moves out of the microbial cells by osmosis, causing them to shrink and die – this prevents the food from spoiling.

(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?

Ans: When food is preserved in high concentrations of salt or sugar:

The surrounding solution becomes hypertonic relative to the microbial cells
By osmosis, water moves out of the bacteria and fungal cells into the surrounding concentrated solution
The microorganisms lose water, their cells shrink (plasmolysis), and they can no longer grow or multiply
Without active microbial growth, the food does not spoil
This is why pickles, jams, and murabbas – which have high salt or sugar content – have a long shelf life

(iii) Suggest a healthy recipe of this kind for food preservation.

Ans: Amla (Indian Gooseberry) Murabba:

Wash and prick amla fruits all over with a fork
Boil briefly to soften slightly
Prepare a sugar syrup (equal weight of sugar and amla)
Add the amla to the syrup along with a small amount of cardamom and saffron
Cook until the syrup thickens
Store in a clean, sterilised glass jar

This preserves amla for months while retaining its Vitamin C content. The high sugar concentration prevents microbial growth through osmosis.

(iv) What are the scientific values addressed in this case?

Ans:

Application of scientific knowledge to solve a practical problem (osmosis used for food preservation)
Sustainability – reducing post-harvest waste and extending shelf life of produce
Innovation – converting perishable produce into value-added products (pickles, sharbat, murabba)
Economic value – increasing farmers’ income and boosting local economy
Food security – making nutritious food available for longer periods
Traditional knowledge – scientific validation of traditional preservation methods passed down through generations