NCERT Class 9 Science Important Chapter 2 Cell: The Building Block of Life English Medium As Per New Syllabus. NCERT Class 9 Science Important Chapter 2 Cell: The Building Block of Life Notes to each chapter is provided in the list so that you can easily browse throughout different chapter NCERT Class 9 Science Important Chapter 2 Cell: The Building Block of Life Question Answer Download PDF and select needs one. CBSE Class 9 Science Additional Solutions.
NCERT Class 9 Science Important Chapter 2 Cell: The Building Block of Life
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Cell: The Building Block of Life
Chapter – 2
| IMPORTANT QUESTION ANSWER |
Short Questions & Answers:
1. Where do some researchers believe life might have originated on early Earth?
Ans: Some researchers believe life may have originated in small water pools with changing environmental conditions, such as hot springs, rather than in the oceans.
2. What is the limit of resolution of the human eye?
Ans: The limit of resolution of the human eye is 0.1 mm, which means if two points are closer than this, they appear as a single point to the unaided eye.
3. Who was the first person to observe a cell?
Ans: Robert Hooke was the first person to observe a cell in 1665 by examining a thin slice of cork under a self-designed microscope.
4. What is the difference between unicellular and multicellular organisms?
Ans: Unicellular organisms consist of only one cell, while multicellular organisms are made up of millions of cells working together.
5. What does the term “selectively permeable” mean in relation to the cell membrane?
Ans: It means the cell membrane allows some specific substances to pass through it while blocking others.
6. What is osmosis?
Ans: Osmosis is the diffusion of water across a selectively permeable membrane from an area with more water (dilute solution) to an area with less water (concentrated solution).
7. How thick is the cell membrane?
Ans: Structurally, the cell membrane is extremely thin, about 7 to 10 nanometres (nm) thick.
8. What is the plant cell wall primarily made of?
Ans: The plant cell wall is primarily made of cellulose, a type of carbohydrate formed by many glucose units linked together.
9. What is a prokaryotic cell?
Ans: A prokaryotic cell lacks a well-defined nucleus and membrane-bound organelles.
10. What is a nucleoid?
Ans: The nucleoid is the region in a prokaryotic cell containing the genetic material as a single circular molecule associated with specific proteins, without a surrounding membrane.
11. Why do mature human Red Blood Cells (RBCs) lack a nucleus?
Ans: The absence of a nucleus provides more space for haemoglobin, allowing it to transport a larger amount of oxygen to all cells of the body.
12. What are the functional segments of DNA called?
Ans: The functional segments of DNA are called genes.
13. What is the main function of ribosomes?
Ans: Ribosomes are tiny structures in the cell that act as the sites of protein synthesis.
14. Why does the Rough Endoplasmic Reticulum (RER) look rough under a microscope?
Ans: It looks rough under an electron microscope because it has ribosomes attached to its surface.
15. Which organelle acts as the cell’s post office?
Ans: The Golgi apparatus acts like the cell’s post office by modifying, sorting, and packaging proteins and lipids into vesicles for transport, secretion, or lysosome formation.
16. Why are lysosomes known as the clean-up system of the cell?
Ans: Lysosomes contain enzymes that break down unwanted waste materials and damaged parts of the cell, keeping it clean and healthy.
17. Which organelles are known as the ‘powerhouses of the cell’?
Ans: Mitochondria are called the ‘powerhouses of the cell’ because they supply the ATP energy needed for most cellular activities.
18. What pigment is present in the chloroplasts?
Ans: Chloroplasts contain a green pigment called chlorophyll, which absorbs sunlight for photosynthesis.
19. What is the function of the large central vacuole in mature plant cells?
Ans: It stores water, minerals, sugars, and waste material, and helps maintain pressure inside the cell to keep the plant firm.
20. What is “contact inhibition”?
Ans: Contact inhibition is a process where cell division stops when cells come into contact with neighbouring cells, which is a control mechanism that cancer cells lose.
Fill in the Blanks:
1. Robert Hooke first observed a __________ in 1665.
Ans: Cell.
2. The plant cell wall is mainly made of __________.
Ans: Cellulose.
3. Ribosomes are the sites of __________ synthesis.
Ans: Protein.
4. Mitochondria are known as the __________ of the cell.
Ans: Powerhouses.
5. Chloroplasts contain the green pigment called __________.
Ans: Chlorophyll.
True or False:
1. Unicellular organisms are made up of only one cell.
Ans: True.
2. A prokaryotic cell has a well-defined nucleus.
Ans: False.
3. The Golgi apparatus modifies, sorts, and packages proteins.
Ans: True.
4. Lysosomes help clean the cell by breaking down waste materials.
Ans: True.
5. Meiosis produces two genetically identical daughter cells.
Ans: False.
Long Questions & Answers:
1. Explain the fluid-mosaic model of the cell membrane.
Ans: The fluid-mosaic model explains the structure of the cell membrane, which is made of lipids and proteins. It consists of a fluid lipid bilayer, meaning two layers of fat molecules with water-attracting heads facing outwards and water-repelling tails facing inwards, where molecules can move sideways, flip, and rotate. It is called a “mosaic” model because various proteins are embedded within this lipid layer like tiles, acting as gatekeepers to help specific substances pass through.
2. Discuss the main differences between plant and animal cells.
Ans: Plant cells have a rigid outer cell wall made of cellulose to help them withstand environmental stresses and maintain their shape, whereas animal cells lack a cell wall and only have a flexible cell membrane. Plant cells contain plastids, like chloroplasts for photosynthesis and leucoplasts for food storage, while animal cells do not. Additionally, a mature plant cell typically has one large central vacuole to maintain cell firmness, whereas animal cells have much smaller, temporary vacuoles. Due to their rigid cell walls, plant cells also do not show contact inhibition during growth, unlike normal animal cells.
3. Describe the structure and function of the nucleus in a eukaryotic cell.
Ans: The nucleus is the house of coded instructions for the cell, enclosed by a double-layered nuclear membrane containing pores for material transfer between the nucleus and the cytoplasm. Inside, it houses the nucleolus, where ribosomal subunits are synthesised, and chromatin material. During cell division, chromatin organises into rod-shaped chromosomes made of DNA and specific proteins. The DNA contains genes, which carry the genetic information required for inheriting characteristics from parents and directing the cell’s activities.
4. How do mitochondria and plastids suggest a shared evolutionary history with bacteria?
Ans: Both mitochondria and plastids, such as chloroplasts, are double-membrane-bound organelles. Similar to certain single-celled bacteria, both of these organelles possess their own DNA and ribosomes. Because they contain their own genetic material and ribosomes, they are able to make some of their own proteins independently of the cell’s main nucleus. These unique, bacteria-like characteristics suggest that both mitochondria and plastids share an evolutionary history with these single-celled organisms.
5. Explain the differences between mitosis and meiosis.
Ans: Mitosis is a type of cell division that is important for normal growth, repair, and maintenance, producing two genetically identical daughter cells with the exact same DNA and number of chromosomes as the parent cell. Meiosis, on the other hand, occurs only in the reproductive organs to produce gametes (sperm and eggs) for sexual reproduction, creating genetic variations. Meiosis is a two-step division process that results in four daughter cells, each containing only half the number of chromosomes as the parent cell, ensuring that the original chromosome number is restored during fertilisation.
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