Class 11 Biology


NCERT Solution

Question 1: Which of the following is not correct?

  1. Robert Brown discovered the cell.
  2. Schleiden and Schwann formulated the cell theory.
  3. Virchow explained that cells are formed from pre-existing cells.
  4. A unicellular organism carries out its life activities within a single cell.

Answer: (a) Robert Brown discovered the cell.

Question 2: New cells generate from

  1. Bacterial fermentation
  2. Regeneration of old cells
  3. Pre-existing cells
  4. Abiotic materials

Answer: (c) Pre-existing cells

Question 3: Match the following

Column IColumn II
(a) Cristae(1) Flat membranous sac in stroma
(b) Cisternae(2) Inflodings in mitochondria
(c) Thylakoids(3) Disc-shaped sacs in Golgi apparatus

Answer: (a) 2, (b) 3, (c) 1

Question 4: Which of the following is correct?

  1. Cells of all living organisms have a nucleus.
  2. Both animal and plant cells have a well defined cell wall.
  3. In prokaryotes, there are no membrane bound organelles.
  4. Cells are formed de novo from abiotic materials.

Answer: (c) In prokaryotes, there are no membrane bound organelles

Question 5: What is a mesosome in a prokaryotic cell? Mention the functions that it performs.

Answer: A special membranous structure is formed by the extensions of plasma membrane into the cell. This is called the mesosome. These extensions are in the form of vesicles, tubules and lamellae. The mesosomes help in cell wall formation, DNA replication and distribution to daughter cells. They also help in respiration, secretion process, to increase the surface area of the plasma membrane and enzymatic content.

Question 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?

Answer: Neutral solutes move across the plasma membrane through passive transport, i.e. by diffusion and osmosis. But polar molecules need a carrier protein of the membrane to be transported across against concentration gradient. This type of transport is dependent on energy and is called active transport.

Question 7: Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labeled diagrams of both.

Answer: Mitochondria and chloroplast are two examples of double-membrane bound cell organelles. These two organelles have self-replicating capabilities. Mitochondria are the site of aerobic respiration. Chloroplasts are the site of photosynthesis.

Question 8: What are the characteristics of prokaryotic cells?

Answer: Membrane bound nucleus is absent in prokaryotic cell. Moreover, membrane bound organelles are also absent in prokaryotic cells. Bacteria are examples of prokaryotic cell.

Question 9: Multicellular organisms have division of labour. Explain.

Answer: In a unicellular organism, a single cell is responsible for all the life processes. This is called cellular level of organization. This can be seen in some simple multicellular organisms as well. But in most of the multicellular organism, there are different groups of cells to carry different functions. Thus, formation of tissues paves the way for division of labour in multicellular organisms.

Question 10: Cell is the basic unit of life. Discuss in brief.

Answer: Unicellular organisms are capable of independent existence and they can perform the essential functions of life. Anything less than a complete cell does not ensure independent living. Hence, cell is called the fundamental structural and functional unit of life.

Question 11: What are nuclear pores? State their function.

Answer: The nuclear membrane is interrupted by minute pores at various places. These are called nuclear pores. These pores provide passage to RNA and protein molecules.

Question 12: Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.

Answer: The functions of lysosomes and vacuoles are coordinated with the functions of other members of the endomembrane system. Hence, they are part of the endomembrane system. As they are different organelles, so their functions are also different. Lysosomes digest various substances, while vacuoles facilitate expulsion of waste products from the cell.

Question 13: Describe the structure of the following with the help of labelled diagrams: (i) Nucleus (ii) Centrosome

Answer: Nucleus: Nucleus is enclosed by a double-membrane nuclear envelope. The space between the two membranes is called the perinuclear space. The perinuclear space forms a barrier between the nucleic materials and cytoplasmic materials. The outer membrane is usually continuous with the endoplasmic reticulum. Ribosomes are present on the outer membrane of nuclear envelope.

Centrosome: Centrosome usually contains two cylindrical structures which are called centrioles. They are surrounded by amorphous pericentriolar materials.

Question 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.

Answer: A chromosome has a primary constriction called centromere. On the basis of position of the centromere, chromosomes can of four types, viz. metacentric, sub-metacentric, acrocentric and telocentric.

When the centromere divides the chromosomes into two identical arms, it is called metacentric chromosome. When the centromere is slightly away from the middle, it is called sub-metacentric chromosome. When the centromere divides the chromosome into a smaller and another much larger arm, it is called acrocentric chromosome. When the centromere is at the tail, it is called telocentric chromosome.