Note 5 |standard 12|Sexual reproduction in flowering plants| structure of pollen grain | notes| NCERT| Biology

 Sexual reproduction in flowering plants (New syllabus chapter 1 / old syllabus Chapter 2)

Structure of Pollen grain|reproduction in flowering plants|short notes| NCERT| standard 12| Biology

Note 5

Pollen grain

> Represent male gametophyte

> Means it contains male gametes

> Haploid 

> Comes in different varieties: size, shape, color, design etc

Structure of Pollen grain

>Shape: spherical

>Size: 25-50 μm (micrometre) in diameter

>2 layered wall: 1) Exine    2) Intine

1) Exine:

>Outer layer

>Non-continous layer

>Hard

>Because it is made up of sporopollenin

->sporopollenin: most resistant organic material known

-->it can withstand high temperature, strong acid and strong alkalis

--> no enzyme found yet - that can degrade sporopollenin 

Q:  Why do you think the exine should be hard? 

A: -because pollen grain disperse in wild nature, 

-no protection for pollen grain from rains and other physical/chemical aspects. 

-Also, it contains very delicate structure -male gamete- which is very much necessary for reproduction and propagation of their generation. 

- if exine not hard, it won't be able to withstand harsh conditions in nature.

>Germ pore: 

-prominent aperture in exine of pollen grain 

-exine is not a continuous layer around the pollen grain, it has little gaps which are called as germ pores

-Sporopollenin absent here

- function of germ pore: pollen tube develops from here (pollen tube will help in transfer of male gametes form pollen grain to ovary for fertilization)

- if sporopollenin present in complete surrounding of pollen grain- pollen tube can't rupture - no fertilization can occur

2) Intine:

> Inner wall

> Continuous layer

> Thin

> Made up of cellulose and pectin

Cytoplasm of pollen grain - surrounded by plasma membrane

Maturation of pollen grain:

>Pollen grain grow in size

>Size of nucleus increase

>Large vacuoles develop

>Asymmetric spindle arise on one side of pollen grain cell

>Division of nucleus and cytoplasm - creates two cells inside a mature pollen grain

1) vegetative cell

-bigger 

-has lot of food storage

-large nucleus

-irregular shape of nucleus

- helps with survival of pollen grain and gamete

2) germinative cell

- small in size

- spindle shapes

- floats in cytoplasm of vegetative cell

- dense cytoplasm

- smaller nucleus

- will function as male gamete in most angiosperms 

>In over 60 % of angiosperms, pollen grains disperse at this 2-celled stage

>In other species, the

generative cell divides mitotically

>give rise to the two male

gametes before pollen grains are shed (3-celled stage)

Happy learning!

Thank you for reading!

Manish Mevada

Urvi Bhanushali