Chapter 2: Cell: The Building Block of Life

(i) Cell membrane vs Cell wall (Permeability): The cell membrane is selectively permeable — it allows only certain substances to pass through, controlling what enters and exits the cell. The cell wall is fully permeable — it allows most substances, including water and dissolved materials, to pass through freely without any selection.

(ii) RER vs SER (Structure): Rough Endoplasmic Reticulum (RER) has ribosomes attached on its outer surface, giving it a rough, granular appearance under the microscope; these ribosomes are involved in protein synthesis. Smooth Endoplasmic Reticulum (SER) has no ribosomes on its surface, giving it a smooth appearance; it is mainly involved in lipid and steroid synthesis.

(iii) Chloroplasts vs Chromoplasts (Pigments): Chloroplasts contain the green pigment chlorophyll (along with other pigments), which is responsible for photosynthesis in plant leaves. Chromoplasts contain coloured pigments other than chlorophyll, such as carotenoids (yellow, orange, red), which give colour to flowers and fruits but do not perform photosynthesis.

✔ Correct Answer: (iii)

Why (iii) is correct: Osmosis is the movement of water from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration) through a selectively permeable membrane. Pure water has a higher water concentration than the cell's contents, so water moves into Cell X, causing it to swell. The concentrated salt solution has a lower water concentration than the cell, so water moves out of Cell Y, causing it to shrink. This is the correct explanation for both observations.

Why other options are wrong:

• (i) Osmosis involves water movement, not salt molecule movement into the cell; salt molecules are too large to cause shrinkage by entering.
• (ii) This option incorrectly suggests salt solution also enters Cell Y; in osmosis, only water moves through the selectively permeable membrane, not the solute.
• (iv) Osmosis is specifically caused by water movement, not solute movement; solutes do not move through the selectively permeable membrane during osmosis.

Based on the typical plant cell diagram (Fig. 2.20), the labels and their matching functions are:

The large central vacuole in plant cells stores water and other materials and provides turgidity (rigidity) to the cell. The Golgi apparatus receives proteins/lipids from the ER, modifies, and packages them for secretion.

✔ Correct Answer: (i)

Why (i) is correct: Leucoplasts are plastids found only in plant cells (they store starch, oils, or proteins in non-photosynthetic plant parts) — so they are present in plant cells but absent in animal cells. The Cell wall is also a structure exclusive to plant cells (and bacteria/fungi) — it is present in plant cells but completely absent in animal cells. Both parts of option (i) are correctly placed.

Why other options are wrong:

• (ii) Mitochondria and ribosomes are present in both plant and animal cells, so they cannot be listed as absent in animal cells.
• (iii) While cell wall is correctly listed as present in plants and absent in animals, the Golgi apparatus is present in animal cells too, making this option incorrect.
• (iv) Both lysosomes and endoplasmic reticulum are present in animal cells, so they cannot be listed as absent in animal cells.

Rohit is correct. Plastids are broadly of three types — chloroplasts (green, for photosynthesis), chromoplasts (coloured, in flowers/fruits), and leucoplasts (colourless, for storage). Roots are underground and do not receive sunlight, so they do not contain chloroplasts or chromoplasts. However, roots do contain leucoplasts, which are colourless plastids that store starch, fats, or proteins, and these are indeed present in root cells. So Renu's claim that ALL plastids are present in roots is incorrect, but it is also wrong to say plastids are completely absent — leucoplasts are present in root cells. Rohit's reasoning that photosynthesis doesn't occur in roots is correct, but he incorrectly concluded that no plastids at all exist in roots.

Similarities between Mitochondria and Chloroplasts:

Both organelles have their own DNA and ribosomes, can produce some of their own proteins, and are involved in energy transformation in the cell.

Differences:

In summary, both organelles are energy-related, double-membraned, and semi-autonomous; but mitochondria release energy through respiration while chloroplasts capture energy through photosynthesis.

✔ Correct Answer: (ii)

Why (ii) is correct: The nucleus is the primary location of DNA in eukaryotic cells, containing the chromosomes made of DNA and proteins. Mitochondria are semi-autonomous organelles that contain their own circular DNA (mitochondrial DNA), which codes for some of their proteins. Both organelles are therefore correctly identified as containing DNA.

Why other options are wrong:

• (i) Chloroplasts do contain DNA, but ribosomes do not contain DNA — ribosomes are made of rRNA and proteins, not DNA.
• (iii) Neither Golgi bodies nor ribosomes contain DNA; Golgi bodies are packaging organelles and ribosomes are protein synthesis sites made of RNA.
• (iv) While the nucleus contains DNA, lysosomes do not contain DNA — they are membrane sacs containing digestive enzymes only.

(i) Hypothesis being tested: The researcher is testing whether the concentration of the surrounding solution affects the water content and firmness of carrot cells through the process of osmosis. Specifically, she hypothesises that a carrot placed in plain water will absorb water and remain firm, while a carrot in concentrated salt solution will lose water and become limp.

(ii) Suggested improvements:

• Use more carrots (multiple samples per condition) to ensure reliability and reduce experimental error.
• Include a control — a carrot kept in the air (no solution) to compare baseline changes.
• Measure and record the mass of each carrot before and after the experiment to quantify water gain or loss.
• Ensure both carrots are of equal size and from the same carrot for a fair comparison.
• Specify the exact concentration of the salt solution for reproducibility.

(iii) Explanation of observations: Carrot cells are surrounded by a selectively permeable membrane. In plain water, the water concentration outside is higher than inside the cells, so water enters the cells by osmosis, making them turgid (swollen) — the carrot stays stiff and crunchy. In concentrated salt solution, the water concentration outside is lower than inside the cells, so water moves out of the cells by osmosis, causing the cells to shrink (plasmolysis) — the carrot becomes rubbery and limp.

Bacteria are prokaryotes and lack all membrane-bound organelles; they have a nucleoid region (not a true nucleus) where their circular DNA is located. Chromoplasts are absent in both bacteria and animal cells because they are plastids found exclusively in plant cells.

(i) Why water gathers in Cup B and Cup C: Cup B contains sugar and Cup C contains salt — both create a high solute concentration inside the hollow of the potato cup. The surrounding water in the beaker has a lower solute concentration. Water moves into the potato cells by osmosis (from outside the potato into the potato), and from the potato cells into the hollowed portion where solute concentration is high. This net movement of water by osmosis causes water to accumulate inside Cup B and Cup C.

(ii) Why Cup A is necessary (Control): Cup A is the control of the experiment — it has no solute added and is made from a raw potato. It shows that without any solute, no water accumulates in the hollow. This helps us confirm that it is the presence of sugar/salt (solute) that causes water to gather in B and C, not any other factor. Without a control, we cannot attribute the observations in B and C specifically to osmosis.

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

• Cup A (empty, raw potato): There is no solute inside the hollow, so there is no osmotic gradient to draw water in — the water concentration inside and outside the cup is similar, so no net water movement occurs into the hollow.
• Cup D (boiled potato with sugar): Boiling kills the cells and denatures the cell membrane proteins, destroying the selectively permeable nature of the membrane. Since the membrane is no longer functional, osmosis cannot occur, and water does not accumulate despite the presence of sugar.

✔ Correct Answer: (ii) — This is the INCORRECT match

Why (ii) is the incorrect match: The Smooth Endoplasmic Reticulum (SER) is involved in the synthesis of lipids and steroids, but it does NOT synthesise cellulose. Cellulose (a carbohydrate/polysaccharide) is the main component of the plant cell wall and is synthesised by enzymes located at the cell membrane (plasma membrane), not by the SER. So the function listed for SER is partially wrong.

Why other options are correctly matched:

• (i) Ribosomes are indeed the sites of protein synthesis — they translate mRNA into proteins; this is correct.
• (iii) Lysosomes contain powerful digestive enzymes and are responsible for digesting foreign agents (like bacteria) and worn-out organelles through phagocytosis; this is correct.

Mitochondria are known as the powerhouse of the cell because they carry out aerobic cellular respiration, converting glucose and oxygen into ATP (adenosine triphosphate) — the primary energy currency of the cell. If all mitochondria are removed, the cell would lose its ability to produce ATP through aerobic respiration, causing a severe energy deficit. The cell might survive briefly on anaerobic respiration (which occurs in the cytoplasm and produces a small amount of ATP), but this is far less efficient and produces lactic acid as a by-product. Without adequate energy, all energy-dependent cellular processes — such as active transport, protein synthesis, cell division, and muscle contraction — would fail, ultimately leading to cell death.

The phenomenon that inhibits tumour formation in the human body is programmed cell death, also called apoptosis. In apoptosis, cells that are damaged, old, or potentially dangerous (such as cells with DNA mutations) are signalled to self-destruct in a controlled manner, preventing uncontrolled cell division (which leads to tumours/cancer). When apoptosis fails or is suppressed, cells can divide without control, forming tumours. Yes, plants can also develop tumours. A well-known example is crown gall disease, caused by the bacterium Agrobacterium tumefaciens, which inserts its DNA into plant cells, causing uncontrolled cell division and the formation of tumour-like growths called crown galls. Unlike animal tumours, plant tumours generally do not metastasize (spread to other parts), but they can still harm the plant significantly.

The cell membrane is composed of lipids and proteins. These components are synthesised by different organelles:

Site of synthesis:

• Lipids are synthesised by the Smooth Endoplasmic Reticulum (SER).
• Proteins are synthesised by ribosomes attached to the Rough Endoplasmic Reticulum (RER).

Pathway from synthesis to cell membrane:

1. Proteins are synthesised on ribosomes of the RER → transported through the RER lumen → packaged into vesicles → sent to the Golgi apparatus.
2. Lipids are synthesised in the SER → also transported via vesicles to the Golgi apparatus.
3. In the Golgi apparatus, proteins are modified (glycosylation, folding), sorted, and packaged into secretory vesicles along with lipids.
4. These vesicles travel to the cell membrane and fuse with it, incorporating proteins and lipids into the membrane — this is called exocytosis.

This pathway is: RER/SER → Golgi apparatus → Secretory vesicles → Cell membrane

Normally, gametes (sperm and egg) are formed by meiosis, which reduces the chromosome number by half (from diploid 2n to haploid n). This ensures that when two gametes fuse during fertilisation, the resulting zygote has the normal diploid chromosome number (2n). If gametes were formed by mitosis instead, they would retain the full diploid number (2n) of chromosomes. When such gametes fuse during fertilisation, the zygote would have double the normal chromosome number (4n), and with each successive generation, the chromosome number would keep doubling, leading to polyploidy. This would result in severe genetic abnormalities, developmental disorders, and would ultimately make sexual reproduction unsustainable for the species over generations.

(i) Scientific concept applied: Farmer Deepa has applied the concept of osmosis in food preservation. By adding high concentrations of salt or sugar to food, she creates a hypertonic environment around the microorganisms. Water moves out of bacterial/fungal cells by osmosis, causing them to lose water and die or become inactive, thereby preserving the food.

(ii) How high salt/sugar prevents microbial growth: When food is surrounded by a highly concentrated salt or sugar solution, the water concentration outside the microbial cells (bacteria and fungi) is much lower than inside them. As a result, water moves out of the microbial cells by osmosis, causing the cells to shrink and undergo plasmolysis (the cell membrane pulls away from the cell wall). This dehydration prevents the microorganisms from growing, reproducing, or causing spoilage. The food is thus preserved naturally without artificial chemicals.

(iii) Healthy recipe for food preservation: Amla Murabba (Indian Gooseberry Preserve):

• Wash and prick amla fruits all over with a fork.
• Boil briefly, then soak in a concentrated sugar syrup (made by dissolving 1 kg sugar in 500 mL water).
• Add cardamom and saffron for flavour.
• Store in sterilised glass jars. The high sugar concentration preserves the amla through osmosis while retaining its Vitamin C content.

(iv) Scientific values addressed:

• Sustainability: Reducing post-harvest food waste by converting perishable produce into preserved products.
• Application of science: Using the scientific principle of osmosis for practical, everyday food preservation.
• Entrepreneurship and economic empowerment: Transforming surplus farm produce into value-added products to increase income.
• Food security: Ensuring availability of nutritious food beyond the harvest season, contributing to community food security.