Question

1. Using the tweezers Sam carefully removes the leaves from the alcohol and dips them in the rinse water. He blots the leaves gently of a piece of paper towel and places them in a different area of light on the plate. Using the tweezers gently spreads the leaves back into their normal shapes. He uses a syringe to flood the leaves with iodine. After two minutes what are the colors of the leaves?

Answer 1

The leaves are placed in an area of light, followed by which iodine was added, let us look for the answer.

Green leaves have chloroplasts within them, which contains enzymatic machinery which helps in carbon dioxide assimilation or fixation. The pathway was discovered by Melvin Calvin, Andrew Benson and James A. Bassham in the early 1950s. The first intermediate product in this pathway is a 3-C product 3-phosphoglycerate which names the cycle as C3 cycle. The enzyme that catalyzes incorporation of CO2 (in plants, 3CO2 molecules at a time) into an organic form is known as rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase). Therefore, 3 molecules of ribulose 1,5-bisphosphate assimilates 3CO2 molecules to form 6 molecules of 3-phosphoglycerate. 3-phosphoglycerate is converted to 1,3-bisphosphoglycerate, followed by conversion to glyceraldehydes-3-phosphate and dihydroxyacetone phosphate (DHAP). Five out of the six molecules of triose phosphates are used to regenerate ribulose 1,5-bisphosphate and the sixth molecule is converted to starch (for storage).

So when iodine is applied on green leaves exposed to light, presence of starch will turn it to blue-black colour.