Extra 20 important long questions and answers from the chapter:5 "The Fundamental Unit of Life" for Class 9 CBSE Science:-
1. Explain the structure and functions of the plasma membrane.
Answer:
Structure:
The plasma membrane is a
thin, flexible membrane
that surrounds the cell.
It is made up of
lipid and protein molecules
(phospholipid bilayer).
It is
semi-permeable
, allowing selective substances to pass.
It contains
protein channels and carriers
for transport.
Functions:
Controls the
entry and exit
of substances (osmosis, diffusion).
Provides
shape and protection
to the cell.
Helps in
cell communication
.
Plays a role in
endocytosis and exocytosis
.
2. Explain the process of diffusion and osmosis. How are they different?
Answer:
Diffusion:
The movement of
molecules (gases, liquids)
from a
higher to lower concentration
.
Does
not require a membrane
.
Example:
Perfume spreading in a room
.
Osmosis:
The movement of
water molecules
from a
higher to lower concentration
.
Occurs
through a semi-permeable membrane
.
Example:
Absorption of water by plant roots
.
Differences:
|
Feature |
Diffusion |
Osmosis |
|---|---|---|
|
Type of substance |
Gases and solutes |
Only water |
|
Membrane needed |
No |
Yes |
|
Energy required |
No |
No |
3. What happens when a plant cell is placed in different types of solutions?
Answer:
Hypotonic solution
(low solute concentration):
Water enters the cell.
Cell
swells and becomes turgid
.
Example:
Plant roots absorbing water
.
Isotonic solution
(equal solute concentration):
No net movement of water.
Cell remains
normal
.
Hypertonic solution
(high solute concentration):
Water moves
out
of the cell.
Cell
shrinks
(plasmolysis occurs).
Example:
Salting vegetables causes shrinkage
.
4. Explain plasmolysis and its significance.
Answer:
Plasmolysis
is the
shrinking of the cytoplasm
when a plant cell loses water in a hypertonic solution.
Water moves
out
of the vacuole due to
exosmosis
.
The
plasma membrane pulls away
from the cell wall.
The cell becomes
flaccid
and may die if prolonged.
Significance:
Helps understand
cell permeability
.
Used in
food preservation
(e.g., pickles).
Explains why
plants wilt in dry conditions
.
5. Why is the nucleus called the control centre of the cell?
Answer:
The
nucleus contains DNA
, which stores genetic information.
It regulates
cell division, growth, and metabolism
.
It controls the
synthesis of proteins
through mRNA.
It plays a role in
heredity and evolution
.
The
nucleolus inside the nucleus
helps in ribosome formation.
6. Describe the structure and function of mitochondria.
Answer:
Structure:
Mitochondria are
double-membraned organelles
.
The
inner membrane is folded
into cristae to increase surface area.
The
matrix contains enzymes, ribosomes, and DNA
.
Functions:
Produces ATP (energy currency)
through respiration.
Helps in
cell metabolism
.
Contains
its own DNA
for self-replication.
Active cells like
muscles and nerves
have more mitochondria.
7. What is the function of the Golgi apparatus?
Answer:
The
Golgi apparatus
is made up of
flattened sacs (cisternae)
.
It
modifies and packages
proteins and lipids.
It
secretes enzymes, hormones, and mucus
.
It
forms lysosomes
.
It
transports materials
inside and outside the cell.
8. What are lysosomes? Why are they called "suicidal bags"?
Answer:
Lysosomes are small vesicles containing
digestive enzymes
.
They
break down waste materials
.
They help in
cell
defence
by digesting harmful substances.
If a cell is damaged,
lysosomes burst and digest the cell
.
This is why they are called
"suicidal bags"
.
9. What is the function of the endoplasmic reticulum (ER)?
Answer:
Rough ER (RER)
– Has ribosomes and helps in
protein synthesis
.
Smooth ER (SER)
– Lacks ribosomes and helps in
lipid synthesis
.
Transports
materials
between organelles.
Detoxifies
harmful substances
(in liver cells).
10. Compare plant and animal cells.
Answer:
|
Feature |
Plant Cell |
Animal Cell |
|---|---|---|
|
Cell Wall |
Present |
Absent |
|
Vacuole |
Large |
Small |
|
Plastids |
Present (e.g., chloroplast) |
Absent |
|
Shape |
Rectangular |
Round |
11. What is the role of the cell wall in plant cells?
Answer:
The
cell wall is a rigid outer covering
made of cellulose in plant cells.
It provides
structural support
and maintains the
shape
of the cell.
It acts as a
protective barrier
against mechanical stress.
It prevents the
cell from bursting
when water enters through osmosis.
It allows
exchange of gases, nutrients, and water
through plasmodesmata.
It helps in
cell communication
and
defence
against pathogens.
It is
fully permeable
, allowing free movement of substances.
12. How do different types of endoplasmic reticulum (ER) function in a cell?
Answer:
The
endoplasmic reticulum (ER) is a network of membranes
inside the cell.
Rough ER (RER)
has
ribosomes
and is responsible for
protein synthesis
.
It transports
proteins to the Golgi apparatus
for further modification.
Smooth ER (SER)
lacks ribosomes and helps in
lipid synthesis
.
It plays a role in
detoxification
of drugs and harmful chemicals.
SER is involved in the
synthesis of steroid hormones
.
It helps in the
transport of materials
within the cell.
13. Explain the concept of cell theory and its significance.
Answer:
Cell theory was proposed by Schleiden and Schwann
and later modified by Virchow.
It states that
all living organisms are composed of cells
.
The
cell is the basic structural and functional unit of life
.
All cells arise from pre-existing cells
(added by Virchow).
It emphasizes the
continuity of life
through reproduction.
It applies to
both unicellular and multicellular organisms
.
The theory helped in
advancing research in microbiology and medicine
.
14. What are the different ways in which substances move across the plasma membrane?
Answer:
Diffusion
– Passive movement of molecules from
higher to lower concentration
.
Osmosis
– Movement of
water molecules
across a semi-permeable membrane.
Facilitated diffusion
– Transport of substances via
carrier proteins
, without energy.
Active transport
– Movement of molecules
against the concentration gradient
using energy (ATP).
Endocytosis
– The cell engulfs large molecules (e.g., phagocytosis and pinocytosis).
Exocytosis
– The process of
expelling
materials from the cell.
Plasmolysis
– Loss of water from a plant cell, causing shrinking of the cytoplasm.
15. How do lysosomes function as the "digestive system" of the cell?
Answer:
Lysosomes are small vesicles
filled with digestive enzymes.
They break down
worn-out organelles and cellular waste
.
They digest
foreign particles like bacteria and viruses
.
If the cell is damaged, lysosomes burst and digest the entire cell (
autolysis
).
They help in
cellular
defence
and immunity
.
They assist in
recycling of cellular components
.
Formed by the
Golgi apparatus
and found mostly in
animal cells
.
16. How does ATP function as the energy currency of the cell?
Answer:
ATP (Adenosine Triphosphate)
is produced in
mitochondria
.
It stores
chemical energy
in phosphate bonds.
When ATP breaks down into ADP + Pi,
energy is released
for cellular activities.
It is used in
muscle contraction, nerve transmission, and biosynthesis
.
ATP powers
active transport, cell division, and movement of organelles
.
It is a
renewable source
since ADP can be converted back to ATP.
Without ATP,
life processes cannot function efficiently
.
17. What are the major differences between unicellular and multicellular organisms?
Answer:
|
Feature |
Unicellular Organisms |
Multicellular Organisms |
|---|---|---|
|
Number of cells |
One cell performs all functions |
Many specialized cells work together |
|
Complexity |
Simple structure |
Complex structure |
|
Size |
Usually microscopic |
Can be large |
|
Reproduction |
Asexual (binary fission) |
Sexual or asexual |
|
Examples |
Bacteria, Amoeba, Paramecium |
Plants, Animals, Humans |
Unicellular organisms
depend only on one cell
for survival.
Multicellular organisms have
differentiated cells
, such as muscle and nerve cells.
18. What is the role of the nucleolus inside the nucleus?
Answer:
The
nucleolus is a small, dense structure
inside the nucleus.
It is the site of
ribosome production
.
It synthesizes and assembles
ribosomal RNA (rRNA)
.
It helps in the
regulation of gene expression
.
It plays a role in
cell growth and division
.
It is involved in the
processing of genetic information
.
Cells that produce large amounts of proteins have
larger nucleoli
.
19. How does the cytoplasm contribute to the functioning of the cell?
Answer:
The cytoplasm is a
jelly-like fluid
inside the cell.
It
holds organelles in place
and provides a medium for biochemical reactions.
It
stores nutrients, enzymes, and waste products
.
It allows the
movement of materials
within the cell (cytoplasmic streaming).
The cytoskeleton in the cytoplasm provides
structural support
.
Many metabolic processes like
glycolysis (sugar breakdown) occur here
.
It acts as a
buffer, maintaining pH and ion balance
.
20. How do cells communicate with each other?
Answer:
Cells communicate using
chemical signals
like hormones and neurotransmitters.
The plasma membrane contains
receptor proteins
that detect signals.
Endocrine
signalling
– Hormones travel through blood to reach target cells.
Paracrine
signalling
– Local signals
affect
nearby cells.
Synaptic
signalling
– Nerve cells release neurotransmitters to send signals.
Direct cell contact
– Cells exchange materials through
plasmodesmata (plants)
or
gap junctions (animals)
.
This communication controls
growth, metabolism, and immune responses
.
