Name: Facilitated Diffusion: Transport in Plants
Uploaded: 2019-07-19
Duration: 18 min 2 s
Description: Facilitated diffusion occurs when important molecules pass through the membranes via special holes called channels. Watch the video to learn more.

Question

Write about the starch hydrolysis theory to explain the mechanism of transpiration.

Accepted Answer

The process of removal of water in the form of water vapor is called transpiration. The starch hydrolysis theory to explain the mechanism of transpiration was first given by Lloyd (1908), later on, it was formulated by J. D. Sayre (1926) and it is afterward modified by Steward (1964). According to this theory,

In the presence of light during daytime the C0₂ is evolved during the process of respiration is used up for the process of photosynthesis and does not store in the guard cells and whereby the pH of the guard cells increases to 7. In that condition, the enzyme helps the formation of glucose 1-phosphate from starch and inorganic phosphate (ip).

In the absence of light, CO₂ evolved in respiration is stored in the cell sap and is dissolved in water to form carbonic acid, whereby the pH of the cell sap decreases to 5 i.e., acidic. At this pH, starch synthesis is favored from glucose-1-phosphate.

Scarth and Shaw (1941) stated that the equilibrium condition in the acidity and alkalinity of the cell content occurs in the following manner- as photosynthesis stops at night, storage of carbon dioxide occurs within the leaf cells, whereby acidity of the guard cell sap increases, causing the decrease of pH value. In dark, a decrease in the pH helps the opposite reaction storing starch within the guard cells. On the other hand, in presence of light, when all the accumulated carbon dioxide is used up for photosynthesis, the acidity decreases, increasing the pH value. Favored by enzymes starch is converted into sugar, whereby the stomata open.

The mechanism of opening and closing of stomata according to starch hydrolysis theory is given below:

Write about the starch hydrolysis theory to explain the mechanism of transpiration.

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