Question

The exine of pollen grain is hard and resistant to enzyme action. Give reason.

Answer 1

Hint: Pollen grains are generally male gametophytes which have male gametes. Its size is about 25-50 Micrometres in diameter. It is covered in a prominent Two-layered wall which is a hard outer layer called exine . Exine consists of Sporopollenin and the intine (inner thin layer).

Complete answer:
Exine is hard and resistant to enzymes because it has a deposition of “Sporopollenin” which is one of the most resistant organic materials. It can withstand high temperature and strong acid and alkalis. It is due to sporopollenin that no enzyme can degrade the pollen grain . This also helps in fossilization. Pollen grains are well-being preserved as fossils because of the presence of sporopollenin. It is hard so that pollen grains are well protected from hazardous environments when they are pollinated by biotic and Abiotic agents.
Exine exhibits a fascinating array of patterns and designs which is of taxonomic significance. It is thin at certain points forming apertures called 'germ pore' where sporopollenin is absent. The germ pore helps in the formation of pollen tubes when anthers reach stigma and release the male gamete during fertilization.
The exine is further differentiated into many sub layers. The exposed surface of the pollen wall shows many types of sculpting making it rough. The roughness of exine is due to tectum and baculum, as these are discontinuous layers.

Note: The sporopollenin present in the exine is a polymer which is highly cross-linked and is composed of carbon, hydrogen, and oxygen which makes it extraordinarily stable. It has been found chemically intact in 500 million year old sedimentary rocks.