Iodine solution turns from orange to a dark purplish or brown color in the presence of starch. This color change can thus be used as an indicator for the presence of starch. You will test various substances for the presence of starch using this test.
Carbohydrates
Plants typically store carbohydrates as energy reserves, and starch is the most commonly used form of storage carbohydrate. Starch is a polymer – a large molecule composed of repeating units of a smaller molecules, which are called monomers. The monomer that makes up starch specifically is one form of glucose (termed alpha glucose). Starch molecules are made out of chains with occasional branches. The chains do not remain straight; instead, take on a helical (corkscrew) shape. Starch is a useful storage molecule because it allows the sugar to be stored compactly, which means that even though starch is polar and hydrophilic, the helical shape reduces the amount of water molecules drawn to interact with it. It is useful because it can remain inactive until the cell needs energy. When energy is needed, sugar monomers (glucose molecules) are removed from the starch and used to power cellular reactions. Stored starch is useful as a food source for animals, which can consume starch and break it down to glucose in order to use it as an energy source. Humans and other animals do not make starch, but we do make glycogen for glucose storage. Glycogen is very similar structurally to starch. Starch does not have a sweet taste because the large molecule does not interact with our neurons that taste sweetness.
While starch is often used to store carbohydrates, plants can also store carbohydrates as simple sugars instead of starch. These storage reserves can be even more desirable as a food source to animals because the sugars are more easily accessed with less digestion. For example, you would likely get a stomachache from eating large amounts of raw starch, but you could eat several tablespoons of sugar without digestive problems. We tend to cook foods high in starch prior to eating them because cooking breaks the starch into smaller more easily digested segments.
We are able to determine whether material contains starch with a simple test. Triiodide ions (which form when iodine is mixed with iodide) react with the helical form of starch molecules and turn from a light orange color to a dark blue, purple or brown color. This reaction does not occur for sugar molecules themselves.
Goals for this exercise:
A hypothesis is a tentative explanation based on available evidence. It is tentative because it may turn out to be incorrect, in which case it would need to be discarded or modified. A prediction is an expectation for what should happen in a particular instance if the hypothesis is correct. If a predication is found to be correct during an experiment, then the hypothesis has received support. However, the hypothesis has not been proven to be correct because a subsequent test could prove it wrong. On the other hand, if a prediction is not supported by the results of an experiment, then it can be concluded with confidence that the hypothesis is at least partially incorrect and needs to be changed.
Create hypotheses and predictions
The first step in this assignment is to create sets of hypotheses and predictions for the following topics:
. For topic 1 - the relative starch content of under-ripe and ripe bananas - consider the following information in forming your hypothesis.
For topic 2 - relative starch content of potato versus onion - consider the following information.
Availability:
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Complete this quiz. When you have done so, you will see instructions for how to perform the iodine test on ripe and unripe bananas as well as on potato and onion.
(Note to instructors: This is set up on Blackboard so that students will not see the instructions for the lab until they have completed the background quiz. In the quiz, they will need to state hypotheses for the amount of starch in potatoes and onions.)
Iodine tincture is sold at pharmacies to sterilize skin. However, some people are allergic to it. It also stains skin and clothes. You must use safety glasses and gloves as a precaution.
Additionally, the scalpel is very sharp - keep your fingers far away from the path of the blade while cutting.
Students turn in results on Padlet as photos of their experiments and interpretation. They may look at other students results on the Padlet to compare/contrast. I set up Padlet so each student has a column (it helps to pre-record their names across the top of the Padlet, so they are in the same order as your gradebook).
This is set up as a quiz on Blackboard LMS with two short-answer questions. It's graded like a normal assignment, just easier to set up as a quiz.
The two questions are as follows: