Chapter 2: Cell: The Building Block of Life (Biology)

(i) Cell membrane is selectively permeable; cell wall is freely permeable. (ii) RER has ribosomes on it; SER has none. (iii) Chloroplasts have green chlorophyll; chromoplasts have other coloured pigments (carotenoids).

Answer: (iii).

This is osmosis: water moves through the selectively permeable cell membrane from a region of higher water concentration to lower water concentration.

• Pure water outside Cell X has more water than the cell's contents (a hypotonic solution), so water enters Cell X and it swells.
• The concentrated salt solution outside Cell Y has less water than the cell (a hypertonic solution), so water leaves Cell Y and it shrinks.

The salt itself does not need to move; it is the movement of water that causes the changes.

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

Fig. 2.20 is a plant cell.

(a) mitochondrion–(ii), (b) nucleus–(i), (c) Golgi apparatus–(vi), (d) chloroplast–(vii), (e) cell membrane–(iv), (f) cell wall–(v), (g) vacuole–(iii).

Answer: (i).

• (i) Leucoplasts (colourless plastids) are present in plant cells, and the cell wall is absent in animal cells. ✓
• (ii) Ribosomes are present in animal cells. ✗
• (iii) Golgi apparatus is present in animal cells. ✗
• (iv) Endoplasmic reticulum is present in animal cells (and lysosomes are rare in plant cells, whose vacuoles do much of this work). ✗

Only option (i) is correct.

Renu is correct.

There are three types of plastids, and not all of them are for photosynthesis:

• Chloroplasts (green, with chlorophyll) carry out photosynthesis and are found in leaves and green stems.
• Chromoplasts (coloured) give colour to flowers and fruits.
• Leucoplasts (colourless) store food such as starch, oils and proteins.

Roots do not need chloroplasts, but their cells do contain leucoplasts that store starch (e.g. in the roots of carrot, radish, sweet potato). So plastids are present in roots too.

Renu is correct: roots lack chloroplasts but contain colourless leucoplasts that store starch; plastids are not only for photosynthesis.

Similarities:

• Both are bounded by a double membrane (two membranes).
• Both have their own DNA and ribosomes, so they can make some of their own proteins.
• Both are concerned with energy: they convert energy from one form into another.
• Both can divide on their own within the cell.

Differences:

Both have double membranes, their own DNA and ribosomes, and handle energy. Mitochondria (in all eukaryotic cells, with cristae) release energy from food by respiration; chloroplasts (only in plants, with chlorophyll and grana) trap light energy to make food.

Answer: (ii) Mitochondria, Nucleus.

The nucleus contains most of the cell's DNA (in chromosomes), and mitochondria have their own small DNA. Ribosomes, Golgi bodies and lysosomes do not contain DNA. (Chloroplasts also have DNA, but ribosomes do not, so option (i) is wrong.)

(ii) Mitochondria and nucleus.

(i) Hypothesis: "Water moves into or out of plant cells by osmosis depending on the concentration of the surrounding solution", i.e. carrot cells gain water in plain water and lose water in a concentrated salt solution.

(ii) Improvements:

• Use carrot pieces of the same size and mass, and weigh them (or measure their length) before and after, to get measurable results.
• Use several pieces in each set-up (repeat the experiment) for reliable results.
• Keep all other conditions the same (temperature, volume of liquid, time).
• Add more set-ups with different salt concentrations to see the trend.

(iii) In plain water, water enters the carrot cells by endosmosis; the cells become turgid (swollen and firm), pressing against the cell walls, so the carrot stays stiff and crunchy. In the concentrated salt solution, water leaves the cells by exosmosis; the cells lose turgor and the cell contents shrink away from the cell wall (plasmolysis), so the carrot becomes soft, rubbery and limp.

(i) Water moves in or out of cells by osmosis depending on the outside solution. (ii) Equal-sized pieces, weighing before and after, repeated trials and controlled conditions. (iii) In water the cells absorb water and become turgid (crunchy); in strong salt solution they lose water and become plasmolysed (limp).

Bacteria have a chromosome (in a nucleoid) but no nucleus, mitochondria, Golgi complex or chromoplasts; animal cells have a chromosome, nucleus, mitochondria and Golgi complex but no chromoplasts.

(i) The sugar (Cup B) and salt (Cup C) dissolve in the little moisture inside the cup and form a concentrated solution. The living potato cells act as a selectively permeable membrane. Water moves from the beaker (higher water concentration) through the potato cells into the hollow (lower water concentration) by osmosis, so water collects in the cups.

(ii) Cup A is the control. It is identical except that nothing is added. It shows that water does not collect just because the potato is placed in water, so the result in B and C is due to the sugar/salt, i.e. to osmosis.

(iii) Cup A: there is no solute in the hollow, so there is no difference in concentration and no net movement of water into it. Cup D: boiling kills the potato cells; dead cells lose their selectively permeable membranes, so osmosis cannot take place even though sugar is present.

(i) A concentrated sugar/salt solution forms in B and C, so water enters through the living potato cells by osmosis. (ii) Cup A is the control for comparison. (iii) A has no concentration difference; D has dead cells (boiled) that cannot carry out osmosis.

Answer: (ii) SER — Lipid and cellulose synthesis is incorrect.

The SER makes lipids (fats, and parts of the cell membrane) and helps detoxify poisons, but it does not make cellulose; cellulose for the plant cell wall is made outside the cell membrane, with the Golgi apparatus supplying other wall materials. Pairs (i) (ribosomes make proteins) and (iii) (lysosomes digest foreign material and worn-out parts) are correctly matched.

(Note: the answer key printed in the book lists (iii); but lysosomes do digest foreign agents such as bacteria, so the incorrect match is (ii).)

(ii) is incorrect: the SER makes lipids, not cellulose.

Mitochondria are the sites of aerobic respiration, where food (glucose) is broken down using oxygen to release most of the cell's energy as ATP. Without mitochondria:

• The cell could get only a little energy through anaerobic breakdown of glucose in the cytoplasm.
• Energy-requiring activities (making proteins, transport across membranes, cell division, movement) would slow down or stop.
• The cell would not be able to survive for long and would die.

(Mitochondria cannot be made from scratch, because they arise only by division of existing mitochondria.)

The cell would lose its main source of energy (aerobic respiration), its activities would stop, and it would eventually die.

Controlled cell division and programmed cell death. In the body, mitosis is tightly controlled: cells divide only when needed and stop when there is contact with neighbouring cells. Damaged or unwanted cells are removed by programmed cell death (apoptosis). When this control fails, cells divide uncontrollably and form a lump, a tumour (and if it spreads, cancer).

Yes, plants can develop tumours. For example, crown gall disease is caused by the bacterium Agrobacterium tumefaciens: it transfers some of its genes into plant cells, making them divide uncontrollably and form a swelling (gall) on the stem or root. Insects and fungi can also cause galls. However, plant tumours do not spread through the body as animal cancers do, because plant cells are held in place by rigid cell walls.

Controlled cell division (regulation of mitosis) and programmed cell death prevent tumours. Plants can form tumours too (e.g. crown gall caused by Agrobacterium), but they do not spread because cell walls hold cells in place.

Organelles involved:

• Ribosomes on the RER make the membrane proteins.
• SER makes the membrane lipids (phospholipids).
• The Golgi apparatus modifies, packs and sends them.
• Vesicles carry them to the cell membrane.

Path: RER (proteins) and SER (lipids) → transport vesicles → Golgi apparatus (modified and packed) → secretory vesicles → fuse with the cell membrane, adding the new proteins and lipids to it. (This process of making new membrane is called membrane biogenesis.)

Ribosomes on the RER make proteins and the SER makes lipids; they travel in vesicles to the Golgi apparatus, are packed into vesicles and delivered to the cell membrane (membrane biogenesis).

• In mitosis, daughter cells have the same number of chromosomes as the parent cell (diploid, 46 in humans).
• Gametes are normally formed by meiosis, which halves the chromosome number (haploid, 23 in humans), so that when a sperm and an egg fuse, the zygote gets the normal number (23 + 23 = 46).
• If gametes were formed by mitosis, each gamete would have 46 chromosomes, and the zygote would have 92. The chromosome number would double in every generation (46 → 92 → 184 ...).
• This would upset the genetic material, and the offspring would not develop normally or survive. Meiosis also creates variation, which would be lost.

The gametes would be diploid, so the zygote would get double the chromosome number, and the number would keep doubling every generation, leading to abnormal or non-viable offspring.

(i) Osmosis (using salt and sugar as preservatives).

(ii) A high concentration of salt or sugar makes the surroundings more concentrated (hypertonic) than the cell sap of bacteria and fungi. Water moves out of the microbial cells by osmosis, so they become dehydrated (plasmolysed). Without water they cannot grow or multiply, and the food does not spoil. The salt or sugar also draws water out of the fruit pieces, leaving less free water for microbes.

(iii) Example: amla candy or jaggery-based amla murabba with less sugar. Wash and steam amla, cut into pieces, coat with jaggery (or a little sugar) and dry in the sun for a few days until dry; store in clean, dry glass jars. Or lemon pickle with salt, turmeric and little oil: cut lemons, mix with salt and turmeric, keep in a sun-dried glass jar in sunlight for 2–3 weeks. (Use moderate salt and sugar for health.)

(iv) Scientific values: applying a scientific principle (osmosis) to a real-life problem; reducing food wastage and post-harvest losses; scientific and traditional knowledge working together; sustainability and food security; entrepreneurship and economic thinking (adding value to produce, agro-processing); hygiene and safe food handling.

(i) Osmosis. (ii) Concentrated salt/sugar draws water out of microbial cells by osmosis, so they dry up and cannot grow. (iii) e.g. sun-dried amla with jaggery, or salt-and-turmeric lemon pickle. (iv) Application of science to daily life, reducing wastage, sustainability, food security and enterprise.