The VIDYANKER team has thoughtfully prepared the NCERT Solutions for Class 11 Biology Chapter 8, "Cell – The Unit of Life" These solutions are designed to help you tackle the NCERT textbook questions with ease. We recommend going through the chapter's theory before diving into the solutions for a deeper understanding. Feel free to share these NCERT Solutions for Class 11 Biology with others—learning is always better when shared!
NCERT TEXTBOOK QUESTIONS SOLVED
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.
| 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 membranous structure in a prokaryotic cell, which is produced by the extensions of the plasma membrane into the cell in the form of vesicles, tubules, and lamellae. Mesosomes are equivalent to mitochondria in eukaryotes, as they carry out aerobic cellular respiration in prokaryotes. It assists in DNA replication and the distribution of genetic material to the daughter cells. Mesosomes aid in respiration, also increasing the surface area of the cell's plasma membrane and enzymatic content and cell wall formation.
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 diffuse across the membrane by simple diffusion along the concentration gradient.
The polar molecules cannot pass through the non-polar lipid bilayer of the membrane, they need a carrier protein to help them cross 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 which involves the use of ATP and is termed as active transport, for example, 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. Mitochondria and chloroplast are double membrane-bound organelles. Mitochondria: They are cylindrical or sausage-shaped cell organelles containing two membranes, outer and inner. Inner compartment is the matrix, a structure which bears DNA, RNA, ribosomes, Kreb's cycle, and so forth; and an outer membrane serves as the single limiting boundary to the organelle. There are numerous invaginations from the inner one that constitute an increase in its surface area which are known as the cristae. Oxysomes occur on the inner mitochondrial membrane. Mitochondria are semiautonomous organelles; they have their own DNA and ribosomes.
8. What are the characteristics of prokaryotic cells?
Ans. Prokaryotic cell is a unicellular organism lacking membrane-bound organelles.
The characteristics of prokaryotic cells are as follows:
→ Most of them are unicellular.
→ They are generally small in size. The size of a prokaryotic cell varies from 0.5 - 5 µm.
→ The nuclear region of a prokaryotic cell is poorly-defined because of the absence of a nuclear membrane. Hence, a prokaryotic cell lacks a true nucleus.
→ The genetic materials of prokaryotic cells are naked. They contain single, circular chromosomes. In addition to the genomic DNA, they have a small, circular plasmid DNA.
→ They have specialised membranous structures called mesosomes. Mesosomes are formed by the invagination of the cell membrane. These extensions help in the synthesis of the cell wall, replication of DNA. They also help in the equal distribution of chromosomes into the daughter cells.
→ Membrane-bound cell organelles such as mitochondria, plastids, and endoplasmic reticulum are absent from a prokaryotic cell.
→ Most prokaryotic cells contain a three-layered structure - outermost glycocalyx, middle cell wall, and the innermost plasma membrane. This structure acts as a protective unit.
9. Multicellular organisms have division of labour. Explain.
Ans. Multicellular organisms are composed of millions and trillions of cells. All these cells perform specific functions. All the cells specialized for performing similar functions are grouped together as tissues in the body. Hence, a particular function is carried out by a group of cells at a definite place in the body. Similarly, different functions are carried out by different groups of cells in an organism. This is commonly referred to as division of labor in multicellular organisms.
10. Cell is the basic unit of life. Discuss in brief.
Ans. Every animal or plant is composed of many systems. Every system consists of many organs. Organs are made of many types of tissues and tissues are made up of many cells. A cell is an autonomous structure and it can carry out various functions itself. A cell can do everything an organism can do. In a sense cell is full of life at the minutest possible level. That is why a cell is called the basic unit of life.
11. What are nuclear pores? State their function.
Ans. Nuclear pores are small holes found in the nuclear membrane of the nucleus. It is essentially formed through the joining of two nuclear membranes.
These holes 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 carry out the function of phagocytosis. Hence, they are also referred to as suicidal bags. Vacuoles are non-cytoplasmic sacs which are covered by a membrane as well. Sap vacuoles have dissolved inorganic and organic substances in sap or water. They keep up osmotic pressure or turgidity in the cell. Thus, both lysosomes and vacuoles are endomembranous structures but vary according to the functions which, they perform.
13. Describe the structure of the following with the help of labelled diagrams.
(a) Nucleus
(b) Centrosome
Ans. Nucleus controls all the cellular activities of the cell. It is spherical in shape. It is composed of the following structures:
• Nuclear membrane It is a double membrane that divides the contents of the nucleus with the cytoplasm. Between the two membranes, there exists a narrow gap called the perinuclear space. Nuclear membrane has tiny pores known as nuclear pores. They permit specific things to be moved into a cell and out of it.
• Nucleoplasm/Nuclear matrix: It is a homogenous granular fluid present inside the nucleus. It contains the nucleolus and chromatin. Nucleolus is a spherical structure that is not bound by any membrane. It is rich in protein and RNA molecules, and is the site for ribosome formation. Chromatin is an entangled mass of thread-like structures. It contains DNA and some basic proteins called histones.
Centrosome
Centrosome is made up of two cylindrical structures called centrioles. They lie perpendicular to each other. Each has a cartwheel-like organization.
Microtubule triplets which are evenly spaced in a ring make a centriole. The adjacent triplets are joined together. In the center of a centriole, a central part is proteinaceous. The hub is attached to the triplets with the help of radial spokes. These centrioles are involved in organizing spindle fibres and astral rays during cell division. They form basal bodies of cilia and flagella.
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. A centromere is the constricted region of a chromosome which takes part in cell division, i.e., mitosis and meiosis. The spindle is the structure that draws the chromatids to opposite poles of the cell during cell division. Each chromosome has only one centromere. Centromeres participate in sister chromatid adhesion as well as their separation, movement of chromosomes, attachment of microtubules, mitotic checkpoint control, etc. On the basis of the location of the centromere, four types of chromosomes are distinguished: metacentric, submetacentric, acrocentric, and telocentric.
