NCERT Solutions Chapter 2 — Cell: The Building Block of Life

Plants need a cell wall because:

• Plants are fixed in one place and cannot move, so they need a rigid structure to withstand environmental stresses like wind, rain, and physical pressure
• The rigid cell wall helps plants stay upright and maintain their shape
• It provides structural support to leaves, flowers, and stems

Animals do not need a cell wall because:

• Animals move from place to place, so they need flexible cells that can change shape easily
• A rigid cell wall would restrict movement and make it difficult for animals to bend, run, or fly
• The flexible cell membrane is sufficient for animals as they can move away from danger or harsh conditions

If the plant cell wall became as flexible as a cell membrane, several problems would occur:

• Loss of structural support: Plants would not be able to stand upright. Stems, leaves, and flowers would collapse
• Cell bursting: When plant cells absorb water through osmosis, the rigid cell wall prevents them from bursting. With a flexible wall, cells would swell and burst in hypotonic solutions
• Loss of shape: Plant cells would lose their fixed box-like shape and become irregular like animal cells
• Unable to withstand pressure: Plants would not survive wind, rain, or any physical stress
• Wilting: Without rigid cell walls, even well-watered plants would appear wilted

Conclusion: The rigid cell wall is essential for plant survival and structure.

This is important for conducting a fair and controlled experiment:

1. Equal size ensures fair comparison:

• If one piece is larger, it will have more cells and absorb or lose more water, making comparison unfair
• Equal sizes mean both pieces start with the same number of cells and surface area

2. Initial weight measurement is necessary:

• Without knowing the starting weight, we cannot measure the change (increase or decrease)
• The change in weight shows whether water entered (weight increase) or left (weight decrease) the cells through osmosis

Scientific principle: In a controlled experiment, only one variable (type of solution) should change. All other factors (size, initial weight) must remain constant to get reliable results.

Yes, white flowers do contain pigments.

Explanation:

• White flowers contain chromoplasts (a type of plastid)
• These chromoplasts contain colourless or white pigments
• They lack chlorophyll (green pigment) and lack the bright coloured pigments found in red, yellow, or orange flowers
• The white appearance comes from the reflection of all wavelengths of light, not from the absence of plastids

Examples: White jasmine, white rose, and white lily all have chromoplasts with colourless pigments in their petals. Learn more about plastids and their types.

Instructions for drawing:

For Plant Cell:

1. Draw a rectangular box-shaped outline (represents cell wall)
2. Inside, draw another boundary slightly inside (represents cell membrane)
3. Draw a large dark round circle on one side (nucleus)
4. Draw a network of lines extending from the nucleus (endoplasmic reticulum)
5. Draw 2-3 small rod-shaped structures (mitochondria)
6. Draw 2-3 green oval rod-shaped structures (chloroplasts)
7. Draw a large central bubble-like structure (vacuole)
8. Draw small dots on the ER network (ribosomes)
9. Draw stacks of flattened sacs near the ER (Golgi apparatus)

For Animal Cell:

1. Draw an irregular round/oval outline (cell membrane only, no cell wall)
2. Draw a large dark round circle in centre (nucleus)
3. Draw ER network extending from nucleus
4. Draw 3-4 rod-shaped mitochondria
5. Small dots for ribosomes
6. Golgi apparatus near nucleus
7. Small circles for lysosomes
8. Note: No chloroplasts, no large vacuole, no cell wall

Cells have many small mitochondria instead of one giant mitochondrion because of the surface area advantage.

Surface Area Concept:

• Mitochondria produce ATP (energy) on their inner membrane, which is folded into cristae
• More surface area = more cristae = more space for energy production

Why many small mitochondria are better:

1. Greater total surface area:
   • 10 small mitochondria have much more combined surface area than 1 large mitochondrion of the same total volume
   • More surface area means more cristae for ATP production
2. Better distribution:
   • Small mitochondria can be distributed throughout the cell
   • Energy (ATP) is available wherever needed in the cell
3. Efficient functioning:
   • If one mitochondrion fails, others continue working
   • A single giant mitochondrion failing would stop all energy production

Example: If you break a large cube into many small cubes, the total surface area increases dramatically, even though the volume stays the same. This same principle applies to mitochondria.

If skin cells divided by meiosis instead of mitosis, the cut would NOT heal properly.

What would happen:

1. Cells with half chromosomes:

• Normal skin cells have 46 chromosomes (in humans)
• Meiosis produces cells with only 23 chromosomes (half the normal number)
• These cells would not function properly as they lack complete genetic information

2. Inability to repair:

• Cells with half chromosomes cannot carry out normal skin cell functions
• They would likely die or malfunction
• The wound would not close

3. Serious consequences:

• Even small cuts would become life-threatening
• Skin could not replace dead cells (which happens constantly)
• The body would not survive

Why mitosis is essential: Mitosis produces genetically identical cells with the full chromosome number (46), which can perfectly replace damaged skin cells and heal wounds.

(i) Cell Membrane vs Cell Wall (Permeability):

• Cell Membrane: Selectively permeable — allows only certain substances to pass through while blocking others
• Cell Wall: Freely permeable — allows water and dissolved minerals to pass through freely

(ii) RER vs SER (Structure):

• RER (Rough Endoplasmic Reticulum): Has ribosomes attached to its surface, giving it a rough appearance under electron microscope
• SER (Smooth Endoplasmic Reticulum): Does not have ribosomes on its surface, so it appears smooth

(iii) Chloroplasts vs Chromoplasts (Pigments):

• Chloroplasts: Contain green pigment called chlorophyll; responsible for photosynthesis
• Chromoplasts: Contain coloured pigments other than chlorophyll (yellow, orange, red); give colour to flowers and fruits

Learn more about these cell organelles.

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

Explanation:

Cell X (in pure water):

• Pure water is a hypotonic solution (less solute concentration outside the cell)
• Water moves into the cell by osmosis
• Cell swells due to water entering

Cell Y (in concentrated salt solution):

• Concentrated salt solution is a hypertonic solution (more solute concentration outside the cell)
• Water moves out of the cell by osmosis
• Cell shrinks due to water leaving

Key point: The cell membrane is selectively permeable — it allows water to pass through but not salt molecules. Water always moves from areas of high water concentration to low water concentration. Read more about osmosis and diffusion.

Labelling (based on Fig. 2.20 in NCERT):

• (a) Cell wall → (v) Provides structural rigidity to the cell
• (b) Cell membrane → (iv) Separates the cell contents from surroundings
• (c) Nucleus → (i) Controlling all the activities of a cell
• (d) Chloroplast → (vii) Helps in manufacturing food
• (e) Vacuole → (iii) Storage organelle that also provides rigidity to the cell
• (f) Mitochondria → (ii) Site of cellular respiration
• (g) Golgi apparatus → (vi) Packs and stores materials received from ER

Note: This is a plant cell diagram as it has cell wall, chloroplast, and large vacuole. Learn more about all cell organelles and their functions.

Correct Answer: Option (i) — Leucoplast (present in plant cells), Cell wall (absent in animal cells)

Analysis of all options:

• Option (i): Leucoplast, Cell wall ✓ CORRECT
  • Leucoplasts are present in plant cells (store starch, oils, proteins)
  • Cell wall is present in plant cells but absent in animal cells
• Option (ii): Mitochondria, Ribosome ✗ INCORRECT
  • Both mitochondria and ribosomes are present in BOTH plant and animal cells
• Option (iii): Cell wall, Golgi apparatus ✗ INCORRECT
  • Cell wall is absent in animal cells (CORRECT)
  • But Golgi apparatus is present in both plant and animal cells (WRONG placement)
• Option (iv): Lysosome, Endoplasmic reticulum ✗ INCORRECT
  • Both are present in animal cells
  • Lysosomes are rare in plant cells but ER is present in both

Renu is partially correct. Rohit's reasoning is incorrect.

Correct explanation:

• Roots DO contain plastids — but they are leucoplasts, not chloroplasts
• Rohit is wrong to say plastids are absent in roots
• Renu is right that roots have plastids, but she should specify the type

Types of plastids in plants:

1. Chloroplasts (green parts):
   • Found in leaves, green stems
   • Contain chlorophyll for photosynthesis
2. Chromoplasts (coloured parts):
   • Found in flowers, fruits
   • Contain coloured pigments (yellow, orange, red)
3. Leucoplasts (storage organs):
   • Found in roots (potato, taro), underground stems
   • Colourless plastids that store starch, oils, proteins
   • Do NOT perform photosynthesis

Conclusion: Roots contain leucoplasts for storage, not chloroplasts for photosynthesis. Rohit's statement that "plastids are absent in roots" is incorrect.

Similarities between Mitochondria and Chloroplasts:

Structural similarities:

1. Both have double membrane (outer and inner membrane)
2. Both have their own DNA
3. Both have their own ribosomes
4. Both can make some of their own proteins
5. Both have an evolutionary link with bacteria

Functional similarities:

• Both are involved in energy-related processes
• Both carry out biochemical reactions

Differences between Mitochondria and Chloroplasts:

Feature	Mitochondria	Chloroplasts
Colour	Colourless	Green (due to chlorophyll)
Function	Cellular respiration — breaks down glucose to release energy (ATP)	Photosynthesis — uses sunlight to make glucose
Where found	Both plant and animal cells	Only plant cells
Inner membrane	Folded into cristae	Contains disc-shaped structures in stroma
Pigments	No pigments	Contains chlorophyll
Energy process	Releases energy from food	Traps sunlight to make food

Summary: Both are double-membrane organelles with DNA, but mitochondria release energy (found in all cells) while chloroplasts capture sunlight to make food (found only in plants).

Correct Answer: (ii) Mitochondria, Nucleus

Explanation:

Organelles that contain DNA:

1. Nucleus: Contains chromosomes made of DNA and proteins; stores most of the cell's genetic information
2. Mitochondria: Have their own DNA separate from nuclear DNA; can make some of their own proteins
3. Chloroplasts: Also have their own DNA (but not mentioned in any option with nucleus)

Why other options are incorrect:

• Ribosomes: Do NOT contain DNA (they are made of RNA and proteins)
• Golgi bodies: Do NOT contain DNA
• Lysosomes: Do NOT contain DNA

Interesting fact: Mitochondria and chloroplasts having their own DNA is evidence that they may have evolved from ancient bacteria.

(i) Hypothesis being tested:

The researcher wants to test osmosis — that water moves across a selectively permeable membrane (cell membrane) from a region of high water concentration to low water concentration.

(ii) Suggestions for improvement:

1. Measure initial and final weight: Record the weight of both carrots before and after the experiment to quantify water gain/loss
2. Use multiple carrots: Repeat the experiment with 3-4 carrots in each solution to get more reliable results
3. Control temperature: Keep both beakers at the same temperature
4. Equal volumes of solutions: Ensure both carrots are fully immersed in equal volumes of their respective solutions
5. Record time intervals: Observe changes every few hours to track the rate of osmosis

(iii) Explanation of observations:

Carrot in plain water (stiff and crunchy):

• Plain water is a hypotonic solution (higher water concentration outside cells)
• Water enters carrot cells by osmosis
• Cells become turgid (full of water)
• The rigid cell wall prevents cells from bursting
• Carrot becomes stiff and crunchy due to turgor pressure

Carrot in concentrated salt solution (rubbery and limp):

• Concentrated salt solution is a hypertonic solution (lower water concentration outside cells)
• Water moves out of carrot cells by osmosis
• Cells lose water and shrink (plasmolysis)
• Loss of turgor pressure makes carrot soft
• Carrot becomes rubbery and limp

Structures in a cell	Bacterial cell	Animal cell
Chromosome	Present (but not in nucleus)	Present (in nucleus)
Nucleus	Absent (has nucleoid instead)	Present
Mitochondria	Absent	Present
Golgi complex	Absent	Present
Chromoplasts	Absent	Absent

Key points:

• Bacterial cells are prokaryotic — no membrane-bound organelles
• Animal cells are eukaryotic — have membrane-bound organelles
• Chromoplasts are found only in plant cells, not in bacterial or animal cells

Experimental setup recap:

• Cup A: Empty (control)
• Cup B: One teaspoon sugar added
• Cup C: One teaspoon salt added
• Cup D: One teaspoon sugar added (but potato is boiled)

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

Water gathers in the hollowed portions because of osmosis:

• Sugar/salt in the hollow creates a hypertonic solution (high solute concentration)
• Potato cells contain water with lower solute concentration
• Water moves from potato cells into the hollow by osmosis (from lower to higher solute concentration)
• The cell membrane of potato cells is selectively permeable — allows water but not large sugar/salt molecules
• Water accumulates in the hollow, dissolving the sugar/salt

(ii) Why Cup A is necessary:

Cup A serves as a control for the experiment:

• It shows what happens when no solute (sugar/salt) is added
• Helps us confirm that water gathering in B and C is due to sugar/salt, not other factors
• Proves that osmosis requires a concentration gradient
• Without a control, we cannot be sure our results are valid

(iii) Why water does NOT gather in Cup A and Cup D:

Cup A (empty):

• No solute added, so no concentration gradient
• No osmosis occurs
• Water stays in cells

Cup D (boiled potato with sugar):

• Boiling kills the potato cells and damages the cell membrane
• Dead cells with damaged membranes cannot control water movement
• The selectively permeable property is lost
• Even though sugar is present, osmosis cannot occur through dead/damaged membranes

Conclusion: Osmosis requires both a concentration gradient AND a living, selectively permeable membrane.

Correct Answer: (ii) SER — Lipid and cellulose synthesis (INCORRECT match)

Explanation:

Why option (ii) is INCORRECT:

• SER (Smooth Endoplasmic Reticulum) is involved in lipid synthesis and hormone synthesis
• SER does NOT synthesise cellulose
• Cellulose synthesis occurs in the Golgi apparatus, not SER
• Cellulose is the main component of cell walls in plants

Why other options are CORRECT:

• (i) Ribosome — Protein synthesis: CORRECT. Ribosomes are the sites where proteins are made
• (iii) Lysosome — Digestion of foreign agents: CORRECT. Lysosomes contain digestive enzymes that break down waste materials and foreign substances

Correct function of SER: Synthesis of lipids (fats) and hormones; detoxification in liver cells. Learn more about ER and other organelles.

If all mitochondria are removed from a eukaryotic cell, the cell would die because it cannot produce energy.

Expected outcomes:

1. No ATP production:
   • Mitochondria are the "powerhouse of the cell" — they produce ATP through cellular respiration
   • Without mitochondria, the cell cannot convert glucose into usable energy (ATP)
2. Cell cannot function:
   • All cellular activities require ATP — protein synthesis, cell division, active transport, muscle contraction, nerve impulses, etc.
   • Without ATP, these processes stop
3. Cell death:
   • The cell cannot maintain its functions without energy
   • Metabolic processes shut down
   • Cell dies within hours

Exception: Some cells like mature red blood cells (RBCs) in humans do not have mitochondria, but they have a very limited lifespan (~120 days) and perform only simple functions. They get energy through a less efficient process called glycolysis.

Phenomenon that inhibits tumour formation: Contact Inhibition

What is contact inhibition?

• In normal animal cells, cell division stops when cells come in contact with neighbouring cells
• This prevents excessive cell division and uncontrolled growth
• Helps maintain proper tissue structure and size

How it prevents tumours:

• When cells touch each other, they receive signals to stop dividing
• This control mechanism prevents cells from piling up and forming tumours
• In cancer cells, contact inhibition is lost — cells keep dividing even after touching neighbours, forming tumours

Can plants develop tumours?

Yes, plants can develop tumours, even though they do not have contact inhibition.

Why plants can develop tumours despite no contact inhibition:

• Plant cells have rigid cell walls, which provide a different type of growth control
• Plants can develop tumours called crown galls caused by bacteria like Agrobacterium tumefaciens
• These bacteria inject their DNA into plant cells, causing uncontrolled cell division
• Plant tumours are less dangerous than animal tumours because plant cells cannot migrate to other parts (due to rigid cell walls)

Key difference: Animal tumours can spread (metastasis) because cells can move. Plant tumours stay localised because cell walls prevent cell movement.

Organelles involved in cell membrane synthesis:

1. RER (Rough Endoplasmic Reticulum): Synthesises proteins for the membrane
2. SER (Smooth Endoplasmic Reticulum): Synthesises lipids for the membrane
3. Golgi Apparatus: Modifies, packages, and transports proteins and lipids

Pathway from synthesis to cell membrane:

For Proteins:

RER (protein synthesis) → Vesicles → Golgi Apparatus (modification and packaging) → Vesicles → Cell Membrane (fusion and insertion)

For Lipids:

SER (lipid synthesis) → Vesicles → Golgi Apparatus (modification and packaging) → Vesicles → Cell Membrane (fusion and insertion)

Step-by-step process:

1. Proteins are made in RER with attached ribosomes
2. Lipids are made in SER
3. Both proteins and lipids are transported to the Golgi apparatus in small membrane-bound sacs called vesicles
4. Golgi apparatus modifies and packages them into new vesicles
5. These vesicles move to the cell membrane
6. Vesicles fuse with the cell membrane, releasing proteins and lipids
7. Proteins and lipids become part of the cell membrane

If gametes were formed by mitosis instead of meiosis, sexual reproduction would fail and offspring would not survive.

What would happen:

1. No reduction in chromosome number:
   • Mitosis produces cells with the same chromosome number as the parent cell
   • In humans: body cells have 46 chromosomes
   • Gametes formed by mitosis would also have 46 chromosomes (instead of 23)
2. Doubling of chromosomes after fertilisation:
   • During fertilisation: Sperm (46) + Egg (46) = Zygote (92 chromosomes)
   • Each generation would double the chromosome number
   • Next generation: 184 chromosomes, then 368, and so on
3. Non-viable offspring:
   • Cells with abnormal chromosome numbers cannot function properly
   • Too many chromosomes cause genetic imbalance
   • Embryo would not develop or would die early
4. No genetic variation:
   • Mitosis produces identical cells
   • No shuffling of genes (which happens in meiosis)
   • Offspring would be clones, lacking genetic diversity
   • Species could not evolve or adapt to changing environments

Why meiosis is essential: Meiosis halves the chromosome number (46 → 23), so when gametes fuse during fertilisation, the original number is restored (23 + 23 = 46), maintaining chromosome stability across generations.

(i) Scientific concept applied: Osmosis

The farmer used the principle of osmosis to preserve food by creating a hypertonic environment that prevents microbial growth.

(ii) How salt and sugar prevent spoilage:

• High concentrations of salt or sugar create a hypertonic environment (very high solute concentration outside bacterial/fungal cells)
• Water moves out of bacterial and fungal cells by osmosis
• Microorganisms lose water, shrink, and become dehydrated
• Without adequate water, bacteria and fungi cannot grow or reproduce
• This prevents food spoilage and extends shelf life

(iii) Healthy recipe suggestion:

Recipe: Lemon-Ginger Honey Preserve (Low Sugar)

Ingredients:

• Fresh lemons — 10 (sliced thin)
• Fresh ginger — 100g (julienned)
• Honey — 500ml (natural preservative with antibacterial properties)
• Rock salt — 1 teaspoon
• Turmeric powder — half teaspoon (antioxidant)

Method:

1. Layer lemon slices and ginger in a sterilised glass jar
2. Pour honey to cover completely
3. Add salt and turmeric
4. Seal jar and store in cool, dry place for 2 weeks before use
5. Can be stored for 6-12 months

Benefits: Uses honey instead of refined sugar; rich in Vitamin C; aids digestion; can be used as immunity booster

(iv) Scientific values addressed:

1. Prevention of food wastage: Farmer converted perishable produce into long-lasting products
2. Application of scientific knowledge: Used osmosis principle for practical benefit
3. Economic sustainability: Created additional income from excess produce
4. Resource optimisation: Made efficient use of farm harvest
5. Food security: Preserved nutritious food for year-round consumption
6. Innovation and problem-solving: Found traditional yet scientific solution to post-harvest loss
7. Environmental consciousness: Reduced food waste that would otherwise decompose