Class 9 Science · Chapter 2

2. Cell: The Building Block of Life

Complete solutions to Revise, Reflect, Refine. CBSE Class 9 Science (NCERT Exploration).

Revise, Reflect, Refine

1Differentiate between the following pairs of terms based on the clues given in parentheses.

(i) Cell membrane and cell wall (permeability)

Cell membrane	Cell wall
It is selectively permeable.	It is freely (fully) permeable.
It allows only some substances to enter or leave the cell.	It allows water and dissolved substances to pass through easily.

(ii) RER and SER (structure)

RER	SER
Ribosomes are attached on its surface.	Ribosomes are absent on its surface.
It looks rough under the electron microscope.	It looks smooth.

(iii) Chloroplasts and chromoplasts (pigments)

Chloroplasts	Chromoplasts
They contain the green pigment chlorophyll.	They contain coloured pigments other than green, like carotenoids.
They give green colour to leaves and help in photosynthesis.	They give yellow, orange and red colours to flowers and fruits.

2Two similar animal cells are placed in two different solutions. Cell X is placed in pure water. Cell Y is placed in a concentrated salt solution. After some time, Cell X swells, and Cell Y shrinks. Which statement provides the correct explanation for the above observations?

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

This is osmosis. Water moves from a dilute solution to a concentrated solution through the cell membrane. Pure water is more dilute than the cell sap, so water enters Cell X. The salt solution is more concentrated, so water leaves Cell Y.

3Look at the diagram of a cell in Fig. 2.20. Identify the parts labelled from (a) to (g) and correctly match them with their functions.

Label	Part	Function
(a)	Mitochondrion	(ii) Site of cellular respiration
(b)	Nucleus	(i) Controlling all the activities of a cell
(c)	Golgi apparatus	(vi) Packs and stores materials received from ER
(d)	Chloroplast	(vii) Helps in manufacturing food
(e)	Cell wall	(v) Provides structural rigidity to the cell
(f)	Cell membrane	(iv) Separates the cell contents from surroundings
(g)	Vacuole	(iii) Storage organelle that also provides rigidity to the cell

4Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories? (Present in the plant cells / Absent in the animal cells)

Answer: (i) Leucoplast — Cell wall

Leucoplasts are present in plant cells. The cell wall is absent in animal cells. In the other options, ribosomes, Golgi apparatus and endoplasmic reticulum are present in animal cells too.

5Renu said that all parts of the plants, even roots, contain plastids. Rohit said plastids are absent in plant roots since roots are underground and do not need to perform photosynthesis. Who is correct? Justify your answer.

Renu is correct.

Roots do not have chloroplasts, but they have leucoplasts. Leucoplasts are colourless plastids. They store food such as starch, oils and proteins. For example, the roots of carrot and radish store food. So plastids are present in roots too.

6Mitochondria and chloroplasts are two important organelles in a plant cell. Discuss how these two organelles are structurally and functionally similar to each other, and different from each other.

Similarities:

Both are bounded by a double membrane.
Both have their own DNA and ribosomes.
Both can make some of their own proteins.
Both are related to energy in the cell.

Differences:

Mitochondria	Chloroplasts
Found in both plant and animal cells.	Found only in green plant cells.
Inner membrane is folded (cristae).	Have stacks of membranes (grana) containing chlorophyll.
No pigment.	Contain green pigment chlorophyll.
Release energy from food by cellular respiration.	Trap light energy to make food by photosynthesis.

7Which of the following pairs of cell organelles contains DNA?

Answer: (ii) Mitochondria, Nucleus

The nucleus contains DNA in chromosomes. Mitochondria have their own DNA. Ribosomes, Golgi bodies and lysosomes do not contain DNA.

8A researcher took two carrots of similar size. She placed one carrot in plain water and the other in concentrated salt solution (Fig. 2.21). After 24 hours she recorded her observations.

(i) What hypothesis does she want to test through this experiment?

Water moves into or out of plant cells by osmosis, depending on the concentration of the solution around them.

(ii) What would you suggest for the improvement of this experiment?

Use carrot pieces of the same mass and size, and weigh them before and after.
Use the same volume of liquid at the same temperature.
Use several carrots in each set and repeat the experiment.
Add a third set with sugar solution or a weaker salt solution for comparison.

(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, water enters the carrot cells by osmosis. The cells become swollen and stiff. So the carrot stays crunchy.

In salt solution, water comes out of the cells by osmosis. The cells shrink and lose their stiffness. So the carrot becomes rubbery and limp.

9Indicate the presence or absence of the following structures in bacterial and animal cells.

Structures in a cell	Bacterial cell	Animal cell
Chromosome	Present (single, in nucleoid)	Present
Nucleus	Absent	Present
Mitochondria	Absent	Present
Golgi complex	Absent	Present
Chromoplasts	Absent	Absent

10Potato cup experiment (Fig. 2.22): Cup A empty; Cup B with sugar; Cup C with salt; Cup D (boiled potato) with sugar. All cups are kept in a beaker of water.

(i) Explain why water gathers in the hollowed portion of Cup B and Cup C.

Sugar and salt make a concentrated solution in the hollow. The potato cells are living and act like a selectively permeable membrane. So water moves from the beaker into the hollow by osmosis.

(ii) Why is Cup A necessary for this experiment?

Cup A is the control. It shows that water does not gather on its own. It proves that sugar or salt is needed for osmosis to take place.

(iii) Explain why water does not gather in the hollowed portions of Cups A and D.

In Cup A, there is no concentration difference, so there is no osmosis.

In Cup D, the potato is boiled. Its cells are dead and their membranes are damaged. Dead cells cannot carry out osmosis. So water does not gather.

11Identify the pair that incorrectly matches the cell organelle with its function.

Answer: (ii) SER — Lipid and cellulose synthesis

SER makes lipids (fats) and hormones. It does not make cellulose. Ribosomes make proteins, and lysosomes digest foreign agents, so (i) and (iii) are correct.

12What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?

Mitochondria release energy from food by cellular respiration. Without them, the cell will get very little energy. It cannot carry out its activities like making proteins, transport and cell division. The cell will soon stop working and die.

13Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.

Contact inhibition prevents tumours. In animal cells, cell division stops when cells touch the neighbouring cells. Cancer cells lose this control and keep dividing, which forms tumours. Programmed cell death also removes unwanted cells.

Yes, plants can develop tumour-like growths (galls), for example due to some bacteria or insects. Plant cells have rigid cell walls and do not show contact inhibition. Their cells cannot move and spread to other parts. So plant tumours stay at one place and do not spread like animal cancers.

14The cell membrane is made up of proteins and lipids. Which cell organelles help in the synthesis of cell membrane? Write the path of these compounds from their site of synthesis to the cell membrane and show this through a labelled diagram.

Organelles involved: rough endoplasmic reticulum (RER), smooth endoplasmic reticulum (SER) and Golgi apparatus.

Proteins are made by ribosomes on the RER.
Lipids are made by the SER.
Vesicles carry them to the Golgi apparatus.
The Golgi apparatus modifies and packs them into vesicles.
These vesicles move to the cell membrane and fuse with it.

Path of membrane proteins and lipids: ER → vesicles → Golgi apparatus → vesicles → cell membrane.

15What would happen if gametes are formed by mitotic divisions?

Mitosis does not reduce the number of chromosomes. So each gamete would have 46 chromosomes instead of 23. On fertilisation, the zygote would have 92 chromosomes. The chromosome number would double in every generation. This would cause abnormal growth, and the organism may not survive. That is why gametes are formed by meiosis.

16Farmer Deepa used salt, sugar or jaggery to make pickles, murabbas and sharbat from her extra amla and lemons, to prevent wastage of post-harvest produce. Answer the following questions.

(i) Which scientific concept has the farmer applied in the preservation of the farm produce?

Osmosis

(ii) How does the addition of high concentrations of salt and sugar create an environment that prevents the growth of spoilage-causing bacteria and fungi?

The concentration of salt or sugar outside is higher than the cell sap of bacteria and fungi. So water comes out of their cells by osmosis. The microbes lose water, shrink and cannot grow. So the food does not spoil.

(iii) Suggest a healthy recipe of this kind for food preservation.

Amla murabba with jaggery: boil amla pieces lightly, add jaggery syrup with a little cardamom and cook till thick. Cool and store in clean, dry glass jars. Lemon pickle with salt and turmeric is another example.

(iv) What are the scientific values addressed in this case?

Applying scientific knowledge (osmosis) to solve a real problem.
Reducing food wastage and conserving resources.
Use of traditional knowledge with scientific reasoning.
Food security, self-reliance and better income.

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