NCERT Class 9 Science Cell: The Building Block of Life Complete Solutions
Short Intro
Cells are the fundamental units of life that perform all essential activities in living organisms. This chapter explains the discovery of cells, structure and functions of cell organelles, types of cells, osmosis, diffusion, mitosis, meiosis, and cell theory. These detailed solutions help students understand concepts easily with step-by-step explanations.
Quick Information Box
| Topic | Details |
|---|---|
| Chapter Name | Cell: The Building Block of Life |
| Subject | Science |
| Grade | 9 |
| Chapter Number | 2 |
| Main Focus | Cell Structure and Functions |
| Important Topics | Cell Organelles, Osmosis, Cell Division |
| Difficulty Level | Moderate |
Concepts Used (Topics Covered)
- Discovery of Cells
- Microscope and Cell Observation
- Cell Membrane and Cell Wall
- Diffusion and Osmosis
- Prokaryotic and Eukaryotic Cells
- Cell Organelles and Their Functions
- Plant Cell vs Animal Cell
- Mitosis and Meiosis
- Cell Theory
- Cell Division and Growth
Important Formulas
Cell Size Estimation
Magnification Formula
Total Magnification=Eyepiece Power×Objective Lens Power
Osmosis Concept
Water moves from dilute solution to concentrated solution through a selectively permeable membrane
Questions & Step-by-step Solutions with Explanation
1. Differentiate Between the Following
(i) Cell Membrane and Cell Wall
| Cell Membrane | Cell Wall |
|---|---|
| Selectively permeable | Freely permeable |
| Thin and flexible | Thick and rigid |
| Present in all cells | Present mainly in plant, fungal and bacterial cells |
| Made of lipids and proteins | Made mainly of cellulose in plants |
(ii) RER and SER
| RER | SER |
|---|---|
| Ribosomes present | Ribosomes absent |
| Rough appearance | Smooth appearance |
| Protein synthesis | Lipid and hormone synthesis |
(iii) Chloroplasts and Chromoplasts
| Chloroplasts | Chromoplasts |
|---|---|
| Contain green chlorophyll pigment | Contain red, yellow or orange pigments |
| Perform photosynthesis | Give colour to fruits and flowers |
2. Animal Cells in Different Solutions
Correct Answer
(iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
Explanation
Cell X in Pure Water
Water enters the cell due to osmosis, causing swelling.
Cell Y in Salt Solution
Water leaves the cell because the outside solution is more concentrated, causing shrinking.
3. Identify the Parts of the Cell Diagram
| Label | Cell Part | Function |
|---|---|---|
| (a) | Cell Wall | Provides rigidity |
| (b) | Cell Membrane | Separates cell from surroundings |
| (c) | Nucleus | Controls cell activities |
| (d) | Vacuole | Storage and rigidity |
| (e) | Chloroplast | Food synthesis |
| (f) | Mitochondria | Cellular respiration |
| (g) | Golgi Apparatus | Packaging and storage |
4. Correct Option of Organelles
Correct Answer
(i) Leucoplast — Cell wall
Explanation
Leucoplasts and cell walls are present in plant cells but absent in animal cells.
5. Discussion on Plastids
Correct Answer
Renu is correct.
Explanation
Roots also contain plastids called leucoplasts that store food materials like starch even though they do not perform photosynthesis.
6. Similarities and Differences Between Mitochondria and Chloroplasts
Similarities
- Both are double membrane-bound organelles.
- Both contain DNA and ribosomes.
- Both can synthesise some proteins independently.
Differences
| Mitochondria | Chloroplast |
|---|---|
| Present in plant and animal cells | Present only in plant cells |
| Perform cellular respiration | Perform photosynthesis |
| Release energy | Prepare food |
7. Organelles Containing DNA
Correct Answer
(ii) Mitochondria, Nucleus
Explanation
Both organelles contain genetic material.
8. Carrot Experiment
(i) Hypothesis
Water movement occurs through osmosis.
(ii) Improvement
Measure carrot weight before and after the experiment.
(iii) Explanation
- Carrot in water absorbs water and stays firm.
- Carrot in salt solution loses water and becomes limp.
9. Presence or Absence of Structures
| Structure | Bacterial Cell | Animal Cell |
|---|---|---|
| Chromosome | Present | Present |
| Nucleus | Absent | Present |
| Mitochondria | Absent | Present |
| Golgi Complex | Absent | Present |
| Chromoplasts | Absent | Absent |
10. Potato Cup Experiment
(i) Why Water Collects in Cups B and C
Sugar and salt create concentrated solutions, causing water to move through osmosis.
(ii) Why Cup A is Necessary
It acts as a control setup for comparison.
(iii) Why Water Does Not Collect in A and D
- Cup A contains no solute.
- Boiled potato in D has damaged membranes, so osmosis cannot occur.
11. Incorrect Match
Correct Answer
(ii) SER — Lipid and cellulose synthesis
Explanation
SER synthesises lipids and hormones, not cellulose.
12. Removal of Mitochondria from Cell
Answer
The cell will not be able to produce sufficient ATP energy and may eventually die.
13. Tumor Prevention
Answer
Contact inhibition prevents uncontrolled cell division.
Can Plants Develop Tumors?
Yes, plants can also develop tumors due to uncontrolled cell growth.
14. Cell Membrane Synthesis
Organelles Involved
- SER synthesises lipids
- RER synthesises proteins
- Golgi apparatus packages them
Pathway
RER/SER→Golgi Apparatus→Vesicles→Cell Membrane
Explanation
Proteins and lipids are transported through vesicles to form the cell membrane.
15. If Gametes Were Formed by Mitosis
Answer
Gametes would contain the full chromosome number. After fertilisation, chromosome numbers would double in every generation causing abnormalities.
16. Food Preservation Case Study
(i) Scientific Concept
Osmosis and food preservation.
(ii) How Salt and Sugar Prevent Spoilage
High salt or sugar concentration removes water from microorganisms causing dehydration and preventing growth.
(iii) Healthy Recipe Suggestion
Lemon pickle with less oil and natural spices.
(iv) Scientific Values
- Scientific thinking
- Resource management
- Food preservation awareness
- Sustainable development
Common Mistakes
- Confusing osmosis with diffusion
- Forgetting functions of organelles
- Mixing up mitosis and meiosis
- Writing incorrect differences between plant and animal cells
- Forgetting that mitochondria contain DNA
Exam Tips
- Practice labelled diagrams regularly
- Learn organelle functions with keywords
- Use tables for comparisons
- Revise osmosis experiments carefully
- Remember definitions of prokaryotic and eukaryotic cells
Practice MCQs
1. Which organelle is known as the powerhouse of the cell?
A. Ribosome
B. Chloroplast
C. Mitochondria
D. Golgi Body
Answer
C. Mitochondria
2. Osmosis involves movement of:
A. Salt
B. Sugar
C. Water
D. Proteins
Answer
C. Water
3. Which organelle contains chlorophyll?
A. Mitochondria
B. Chloroplast
C. Vacuole
D. Nucleus
Answer
B. Chloroplast
4. Which cell lacks a true nucleus?
A. Plant Cell
B. Animal Cell
C. Bacterial Cell
D. Fungal Cell
Answer
C. Bacterial Cell
5. Mitosis produces:
A. Four daughter cells
B. Two identical daughter cells
C. Gametes
D. Spores
Answer
B. Two identical daughter cells
FAQ Section
Q1. Why is the cell called the basic unit of life?
Because all living organisms are made up of cells and all life processes occur within cells.
Q2. What is osmosis?
Osmosis is the movement of water through a selectively permeable membrane from dilute to concentrated solution.
Q3. What is the function of mitochondria?
Mitochondria produce ATP energy through cellular respiration.
Q4. Why do plant cells have a cell wall?
The cell wall provides rigidity, protection, and shape to plant cells.
Q5. What is the difference between mitosis and meiosis?
Mitosis forms two identical cells, while meiosis forms four gametes with half chromosome number.
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