NCERT Solutions for Class 9 Science Chapter 5 – The Fundamental Unit of Life
Class 9 Science Chapter 5 – The Fundamental Unit of Life introduces students to the cell — the basic structural and functional unit of all living organisms. The chapter covers the discovery of the cell by Robert Hooke, the cell theory proposed by Schleiden and Schwann, the structure and functions of the plasma membrane, cell wall, nucleus, cytoplasm, and cell organelles such as mitochondria, plastids, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles. The chapter also discusses the differences between plant and animal cells and between prokaryotic and eukaryotic cells.
This chapter is a fundamental part of the CBSE Class 9 Science syllabus and carries significant weight in school examinations. Questions from this chapter test students' understanding of cell structure, the functions of various organelles, osmosis and diffusion, and the differences between different types of cells. A thorough understanding of this chapter is also essential for Class 10 Biology and for competitive examinations like NEET.
NCERT Solutions for Class 9 Science Chapter 5 – The Fundamental Unit of Life
The Fundamental Unit of Life
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The Fundamental Unit of Life
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The Fundamental Unit of Life
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The Fundamental Unit of Life
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The Fundamental Unit of Life
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The Fundamental Unit of Life
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The Fundamental Unit of Life
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The Fundamental Unit of Life
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The Fundamental Unit of Life
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The Fundamental Unit of Life
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Question 1. Who discovered the cell and how?
Answer:
The cell was discovered by Robert Hooke in 1665. He used a self-designed microscope to observe a thin slice of cork — the dead outer bark of a tree. He noticed small, box-like compartments which he called "cells" because they reminded him of the small rooms in a monastery called cellula. This was the first time the word "cell" was used in a biological context.
Question 2. Why is the cell called the structural and functional unit of life?
Answer:
The cell is called the structural and functional unit of life because all living organisms are made up of cells, and all the basic life processes — nutrition, respiration, excretion, reproduction, and growth — are carried out at the cellular level. Just as a brick is the basic unit of a building, the cell is the basic unit of all living organisms. Every function that a living being performs is ultimately a result of the coordinated activities of its cells.
Question 3. How does a cell maintain its shape and protect itself?
Answer:
In plant cells, the cell wall — made of cellulose — provides rigidity, shape, and protection to the cell. It is a tough outer covering that lies outside the plasma membrane. In animal cells, which do not have a cell wall, the plasma membrane alone maintains the shape of the cell. The plasma membrane is a flexible, selectively permeable covering made of lipids and proteins that regulates what enters and leaves the cell, thereby protecting its internal environment.
Question 4. What is the plasma membrane? What is its function?
Answer:
The plasma membrane, also called the cell membrane, is the outermost covering of animal cells and lies just inside the cell wall in plant cells. It is a thin, flexible, and selectively permeable membrane made of lipids and proteins.
Its main functions are to separate the contents of the cell from the external environment, to control the movement of substances into and out of the cell, and to allow communication between cells. It allows the passage of necessary substances like water and gases while blocking harmful substances.
Question 5. What is osmosis?
Answer:
Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration through a selectively permeable membrane. In living cells, water moves in or out of the cell through the plasma membrane depending on the concentration of the solution outside. If a cell is placed in a hypotonic solution, water moves into the cell and it swells. If placed in a hypertonic solution, water moves out and the cell shrinks. In an isotonic solution, there is no net movement of water.
Question 6. What is the cell wall? What is its function?
Answer:
The cell wall is a rigid outer covering found in plant cells, fungal cells, and bacterial cells but absent in animal cells. In plants, it is made of cellulose. The cell wall lies outside the plasma membrane and provides structural support, rigidity, and shape to the cell. It also protects the cell from mechanical damage and infections. It allows the cell to withstand osmotic pressure without bursting and facilitates cell-to-cell communication through small pores called plasmodesmata.
Question 7. What is the nucleus? What are its functions?
Answer:
The nucleus is the control centre of the cell. It is typically a large, spherical organelle found in eukaryotic cells, surrounded by a double-layered nuclear membrane with small pores called nuclear pores. Inside the nucleus is the nucleoplasm, which contains chromatin — the genetic material made of DNA and proteins. The nucleus also contains a dense structure called the nucleolus.
The functions of the nucleus include controlling all the metabolic activities of the cell, containing the genetic information (DNA) that is passed from one generation to the next, directing cell division, and controlling protein synthesis through the production of RNA.
Question 8. What is the difference between prokaryotic and eukaryotic cells?
Answer:
Prokaryotic cells are primitive, simple cells that do not have a well-defined nucleus — their genetic material lies freely in the cytoplasm without a nuclear membrane. They also lack membrane-bound organelles. Bacteria and blue-green algae are examples of prokaryotic organisms.
Eukaryotic cells are advanced, complex cells that have a well-defined nucleus enclosed by a nuclear membrane. They also contain membrane-bound organelles like mitochondria, endoplasmic reticulum, and Golgi apparatus. All plant and animal cells are eukaryotic.
Question 9. What is the endoplasmic reticulum? What are its types and functions?
Answer:
The endoplasmic reticulum (ER) is a large network of membrane-bound tubules and sheets that extends throughout the cytoplasm of eukaryotic cells. It is connected to the nuclear membrane on one side and to the plasma membrane on the other.
There are two types of endoplasmic reticulum. The rough endoplasmic reticulum (RER) has ribosomes attached to its surface and is involved in the synthesis and transport of proteins. The smooth endoplasmic reticulum (SER) has no ribosomes and is involved in the synthesis of lipids and detoxification of certain substances.
Question 10. What is the Golgi apparatus? What are its functions?
Answer:
The Golgi apparatus, also called the Golgi body or Golgi complex, is a stack of flattened membrane-bound sacs found near the nucleus. It was first described by Camillo Golgi in 1898.
Its main functions include the processing, packaging, and dispatching of materials synthesised in the endoplasmic reticulum to their final destinations inside or outside the cell. It is also involved in the formation of lysosomes and in the modification of proteins and lipids.
Question 11. What are mitochondria? Why are they called the powerhouse of the cell?
Answer:
Mitochondria are double-membrane-bound organelles found in eukaryotic cells. They have an outer smooth membrane and an inner membrane that is folded into finger-like projections called cristae, which increase the surface area for chemical reactions. The inner space is called the matrix and contains its own DNA and ribosomes.
Mitochondria are called the powerhouse of the cell because they are the sites of cellular respiration — the process by which food is broken down in the presence of oxygen to release energy in the form of ATP (adenosine triphosphate). This energy is used for all the activities of the cell.
Question 12. What are plastids? What are their types and functions?
Answer:
Plastids are large organelles found only in plant cells. They are of three types. Chloroplasts contain the green pigment chlorophyll and are the sites of photosynthesis — the process by which plants make their own food using sunlight, water, and carbon dioxide. Chromoplasts contain coloured pigments other than green and give flowers and fruits their colours, helping to attract pollinators. Leucoplasts are colourless plastids that store starch, oils, or proteins.
Question 13. What are lysosomes? Why are they called suicide bags?
Answer:
Lysosomes are small, single-membrane-bound organelles found in animal cells. They contain powerful digestive enzymes that can break down all types of biological polymers — proteins, lipids, carbohydrates, and nucleic acids.
Lysosomes are called suicide bags because during cellular damage or when a cell has outlived its purpose, the lysosomes burst and release their digestive enzymes into the cell, digesting the entire cell from within. This process of self-destruction is called autolysis and is important for removing damaged or old cells from the body.
Question 14. What are vacuoles? How do they differ in plant and animal cells?
Answer:
Vacuoles are membrane-bound storage organelles found in both plant and animal cells. They store water, food, waste products, and other substances.
In plant cells, vacuoles are very large — a single central vacuole may occupy up to 90% of the cell's volume. It stores cell sap, maintains the turgor pressure of the cell, and helps keep the cell rigid. In animal cells, vacuoles are much smaller and more numerous, and they mainly store food or waste products temporarily.
Question 15. What are the differences between plant cells and animal cells?
Answer:
Plant cells have a cell wall made of cellulose while animal cells do not have a cell wall. Plant cells contain chloroplasts for photosynthesis while animal cells do not. Plant cells have a large central vacuole while animal cells have small vacuoles or none at all. Plant cells do not have centrioles while animal cells do. Plant cells are generally larger and have a fixed rectangular shape while animal cells are smaller and have irregular shapes.
FAQs – Chapter 5 The Fundamental Unit of Life
Q1. What is the cell theory? The cell theory was proposed by Matthias Schleiden and Theodor Schwann and later modified by Rudolf Virchow. It states that all living organisms are composed of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells. This theory is one of the most fundamental principles of biology.
Q2. What is the difference between a cell membrane and a cell wall? The cell membrane is a thin, flexible, and selectively permeable layer made of lipids and proteins found in all cells — both plant and animal. The cell wall is a rigid outer covering made of cellulose found only in plant cells, lying outside the cell membrane. The cell wall provides structural support and rigidity while the cell membrane controls the movement of substances in and out of the cell.
Q3. Why do plant cells not burst when placed in a hypotonic solution? When a plant cell is placed in a hypotonic solution, water enters the cell through osmosis. However, the plant cell does not burst because its rigid cell wall provides a counterforce — called wall pressure — that resists the expansion of the cell. The cell becomes turgid but remains intact. Animal cells, which lack a cell wall, would burst under the same conditions.
Q4. What is the function of ribosomes? Ribosomes are tiny organelles found either free in the cytoplasm or attached to the rough endoplasmic reticulum. They are the sites of protein synthesis in the cell. They read the instructions carried by messenger RNA (mRNA) and assemble amino acids into proteins. Ribosomes are found in both prokaryotic and eukaryotic cells.
Q5. Why is the nucleus considered the control centre of the cell? The nucleus is considered the control centre of the cell because it contains the DNA — the genetic blueprint that carries all the instructions for the cell's activities. It controls all metabolic processes, directs cell division, and regulates protein synthesis through RNA. Without the nucleus, the cell cannot carry out its functions properly and will eventually die.