Draw a labelled two-celled structure of male gametophyte of an angiosperm. Name the three layers that surround the cytoplasm of a male gametophyte starting from innermost to outermost layer. (iii) Which organic material makes the outermost layer? Mention its advantage. (iv) Why is the outermost layer of male gametophyte not continuous ?
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To remember the functions of a pollen grain's walls, think of them this way: the outer Exine wall provides Extreme environmental protection using sporopollenin, while the inner Intine wall grows Inward and outward to build the pollen tube through a germ pore.
Step 1: Understanding the Question:
The question requires drawing a clear diagram of a mature, 2-celled pollen grain (male gametophyte) in angiosperms. It also asks to list the layers surrounding its cytoplasm from the inside out , identify the chemical component of the outer wall along with its biological advantages , and explain why this outer wall has structural gaps. Step 2: Key Formula or Approach: Two-Celled State: Consists of a large vegetative cell with an asymmetric nucleus and a small generative cell floating within its cytoplasm. Wall Envelope (Sporoderm): Composed of an inner intine layer and an outer exine layer. Chemical Resistance: Powered by sporopollenin, one of the most chemically resistant organic materials known. Step 3: Detailed Explanation: (i) Drawing of a Two-Celled Male Gametophyte: A sketch of a mature 2-celled pollen grain must clearly show:
1. A large Vegetative Cell containing abundant food reserves and a large, irregular or amoeboid-shaped nucleus.
2. A small, spindle-shaped Generative Cell with dense cytoplasm and a distinct nucleus, floating freely inside the vegetative cell's cytoplasm.
3. The inner wall layer (Intine) and the sculpted outer wall layer (Exine), along with small openings called Germ Pores. (ii) Three Layers Surrounding the Cytoplasm: Starting from the innermost layer next to the plasma membrane and moving outward, the structural layers are:
1. Plasma Membrane: The selective lipid bilayer that encloses the cytoplasm.
2.
Intine: The inner wall layer, which is thin, flexible, and composed of a mixture of cellulose and pectin.
3.
Exine: The outermost wall layer, which is thick, rigid, and highly sculpted. (iii) Organic Material of the Outermost Layer (Exine): The exine layer is composed of a specialized organic polymer called Sporopollenin.
Advantages: Sporopollenin is one of the most chemically resistant organic materials known in the biological world. It can withstand extremely high temperatures, strong laboratory acids, and harsh alkaline treatments. Furthermore, no known biological enzyme can degrade or digest sporopollenin. This incredible chemical resistance protects the male gametes from drying out or being damaged by the environment during wind or insect transport, allowing pollen grains to remain well-preserved as fossils for millions of years. (iv) Why the Outermost Layer is Not Continuous: The exine layer is not continuous because it contains small, circular gaps or apertures where sporopollenin is completely absent. These openings are called Germ Pores. Because sporopollenin is an impenetrable chemical barrier, a continuous outer wall would trap the cells inside forever. The germ pores provide designated exit points that allow the intine layer to grow outward and form a pollen tube during germination on a compatible flower stigma, enabling the male gametes to travel down to the ovary for fertilization. Step 4: Final Answer:
(i) The student should sketch a pollen grain showing the large vegetative cell and the spindle-shaped generative cell enclosed within the intine and exine walls.
(ii) From inside to outside, the layers are the plasma membrane, the intine, and the exine.
(iii) The exine is made of sporopollenin, which protects the pollen from environmental stress, heat, and enzymes, allowing it to survive fossilization.
(iv) The exine has gaps called germ pores where sporopollenin is absent, which allows the pollen tube to emerge during germination.
