Question 1 (10 marks)
Water is essential to plant’s life. It is absorbed from soil by the plant’s root and then transported within the whole plant’s body. Write an essay on the journey of a water molecule from the root to the leave in a terrestrial plant. In the essay, discuss the ways and routes taken by the water molecule at cell, tissue and organ levels of the plant. (10 marks)

Ans:
The ‘soup of life’ in the living system or the cytoplasm is an aqueous environment enclosed within the limits of the plasma membrane. Thus, for metabolism, all the enzymatic reactions occur in an aqueous environment, and hence water is a quintessential requirement for any living system. Water acts as a solvent, and all the necessary nutrients such as sugar and minerals have to be transported dissolved in water. Similarly, the waster products need to be excreted through the water. Approximately 80-95% of the total mass of a plant is constituted by water. Plant cells have a large vacuole that contains maximum proportion of water and the protoplasm is less in volume as compared. The presence of cell wall together with a large vacuole allows plant cell to regulate osmotic balance with the cell wall providing an ability to bear large turgor pressure. The requirement of water balance is necessitated by the fact that approximately half a litter of water is absorbed by roots just to produce 1 gm of the organic matter. This water is transported via xylem vessels, and is lost to the atmosphere through leaves continuously, and thus a delicate balance between water absorption and water transpiration is maintained in a plant.

In roots, the water is absorbed by root hairs present in the apical region. These root hairs substantially increase the surface area for absorption. The water enters the root and is transported to the xylem via three pathways:

a) Transmembrane pathway
b) Apoplast pathway and
c) Symplast pathway.

In the transmembrane pathway, the water flows from one cell to another by entering one cell and exiting to the other side to enter the other cell. So to pass from one cell to the other, the water has to cross at least two membranes.

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