Chemical Composition of Cells, Test for Starch

642 words | 3 page(s)

The purpose of this experiment is to investigate if different solutions (unknown, glucose, starch, potato juice, and onion juice) contain starch by using iodine as an indicator. The hypothesis is that of the solutions potato juice will contain starch.

Carbohydrates are molecules which are found in a wide variety of foods. There are several main types including sugars, fibers and starches. Humans can digest sugars and starches. Sugar, including glucose and sucrose, is made up of monosaccharides whereas starches are made up of longer chains of single sugar molecules known as polysaccharides. Starch is found in food such as potatoes, bread and cereals. Sugars can be directly used by the body for energy, however starches must first be broken down into their single monosaccharide components before it can be used (Slavin & Carlson, 2014).

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When conducting an experiment, it is important that controls be included. The purpose of controls is to ensure and eliminate the possibility of a false positive or false negative result. A negative control is a control which contains all components except the component being measured. The negative control confirms that there is no background signal and reduces the chance of observing a false positive result (Lipsitch, Tchetgen, & Cohen, 2010). In this instance, the negative control would be water and iodine solution. Water is used as the negative control because it is the solvent of the juices and the glucose solutions.

• Test Tubes
• Test Tube Rack
• Iodine
• Dropper
• Glucose Solution
• Starch Suspension
• Distilled Water
• Onion Juice
• Potato Juice
Six separate test tubes were placed into the test tube rack and labeled. To each tube 1.5ml of the solutions were added as follows: 1- glucose, 2- starch, 3- unknown solution, 4- water, 5- onion juice, 6- potato juice. The initial appearance of each of the tubes was then recorded. Next 10 drops of iodine were then added to each tube. The tubes were then swirled to ensure mixing of the iodine with the test solutions. The results of the color changes for each tube was then recorded. As well if the test was positive (black) or negative (no change) was also recorded.

The results for each of the 6 tubes are shown below in table 1. Of the 6 tubes only 2 tubes, starch (2) and the unknown tube (3), were positive for the presence of starch. All other tubes either had only a slight color change, potato juice (6), or no color change.

Table 1


Summary of the results for detection of the presence of starch using iodine


Tube # Solution Initial Appearance Final Appearance Test Result
1 Glucose Clear Very Orange Negative
2 Starch Clear Black Positive
3 Unknown Clear but slightly cloudy. Black Positive
4 Water Very clear Orange Negative
5 Onion Juice Very cloudy Yellow Negative
6 Potato Juice Orange Brown Negative

Of the difference substances tested both starch and the unknown tested positive for starch. Iodine reacts with the starch and changes color from a orange/brown to a dark black color. The results for the starch tube is not surprising as it is the positive control and should change color. For the other tubes (glucose, water and onion juice), none of these solutions contain starch, therefore the tubes are negative. There was one abnormal result, that of potato juice, unlike onion juice which does not contain starch, potato juice should contain starch as potatoes are very starchy. However, there was only a slight color change in the potato juice. This indicates that the starch level was low. Therefore, this result should be repeated with a separate solution as potato juice should have high levels. Overall, the results of this experiment show that different types of food contain different types of carbohydrates.

  • Lipsitch, M., Tchetgen, E. T., & Cohen, T. (2010). Negative controls: a tool for detecting confounding and bias in observational studies. Epidemiology (Cambridge, Mass.), 21(3), 383.
  • Slavin, J., & Carlson, J. (2014). Carbohydrates. Advances in Nutrition, 5(6), 760-761.

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