5. The Fundamental Unit of Life Class 9 Science Chapter 5 NCERT Solutions (CBSE)

CBSE Class 9 Science Chapter 5 : The Fundamental Unit of Life – Full NCERT Textbook Solutions

Chapter 5. The Fundamental Unit of Life

Internal Question and Answer :

1. Who discovered cells, and how ?

Answer: Cells were discovered by Robert Hooke in 1665. He used a compound microscope to examine a thin slice of cork and observed tiny compartments that he called "cells," resembling the cells of a honeycomb.

2. Why is the cell called the structural and functional unit of life ?

Answer: The cell is called the structural and functional unit of life because it is the basic building block of all living organisms. Cells perform essential functions, such as metabolism, reproduction, and responding to stimuli, and collectively, they form the foundation of biological structure and function.

Internal Question and Answer :

1. How do substances like  and water move in and out of the cell ? Discuss.

Answer: The substances like  and water move in and out of cells through diffusion. They pass through the cell membrane, from areas of high concentration to low concentration, to maintain balance inside and outside the cell.

2. Why is the plasma membrane called a selectively permeable membrane?

Answer: The plasma membrane is called selectively permeable because it allows certain substances to pass through while blocking others. It selectively regulates the entry and exit of molecules to maintain cell function.

Internal Question and Answer :

1. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.

Prokaryotic Cell	Eukaryotic Cell
1. Size : generally small ( 1-10 μm)  ;	1. Size: generally large ( 5-100 μm)
2. Nuclear region: ___  ___ and known as__	2. Nuclear region: well defined and surrounded by a nuclear membrane
3. Chromosome: single	3. More than one chromosome
4. Membrane-bound cell organelles absent	4. ___  ___  ____

Answer: The completed table is given below:

Prokaryotic Cell	Eukaryotic Cell
1. Size: generally small (1–10 μm)	1. Size: generally large (5–100 μm)
2. Nuclear region: not well defined, not surrounded by nuclear membrane and known as nucleoid	2. Nuclear region: well defined and surrounded by a nuclear membrane
3. Chromosome: single	3. More than one chromosome
4. Membrane-bound cell organelles absent	4. Membrane-bound cell organelles present

Internal Question and Answer :

1. Can you name the two organelles we have studied that contain their own genetic material ?

Answer:  The two organelles that contain their own genetic material are mitochondria and chloroplasts .
2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

Answer:If the organisation of a cell is destroyed due to any physical or chemical influence, the cell will not be able to perform its normal functions properly. As a result, the cell may die.
3. Why are lysosomes known as suicide bags?

Answer: Lysosomes are known as "suicide bags" because they contain powerful digestive enzymes capable of breaking down all organic material. When cells get damaged or disrupted, lysosomes may burst, causing these enzymes to digest their own cell, leading to cell self-destruction.
4. Where are proteins synthesized inside the cell?

Answer: Proteins are synthesized inside the cell on ribosomes, which can be found in the cytoplasm and attached to the endoplasmic reticulum.

Exercises of Chapter 5: The Fundamental Unit of Life

1. Make a comparison and write down ways in which plant cells are different from animal cells.

Answer: The comparison between plant cells and animal cells is given below :

Characteristic	Plant Cells	Animal Cells
Cell Wall	Present	Absent
Chloroplasts	Present	Absent
Vacuole	Large central vacuole	Small or multiple vacuoles
Shape and Size	Often rigid and fixed	Usually flexible and varied
Locomotion	Lacks mobility	May have mobility structures
Food Storage	Store starch	Store glycogen
Nucleus Location	Periphery	Center
Centrioles	Generally absent	Present (in some)
Lysosomes	Rarely present	Commonly present

2. How is a prokaryotic cell different from a eukaryotic cell?

Answer: Prokaryotic cells and eukaryotic cells have several differences:

Characteristic	Prokaryotic Cell	Eukaryotic Cell
Nucleus	No true nucleus	True nucleus present
Membrane-bound Organelles	Few or none	Numerous organelles
DNA Location	In the nucleoid region	Enclosed in the nucleus
Size	Usually smaller	Usually larger
Complexity	Simple	Complex
Example	Bacteria, Archaea	Animals, Plants, Fungi

3. What would happen if the plasma membrane ruptures or breaks down?

Answer: If the plasma membrane ruptures or breaks down, the cell loses its structure and control over what enters and exits. This can lead to cell death and potentially harm neighboring cells.

4. What would happen to the life of a cell if there was no Golgi apparatus?

Answer: Without a Golgi apparatus, the cell would have difficulty processing, modifying, and packaging proteins and other molecules for transport. This could disrupt essential cellular functions and affect the cell's survival.

5. Which organelle is known as the powerhouse of the cell? Why?

Answer: Mitochondria are known as the powerhouses of the cell because they release the energy required for various chemical activities needed for life in the form of ATP (Adenosine triphosphate) molecules. ATP is known as the energy currency of the cell. The body uses energy stored in ATP to make new chemical compounds and for mechanical work.

6. Where do the lipids and proteins constituting the cell membrane get synthesised?

Answer: The lipids and proteins that make up the cell membrane are primarily synthesized in the endoplasmic reticulum (ER). The rough ER, which has ribosomes attached to its surface, synthesizes proteins, while the smooth ER is involved in lipid synthesis, including the production of phospholipids for the cell membrane.
7. How does an Amoeba obtain its food?

Answer: An amoeba obtains food through a process called endocytosis. It surrounds its food with its cell membrane, creating a small food vacuole. Then, it digests the engulfed food inside the vacuole to extract nutrients and energy for its survival and growth.
8. What is osmosis?

Answer: Osmosis is a process in which solvent molecules move from an area of lower solute concentration to an area of higher solute concentration through a semipermeable membrane to equalize the concentrations.
9. Carry out the following osmosis experiment:
Take four peeled potato halves and scoos each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now,
(a) Keep cup A empty
(b) Put one teaspoon sugar in cup B
(c) Put one teaspoon salt in cup C
(d) Put one teaspoon sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups and answer the following:
(i) Explain why water gathers in the hollowed portion of B and C.
(ii) Why is potato A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed out portions of A and D.

Answer: (i) Water gathers in the hollowed portions of B and C because the sugar and salt solutions inside the cups are more concentrated than the surrounding water. Due to osmosis, water moves from the region of higher water concentration (outside) to lower water concentration (inside), so water collects in the cups.

(ii) Potato A is necessary as a control. It shows that without any solute (sugar or salt), water does not move into the cavity, helping us compare the results.

(iii) Water does not gather in A because there is no concentration difference to cause osmosis. In D (boiled potato), the cells are dead and the semi-permeable membrane is destroyed, so osmosis cannot take place.

10. Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?

Answer: For growth and repair of the body, mitosis is required. It produces identical daughter cells and helps in increasing cell number.

For the formation of gametes (sex cells), meiosis is involved. It reduces the chromosome number to half, which is necessary for sexual reproduction.

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