NCERT Solutions for Class 11 Biology Chapter 8: Cell – The Unit of Life
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 discovered the cell.
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
3. Match the following:
|Column I||Column II|
|(a) Cristae||(i) Flat membranous sacs in stroma|
|(b) Cisternae||(ii) Infoldings in mitochondria|
|(c) Thylakoids||(iii) Disc-shaped sacs in Golgi apparatus|
Ans. The correct match is as follows:
|Column I||Column II|
|(a) Cristae||(ii) Infoldings in mitochondria|
|(b) Cisternae||(iii) Disc-shaped sacs in Golgi apparatus|
|(c) Thylakoids||(i) Flat membranous sacs in stroma|
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.
5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
Ans. Mesosome is a special membrane bound structure, which is formed by the extension of the plasma membrane in a prokaryotic cell.
Functions of Mesosome:
(a) It helps in cell wall formation, DNA replication and its distribution to the daughter cells.
(b) It also helps in respiration, secretion possesses, increases the surface area of the plasma membrane and enzymatic content.
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 solutes may move across the membrane by the process of simple diffusion along the concentration gradient.
The polar molecules cannot pass through the non-polar lipid bilayer of the membrane, they require a carrier protein to facilitate their transportation across the membrane. A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to the higher concentration. Such
a transport is an energy dependent process, in which ATP is utilised and is known as active transport, e.g., Na+/K+ pump.
7. Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions
and draw labelled diagrams of both.
Ans. Chloroplasts and mitochondria are double membrane bound organelles.
Characteristics of Mitochondria:
(a) The mitochondria are sausage-shaped or cylindrical in shape having a diameter of 0.2-1.0 μm, average diameter of about 0.5 μm and length of 1.0-4.1 μm.
(b) The inner compartment is called the matrix and the outer membrane of mitochondria forms the continuous limiting boundary of the organelle.
(c) The inner membrane forms a number of infoldings called the cristae (single crista) towards the matrix. The cristae increases the surface area for absorption.
(d) The two membranes have their own specific enzymes associated with the mitochondrial function. The matrix of mitochonrdria also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70 S) and the components required for the synthesis of proteins.
Functions of Mitochondria:
(a) Mitochondria are sites of aerobic respiration.
(b) They produce cellular energy in the form of ATP hence, they are called as power house of the cell.
Characteristics of chloroplasts:
(a) A number of organized flattened membranous sacs called thylakoids are present in the stroma.
(b) The space bounded by the inner membrane of the chloroplast is called the stroma.
(c) Thylakoids are arranged in stacks-like the piles of coins called grana.
(d) In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.
(e) The stroma of the thy lakoids enclose a space called a lumen.
Functions of Chloroplasts:
(a) Chloroplasts are plant cell organelles that convert light energy into chemical energy via the process of photosynthesis. By doing so, they sustain life on Earth.
(b) Chloroplasts also provide diverse metabolic activities for plant cells, including the synthesis of fatty acids, lipids, etc.
8. What are the characteristics of prokaryotic cells?
Ans. Characteristics of prokaryotic cells:
(a) A prokaryotic cell is surrounded by a cell membrane. The cell wall in turn is surrounded by a slimy layer.
(b) It lacks membrane bound cell organelles such as chloroplast, mitochondria and golgi apparatus.
(c) The true nucleus with nuclear membrane and nucleolus are absent.
(d) The cytoplasm is filled with dense granules. Most of these granules are ribosomes.
9. Multicellular organisms have division of labour. Explain.
Ans. In multicellular organisms, cell is the basic structural and functional unit. The cells are organised to form tissues of blood, bone, etc. These tissues are further organised to form organs like heart, kidney, etc. The organs then organise to form organ systems such as digestive system, reproductive system and respiratory system, and the various organ systems of organism get arranged to form a complete individual. Hence, a specific group of cells performs the specific functions. Similarly, there are different functions that are carried out by different groups of cells and this is known as division of labour in multicellular organisms.
10. Cell is the basic unit of life. Discuss in brief.
Ans. The cell is the structural and functional unit of life because certain organisms complete their life cycle as a single cell. They are called unicellular or acellular organisms, e.g., Amoeba, Chlamydomonas, bacteria and yeast. In other organisms, the single cell undergoes divisions to form multicellular body. Body of human being, is made up of trillion of cells. All the cells of an organism carry the same genetic material, develop from same pre-existing cells and possess several organelles to perform various life activities. Thus the cells are therefore, basic unit of life.
11. What are nuclear pores? State their function.
Ans. Nuclear pores are tiny holes present in the nuclear membrane of the nucleus. They are formed by the fusion of two nuclear membranes.
These nuclear pores allow specific substances to be transferred into a cell and out from it. They allow molecules such as RNA and proteins to move in both directions, between the nucleus and the cytoplasm.
12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
Ans. Both lysosomes and vacuoles are covered by a single membrane. Both of them perform different types of functions. Lysosomes contain hydrolysing enzymes and can hydrolyse all types of organic substances, except cellulose.
They perform phagocytic function. Therefore, they are also known as suicidal bags. The vacuoles are non-cytoplasmic sacs which are also covered by a membrane. The sap vacuoles store sap or water with dissolved organic and inorganic substances. They maintain osmotic pressure or turgidity of the cell. Thus, both lysosomes and vacuoles are endomembranous structures but differ from each other in the functions which, they perform.
13. Describe the structure of the following with the help of labelled diagrams.
Ans. (a) Nucleus: The nucleus is a double membrane bound structure. It regulates the cellular activities of the cell. It constitutes the following parts:
1. Nuclear membrane: It is a double membrane separating the contents of the nucleus from the cytoplasm. The narrow space between the two membranes is called the perinuclear space. The nuclear membrane has tiny holes called nuclear pores. These holes allow specific substances to be transferred into the nucleus and out of it.
2. Nucleoplasm/Nuclear matrix: It is a homogenous granular fluid present inside the nucleus. It contains the nucleolus and chromatin. The nucleolus is a spherical structure that is not bound by any membrane. It is rich in protein and RNA molecules and is the site of ribosome formation. Chromatin is thread-like structures containing DNA and some basic proteins called histones.
(b) Centrosome: It is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like that of the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin. There is a proteinaceous hub in the central part of a centriole which is connected with tubules of the peripheral triplet by radial spokes made of protein. The centriole form the basal bodies of cilia or flagella and spindle fibres that give rise to spindle apparatus during cell division in animal life.
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. The centromere is a constriction present on the chromosomes where the chromatids are held together. Chromosomes are divided into four types based on the position of the centromere.
(a) Metacentric chromosome: The chromosomes in which the centromere is present in the middle and divides the chromosome into two equal arms is known as a metacentric chromosome. During anaphase, they appear V-shaped.
(b) Sub-metacentric chromosome: The chromosome in which the centromere is slightly away from the middle region is known as a sub-metacentric chromosome. In this, one arm is slightly longer than the other. During anaphase, they appear L-shaped.
(c) Acrocentric chromosome: The chromosome in which the centromere is located close to one of the terminal ends is known as an acrocentric chromosome. In this, one arm is extremely long and the other is extremely short. During anaphase, they appear J-shaped.
(d) Telocentric chromosome: The chromosome in which the centromere is located at one of the terminal ends is known as a telocentric chromosome. During anaphase, they appear I-shaped.