The Fundamental Unit of Life — Previous Year Questions (Class 9 Science)
Cells are the basic structural and functional units of life. Learn about prokaryotic and eukaryotic cells, their structures, and organelles.
TL;DR: Cells are the basic structural and functional units of life. Learn about prokaryotic and eukaryotic cells, their structures, and organelles.
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Cells are the basic structural and functional units of life. Learn about prokaryotic and eukaryotic cells, their structures, and organelles.
The Fundamental Unit of Life — Previous Year Questions with Solutions
Q (2023, 3 marks): Define cell and state the cell theory.
Answer: Cell: The smallest unit of life that can perform all life functions independently.
Cell Theory (3 postulates):
1. All living organisms are composed of cells
2. Cell is the basic unit of life
3. All cells arise from pre-existing cells (omnis cellula e cellula)
Historical context: Formulated by Schleiden (plants), Schwann (animals), and Virchow (origin)
Q (2022, 5 marks): Compare prokaryotic and eukaryotic cells in terms of nucleus, organelles, and size.
Answer: Prokaryotic Cells (bacteria, archaea):
- No true nucleus; genetic material in nucleoid region
- No membrane-bound organelles
- Smaller (1-10 μm)
- Cell wall (in bacteria: peptidoglycan)
- Single circular chromosome
- Examples: E. coli, cyanobacteria
Eukaryotic Cells (animals, plants, fungi):
- True nucleus with nuclear envelope
- Membrane-bound organelles (mitochondria, ER, Golgi, etc.)
- Larger (10-100 μm)
- Cell wall (in plants: cellulose)
- Multiple linear chromosomes
- Examples: Human cells, plant cells
Comparison table:
Feature | Prokaryotic | Eukaryotic
Size | 1-10 μm | 10-100 μm
Nucleus | No | Yes
Organelles | No | Yes
Cell wall | Yes (bacteria) | Yes (plants)
Chromosomes | 1 (circular) | Many (linear)
Q (2023, 3 marks): Name three membrane-bound organelles and state their main functions.
Answer: Three membrane-bound organelles and their functions:
1. Mitochondria: Energy production (ATP synthesis) through aerobic respiration; called 'powerhouse of cell'
2. Endoplasmic Reticulum (rough): Protein synthesis (ribosomes attached); transport of proteins
3. Golgi Apparatus (Golgi body): Processing, modification, and packaging of proteins and lipids for secretion
Other examples: Lysosomes (digestion), Vacuoles (storage)
Q (2021, 2 marks): Explain why mitochondria is called the powerhouse of the cell.
Answer: Mitochondria is called powerhouse because:
1. It carries out aerobic respiration
2. Produces ATP (adenosine triphosphate) through cellular respiration
3. ATP is the energy currency used for all cellular activities
4. The process: Glucose + O2 → CO2 + H2O + Energy (ATP)
5. One glucose molecule produces ~30-32 ATP molecules
Therefore, mitochondria is the main site of energy production in the cell
Q (2022, 3 marks): Distinguish between plant and animal cells. Mention at least 4 differences.
Answer: Differences:
1. Cell wall: Present in plants (cellulose), absent in animals
2. Plastids: Present in plants (chloroplasts for photosynthesis), absent in animals
3. Vacuole: Large (covering 90% of volume) in plants, small/absent in animals
4. Centrioles: Absent in plants, present in animals
5. Shape: Plants - fixed/rigid shape; animals - round/irregular
6. Plasmodesmata: Present in plant cells; gap junctions in animal cells
Both have: Nucleus, cell membrane, mitochondria, ER, Golgi, ribosomes, cytoplasm
Q (2023, 5 marks): Draw and label a eukaryotic cell with at least 8 organelles. State one function for each.
Answer: A eukaryotic cell diagram should include:
1. Nucleus - contains DNA; controls cell activities
2. Nucleolus - site of ribosomal RNA synthesis
3. Mitochondria - ATP production (energy)
4. Rough ER - protein synthesis
5. Smooth ER - lipid synthesis; no ribosomes
6. Golgi apparatus - protein packaging and modification
7. Ribosomes - protein synthesis sites
8. Lysosomes - intracellular digestion (animal cells)
9. Centrioles - involved in cell division (animal cells)
10. Cell membrane - selectively permeable boundary
(A proper diagram should show these with labels and relative positions)
Frequently Asked Questions
What is the difference between rough and smooth endoplasmic reticulum?
Rough ER has ribosomes attached to its outer surface, making it appear grainy under the microscope. It synthesizes proteins. Smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification. Both are continuous with the nuclear envelope and Golgi apparatus.
Why do plant cells have cell walls while animal cells do not?
Plant cells need cell walls to provide structural support, rigidity, and protection. They allow plants to stand upright. Animal cells don't have cell walls because animals can move and their bodies are supported by the skeletal system. Cell membranes in animals provide flexibility needed for movement.
More Class 9 Science PYQs
- Chemical Reactions and Equations
- Light (Reflection and Refraction)
- Electricity
- Acids, Bases and Salts
- Metals and Non-metals
- Life Processes
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