Iodine Test for Starch

Protocol for Iodine test for starch

Overview

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.

Background

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:

  1. Make two sets of hypotheses & predictions about which materials will contain starch.
  2. use the iodine test to test your hypotheses
  3. Qualitatively assess amount of starch based on darkness of color change

Hypothesis testing

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:

  1. relative starch content of under-ripe and ripe bananas.
  2. relative starch content of potato versus onion

. For topic 1 - the relative starch content of under-ripe and ripe bananas - consider the following information in forming your hypothesis.

  1. If you have eaten bananas at different stages of ripeness, you have probably noticed that bananas become softer and sweeter as they ripen.
  2. Bananas, like other fleshy fruits, are adaptations to encourage animals to eat fruit in order to disperse the seeds (we no longer have seeds in our bananas, but that is still the explanation of why banana plants experienced selection to make fruit). Mammals generally have a hard time digesting starch, so a high starch content at the time of ripeness might make mammals less likely to consume the fruit and disperse the seeds.

For topic 2 - relative starch content of potato versus onion - consider the following information.

  1. Both onions and potatoes are storage organs. There are varieties of onions that humans eat raw (red and white onions as well as spring onions), but there are not varieties of potato that humans generally eat raw.
  2. There are varieties of onions that we call sweet onions (like Vadalia). However, there is no variety of true potato that we call sweet (this is confusing - "sweet potatoes" are not really potatoes; instead, they are in a different plant family altogether.
  • In the Starch & hypothesis testing background quiz, you will be asked to answer questions about this background, and create hypotheses and predictions for each topic. After taking the quiz, you will be given directions to test your hypotheses in the document "Instructions to perform iodine test for starch". Be sure to use "Instructions to perform iodine test for starch" to conduct the laboratory!


Starch & hypothesis testing background quiz

Availability:

Item is hidden from students. It was last available on Mar 11, 2020 11:59 PM.

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.)

Directions for Iodine Test for Starch

Safety

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.

Supplies provided to you:

  1. Iodine tincture in squeeze bottles (for some of you this was mailed in a padded envelope).
  2. Protective non-latex gloves
  3. Safety glasses
  4. Scalpel

Supplies you provide

  1. Plant material to test: under-ripe and ripe banana; potato and onion
  2. Cutting board (or plate)
  3. Knife (or use the scalpel)

Instructions

  1. Wear safety glasses and gloves
  2. For the under-ripe and ripe bananas, cut one disk of each such that the flesh is exposed.
  3. Put them on a sheet of paper with labels ("under-ripe" and "ripe")
  4. Dry off damp surfaces with a paper towel or tissuel
  5. Use the dropper to drop 4 drops of iodine tincture onto the exposed surface of each item.
  6. Wait 30 seconds for color change to occur.
  7. Photograph results (include stickers in photograph) and qualitatively describe results (see below)
  8. Cut a slice of potato and onion - being careful to not cut fingers.
  9. Put them on a sheet of paper with labels ("potato", "onion")
  10. Dry the slices with a paper towel or tissue.
  11. Use the dropper to drop 4 drops of iodine tincture onto the exposed surface of each item.
  12. Wait 30 seconds for color change to occur.
  13. Photograph results (include stickers in photograph) and qualitatively describe results (see below)

What to turn in

  1. Use the Padlet page, to turn in photographs and explanations of your tests in this order: Post #1. banana photo;Post #2. potato/onion photo. Make sure your identifying sticker is present in each photo. Upload the banana photo first, then the potato/onion.
  2. TAKE the starch and hypothesis testing final quiz to record your explanation of whether the outcome matched your predictions and whether the hypotheses were tentatively supported or rejected.


Note to Instructors:

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).

Interpretation

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:

  1. For your comparison of the results of the iodine test for starch for the under-ripe and ripe banana, what was the outcome? Did this outcome match your prediction (if it did not match, explain what parts did not do so)? Based on this, what can you conclude about your hypothesis - was it tentatively supported or refuted)?
  2. For your comparison of the results of the iodine test for starch for the onion and potato, what was the outcome? Did this outcome match your prediction (if it did not match, explain what parts did not do so)? Based on this, what can you conclude about your hypothesis - was it tentatively supported or refuted)?