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.
