Biology Class 11 NCERT Solutions: Chapter 8 Cell the Unit of Life Part 3

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

Answer:

Nuclear pores are tiny holes present in the nuclear membrane of the nucleus. They are formed by the fusion 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.

Image shows the function of nuclear pole

Image Shows the Function of Nuclear Pole

Image shows the function of nuclear pole

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

Answer:

Lysosomes are membrane-bound vesicular structures holding a variety of enzymes such as lipases, proteases, and carbohydrases. The purpose of lysosomes is to digest worn out cells. They are involved in the intracellular digestion of foreign food particles and microbes. Sometimes, they also act as suicidal bags. They are involved in the self-digestion of cells. They are a kind of waste disposal systems of a cell. On the other hand, vacuoles are storage sacs found in cells. They might store the waste products of cells. In unicellular organisms, the food vacuole contains the consumed food particles. It also plays a role in expelling excess water and some wastes from the cell.

Image shows the endomembrane structures

Image Shows the Endomembrane Structures

Image shows the endomembrane structures

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

(i) Nucleus (ii) Centrosome

Answer:

(i) Nucleus

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 separating the contents of the nucleus from the cytoplasm. The narrow space between the two membranes is called the perinuclear space. Nuclear membrane has tiny holes called nuclear pores. These holes allow specific substances to be transferred into a cell and out from 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.

Nucleus of the cell

Nucleus of the Cell

Nucleus of the cell

(ii) Centrosome

Centrosome consists of two cylindrical structures called centrioles. Centrioles lie perpendicular to each other. Each has a cartwheel-like organisation.

A centriole is made up of microtubule triplets that are evenly spaced in a ring. The adjacent triplets are linked together. There is a proteinaceous hub in the central part of a centriole. The hub is connected to the triplets via radial spokes. These centrioles help in organising the spindle fibres and astral rays during cell division. They form the basal body of cilia and flagella.

Centrosome and centriole

Centrosome and Centriole

Centrosome and centriole

Q: 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:

Centromere is a constriction present on the chromosomes where the chromatids are held together.

The basis of classification of chromosomes.

The Basis of Classification of Chromosomes.

The basis of classification of chromosomes.

Chromosomes are divided into four types based on the position of the centromere.

(i) Metacentric chromosome

The chromosome in which the centromere is present in the middle and divides the chromosome into two equal arms is known as a metacentric chromosome.

Q 14 Image of Metacentric Chromosome

Q 14 Image of Metacentric Chromosome

Q 14 Image of Metacentric Chromosome

(ii) 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.

Q 14 ii Image of Sub-Metacentric Chromosome

Q 14 Ii Image of Sub-Metacentric Chromosome

Q 14 ii Image of Sub-Metacentric Chromosome

(iii) 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.

Q 14 iii Image of Acrocentric Chromosome

Q 14 Iii Image of Acrocentric Chromosome

Q 14 iii Image of Acrocentric Chromosome

(iv) Telocentric Chromosome

The chromosome in which the centromere is located at one of the terminal ends is known as a telocentric chromosome.

Q 14 iv Image of Telocentric Chromosome

Q 14 Iv Image of Telocentric Chromosome

Q 14 iv Image of Telocentric Chromosome