NCERT Solutions For Class 11 | Biology | Chapter 8 | Cell

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NCERT Solutions for Class 11 Biology Chapter 8 Cell: The Unit Of Life is the essential study material needed to perfect Cell: The Unit of Life topics.

NCERT Class 11 Biology solutions provided here have correct answers to NCERT textbook questions. Solutions curated in a comprehensive manner will help students to understand the subtopics in this chapter in a better way.

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.

Answer –

Option (a) is incorrect. Robert Brown discovered the nucleus in the cell.

2. New cells generate from

(a) bacterial fermentation
(b) regeneration of old cells
(c) pre-existing cells
(d) abiotic materials

Answer –

Option (c) pre-existing cells is the correct answer.

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

Answer –

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

Answer –

Option (c) In prokaryotes, there are no membrane-bound organelles is correct.

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

Answer – A special membranous structure formed by the extensions of the plasma membrane into the cell is called a mesosome. They are in the form of vesicles, tubules and lamellae. They perform the following functions:

Mesosomes help in the cell wall formation, DNA replication and distribution to daughter cells.
They help in respiration, and secretion, increasing the surface area of the plasma membrane and the 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?

Answer – Neutral solutes dissolve in fat, i.e. lipid-soluble and thus pass through the lipid bilayer. No, polar molecules cannot cross the plasma membrane in the same way as neutral solutes; they require specific hydrophilic regions for their passage. They are transported by three main types of transport mechanisms – ionic, channel, permeable, and active transport using 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.

Answer – Cell organelles that are double-membrane are chloroplast and mitochondria.

Credits – NCERT

CHARACTERISTICS OF MITOCHONDRIA:

They are double-membrane with an outer membrane and inner membrane dividing its lumen into two aqueous compartments.
The inner compartment is called the matrix whereas the outer membrane forms infoldings called cristae towards the matrix that increase the surface area.
They are semi-autonomous. They contain genetic material in their matrix and can divide by fission.
FUNCTIONS OF MITOCHONDRIA:
- They are sites for aerobic respiration.
- They are called powerhouses of the cell as they produce cellular energy in the form of ATP.

Credits – NCERT

CHARACTERISTICS OF CHLOROPLASTS:

They are found in plants and euglenoids, having different shapes in lower plants and discs in higher plants.
They have specific pigments that give leaves their colour.
Chloroplasts have their own DNA and ribosomes. Consists of flattened membranous sacs called thylakoids located in their stroma. In some places, the thylakoids are stacked and called grana.
FUNCTIONS OF CHLOROPLASTS:
- They have enzymes that are used to synthesize carbohydrates and proteins.
- The pigments present in the chloroplasts help in trapping the sunlight used for photosynthesis.

8. What are the characteristics of prokaryotic cells?

Answer –

CHARACTERSITICS OF PROKARYOTIC CELLS:

They lack a nuclear membrane and the genetic material is found floating in the cytoplasm without any nuclear envelope. The genetic material is called nucleoid or genophore.
They lack membrane-bound organelles.
Many procaryotic cells have a smaller circular DNA in addition to the genetic DNA. This smaller DNA is called plasmids which provide certain phenotypic characteristics to the cell.
They contain mesosome which is essentially the infoldings of the plasma membrane. Mesosome helps in respiration.
They contain ribosomes that are made of two sub-units- 50S and 30S units making together 70S ribosomes.
Reserve material is stored in the cytoplasm inside inclusion bodies.

9. Multicellular organisms have division of labour. Explain.

Answer – Cells are organized to form tissues that form organs and organ systems. A cell is an autonomous structure capable of performing functions on its own. Therefore, division of labour is needed to perform different functions for increased efficiency and better survivability.

10. The cell is the basic unit of life. Discuss in brief.

Answer – All plant and animal cells are made up of organ systems and organ systems are made up of organs. Organs are made up of tissues and tissues are made up of a collection of cells. Since the cell is the smallest unit which is an autonomous structure capable of performing functions on its own, the cell is considered the basic unit of life.

11. What are nuclear pores? State their function.

Answer – In the nuclear membrane, there are several places where minute pores could be seen due to the fusion of two membranes. These perforations on the nuclear membrane are called nuclear pores.

FUNCTIONS OF NUCLEAR PORES ARE- They provide the passage for the movement of RNA and protein molecules takes place in both directions between the nucleus and cytoplasm.

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

Answer – The endomembrane system contains Endoplasmic Reticulum, Golgi complex, lysosomes and vacuoles. They work in a coordinated manner to perform functions. Lysosomes are membrane-bound vesicular structures formed during the packaging process of the Golgi apparatus. They contain lytic enzymes to digest worn-out cells. While vacuoles are membrane-bound space found in the cytoplasm that contains water, sap, excretory products, etc. They help in the osmoregulation of the cell and maintain its shape.

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

(i) Nucleus (ii) Centrosome

Answer –

(i) Nucleus –

Credits – NCERT

Nucleus was first described by Robert Brown.
The nucleus is the cell organelle having a double membrane and containing genetic material. The five parts of a nucleus are the nuclear envelope, nucleolus, chromatin, nuclear matrix, and nucleoplasm.
Nuclear Envelope – The nucleus is surrounded by the double membrane nuclear envelope with tiny pores called nuclear pores in the membrane which serve as the channel for substance exchange from outside the nucleus. The rough endoplasmic reticulum is attached to the outer membrane and helps in protein synthesis.
Nucleoplasm – The fluid present in the nuclear region containing enzymes, nucleosides and proteins important for the normal functioning of genetic material. It also contains chromatin and nucleolus.
Chromatin – They are the fine network of DNA and a few RNA, histone and non-Histone proteins. They condense during cell division into chromosomes.
Nucleolus – They are irregular, naked structures that produce ribosome units and are attached with the chromatin. They produce proteins.

(ii) Centrosome –

Credits – NCERT

Centrosome is an organelle containing two cylindrical structures called centrioles.
They are surrounded by amorphous pericentriolar materials.
They are made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibrils is a triplet.
The proteinaceous central part of the proximal region of the centriole is called a hub, which is attached to triplets through radial spokes.
It is an important organelle during cell division as it organizes the spindle fibres.

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.

Answer –

The centromere is the primary constriction on the sides of the chromosome. They hold two chromatids of the chromosome.
On the basis of the position of the centromere, there are four types of chromosomes –

METACENTRIC – They have two equal arms of the chromosomes with the centromere present in the middle.
SUB-METACENTRIC – They have one shorter arm and one long arm with the centromere present a little away from the middle.
ACROCENTRIC – Centromere is located close to the end which divides the chromosome into one extremely short and one very long arm.
TELOCENTRIC – They have terminal centromere.

Credits – NCERT

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