The pollen grain as the male gametophyte · Biology… | StemPy Lab
Course 19.1·Lesson 3 of 3
The pollen grain as the male gametophyte
A pollen grain is a haploid structure that makes the male gametes, so it is called the male gametophyte. It is about 25 to 50 micrometres wide. Its outer wall, the exine, is made of sporopollenin, one of the toughest organic materials known, and has gaps called germ pores. Its inner wall, the intine, is made of cellulose and pectin.
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A very small plant
You cannot see a single pollen grain without a microscope. It is about 25 to 50 micrometres across, which is the width of a thin hair or less. Yet this tiny grain has its own structure, its own two cells and its own job.
Two drawings of a pollen grain, one above the other. Both show an outer yellow-brown wall, the exine, with a spiny pattern and gaps called germ pores where the exine is missing. Inside it is a thin blue wall, the intine, which bulges out at the germ pores. In the two-celled stage, the big vegetative cell holds a large irregular vegetative nucleus, and a small spindle-shaped generative cell floats in its cytoplasm. In the three-celled stage, the generative cell has divided into two elongated male gametes.
The two walls
A pollen grain has two walls.
The exine is the hard outer wall. It is made of sporopollenin, one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkalis. NCERT says that no enzyme that breaks down sporopollenin is known.
The intine is the thin, continuous inner wall. It is made of cellulose and pectin.
The exine has gaps where there is no sporopollenin. They are called germ pores. When the grain germinates, the pollen tube comes out through one of them.
The exine also has a rich pattern of ridges, spines and pits. The pattern differs from one species to another, so scientists can name a plant from its pollen. Because sporopollenin does not decay, the exine of pollen is often found well preserved in fossils.
The cytoplasm and the two cells
Inside the walls, the cytoplasm is surrounded by a plasma membrane. A mature grain has two cells.
The vegetative cell is bigger, holds a food reserve and has a large, irregularly shaped nucleus.
The generative cell is small and spindle shaped, with dense cytoplasm and a nucleus. It floats in the cytoplasm of the vegetative cell.
At the three-celled stage the generative cell has divided into two male gametes. These male gametes do not swim. In flowering plants they are carried to the egg by the pollen tube.
Why a pollen grain is a gametophyte
Plants have two phases in their life cycle. The sporophyte is the diploid plant, with 2n chromosomes, that makes spores by meiosis. The gametophyte is a haploid phase, with n chromosomes, that grows from a spore and makes gametes by mitosis.
Now look at the pollen grain.
It starts as a microspore, which was made by meiosis from a pollen mother cell. So it is a spore.
It is haploid.
It divides by mitosis to make male gametes.
So the pollen grain is the male gametophyte. It is very much reduced. It has only two or three cells and lives only a short time. The big plant that you see is the sporophyte.
Pollen in daily life
This section is not in the NEET syllabus text, but school boards ask it, and A-Level and IB use pollen in ecology and in plant breeding.
Viability. Pollen of rice and wheat stays viable only for about 30 minutes after it is released. In some plants of the families Rosaceae, Leguminosae and Solanaceae it stays viable for months. Viable means able to germinate.
Pollen banks. Pollen can be stored for years in liquid nitrogen at −196∘C. Such cryopreserved pollen can be used in crop breeding.
Allergy. The pollen of many plants causes allergy and breathing problems in sensitive people. Parthenium hysterophorus, called carrot grass, spread widely in India and causes pollen allergy.
Food. Pollen is rich in nutrients, and pollen tablets and syrups are sold. The claim that they improve the performance of athletes is not settled.
You should find that the pollen with the shorter half-life falls to 10 per cent of the viable pollen much sooner. A pollen bank stops this decay by keeping the pollen very cold.
Try it in Python
The program uses the model of the simulation to give the viability after some time.
Practise with new numbers
Common mistakes
Swapping the walls. The exine is outside and is made of sporopollenin. The intine is inside and is made of cellulose and pectin.
Saying that the germ pore is a hole in the intine. It is a gap in the exine, where the intine can bulge out.
Calling the pollen grain a gamete. It is a gametophyte. The male gametes are formed inside it.
Saying that the pollen grain is diploid. It is haploid.
Thinking that the male gametes swim. They are carried by the pollen tube.
Remember these
A pollen grain is 25 to 50 micrometres wide. It is the male gametophyte, haploid and reduced to two or three cells.
Exine: outer, hard, sporopollenin, with germ pores. Intine: inner, thin, cellulose and pectin.