Chapter 5: The Fundamental Unit of Life Long Questions | science Class 9 CBSE

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Chapter 5: The Fundamental Unit of Life Long Questions | science Class 9 CBSE | ByteNotes.in

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Question 1

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Q1. Describe the historical development and major discoveries related to the cell.

• Robert Hooke first observed cells in 1665 while examining cork slices under a microscope.
• Anton van Leeuwenhoek discovered free-living cells in pond water in 1674.
• Robert Brown discovered the nucleus in 1831.
• Purkinje coined the term 'protoplasm' in 1839 for the cell fluid.
• Schleiden and Schwann proposed the Cell Theory in 1838–1839, stating that all plants and animals are composed of cells.
• Virchow in 1855 suggested that all cells arise from pre-existing cells.
• The electron microscope, developed in 1940, allowed observation of detailed cell structures and organelles.

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Question 2

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Q2. Why is the cell considered the fundamental unit of life?

• All living organisms are composed of cells, making it the basic structural unit.
• A cell performs essential life functions such as nutrition, respiration, excretion, and reproduction.
• Cells divide to form new cells, enabling growth and replacement of dead or damaged cells.
• Specialized components (organelles) inside the cell allow division of labor within the cell.
• Cells contain genetic material (DNA) that controls all life processes.
• Both unicellular and multicellular organisms depend on cell function for survival.
• Without cells, no form of life could exist.

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Question 3

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Q3. Describe the structure and function of the plasma membrane.

• The plasma membrane is the outermost covering of the cell.
• It is composed of lipids and proteins and is selectively permeable.
• It regulates movement of substances in and out of the cell.
• Gaseous exchange (e.g., CO₂ and O₂) occurs through diffusion.
• Water moves across the membrane via osmosis.
• Its flexibility allows processes like endocytosis (e.g., Amoeba engulfing food).
• The plasma membrane is essential for maintaining cellular homeostasis.

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Question 4

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Q4. What is osmosis? Describe its importance in cells with suitable examples.

• Osmosis is the diffusion of water through a selectively permeable membrane.
• It occurs from a region of higher water concentration to lower concentration.
• In hypotonic solutions, cells gain water and swell.
• In isotonic solutions, water movement is balanced; cells retain shape.
• In hypertonic solutions, cells lose water and shrink.
• Example: Raisins swell in water and shrink in concentrated sugar solution.
• Osmosis is crucial for water absorption by plant roots and cellular fluid balance.

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Question 5

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Q5. What is a cell wall? How does it differ from the plasma membrane?

• The cell wall is a rigid outer covering found in plant cells, fungi, and bacteria.
• It lies outside the plasma membrane and is composed mainly of cellulose.
• Provides structural support and protection to the cell.
• Prevents bursting of cells in hypotonic environments by exerting turgor pressure.
• Unlike the plasma membrane, it is freely permeable.
• Animal cells lack a cell wall.
• Cell wall contributes to shape and rigidity of plant cells.

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Question 6

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Q6. Describe the structure and functions of the nucleus.

• The nucleus is a large, spherical organelle usually present in the center of the cell.
• It is surrounded by a double-layered nuclear membrane with pores.
• Contains chromatin material (DNA + proteins) that condenses to form chromosomes during cell division.
• Chromosomes contain genes responsible for inheritance and controlling cell functions.
• Nucleus regulates cell growth, metabolism, and reproduction.
• Nucleoplasm (nuclear sap) and nucleolus are also present inside.
• In prokaryotes, a well-defined nucleus is absent; the nuclear region is called nucleoid.

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Question 7

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Q7. Differentiate between prokaryotic and eukaryotic cells.

• Prokaryotic cells are generally smaller (1–10 µm), while eukaryotic cells are larger (5–100 µm).
• Prokaryotes lack a true nucleus; eukaryotes have a well-defined nucleus with a nuclear membrane.
• Membrane-bound organelles are absent in prokaryotes but present in eukaryotes.
• Prokaryotes have a single, circular chromosome; eukaryotes have multiple linear chromosomes.
• Examples of prokaryotes: Bacteria, blue-green algae.
• Examples of eukaryotes: Animals, plants, fungi.
• Ribosomes are smaller in prokaryotes compared to eukaryotic cells.

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Question 8

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Q8. What is cytoplasm? State its importance in the cell.

• Cytoplasm is the jelly-like fluid enclosed by the plasma membrane.
• It contains water, enzymes, salts, organelles, and other organic molecules.
• Site for many metabolic reactions essential for life.
• Supports and suspends organelles within the cell.
• Acts as a medium for movement of materials within the cell.
• In prokaryotic cells, cytoplasm performs functions of organelles as they are absent.
• Plays a role in maintaining cell shape and internal pressure.

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Question 9

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Q9. Explain the structure and function of endoplasmic reticulum (ER).

• ER is a network of membrane-bound tubules and vesicles in the cytoplasm.
• Two types: Rough ER (with ribosomes) and Smooth ER (without ribosomes).
• Rough ER synthesizes proteins and transports them within the cell.
• Smooth ER synthesizes lipids and helps in detoxification of drugs (especially in liver cells).
• Helps in membrane biogenesis by supplying lipids and proteins.
• Acts as a transport channel between cytoplasm and nucleus.
• Provides a framework for biochemical activity.

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Question 10

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Q10. Describe the structure and functions of the Golgi apparatus.

• Golgi apparatus consists of flattened membrane-bound sacs called cisterns.
• Found close to the nucleus and often connected to ER membranes.
• Modifies, stores, and packages substances from ER.
• Forms secretory vesicles for transport within and outside the cell.
• Synthesizes complex sugars from simple ones.
• Plays a role in formation of lysosomes.
• Essential for intracellular transport and secretion.

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Chapter 5: The Fundamental Unit of Life | Long Questions

Q1. Describe the historical development and major discoveries related to the cell.

Q2. Why is the cell considered the fundamental unit of life?

Q3. Describe the structure and function of the plasma membrane.

Q4. What is osmosis? Describe its importance in cells with suitable examples.

Q5. What is a cell wall? How does it differ from the plasma membrane?

Q6. Describe the structure and functions of the nucleus.

Q7. Differentiate between prokaryotic and eukaryotic cells.

Q8. What is cytoplasm? State its importance in the cell.

Q9. Explain the structure and function of endoplasmic reticulum (ER).

Q10. Describe the structure and functions of the Golgi apparatus.

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