NCERT Solutions for Class 9 Science Chapter 2 The Building Blocks of Life
Table of Contents
Page No. 8
Think It Over
Question 1:
Where does a cell come from?
Answer
According to the Cell Theory expanded by Rudolf Virchow in 1855, all cells arise from pre-existing cells. This means that cells do not appear out of nowhere; they are formed through the process of cell division from cells that already exist.
Question 2:
How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye?
Answer
Technology has played a huge role in studying tiny structures like cells:
(1) The invention of the microscope allowed scientists to see cells for the first time.
(2) Advanced microscopes (like electron microscopes) show detailed structures inside cells.
(3) Techniques like staining, imaging, and digital analysis help scientists observe cell functions clearly.
Because of these tools, we now understand things like cell structure, microorganisms, and how diseases work.
Question 3:
How is the cell structural and functional unit of life?
Answer
A cell is called the structural and functional unit of life because:
(1) Structural unit: All living organisms are made up of cells (from single-celled organisms to complex multicellular ones like humans).
(2) Functional unit: All life processes (like respiration, digestion, excretion, growth) occur inside cells.
So, cells are the basic building blocks that perform all essential functions of life.
Question 4:
How does a cell multiply?
Answer
Cells multiply through a process called cell division. There are two main types:
(1) Mitosis:
\(\quad\quad\quad\) a. Produces two identical daughter cells
\(\quad\quad\quad\) b. Helps in growth and repair
(2) Meiosis:
\(\quad\quad\quad\) a. Produces cells with half the number of chromosomes.
\(\quad\quad\quad\) b. Important for reproduction (formation of gametes like sperm and egg).
In simple terms, one cell divides to form two new cells.
NCERT Solutions for Class 9 Science Chapter 2 The Building Blocks of Life
Page No. 14
Pause and Ponder
Question 1:
What argument would you give for the necessity of a cell wall in plants usually fixed in one place versus in animals usually moving from one place to the other?
Answer
Plant cells usually remain fixed in one place, so they need extra support and protection. The cell wall provides:
(1) Rigidity and strength to maintain shape
(2) Protection from mechanical damage
(3) Ability to withstand internal pressure (turgor pressure) without bursting
In contrast, animal cells:
(1) Move from place to place
(2) Need flexibility, not rigidity
So, they only have a cell membrane, which allows movement and shape change.
Question 2:
Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?
Answer
This is important for a fair experiment:
(1) Equal size ensures same surface area and volume, so both pieces absorb or lose water similarly.
(2) Measuring initial weight helps compare accurate changes after osmosis.
(3) It ensures that any weight change is due to the liquid, not size differences
Without this, the results would not be reliable.
Question 3:
What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?
Answer
If the cell wall lost its rigidity:
(1) The cell would lose its fixed shape.
(2) It could burst easily due to water entering (osmotic pressure)
(3) The plant would lose support and strength, becoming weak or wilted
(4) Overall, plants would not be able to stand upright.
So, the rigid cell wall is essential for plant survival and structure.
NCERT Solutions for Class 9 Science Chapter 2 The Building Blocks of Life
Page No. 19
Pause and Ponder
Question 4:
Do white flowers contain any pigment? Give reasons.
Answer
Question 5:
Draw a well-labelled schematic diagram of a plant or an animal cell using these clues —
(i) Nucleus appears as a dark and round body inside the cell.
(ii) ER spreads like a network of extended nuclear envelope.
(iii) Mitochondria and chloroplasts are rod shaped.
Answer
NCERT Solutions for Class 9 Science Chapter 2 The Building Blocks of Life
Page No. 22
Pause and Ponder
Question 6:
Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area?
Question 7:
If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to a cut on the skin?
NCERT Solutions for Class 9 Science Chapter 2 The Building Blocks of Life
Page No, 24
Revise, Reflect, Refine
Question 1:
Differentiate between the following pairs of terms based on the clues given in parentheses:
(i) Cell membrane and cell wall (permeability)
(ii) RER and SER (structure)
(iii) Chloroplasts and chromoplasts (pigments)
Question 2:
Two similar animal cells are placed in two different solutions:
(i) Cell X is placed in pure water.
(ii) Cell Y is placed in a concentrated salt solution
Cells are observed after some time. Cell X swells, and Cell Y shrinks.
Which statement provides the correct explanation for the above observations?
(i) Salt molecules moved into Cell Y, causing it to shrink.
(ii) Water moved into Cell X and more water moved out of Cell Y than the salt solution entered in it.
(iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
(iv) Solute movement caused osmosis in both cells.
Question 3:
Look 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 given below:
(i) Controlling all the activities of a cell.
(ii) Site of cellular respiration.
(iii) Storage organelle that also provides rigidity to the cell.
(iv) Separates the cell contents from surroundings.
(v) Provides structural rigidity to the cell.
(vi) Packs and stores materials received from ER.
(vii) Helps in manufacturing food.
NCERT Solutions for Class 9 Science Chapter 2 The Building Blocks of Life
