NCERT Class 11-Biology: Chapter – 8 the Unit of Life Part 3 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 6:

What is the feature of a metacentric chromosome?


The metacentric chromosome has middle centromere forming two equal arms of chromosome.

Question 7:

What is refered to as satellite chromosome?


Sometimes a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite. Such chromosomes are called satellite chromosomes.

Short Answer Type Questions

Question 1:

Discuss briefly the role of nucleolus in the cells actively involved in protein synthesis.


  • The nucleoplasma contains small spherical bodies which are known as the nucleolus. It is not a membrane bound structure.
  • It is a site for active ribosomal RNA synthesis.
  • It also receives and stores ribosomal proteins formed in the cytoplasm.

Question 2:

Explain the association of carbohydrate to the plasma membrane and its significance.


Carbohydrates form glycoproteins and glycolipids.

Question 3:

Comment on the cartwheel structure of centriole.


  • Centromere is an organelle usually containing two cylindrical structures called centrioles.
  • Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel.
  • They are made of nine evenly spaced peripheral fibrils of tubulin protein.
  • Each of the peripheral fibril is a triplet.
  • The adjacent triplets are also linked.
  • The central part of the proximal region of the centriole is also proteinaceous and called the hub.
  • The hub is connected with the tubules of the peripheral triplets by radial spokes made up of protein.

Question 4:

Briefly describe the cell theory.


Cell theory was given by Schleiden and Schwann which stated –

(a) Bodies of all the plants and animals are made up of cells.

(b) The cells are structural and functional units of life.

But it failed to state the origin of life.

Few years later, Rudolf Virchow stated that cells divide to produce new cells. So the modern cell theory is –

(a) All living organisms are composed of cells and products of cells.

(b) all cells arise from pre-existing cells.

Question 5:

Differentiate between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER) .


Rough Endoplasmic Reticulum
S. NoRough Endoplasmic Reticulum (RER)Smooth Endoplasmic Reticulum (SER)
1Has ribosomes attached to its SurfaceHas no ribosomes attached to its surface.
2.Helps in synthesis of proteins and enzymes.Helps in synthesis of glycogen, lipids
3It is mainly formed of cisternae and few tubules.It is formed mainly of vesicles and tubules.
4RER is internalSER is peripheral.
5It is connected with nuclear envelope.It is connected with Plasmalemma.
6.Enzymes for detoxification are absentEnzymes for detoxification are present.
7Develops from nuclear envelopeDevelops from RER.

Question 6:

How Give the biochemical composition of plasma membrane. How are lipid molecules arranged in the membrane?


Plasma membrane is composed of proteins, lipids, water and carbohydrates. The ratio of lipids and proteins varies considerably in different types cells.

The major lipids are phospholipids that are arranged in a bilayer. In addition to phospholipids membrane also contains cholesterol

Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic (water-repellent) tails towards the inner part. This ensures that the non-polar tail of saturated hydrocarbons is protected from the aqueous environment.

Question 7:

What are plasmids? Describe their role in bacteria?


In bacteria, there՚s single chromosome/circular DNA which is called genomic DNA but many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids.

Role of plasmids in bacteria –

The plasmid DNA grants certain phenotypic characters like resistance to antibiotics to such bacteria. DNA is used to monitor bacterial transformation with foreign DNA.

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