NCERT Class 11-Biology: Chapter – 8 the Unit of Life Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 8:

What are histones? What are their functions?


In eukaryotes, chromatin contains DNA and some basic proteins called histones, non-histone proteins and also RNA.

Histones are highly alkaline proteins that are found in nuclei of eukaryotes. They package and order DNA into structural units. Since DNA wraps around histones, they also play role in gene regulation (increase or decrease the production of specific gene products) .


Long Answer Type Questions

Question 1:

What structural and functional attributes must a cell have to be called a living cell?


A cell must have the following structural and functional attributes to be a called a living cell

(a) Cell membrane – It is the outer membrane of the cell which is made up of phospholipids primarily. This membrane is selectively permeable as it allows only selected material like water, food to pass through it.

(b) Cytoplasm – The liquid containing the controlled concentration of organic and inorganic compounds. It functions as a site for metabolism and provide energy and material for growth and reproduction.

(c) Nucleus – It contains DNA, the nuclear matrix or the nucleoplasm containing nucleolus and chromatin. It is also called the brain of the cell as it controls all the activities carried out by the cell. It also transfers the genetic material required for reproduction and growth. In eukaryotic cells, it is membrane bound but not in prokaryotic cells

The nucleus, cell membrane, cytoplasm are common in both prokaryotic (primitive) cells and eukaryotic cells. But eukaryotic cells have other things like Golgi apparatus, endoplasmic reticulum, plastids, mitochondria, etc. which are all membrane bound. The prokaryotes don՚t have membrane bound organelles.

The Nucleus, Cell Membrane, Cytoplasm

Question 2:

Briefly give the contributions of the following scientists in formulating the cell theory

a. Robert Virchow

b. Schielden and Schwann


a. Rudolf Virchow first explained that the cells divide and new cells are formed from the pre-existing cells. He gave the modern cell theory by modifying the old one given by Schleiden and Schwann –

  • All living organisms are composed of cells and products of cells.
  • All cells arise from pre-existing cells.

b. In 1838, Schleiden conducted tests on different plants and concluded that all the plant are made up of different types of cells which together form plant tissues.

  • In 1839, Schwann studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’ .
  • He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells.
  • So the cell theory with the contribution of the two stated that the bodies of animals and plants are composed of cells and products of cells.

Question 3:

Comme Is extra genomic DNA present in prokaryotes and eukaryotes? If yes, indicate their location in both the types of organisms.


Yes, extra genomic DNA is present in prokaryotes and eukaryotes.

In eukaryotes, the extra genomic DNA is present in mitochondria and plastids.

DNA is Present in Mitochondria and Plastids

In many prokaryotic bacteria, small circular DNA called plasmids are present outside genomic DNA which helps the cell develop some phenotypic characteristics like resistance to anti-biotics