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.

Solution:

The answer is (a) Robert Brown discovered the cell

It is incorrect because Robert brown discovered nucleus in the cell.

2. New cells generate from

(a) bacterial fermentation (b) regeneration of old cells

(c) pre-existing cells (d) abiotic materials

Solution:

The answer is (c) pre-existing cells

3. Match the following

Solution:

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

Solution:

The answer is (c) In prokaryotes, there are no membrane-bound organelles.

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

Solution:

A mesosome is a unique membranous structure formed by the extensions of plasma membrane into the cell. It it is found attached to the nucleoid, it is known as septal mesosome and is known as lateral mesosome if the mesosome is free from the nucleoid.

Functions of mesosome are as follows

• They help in cell-wall formation
• They help in DNA replication and distribution to daughter cells.
• They also help in respiration, secretion processes, to increase 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?

Solution:

The neutral solutes are lipid-soluble hence they pass through lipid bilayer. No polar molecules cannot move across the plasma membrane in the same way as neutral solutes; they require particular hydrophilic areas for their passage. They get transported by three types of transport mechanisms – ions, channels, permeases and active transport utilising ATP. 

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.

Solution:

Two organelles that are double membrane-bound are i) Mitochondria ii) Chloroplasts

Characteristics of mitochondria

The mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e., the outer compartment and the inner compartment.

• Mitochondria is semi-autonomous as they possess their own DNA.
• Mitochondria is the place of aerobic respiration.

Characteristics of Chloroplasts

• They are found in plants and euglenoids, variously shaped in lower plants and disc-shaped in higher plants
• They have specific pigments which imparts colour to the leaves of the plants
• Chloroplasts possess their own DNA.
• Pigments help to trap sunlight thereby helps in photosynthesis.
• Consists of membrane-flattened sacs known as thylakoids located in their matrix. At some locations, thylakoids are stacked and are known as gana.

8. What are the characteristics of prokaryotic cells?

Solution:

• The nuclear membrane is absent and the genetic material is not enclosed by an envelope. The naked DNA is found to be coiled in the cytoplasm and is referred to as a genophore or nucleoid
• They lack membrane bound organelles and membrane bound nucleus
• Cell lumen is filled with a fluid known as cytoplasm
• Prokaryotic DNA is situated in the nucleoid of the cell.
• Prokaryotic cellwall acts as alyer of protection and helps in maintaining cell shape.
• Ribosome is the only cytoplasmic organelle that is found in prokaryotic cells

9. Multicellular organisms have division of labour. Explain.

Solution:

Cells are organized to form tissues which make organ and organ system. A cell is an autonomous structure which is capable of carrying out functions on its own. Hence division of labour is essential to carry out different functions for increased efficiency and higher survival. 

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

Solution:

Every plant and animal cells is made of organ systems and organ systems are made up of organs. Organs are constructed by tissues, and tissues are made up of cluster of cells. Sicne cells are autonomous structures capable of carrying out functions on their own, it is said that cell is the basic unit of life.

11. What are nuclear pores? State their function.

Solution:

At several places nuclear membrane are surrounded by minute pores formed by the fusion of two membranes called as nuclear pore. They are simple perforations on the nuclear envelope.

Functions:

• Retains the shape of the nucleus
• Known to preserve the stability of the genetic material by safeguarding it from respiratory breakdown occurring in the cytoplasm
• Responsible for the movement of RNA and protein molecules 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.

Solution:

The endomembrane is an intercellular system reponsible for the flow of materials from one to another part through vesciles. Some of its components are vacuoles, plasma membrane, endoplasmic reticulum, lysosomes and golgi apparatus. Lysosomes are membrane-bound organelles that release lytic enzymes to digest worn-out cells hence they are known as suicidal bags. On the other hand vacuoles help cells to maintain their shape. Vacuoles also store food, water and waste products.

13. Describe the structure of the following with the help of labelled diagrams.

(i) Nucleus (ii) Centrosome

Solution:

i) The nucleus is a oval shaped or round structure having a double membrane containing the genetic material. It consists of five parts, namely – nuclear envelope, nuclear matrix, nucleoplasm, nucleolus and chromatin.

Nuclear envelope – The nucleus is constrained by the nuclear pore which is a double membrane nuclear envelope having tiny pores in the membrane which serve as channel for substances to pass in and out of the nucleus. The endoplasmic reticulum is connected to the outer membrane which also contains the ribosomes.

Nucleoplasm – it is the fluid filled in the nucleus containing enzymes, nucleosides, proteins and other factors responsible for the functioning of the genetic material. It also embeds the chromatin fibers and nucleolus.

Chromatin – found inside the nucleus, they are a fine network of thread-like structures containing DNA and a few basic proteins such as histones, RNA and non-histone proteins. These chromatin fibers during the process of cell division condense to form the chromosomes.

Nucleolus – They are attached with the chromatin and are round, lightly irregular, naked structures that produce the ribosome subunits. They produce proteins.

(ii) Centrosome

• Consists of two cylindrical structures known as centrioles which lie perpendicular to one another, organized as a catrwheel
• Engirdled by amorphous pericentriolar materials
• Consists of 9 even spaced peripheral fibrils of the tubulin protein, wherein each is a triplet and adjacent triplets are linked to each other
• The centre of the centriole is a proteinaceous hub attached to the triplets through radial spokes
• It is critical during cell division as it organizes the spindle fibers and astral rays

14. What is centromere? How does the position of centromere form the basis of classification of chromosomes? Support your answer with a diagram showing the position of the centromere on different types of chromosomes.

Solution:

Primary constriction present in a chromosome is called the centromere.

Centromere holds two chromatids of a chromosome.

Based on the position of the centromere, the chromosomes can be classified into four types:

• Metacentric has two middle centromeres forming two equal arms of the chromosomes.
• Submetacentric chromosomes have centromere slightly away from the middle of the chromosomes resulting in one shorter arm and one more extended arm.
• Acrocentric chromosomes have centromere situated close to its end forming one extremely short and one very long arm.
• Telocentric chromosome has a terminal centromere.