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NCERT Class 11 Biology Chapter 8 Cell
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Cell
Chapter: 8
BIOLOGY
TEXTUAL QUESTIONS ANSWERS
Q.1. Which of the following is not correct?
(a) Robert Brown discovered the cell.
(b) Schleiden and Schwann formulated the cell theory.
(c) Virchow explained that cells are formed from pre-existing cells.
(d) A unicellular organism carries out its life activities within a single cell.
Ans. (a) Robert Brown did not discover the cell.
Q.2. New cells generate from
(a) Bacterial fermentation.
(b) Regeneration of old cells.
(c) Pre-existing cells.
(d) Abiotic materials.
Ans. (c) Pre-existing cells.
Q.3. Match the following
| (a) Cristae. | (i) Flat membranous sacs in stroma. |
| (b) Cisternae. | (ii) Infoldings in mitochondria. |
| (c) Thylakoids. | (iii) Disc-shaped sacs in Golgi apparatus. |
Ans.
| (a) Cristae. | (ii) Infoldings in mitochondria. |
| (b) Cisternae. | (iii) Disc-shaped sacs in Golgi apparatus. |
| (c) Thylakoids. | (i) Flat membranous sacs in stroma. |
Q.4. Which of the following is correct:
(a) Cells of all living organisms have a nucleus.
(b) Both animal and plant cells have a well defined cell wall.
(c) In prokaryotes, there are no membrane bound organelles.
(d) Cells are formed de novo from abiotic materials.
Ans. (c) In prokaryotes, there are no membrane bound organelles.
Q.5. What is a mesosome in a prokaryotic cell? Mention the be functions that it performs.
Ans. Mesosome is membranous structure formed in prokaryotic cell by the invagination of the plasma membrane. Mesosome helps in the synthesis of the cell wall, replication of DNA and equal distribution of chromosomes into the daughter cells. It increases the surface area of the plasma membrane to do enzymatic activities. It also performs aerobic respiration.
Q.6. How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
Ans. Neutral molecules move across the plasma membrane by passive diffusion. Movement of polar molecules across the cell membrane occurs through integral proteins/tunnel proteins which form channels for the passage of water and soluble substances.
Q.7. Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their function and draw labelled diagrams of both.
Ans. Mitochondria and chloroplasts are the two organelles that are double membrane bound. Mitochondrion contains two membranes and two chambers, outer and inner. The two membranes form the envelope of the mitochondrion.
(a) Inner Membrane-
(i) The inner membrane is infolded to form involutions called cristae
(ii) They are meant for increasing the physiologically active area of inner membrane.
(iii) The inner membrane and cristae has small tennis racket like particles called elementary particles, F₀ – F₁ particles or oxysomes (=oxisomes).
(iv) Elementary particles function as ATP-ase. They are centre’s of ATP synthesis during oxidative phosphorylation.
(b) Outer Chamber-
(i) Outer chamber is the space that lies between outer and inner membrane of mitochondrial envelope.
(ii) The chamber contains a fluid having a few enzymes.
(c) Inner chamber-
(i) It forms the core of mitochondrion.
(ii) The inner chamber contains a semi-fluid matrix.
(iii) The matrix has protein particles, ribosomes, RNA, DNA, enzymes of Krebs or TCA cycle. Mitochondrial ribosomes are 55 S to 70 S in nature. DNA is naked. It is commonly circular.
(d) Chloroplasts-
(i) They are greenish plastids which possess photosynthetic pigments, chlorophyll and carotenoids and take part in the synthesis of food from inorganic raw materials in the presence of radiation energy.
(ii) A chloroplast has three parts envelope, matrix and thylakoids.
(e) Matrix:
(i) The ground substance of a chloroplast is known as matrix or stoma.
(ii) It is semifluid made of proteins, DNA, RNA, ribosomes, and enzymes.
(iii) Chloroplast DNA is naked and circular.
(iv) DNA makes chloroplast genetically autonomous because it can replicate & transcribe to form RNA.
(v) Chloroplast ribosomes are 70 S.
(f) Thylakoids:
(i) Are membrane lined flattened sacs in the stroma or matrix of the chloroplast.
(ii) They are stacked at places to form grana.
(iii) Thylakoids are structural elements of chloroplast.
(iv) Their membrane have photosynthesis pigments and coupling factors involved in ATP synthesis.
Q.8. What are the characteristics of prokaryotic cells?
Ans. Prokaryotic Cells-
1. One envelope system: single membrane surrounds the cell and membrane bound cell organelles are absent.
2. Flagella is single stranded and without differentiation into axoneme and sheath.
3. Organized nucleus is absent. Nucleoid or pro chromosome is found.
4. DNA is circular, naked without histones.
5. DNA lies freely in cytoplasm.
6. The ratio of A +T/G + C is low, < 1.
7. Ribosomes are 70 S.
8. True vacuoles are absent.
9. Cell wall has muramic acid.
Q.9. Multicellular organisms have division of labour. Explain.
Ans. Multicellular organisms have cells organised into tissues and organs to perform a specific function like alimentary canal for digestion, heart to pump blood etc.
Q.10. Cell is the basic unit of life. Discuss in brief.
Ans. All living beings are made of cell and their products. Cell is the basic unit of life. Cell is structural and functional unit of an organism. Cell is the smallest unit capable of independent existence. All cells have a fundamental similarity in their structure, chemical composition and metabolism. New cell develop from preexisting cell through division.
Q.11. What are nuclear pores? State their function.
Ans. Nuclear envelope contains a large number of pores or perforations called nuclear pores.
They allow molecules like RNA and proteins to move in both directions between the nucleus and the cytoplasm.
Q.12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
Ans. Lysosomes-
(i) Lysosomes are small vesicles bounded by a single membrane.
(ii) They contain hydrolytic enzymes called acid hydrolases because these digestive enzymes usually function in acidic medium of pH of 4-5. Acidic conditions are maintained inside the lysosomes.
(iii) They take part in digestion, autophagy.
Vacuoles-
(i) Vacuoles are separated from cytoplasm by a membrane called tonoplast.
(ii) They are of four types:
1. Sap Vacuoles-
(a) They are fluid filled vacuoles which are separated from the cytoplasm by a selectively permeable membrane called tonoplast.
(b) In mature plant cell, small vacuoles fuse to form a single large central vacuole which occupies 90% volume of cell.
(c) The fluid present in the sap vacuoles is called sap or vacuolar sap. It contains minerals, waste products and water soluble pigments.
(d) The sap vacuoles store and concentrate waste products.
2. Contractile Vacuoles-
(a) They occur in Protista and algal cells.
(b) Contractile vacuoles take part in osmoregulation and excretion.
3. Food Vacuoles-
(a) They occur in protozoans.
(b) A food vacuole is formed by fusion of phagosome and a lysosome.
4. Air Vacuoles/ Gas vacuoles-
(a) They occur only in prokaryotes.
(b) It consists of small vesicles. Each vesicle encloses metabolic gases.
Q.13. Describe the structure of the following with the help of labelled diagrams.
(i) Nucleus.
Ans. Nucleus is a specialized double membrane cell organelle which contains all genetic information. Nucleus is the largest cell organelle. It is differentiated into five parts – nuclear envelope, nucleoplasm, nuclear matrix, chromatin and nucleolus.
(ii) Centrosome.
Ans. Centrosome The complex formed of centrioles and centrosphere is called centrosome. They are found in all eukaryotic animal cells. They are capable of replication.
Q.14. What is a centromere? How does the position of centromere form the basis of classification of chromosomes? Support your answer with a diagram showing the position of centromere on different types of chromosomes.
Ans. There are two chromosome halves or chromatids attached to each other by a narrow area called centromere or primary constriction.Based on the position of centromere, chromosomes are of four types-
(i) Telocentric – Centromere terminal [in the area of telomere]
(ii) Acrocentric – Centromere subterminal.
(iii) Submetacentric – Centromere submedian or slightly away from middle.
(iv) Metacentric – Centromere in middle.
