Answer: Plants are stationary and need a rigid structure to withstand environmental stresses like wind and rain. The cell wall provides this rigidity and helps plants stay upright. Animals, being mobile, need flexibility to move. Animal cells lack a cell wall, allowing them to change shape easily and support overall movement.

Answer: If the cell wall became flexible, plant cells would not be able to maintain their shape. The plant would lose its rigidity, leaves and stems would droop, and the plant would be unable to stand upright against gravity and wind. The cell wall's rigidity is essential for structural support.

Answer: Cutting equal-sized pieces and measuring initial weight ensures a fair comparison. Any change in weight can then be attributed to osmosis, not to differences in initial mass or surface area. This makes the experiment valid and reliable.

Answer: White flowers do not contain pigments like chlorophyll or chromoplasts. The white colour is due to the reflection of all wavelengths of light from air spaces within the petal cells. They may contain leucoplasts (colourless plastids) but no coloured pigments.

Answer: [Diagram to be drawn by student with labelled parts: Nucleus (dark round body), ER (network extending from nuclear envelope), Mitochondria (rod-shaped), Chloroplasts (rod-shaped in plant cells), and other organelles as shown in Fig. 2.10 of the textbook.]

Answer: A single giant mitochondrion would have a lower surface area-to-volume ratio, reducing efficiency of nutrient exchange and energy production. Many small mitochondria provide a larger total surface area for cellular respiration, allowing faster diffusion of oxygen and glucose, and more efficient ATP production.

Answer: If skin cells divided by meiosis instead of mitosis, the cut would not heal properly. Meiosis produces four genetically different daughter cells with half the chromosome number, not identical cells needed for repair. The wound would not be closed, and normal tissue regeneration would fail.

Answer:
(i) Cell membrane vs Cell wall: Cell membrane is selectively permeable; allows only certain substances to pass. Cell wall is fully permeable; allows water and dissolved substances to pass freely.
(ii) RER vs SER: Rough Endoplasmic Reticulum (RER) has ribosomes attached to its surface; looks rough under microscope. Smooth Endoplasmic Reticulum (SER) lacks ribosomes; looks smooth.
(iii) Chloroplasts vs Chromoplasts: Chloroplasts contain green pigment chlorophyll for photosynthesis. Chromoplasts contain yellow, orange, or red pigments for flower and fruit colouration.

Answer: (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane. Pure water is hypotonic, so water enters cell X by osmosis. Concentrated salt solution is hypertonic, so water leaves cell Y by osmosis.

Answer:
(a) Nucleus – Controlling all the activities of a cell.
(b) Mitochondrion – Site of cellular respiration.
(c) Vacuole – Storage organelle that also provides rigidity to the cell.
(d) Cell membrane – Separates the cell contents from surroundings.
(e) Cell wall – Provides structural rigidity to the cell (in plant cells).
(f) Golgi apparatus – Packs and stores materials received from ER.
(g) Chloroplast – Helps in manufacturing food.

Answer: (i) Leucoplast – Cell wall. Both are present in plant cells but absent in animal cells.

Answer: Renu is correct. All plant cells contain plastids. Roots contain leucoplasts (colourless plastids) that store starch, oils, or proteins. They do not contain chloroplasts because roots are underground and do not perform photosynthesis, but they still have plastids.

Answer:
Similarities: Both are double-membrane bound organelles, have their own DNA and ribosomes, and can synthesise some of their own proteins. They are semi-autonomous organelles.
Differences: Mitochondria are found in all eukaryotic cells and produce energy (ATP) through cellular respiration. Chloroplasts are found only in plant cells and perform photosynthesis, converting light energy into chemical energy (glucose).

Answer: (ii) Mitochondria, Nucleus. Both contain DNA – nucleus contains nuclear DNA, mitochondria contain mitochondrial DNA.

Answer:
(i) What hypothesis does she want to test through this experiment? The researcher wants to test whether water moves into or out of plant cells by osmosis when placed in solutions of different concentrations.
(ii) What would you suggest for the improvement of this experiment? Use three carrots instead of two (plain water, salt solution, and a control with no treatment). Also measure the weight and firmness before and after to quantify changes.
(iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp? In plain water (hypotonic), water enters the cells by osmosis, making them turgid and firm. In concentrated salt solution (hypertonic), water leaves the cells, making them flaccid and limp.

Structures in a cell	Bacterial cell	Animal cell
Chromosome	Present (single circular)	Present (multiple linear)
Nucleus	Absent (nucleoid region)	Present
Mitochondria	Absent	Present
Golgi complex	Absent	Present
Chromoplasts	Absent	Absent

Answer: Cup A is empty, so no concentration gradient exists to drive osmosis. Cup D is made from boiled potato, in which the cells are dead and their membranes are denatured. Since osmosis requires living, selectively permeable membranes, no water movement occurs in Cup D.

Answer: (ii) SER — Lipid and cellulose synthesis. SER synthesises lipids but not cellulose; cellulose is synthesised by the cell membrane.

Answer: The cell would not be able to produce sufficient ATP (energy). Cellular respiration would stop, and the cell would eventually die due to lack of energy for essential functions.

Answer: Contact inhibition inhibits tumour formation in humans. When normal cells come in contact with neighbouring cells, they stop dividing. Cancer cells lose this control. Plants can develop tumours (galls) caused by bacteria, fungi, or insects, but they do not have contact inhibition due to their rigid cell walls.

Answer: Ribosomes (attached to RER) synthesise proteins. SER synthesises lipids. These are transported to the Golgi apparatus for modification and packaging into vesicles, which then fuse with the cell membrane to deliver their contents.

Answer: If gametes were formed by mitosis, they would have the same number of chromosomes as the parent cell. Upon fertilisation, the chromosome number would double in each generation, leading to genetic instability and likely preventing normal development.

Answer:
(i) Which scientific concept has the farmer applied? Osmosis – preservation of food using high concentrations of salt or sugar.
(ii) How does the addition of high concentrations of salt and sugar prevent growth of spoilage-causing bacteria and fungi? High salt/sugar concentration creates a hypertonic environment, causing water to leave microbial cells by osmosis. This dehydrates and kills them, preventing spoilage.
(iii) Suggest a healthy recipe of this kind for food preservation. Lemon pickle: Cut lemons, add salt and spices, store in a glass jar. The salt prevents microbial growth.
(iv) What are the scientific values addressed in this case? Application of scientific knowledge (osmosis) for food preservation, reducing waste, sustainable farming, and boosting local economy.