Cell :The Building Block of Life — NCERT Solutions

Normal cells grow, age and die in a controlled manner. Sometimes this system breaks down, and abnormal cells start growing and dividing uncontrollably. This leads to the formation of tumours, which may be benign or malignant. Cancerous tumours can invade nearby tissues and even spread to other parts of the body to form new tumours.

The two cells are not similar in structure. The chapter says onion peel or Rhoeo leaf peel cells are box-shaped and regularly arranged, whereas cheek cells are irregularly arranged.

Yes. The chapter states that onion peel cells or Rhoeo leaf peel cells are box-shaped and regularly arranged, whereas cheek cells are irregularly arranged.

This difference exists because plant cells have a rigid cell wall outside the cell membrane, which gives them a regular box-like shape. Cheek cells are animal cells and do not have a cell wall, so they can change shape easily and appear irregularly arranged.

The chapter says cell division occurs in stages and asks which stage comes first. The first stage before division is interphase.

• Cell membrane: selectively permeable; allows some substances to pass, blocks others.
• Cell wall: permeable; water and some dissolved minerals can pass through it.
• RER: rough because ribosomes are attached; mainly involved in protein synthesis and secretion.
• SER: smooth because ribosomes are absent; involved in synthesis and storage of fats and hormones.
• Chloroplasts: contain chlorophyll.
• Chromoplasts: contain pigments other than chlorophyll, such as yellow, orange or red pigments.

Cell X is in pure water, so water moves into it by osmosis, making it swell. Cell Y is in concentrated salt solution, so water moves out of it by osmosis, making it shrink. Therefore, the correct statement is (iii).

From the chapter:

• Leucoplast is present in plant cells and cell wall is absent in animal cells, so (i) is correct.
• Mitochondria and ribosomes are present in both plant and animal cells, so (ii) is incorrect.
• Cell wall is present in plant cells and Golgi apparatus is present in both, so (iii) is correct.
• Lysosome is not absent in animal cells, and ER is present in both, so (iv) is incorrect.

Thus, the correct options are (i) and (iii).

Renu is correct. The chapter says that plastids are present in plant cells, and some plastids are found even in root cells as leucoplasts, which store food materials like starch, oils or proteins. Roots may not do photosynthesis, but they still contain plastids for storage. So Rohit is not correct in saying roots do not have plastids just because they are underground.

Similarities:

• Both mitochondria and chloroplasts are double-membrane-bound organelles.
• Both have their own DNA and ribosomes.
• Both can make some of their own proteins.

Differences:

• Mitochondria are the powerhouses of the cell; they release energy during cellular respiration and store it as ATP.
• Chloroplasts are found in plant cells and are the site of photosynthesis; they contain chlorophyll and help make food.
• Mitochondria are present in both plant and animal cells, while chloroplasts are present in plant cells.

So, they are structurally similar in being double-membraned and having their own genetic material, but functionally one releases energy and the other prepares food.

The chapter says that mitochondria have their own DNA and that the nucleus contains DNA in chromosomes. Therefore, the correct pair is (ii).

The experiment shows the effect of osmosis on plant cells.

(i) The researcher wants to test the hypothesis that water moves into plant cells in a hypotonic medium and moves out in a hypertonic medium.
(ii) The experiment can be improved by taking more than one carrot for each condition, keeping the carrots of exactly equal size, measuring mass before and after, and controlling the time and temperature carefully.
(iii) The carrot in plain water stays stiff and crunchy because water enters its cells by osmosis, making them turgid. The carrot in concentrated salt solution becomes rubbery and limp because water leaves its cells by osmosis, so the cells lose firmness.

For the table:

• Chromosome: present in bacterial cell; present in animal cell
• Nucleus: absent in bacterial cell; present in animal cell
• Mitochondria: absent in bacterial cell; present in animal cell
• Golgi complex: absent in bacterial cell; present in animal cell
• Chromoplasts: absent in bacterial cell; absent in animal cell

The incorrect match is SER — Lipid and cellulose synthesis. The chapter says SER is involved in the synthesis and storage of fats and hormones, not cellulose. So (ii) is incorrect.

If all the mitochondria are removed from a eukaryotic cell, the cell will not be able to carry out cellular respiration properly and will not produce enough ATP. Since ATP is the energy currency of the cell, the cell would fail to perform its normal activities and may die.

The phenomenon that inhibits tumour formation in many animal cells is contact inhibition. In the chapter, it is stated that when animal cells touch neighbouring cells, cell division usually stops. Plants can also develop tumours, but plant cells behave differently because of their rigid cell wall and they do not show contact inhibition in the same way. Cancerous growth can still occur in plants, though the chapter mainly discusses tumours in animals and the different growth pattern of plant cells.

The organelles that help in the synthesis of the cell membrane are the endoplasmic reticulum and the Golgi apparatus.

Path:

• Proteins are made by ribosomes on the RER.
• SER helps in lipid synthesis.
• These materials move through the ER.
• They are modified, sorted and packaged by the Golgi apparatus.
• Vesicles carry them to the cell membrane and help in membrane formation.

So, the path is: ER → Golgi apparatus → vesicles → cell membrane. A labelled diagram should show the nucleus connected to ER, ER to Golgi, and Golgi to membrane via vesicles.

If gametes were formed by mitotic divisions instead of meiosis, the gametes would have the same number of chromosomes as body cells. After fertilisation, the chromosome number would keep doubling in every generation. This would disturb the stability of the species and prevent proper sexual reproduction and genetic balance.

(i) The farmer applied the scientific concept of osmosis and food preservation by high solute concentration.

(ii) High concentrations of salt, sugar or jaggery make the surroundings hypertonic for bacteria and fungi. Water moves out of their cells by osmosis, so their cells shrink and they cannot grow properly.

(iii) A healthy recipe could be fruit pieces preserved with less sugar and more natural ingredients, such as a lightly sweetened lemon or amla preserve using a moderate amount of sugar and hygienic storage.

(iv) The scientific values shown are use of traditional knowledge, innovation, value addition, food preservation, waste reduction, entrepreneurship, and support for food security and local economy.

The cell membrane allows only some substances to pass through it, so it is selectively permeable. The cell wall lets water and some dissolved minerals pass through it, so it is permeable.

Rough Endoplasmic Reticulum (RER) looks rough because ribosomes are attached to its surface, while Smooth Endoplasmic Reticulum (SER) has no ribosomes on its surface, so it looks smooth.

Chloroplasts are green plastids because they contain chlorophyll. Chromoplasts contain pigments other than chlorophyll, such as yellow, orange or red pigments, which give bright colours to flowers and fruits.

Cell X is in pure water, so water enters the cell by osmosis and it swells. Cell Y is in a concentrated salt solution, so water moves out of the cell by osmosis and it shrinks. Therefore, the correct explanation is that water moved into Cell X and moved out of Cell Y through the cell membrane.

(i) She wants to test the hypothesis that water moves into or out of carrot cells by osmosis depending on the concentration of the surrounding solution.

(ii) To improve the experiment, she should use carrots of the same size and initial weight, keep equal volumes of water and salt solution, and maintain the same time and conditions for both samples.

(iii) The carrot in plain water stays stiff and crunchy because its cells absorb water by osmosis, so they become turgid. The carrot in concentrated salt solution becomes rubbery and limp because water moves out of its cells by osmosis, so the cells lose turgidity and become flaccid.

(i) The farmer has applied the concept of osmosis.

(ii) A high concentration of salt or sugar makes the surrounding medium hypertonic. Water moves out of the cells of bacteria and fungi by osmosis, so they lose water and cannot grow properly. This prevents spoilage.

(iii) One healthy recipe is fruit preserve with a controlled amount of sugar and no extra artificial additives, or pickles made with moderate salt and clean hygienic preparation.

(iv) The scientific values shown are innovation, scientific temper, food preservation, problem-solving, reduction of waste, and entrepreneurship for food security and livelihood.

Cell X in pure water swells because water enters by osmosis. Cell Y in concentrated salt solution shrinks because water moves out by osmosis. So the correct statement is that water moved into Cell X and moved out of Cell Y through the cell membrane.

From the chapter, the likely organelles and functions are:

• Nucleus — controlling all the activities of a cell
• Mitochondrion — site of cellular respiration
• Vacuole — storage organelle that also provides rigidity to the cell
• Cell membrane — separates the cell contents from surroundings
• Cell wall — provides structural rigidity to the cell
• Golgi apparatus — packs and stores materials received from ER
• Chloroplast — helps in manufacturing food

If the labelled figure is shared, the exact matching can be done.

She wants to test the hypothesis that placing carrot cells in different solutions changes their water content by osmosis—carrots in plain water gain water, while carrots in concentrated salt solution lose water.

To improve the experiment, use carrots of equal size and initial weight, keep the same amount of liquid in both setups, and maintain the same duration and temperature for fair comparison.

The carrot in plain water stays stiff and crunchy because its cells absorb water by osmosis and become turgid. The carrot in concentrated salt solution becomes rubbery and limp because water moves out of its cells by osmosis, making them flaccid.