Chemical Composition of Cells

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BIOL 140 Lab—The Chemical Composition of Cells
Name ____________________________ I) Introduction All cells contain four major types of macromolecules: carbohydrates, lipids, nucleic acids, and proteins. In today’s lab, we will be studying three of the four-proteins, carbohydrates and lipids. Various chemical tests can be used to detect the presence of each of these molecules. Most of the tests involve a color change visible to the eye. If a color change is observed, the test is considered positive. If the color change is not observed, the test is negative, indicating that a particular molecule is not present. In all the chemical tests we will be performing, we will also be using a control. In most cases, the control will be a sample of distilled water (which should not contain any macromolecules). If your control gives you a positive result, you know your test is invalid. Another important aspect of the chemical composition of cells is the pH of their environment. Most cells operate in a narrow pH range. That is, pH values above or below a certain value may cause a cell distress or may even be fatal. To help maintain relatively constant pH levels, living systems use buffers. A buffer is a combination of a weak acid and a weak base that function together to minimize changes in the pH of a solution. In today’s lab, we will learn how to measure pH and demonstrate how buffers work. II) Procedure A) Carbohydrates The basic building blocks for carbohydrates are sugars, also called monosaccharides. These molecules are often linked together to form medium-length chains called oligosaccharides, or very long chains called polysaccharides. The chemical and physical properties of monosaccharides and polysaccharides are different and can be detected with specific chemical tests. Glucose is an example of a monosaccharide that can be linked together in long chains to make a polysaccharide called starch. Even though starch is made up of glucose, in chemical tests the two molecules will react differently. 1. Benedict's Test for Reducing Sugars The Benedict’s test detects the presence of sugars known as reducing sugars. Reducing sugars are usually either single sugars or disaccharides which have a free aldehyde group that can be oxidized. Generally, large polysaccharides are non-reducing sugars. When heated to boiling in the presence of Benedict’s reagent, reducing sugars will turn the reagent, green, yellow, orange or red-orange, depending on the concentration of reducing sugars present. In general, greenish shades indicate a low concentration, while high concentrations yield brick red colored solutions. If no reducing sugar is present, the Benedict's reagent stays blue. a. Using a hot plate and a beaker filled with water, heat the water to boiling. b. Obtain six clean test tubes and number them 1-6, placing 1.0 ml of each of the following solutions in the corresponding test tubes 1 = water, 2 = glucose, 3 = sucrose, 4 = starch, 5 = onion juice, 6 = potato juice.

c. Now add 2.0 ml of Benedict’s reagent to each tube. Heat all six tubes 2-3 minutes in the boiling water bath. d. Record all color changes in the table below, and answer the following questions. Table 1. Benedict's Test Results Test tube 1 2 3 4 5 6 Color positive or negative?

e. Please answer the following questions: What was the purpose of test tube #1?

Did you expect starch to give you a positive or negative result? Why?

How are glucose and sucrose different from one another?

2. The Iodine Test for Starches The iodine test is a carbohydrate test that identifies the presence of starch. Starch molecules are actually polymers of glucose commonly found in plants. In the presence of starch, iodine solution (IKI) changes from yellow to blue-black. If the solution stays yellow, the test is negative for starch. a. Obtain a ceramic spot plate from the front lab bench. b. Add 2 drops of each of the following solutions to the ceramic plate: water, glucose, sucrose, and...
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