Pollen Grain: Definition, Structure, Functions | Microsporogenesis - YB Study -->

Pollen Grain: Definition, Structure, Functions | Microsporogenesis

What are Pollen Grains? Definition,  Structure, and Formation of Pollen grains: 

Definition of Pollen grain :

  • Pollen grains microscopic granular structure present on anther and gives rise to the male gametophyte of a seed plant.

  • The pollen grain is a microscopic, non-motile, haploid, unicellular body with a single nucleus. 


What are Pollen grains?

  • The pollen grain is a microscopic, non-motile, haploid, unicellular body with a single nucleus. 
  • It is also known as microspores.
  • The study of pollen is known as palynology.
  • The pollen grain is produced in the anther of the flower, which is the terminal portion of the stamens.
  • In anther, each microspore mother cell divides meiotically to form a tetrad of haploid microspores (pollen grains) this process is known as microsporogenesis.
  • The pollen grain is essential for the reproduction of gymnosperms and angiosperms.
  • Pollen is the set of tiny grains produced by the flowers of angiosperms, which are the male reproductive elements or microgametophytes, where the gametes that will fertilize the eggs are found, which will later turn into seeds. 
  • Pollen is often found in fossils because sporopollenin in the outer membrane remains as fossils, and the plant species can be identified by the fact that the surface mesh, radial pattern, and shape are different for each pollen. 
  • Some insects feed on pollen. 
  • Pollen grains are high in protein, 16 to 30%, fat in 3 to 10%, and starch in 1 to 7%, and these insects seem to prefer pollen with high-fat content.


Structure of Pollen grains

Pollen and pollen grain structure and function

  • Pollen grains are microscopic, haploid, unicellular bodies with a single nucleus. 
  • The size of pollen grains generally varies from species to species.
  • Most pollen grains are 10–70 µm in diameter. The largest is about 200 microns, such as pumpkin, and the smallest pollen grain, that of the forget-me-not (Myosotis spp.), is 2.5-5 µm (0.005 mm) in diameter.
  • The shape of the pollen grain is commonly found in round, oval, triangular, elliptical, or bean shapes.
  • The natural color of pollen grains is white, which may also vary depending on the plant species. Some are yellow in color, orange, cream, and so on.
  • It is surrounded by a two-layered wall called sporoderm. 
  • The outer layer of exine is thick and made up of a complex, non-biodegradable, substance called sporopollenin. 
  • Sporopollenin is a macromolecular substance that is difficult to decompose and is insoluble in both alkalis and acids, and pollen and microspores are preserved as fossil records for many years even under conditions of low oxygen content in the soil.
  • It may be smooth or with a sculptured pattern (characteristic of the species). It is resistant to chemicals. 
  • In some places, exine is very thin showing thin areas known as germ-pores. 
  • These are meant for the growth of emerging pollen tubes during the germination of the pollen grain. 
  • The inner wall is generally thinner, softer, and more elastic than the outer wall, except the germination hole is thicker.
  • The inner wall layer, intine consists of cellulose and pectin.
  • Mature pollen grains containing only vegetative cells and germ cells are called binucleate pollen grains; while before maturation, germ cells undergo a mitotic division to form two sperm, called trinucleate pollen grains.

Microsporogenesis/ Formation of Pollen grain
Microsporogenesis

  • The male reproductive whorl of the flower is called androecium. An individual member of the androecium is called the stamen. The stamen consists of filament, connective, and anther.
  • The anther is generally dithecous (having two lobes) and tetrasporongiate. Each monothecous anther contains two pollen sacs.
  • In these pollen sacs, pollen grains are produced.
  • Anther Sporogenous cell forms sporogenous tissue. This tissue produces microspores mother cell or pollen mother cell.
  • Each microspore mother cell divides meiotically to form a tetrad of haploid microspores (pollen grains).
  • The formation of pollen grains from the microspore mother cell is termed microgametogenesis.
  • Microspores' mother cell (MMC) undergoes first mitotic division to produce bigger, naked vegetative cells and small, thin-walled generative cells. 
  • The vegetative cell is rich in food and has an irregularly shaped nucleus. 
  • The generative cell floats in the cytoplasm of the vegetative cell.
  • The second mitotic division is concerned with generative cells only and gives rise to two non-motile male gametes. 
  • The mitotic division of generative cells takes place either in the pollen grain or in the pollen tube. The pollen grains are shed from the anther, at this two-celled stage in most of the angiosperms.
  • After the pollen grains reach the stigma, they germinate to form pollen tubes, which is the beginning of the fertilization process.


Frequently Asked Questions on Pollen grains:

1. What is the difference between pollen and pollen grains?

Answer: Pollen is the powdery substance (pollen grain) in the anthers of plant male flowers. It is the male reproductive cell of the plant and the sperm of the plant. Its function is to combine with the plant egg cell to form a fertilized egg and develop into a seed. The material basis is the essence of the plant body. But male gametophyte refers to mature pollen grains, which in turn develop from haploid microspores, which are immature male gametophytes.


2. Where pollen grains are produced?

Answer: In gymnosperms, pollen grain is produced in strobiles while in angiosperms pollen grains are produced in the anther of flowers. 


3. What is pollination?

Answer: Pollination is the name given to the transfer of pollen grains from anther to the stigma of the same flower or different flower.  This transfer can happen with the action of various agents, such as wind, water, Insects and animals, etc.


4. What does pollen look like? Is it granular?

Answer: Yes, All objects have shapes, and pollen is no exception.


5. Pollen grains are male gametes or female gametes?

Answer: Many people think, the pollen grain is not the male gamete in gymnosperms and angiosperms. We can define the pollen grain as the microspore that contains the male gametophyte.


6. Why is pollen grain resistant?

Answer: The pollen grain is extremely resistant and guarantees great reproductive efficiency. It is because of their emergence that gymnosperms and angiosperms do not depend on water for reproduction.


7. 10 pollen mother cells of plants can form?

Answer: 10 microspore mother cells of plants can form 40 pollen grains.


8. 10 Megaspore mother cell produces?

Answer: 10 Megaspore mother cells can form 10 embryo sacs. 10 egg cells, 20 polar nuclei, 20 synergetic cells, and 30 antipodal cells.


9. Why can pollen grains remain well preserved as fossils?

Answer: Sporopollenin is a macromolecular substance that is difficult to decompose and is insoluble in both alkalis and acids, and pollen and microspores are preserved as fossil records for many years even under conditions of low oxygen content in the soil.

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