PAUSE AND PONDER (In-chapter Questions)
Q1. What argument would you give for the necessity of a cell wall in plants (fixed) vs animals (moving)?
Plants are stationary and face mechanical stresses like wind, rain, and gravity. The rigid cell wall provides structural support to withstand these forces and maintain upright posture. Animals move actively, so their cells need flexibility to change shape — a rigid wall would hinder movement of tissues like muscles. Thus, a cell wall is essential for plants but would be a disadvantage for animals.
Q2. What consequences if a plant cell’s cell wall became as flexible as the cell membrane?
- The cell would lose its fixed shape and rigidity.
- Plants would become unable to stand upright and would wilt permanently.
- Leaves and flowers would collapse.
- When placed in hypotonic solution, the cell could burst (like animal cells) since the wall wouldn’t resist excessive water entry.
- Overall structural integrity of the plant body would be lost.
Q3. Why is it important to cut potato pieces in roughly equal size and measure initial weight?
This ensures a fair and controlled experiment. If sizes differ, the surface area exposed to solution differs, affecting the rate of osmosis. Measuring initial weight allows accurate calculation of water gain or loss. Without these controls, results cannot be reliably compared between the two beakers.
Q4. Do white flowers contain any pigment? Give reasons.
Yes, white flowers do contain pigments, but they reflect all wavelengths of visible light rather than absorbing any specific colour, which makes them appear white. Some white flowers contain flavonoids or other colourless/UV-absorbing pigments. The whiteness may also result from air spaces in petals that scatter light. So white does not mean absence of all pigments.
Q5. Draw a well-labelled schematic diagram of a plant or animal cell.
(This is a drawing-based question — draw referring to Fig. 2.10 in the textbook.)
For a plant cell, include: cell wall, cell membrane, nucleus (dark round body), cytoplasm, ER (network extending from nuclear envelope), mitochondria (rod-shaped), chloroplasts (rod-shaped, green), vacuole (large, central), Golgi apparatus, ribosomes.
For an animal cell, include: cell membrane, nucleus, cytoplasm, ER, mitochondria (rod-shaped), Golgi apparatus, lysosomes, ribosomes, small vacuoles.
Q6. Why does a cell not have one giant mitochondrion instead of many small ones? (Surface area concept)
Many small mitochondria provide a much greater total surface area than one large mitochondrion of the same combined volume. The inner membrane (with cristae) is where ATP production occurs. More surface area means more space for chemical reactions, leading to greater and more efficient energy production. A single giant mitochondrion would have far less surface-area-to-volume ratio and would be metabolically inefficient.
Q7. If skin cells divide by meiosis instead of mitosis, what would happen to a skin cut?
Meiosis produces cells with half the chromosome number. If skin cells used meiosis for repair:
- Daughter cells would have only half the genetic information needed.
- They would be genetically different from the parent skin cells.
- Proper healing would not occur since the new cells would be abnormal and non-functional.
- The wound would not heal correctly, and skin integrity would be permanently compromised.
REVISE, REFLECT, REFINE (End-chapter Questions)
Q1. Differentiate between the following:
(i) Cell membrane vs Cell wall (permeability)
| Cell Membrane | Cell Wall |
|---|---|
| Selectively permeable — allows only certain substances | Fully permeable — water and dissolved minerals pass freely |
| Present in all cells | Present only in plant, fungal, and bacterial cells |
| Made of lipids and proteins | Made primarily of cellulose (in plants) |
| Thin, flexible | Thick, rigid |
(ii) RER vs SER (structure)
| RER (Rough ER) | SER (Smooth ER) |
|---|---|
| Has ribosomes attached on its surface | No ribosomes on surface |
| Appears rough under electron microscope | Appears smooth under electron microscope |
| Involved in protein synthesis and secretion | Involved in synthesis and storage of fats and hormones |
(iii) Chloroplasts vs Chromoplasts (pigments)
| Chloroplasts | Chromoplasts |
|---|---|
| Contain green pigment chlorophyll | Contain yellow, orange, or red pigments (not chlorophyll) |
| Found in green parts of plants | Found in flower petals and fruits |
| Responsible for photosynthesis | Responsible for bright colours attracting pollinators/animals |
Q2. Cell X in pure water swells; Cell Y in concentrated salt solution shrinks. Which statement is correct?
Answer: (iii) — Water moved into Cell X and moved out of Cell Y through the cell membrane.
Explanation: The cell membrane is selectively permeable and allows water (not salt/sugar molecules) to move by osmosis. Pure water (hypotonic outside) causes water to enter Cell X → it swells. Concentrated salt solution (hypertonic outside) causes water to leave Cell Y → it shrinks. Option (i) is wrong because salt molecules don’t move in. Option (ii) is partially wrong. Option (iv) is wrong because osmosis involves water movement, not solute movement.
Q3. Identify parts (a) to (g) in Fig. 2.20 (plant cell diagram) and match functions:
Based on a standard plant cell diagram with labels (a) through (g):
| Label | Organelle | Function |
|---|---|---|
| (a) | Chloroplast | (vii) Helps in manufacturing food |
| (b) | Nucleus | (i) Controlling all activities of a cell |
| (c) | Endoplasmic Reticulum | (transport network) |
| (d) | Cell Wall | (v) Provides structural rigidity |
| (e) | Cell Membrane | (iv) Separates cell contents from surroundings |
| (f) | Golgi Apparatus | (vi) Packs and stores materials received from ER |
| (g) | Vacuole | (iii) Storage organelle that also provides rigidity |
| Mitochondria | Mitochondria | (ii) Site of cellular respiration |
(Exact matching depends on the actual figure; match accordingly using the functions above.)
Q4. Which option correctly pairs cell organelles present in plant cells / absent in animal cells?
Answer: (i) — Leucoplast (present in plant cells) and Cell wall (absent in animal cells).
Explanation: Leucoplasts are plastids found only in plants. Cell wall is present in plants but absent in animals. Option (ii) is wrong — both mitochondria and ribosomes are present in animal cells. Option (iii) is wrong — Golgi apparatus is present in animal cells. Option (iv) is wrong — both lysosomes and ER are present in animal cells.
Q5. Renu says plastids are in all plant parts including roots. Rohit says roots have no plastids. Who is correct?
Rohit is partially correct, but Renu is more accurate overall.
Plastids are indeed present in root cells, but they are leucoplasts (colourless, storage plastids), not chloroplasts. Roots do not receive sunlight, so chloroplasts are absent. However, saying plastids are entirely absent in roots is incorrect. Leucoplasts in root cells store starch and other food materials. So Renu is correct that plastids are present in roots, but they are non-photosynthetic leucoplasts rather than chloroplasts.
Q6. Compare mitochondria and chloroplasts — similarities and differences:
Similarities:
- Both are double-membrane-bound organelles.
- Both contain their own DNA and ribosomes, so they can synthesise some proteins independently.
- Both are involved in energy-related processes.
- Both are rod-shaped.
- Both show evolutionary relationship with ancient bacteria (endosymbiotic theory).
Differences:
| Mitochondria | Chloroplasts |
|---|---|
| Found in both plant and animal cells | Found only in plant cells |
| Inner membrane folded into cristae | Has stroma with disc-shaped thylakoid membranes containing chlorophyll |
| Carries out cellular respiration — breaks down glucose to release ATP | Carries out photosynthesis — converts light energy into glucose |
| Uses oxygen, releases CO₂ | Uses CO₂, releases oxygen |
| Active in all living cells 24 hours | Active only in the presence of light |
Q7. Which pair of cell organelles contains DNA?
Answer: (ii) Mitochondria and Nucleus
Explanation: The nucleus contains chromosomal DNA carrying the cell’s genetic information. Mitochondria have their own circular DNA (inherited maternally). Chloroplasts also have DNA, but they are not listed here. Ribosomes, Golgi bodies, and lysosomes do not contain DNA.
Q8. Researcher’s carrot experiment (plain water vs concentrated salt solution):
(i) Hypothesis she wants to test:
The hypothesis is: Cell membranes are selectively permeable, and water moves by osmosis from a region of lower solute concentration to higher solute concentration. Specifically: a carrot in plain water will remain firm (water entry) while a carrot in salt solution will become limp (water loss).
(ii) Suggestions for improvement:
- Use carrots of identical size, shape, and weight to ensure a controlled experiment.
- Record initial and final weights to quantify water gain/loss.
- Specify exact concentration of salt solution for reproducibility.
- Use multiple trials and calculate average results.
- Keep temperature constant throughout the experiment.
(iii) Why does carrot in plain water stay stiff but carrot in salt solution become rubbery?
In plain water (hypotonic solution), water enters the carrot cells by osmosis (water concentration outside > inside), making cells turgid → carrot stays stiff and crunchy. In concentrated salt solution (hypertonic), water moves out of carrot cells by osmosis (solute concentration outside > inside), cells lose turgor pressure → carrot becomes flaccid, rubbery, and limp.
Q9. Presence/absence of structures in bacterial and animal cells:
| Structure | Bacterial Cell | Animal Cell |
|---|---|---|
| Chromosome | Present (circular, in nucleoid, no membrane) | Present (linear, enclosed in nucleus) |
| Nucleus | Absent (nucleoid present instead) | Present (well-defined, membrane-bound) |
| Mitochondria | Absent | Present |
| Golgi complex | Absent | Present |
| Chromoplasts | Absent | Absent |
Q10. Potato cup experiment (Cups A, B, C, D):
(i) Why does water gather in Cup B (sugar) and Cup C (salt)?
Sugar and salt create a concentrated solution inside the potato cup. The surrounding potato cells have a lower solute concentration inside. By osmosis, water moves from the potato cells (higher water concentration) into the hollow cup (lower water concentration / higher solute concentration). This is why water accumulates in the hollow of Cup B and Cup C.
(ii) Why is Cup A (empty) necessary?
Cup A is the control. It shows that water does not enter an empty potato cup by itself, confirming that the presence of solute (sugar/salt) is what drives water movement. Without a control, we cannot attribute the water gathering to osmosis specifically.
(iii) Why does water not gather in Cup A and Cup D?
- Cup A (empty): No solute inside the cup, so no concentration gradient exists. No osmosis occurs; water has no driving force to enter.
- Cup D (boiled potato with sugar): Boiling kills the cells and denatures the cell membrane proteins, destroying selective permeability. The cell membranes are no longer functional, so osmosis cannot occur even though sugar is present. Therefore, water does not accumulate.
Q11. Identify the incorrectly matched pair:
Answer: (ii) SER — Lipid and cellulose synthesis
Explanation: SER is involved in synthesis of lipids and hormones — not cellulose. Cellulose synthesis occurs at the cell wall level involving Golgi apparatus. Options (i) and (iii) are correct: ribosomes do synthesise proteins, and lysosomes do digest foreign agents and waste materials.
Q12. What if all mitochondria are removed from a eukaryotic cell?
- Mitochondria are the powerhouses that produce ATP through cellular respiration.
- Without mitochondria, ATP production would stop almost entirely.
- The cell would have no energy currency to carry out any activity — protein synthesis, active transport, movement, division, etc.
- The cell would quickly become non-functional and die.
- Some minimal anaerobic energy production (glycolysis in cytoplasm) might occur briefly, but it would be insufficient to sustain the cell.
Q13. Which phenomenon inhibits tumour formation in humans? Can plants develop tumours?
Phenomenon: Contact Inhibition. When normal animal cells come in contact with neighbouring cells, they stop dividing. This prevents uncontrolled growth and tumour formation. Cancer cells lose this property and keep dividing uncontrollably.
Can plants develop tumours? Yes, plants can develop tumours, but through a different mechanism. Plant cells do not show contact inhibition because their rigid cell walls regulate growth differently. A well-known example is Crown Gall Disease, caused by the bacterium Agrobacterium tumefaciens, which inserts its DNA into plant cells causing uncontrolled cell proliferation and tumour formation. So plants can have tumours, but the regulatory mechanism differs from animals.
Q14. Which organelles help synthesise cell membrane? Trace the path of synthesis:
Cell membrane is made of proteins and lipids.
Organelles involved:
- Ribosomes (on RER) — synthesise proteins
- SER — synthesises lipids
- Golgi apparatus — modifies, packages, and transports both
Path:
- Proteins are synthesised on ribosomes attached to RER.
- Lipids are synthesised in SER.
- Both are transported to the Golgi apparatus via vesicles.
- Golgi apparatus modifies, sorts, and packages them into vesicles.
- These vesicles move toward and fuse with the cell membrane, adding new lipid and protein components to it.
(Draw a labelled diagram showing: Ribosome → RER → Vesicle → Golgi → Vesicle → Cell membrane, with SER also contributing lipids to Golgi.)
Q15. What would happen if gametes are formed by mitotic divisions?
Normally, gametes are formed by meiosis, which halves the chromosome number. If gametes were formed by mitosis instead:
- Each gamete would have the full (diploid) chromosome number instead of half (haploid).
- Upon fertilisation, the zygote would have double the normal chromosome number (tetraploid).
- With each successive generation, chromosome numbers would keep doubling.
- This would cause severe genetic abnormalities, developmental disorders, and would disrupt the normal chromosome balance essential for proper gene function.
- Genetic diversity (normally created by meiosis through crossing over and random segregation) would also be eliminated.
Q16. Farmer Deepa’s amla and lemon preservation:
(i) Which scientific concept has she applied?
She has applied the concept of osmosis and hypertonic solutions. Adding high concentrations of salt, sugar, or jaggery creates a hypertonic environment. By osmosis, water is drawn out of the cells of microorganisms (bacteria, fungi) that cause spoilage, dehydrating and killing them, thereby preserving the produce.
(ii) How does high salt/sugar prevent microbial growth?
High concentrations of salt or sugar create a hypertonic external environment around microbial cells. Water moves out of the microbial cells by osmosis (plasmolysis), causing them to shrink and dehydrate. Without adequate water, enzymes of spoilage microorganisms cannot function, their metabolic processes stop, and they cannot grow or reproduce. This is called osmotic preservation.
(iii) Suggest a healthy preservation recipe:
Raw Mango Preserve (Aam ki launji): Take raw mango pieces, add jaggery (natural sweetener, rich in iron) and a small amount of salt. Cook gently until jaggery dissolves and coats the mango. Store in a clean glass jar. The high sugar/jaggery concentration preserves the fruit while providing nutritional value. Alternatively, lemon in salt brine (lemon pieces immersed in concentrated salt water) preserves lemons for months and serves as a healthy condiment.
(iv) Scientific values addressed:
- Observation and inquiry — noticing post-harvest loss and seeking solutions.
- Application of scientific knowledge — using osmosis practically.
- Sustainability — reducing food waste and environmental impact.
- Economic thinking — converting waste into value-added products.
- Traditional knowledge integration — combining indigenous methods with scientific understanding.
- Food security — ensuring produce reaches consumers rather than being wasted.