My full question.
A leaf was plucked from the garden. The procedure for testing for starch in the leaf was carried out. Part of the leaf turned blue-black and part of the leaf just turned brown.
Question : Why do you think the other part of the leaf did not contain starch?
The test for starch is to add I2. The presence of starch is indicated by the dark-blue color formed upon the addition of the I2. The part that didn't turn blue-black has stopped making carbohydrates (starch) due to injury or natural dying of the leaf or due to seasonal changes (summer to fall).
To understand why one part of the leaf turned blue-black while the other part turned brown, we need to understand the procedure for testing for starch in a leaf.
Typically, the procedure involves a chemical reaction between the leaf and a reagent known as iodine solution (also called Lugol's solution). Iodine solution reacts with starch molecules to produce a blue-black color, indicating the presence of starch.
Now, let's analyze why only one part of the leaf turned blue-black while the other part turned brown.
The key factor here is the presence or absence of starch in different regions of the leaf. When the iodine solution comes into contact with starch, it reacts specifically with these starch molecules and produces the characteristic blue-black color.
When the leaf turns brown in some areas, it indicates the absence of starch. This could be due to a variety of reasons:
1. Starch Storage: Leaves primarily produce and store starch in certain regions, such as the chloroplasts in mesophyll cells. If the brown parts of the leaf were from regions lacking starch storage, then there would be no starch for the iodine solution to react with, resulting in a brown color.
2. Metabolic Activity: It's also possible that the brown parts of the leaf were metabolically active and used up the available starch. During active metabolic processes like respiration or photosynthesis, starch is broken down to provide energy and carbon sources. Consequently, there may not have been enough remaining starch for the iodine solution to produce a blue-black color.
3. Time of Testing: It is important to note that the chemical reaction may take some time to fully develop. If the blue-black color was observed in a shorter period, it might be because the iodine solution had enough time to react with the starch in that particular region.
To determine the exact reason for the discrepancy between the blue-black and brown parts of the leaf, further investigation and evaluation of the leaf's anatomy, metabolic activity, and storage sites would be necessary.