Chapter 2: Cell - NCERT Solutions

Question 1 Differentiating Cell Structure Pairs

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)

Solution

(i) Cell membrane vs Cell wall (based on permeability):

Cell Membrane (Plasma Membrane): It is selectively permeable (semi-permeable), allowing only specific solute molecules and water to enter or exit while blocking others.
Cell Wall: It is freely permeable, allowing water, minerals, and dissolved gases to pass through without restriction.

(ii) RER and SER (based on structure):

Rough Endoplasmic Reticulum (RER): Its surface is rough due to the attachment of numerous ribosomes, and it consists mainly of flattened sac-like structures called cisternae.
Smooth Endoplasmic Reticulum (SER): Its surface is smooth because it lacks ribosomes, and it consists mainly of a network of tubular structures.

(iii) Chloroplasts and Chromoplasts (based on pigments):

Chloroplasts: Contain the green pigment chlorophyll (along with carotenoids) essential for photosynthesis.
Chromoplasts: Contain non-green pigments such as yellow, orange, or red carotenoids (xanthophylls and carotenes) which give bright colors to flowers and fruits to attract pollinators.

Question 2 Animal Cells in Water and Salt Solution

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.

Solution

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

Detailed Explanation:

Cell X (in pure water / hypotonic solution): Water concentration outside Cell X is higher than inside. By endosmosis, water molecules pass into Cell X through its selectively permeable cell membrane, causing Cell X to swell.
Cell Y (in concentrated salt solution / hypertonic solution): Water concentration inside Cell Y is higher than outside. By exosmosis, water molecules leave Cell Y through the cell membrane, causing Cell Y to shrink (crenate).
Osmosis involves only the movement of water (solvent) molecules across the membrane, not salt (solute) molecules. Thus option (iii) is the accurate scientific explanation.

Question 3 Identification and Function of Cell Organelles (Fig. 2.20)

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.

Solution

Based on the cell organelle labels in Fig. 2.20:

Label	Identified Organelle / Structure	Function Match
(a)	Mitochondrion	(ii) Site of cellular respiration (ATP synthesis).
(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 through photosynthesis.
(e)	Cell Wall	(v) Provides structural rigidity to the cell.
(f)	Cell Membrane (Plasma Membrane)	(iv) Separates the cell contents from surroundings.
(g)	Vacuole (Large Central Vacuole)	(iii) Storage organelle that also provides rigidity to the cell (turgidity).

Question 4 Correct Organelle Presence/Absence Category

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

Solution

Correct Option: (i) Leucoplast | Cell wall

Justification:

Leucoplast: Present in plant cells (stores starch, oils, proteins).
Cell wall: Present in plant cells, but completely absent in animal cells.
Options (ii), (iii), and (iv) list Ribosomes, Golgi apparatus, and Endoplasmic reticulum as absent in animal cells, which is incorrect because animal cells contain all three organelles.

Question 5 Discussion on Plastids in Plant Roots

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.

Solution

Renu is correct.

Justification:

Plastids are not limited to green, photosynthetic chloroplasts. Plastids exist in three primary types: Chloroplasts (green), Chromoplasts (colored), and Leucoplasts (colorless).
While underground roots do not contain chloroplasts because they are not exposed to sunlight, root cells contain leucoplasts (such as amyloplasts and elaioplasts).
Leucoplasts in roots play a vital role in storing nutrients like starch, proteins, and lipids synthesized by the plant (e.g., starch in potato tubers and sweet potato roots).
Therefore, all living plant cells, including root cells, possess plastids.

Question 6 Comparison of Mitochondria and Chloroplasts

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.

Solution

1. Structural and Functional Similarities:

Double Membrane: Both organelles are bounded by two membranes (an outer membrane and an inner membrane).
Semiautonomous Nature: Both contain their own circular DNA and 70S ribosomes, allowing them to synthesize some of their own proteins and self-replicate.
ATP Energy Involvement: Both are involved in energy transformation processes that produce ATP.

2. Differences:

Feature	Mitochondria	Chloroplasts
Occurrence	Found in almost all eukaryotic cells (plant & animal).	Found only in plant cells and green algae.
Pigments	Pigments are absent.	Contain green pigment (chlorophyll) and carotenoids.
Inner Structure	Inner membrane is folded into cristae.	Contains stacked disc-like thylakoids (grana) in a fluid matrix (stroma).
Primary Function	Cellular respiration: breaks down glucose to release energy ($E_k / \text{ATP}$).	Photosynthesis: traps solar energy to synthesize glucose food.

Question 7 Organelles Containing DNA

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

Solution

Correct Option: (ii) Mitochondria, Nucleus (Chloroplasts also contain DNA).

Reason:

The Nucleus contains nuclear DNA stored in chromosomes. Mitochondria (and Chloroplasts) are semiautonomous organelles that possess their own extranuclear circular DNA. Ribosomes, Golgi bodies, and Lysosomes do not contain DNA.

Question 8 Carrot Osmosis Experiment (Fig. 2.21)

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?

Solution

(i) Hypothesis being tested:

"Water moves across plant cell membranes by osmosis depending on the solute concentration gradient of the surrounding liquid, affecting cell turgidity."

(ii) Suggestions for improvement:

Measure and record the initial mass/weight and initial length/diameter of both carrots before placing them in liquids.
Use multiple replicates (3–5 carrots in each liquid) to minimize experimental error and calculate average percentage mass change.
Ensure identical environmental temperature and light conditions for both beakers.

(iii) Scientific Explanation:

Carrot in Plain Water (Hypotonic Medium): Water enters the carrot cells by endosmosis. The central vacuoles fill with water, exerting turgor pressure against the cell walls, making the carrot stiff and crunchy (turgid).
Carrot in Salt Solution (Hypertonic Medium): Water moves out of the carrot cells into the concentrated salt solution by exosmosis. The cells lose turgor pressure (plasmolysis), causing the carrot to become rubbery and limp (flaccid).

Question 9 Presence/Absence of Structures in Bacterial & Animal Cells

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

Solution

Structures in a cell	Bacterial cell (Prokaryote)	Animal cell (Eukaryote)
Chromosome	Present (single circular nucleoid DNA)	Present (multiple linear chromosomes inside nucleus)
Nucleus	Absent (undefined nucleoid without nuclear membrane)	Present (well-defined double-membraned nucleus)
Mitochondria	Absent	Present
Golgi complex	Absent	Present
Chromoplasts	Absent	Absent (found only in plant cells)

Question 10 Potato Osmoscope Experiment (Fig. 2.22)

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.

Solution

(i) Why water gathers in Cups B and C:

The living potato cells of Cups B and C act as a selectively permeable membrane. Adding sugar (in B) or salt (in C) inside the cup creates a hypertonic environment in the hollowed cavity compared to the water in the outer beaker. Water moves from the beaker through the living potato cells into the hollow cavity by osmosis (endosmosis), accumulating as a liquid.

(ii) Why Cup A is necessary:

Cup A serves as a control experiment. It proves that water movement into hollow cavities requires a solute concentration gradient (sugar/salt) and does not occur spontaneously in an empty cup.

(iii) Why water does not gather in Cups A and D:

Cup A: Lacks any solute inside the hollow cavity, so there is no concentration difference to drive osmosis.
Cup D: Boiling the potato kills its cells and denatures cell membranes, destroying selective permeability. Dead cell membranes cannot perform osmosis, so water does not accumulate.

Question 11 Organelle-Function Matching Error

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

Solution

Incorrectly Matched Pair: (ii) SER — Lipid and cellulose synthesis

Correction:

Smooth Endoplasmic Reticulum (SER) synthesizes lipids, phospholipids, and steroid hormones (and detoxifies drugs). Cellulose is synthesized by enzymes in the plasma membrane, not by SER.

Question 12 Removal of All Mitochondria

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

Solution

If all mitochondria are removed from a eukaryotic cell:

Aerobic cellular respiration will cease entirely because the Krebs cycle and electron transport chain take place inside mitochondria.
Production of ATP (energy currency) will drop drastically (the cell will yield only 2 ATP molecules via anaerobic glycolysis instead of 36-38 ATP).
Active transport, protein synthesis, muscle contraction, and organelle functions will halt due to energy starvation.
The cell will ultimately undergo metabolic shutdown and die.

Question 13 Tumor Inhibition & Plant Tumors

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

Solution

1. Phenomenon Inhibiting Tumors:

Contact Inhibition and Programmed Cell Death (Apoptosis / PCD) inhibit tumor formation in humans. Contact inhibition stops animal cell division when cells come in contact with adjacent cells, preventing overcrowding.

2. Can plants develop tumors?

Yes, plants can develop tumors.

Explanation: Plant cells lack contact inhibition due to their rigid cell walls. Infection by pathogens like the soil bacterium Agrobacterium tumefaciens transfers bacterial T-DNA into plant cells, inducing uncontrolled cell division and forming tumors known as Crown Gall disease.

Question 14 Cell Membrane Synthesis Pathway

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.

Solution

1. Organelles Involved:

Rough Endoplasmic Reticulum (RER): Synthesizes membrane proteins via ribosomes.
Smooth Endoplasmic Reticulum (SER): Synthesizes membrane lipids (phospholipids).
Golgi Apparatus: Modifies, packages, and routes proteins and lipids into transport vesicles.

2. Pathway of Synthesis (Membrane Biogenesis):

$$\text{RER (Proteins) / SER (Lipids)} \xrightarrow{\text{Transport Vesicles}} \text{Golgi Apparatus (Modification)} \xrightarrow{\text{Secretory Vesicles}} \text{Plasma Membrane (Fusion)}$$

3. Biogenesis Mechanism: Vesicles containing modified glycoproteins and glycolipids fuse with the plasma membrane, incorporating new lipids and proteins directly into the membrane structure.

Question 15 Gamete Formation via Mitosis Consequence

What would happen if gametes are formed by mitotic divisions?

Solution

If gametes were formed by mitosis instead of meiosis:

Gametes would be diploid ($2n$) instead of haploid ($n$).
Upon fertilization ($\text{Sperm } 2n + \text{Egg } 2n$), the resulting zygote would be tetraploid ($4n$).
With every subsequent generation, the chromosome number would double continuously ($2n \to 4n \to 8n \to 16n$).
This massive chromosomal instability would cause severe genetic abnormalities, organismal lethality, and disruption of species identity.

Question 16 Post-Harvest Food Preservation Case Study

A farmer, Deepa, was very happy with the harvest of amla (Indian Gooseberry) and lemons on her farm. However, she could sell only one-fourth 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?

Solution

(i) Scientific Concept Applied:

The farmer applied the principle of Osmosis (Exosmosis) and Dehydration using hypertonic preservatives (high salt/sugar concentrations).

(ii) Mechanism preventing microbe growth:

Adding high concentrations of salt or sugar creates a strongly hypertonic medium surrounding microbial cells. When bacteria or fungi land on the pickle/sharbat, water moves out of the microbial cells into the preservative medium by exosmosis (plasmolysis). The loss of water inhibits bacterial metabolic enzymes, preventing cell growth and decay.

(iii) Healthy Preservation Recipe (Amla-Honey Murabba):

Wash fresh amla, prick with a fork, steam lightly for 10 minutes, and steep in pure organic honey or jaggery syrup with cardamom. Honey acts as a natural hypertonic preservative rich in antioxidants.

(iv) Scientific Values Addressed:

Problem-Solving & Innovation: Applying scientific concepts to prevent post-harvest food waste.
Sustainability & Agro-Entrepreneurship: Creating economic value from surplus agricultural produce.
Resource Conservation: Reducing food loss to support community food security.

NCERT SOLUTIONS • CHAPTER 2