What is transpiration? Explain stomatal opening and closing mechanism with reference to starch-sugar interconversion theory.

Answer:

Transpiration is the loss of water in the form of vapour from the aerial parts of a plant, mainly through stomata.

Starch–sugar interconversion theory (Steward / Sayre):

• Opening (in light): photosynthesis in guard cells uses CO₂, so the pH of guard cells rises (alkaline, about pH 7). At high pH the enzyme phosphorylase converts starch into glucose-1-phosphate and then glucose. The osmotic concentration of guard cells increases, water enters them from neighbouring cells by endosmosis, they become turgid and bulge out, and the stoma opens.
• Closing (in dark): respiration accumulates CO₂, the pH falls (acidic, about pH 5), and phosphorylase converts glucose back into starch. The osmotic concentration falls, water leaves the guard cells by exosmosis, they become flaccid and the stoma closes.

Starch (insoluble) ⇌ Glucose-1-phosphate ⇌ Glucose (soluble), favoured to the right at high pH (light) and to the left at low pH (dark).