Exercise 3.1
How is nitrogen useful for plants? Remember that you learnt this in the chapter ‘The Invisible Living World: Beyond Our Naked Eyes’, in your Grade Science textbook.
The book prints no answer for this
NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.
Plants need nitrogen to build proteins and grow, but they cannot use the nitrogen gas of the air directly.
Nitrogen-fixing bacteria — in the soil and in the root nodules of leguminous plants — change atmospheric nitrogen into compounds that roots can absorb.
That is why nitrogen, though it makes up $\displaystyle 78$% of the atmosphere, reaches plants only through these tiny living helpers.
Check your Science book
This chapter only tells you that nitrogen is one of the two most abundant gases and is essential to life. It sends you back to 'The Invisible Living World: Beyond Our Naked Eyes' in your Grade $\displaystyle 8$ Science textbook — read that chapter for the exact wording your teacher will expect.