Lab #1- Measuring the Carbohydrate Content
Objective: to measure the concentration of glucose and starch in different stages of ripening bananas.
Introduction
Fresh pineapple, chilled watermelon, and an icy cold soft drink on a hot summer's day. What do these
things have in common? They all taste so good because of the sweetness the sugar in them provides.
When you think of sugar, you probably picture the white granules you put in cookies, that your parents
put in their coffee, or that you put on your cereal. Actually, this is just one kind of sugar,
called sucrose, which is extracted from sugar cane or sugar beets. Technically, sugar is
a carbohydrate that occurs naturally in every fruit and vegetable. It is the major product of
photosynthesis, the process by which plants transform the Sun's energy into food.
Glucose is another type of sugar and is a very important biochemical. For one thing, glucose is the only
fuel used by brain cells. Glucose is also an important source of energy for muscles and other tissues in
the body.
The glucose in your blood comes from the food you eat. Complex carbohydrates, found in pasta or
cereal, for example, are long chains of sugar molecules that are broken down by enzymes to simple
sugars, such as glucose. Sucrose, or table sugar, is also broken down to form glucose. Because
carbohydrates and sucrose in food are broken down to form glucose, the level of glucose in your blood
goes up after you eat.
Like most of the chemicals in your blood, the level of glucose must be tightly controlled. The level of
glucose in your blood is controlled by insulin, a hormone made in the pancreas. Too little glucose, and
your brain and other organs will not have the energy they need to function. Too much glucose in the
blood can cause diabetes, which is a serious and growing health problem in the United States.
Ripening is a process in fruits that causes them to become sweeter, softer, and less green. The process of
ripening is controlled by the plant hormone called ethylene, which is a gas created by plants from
the amino acid called methionine. A plant hormone is a chemical that regulates growth and other
processes. Storing fruit in a closed container keeps the ethylene from drifting away and can increase the
rate at which the fruit ripens. Ethylene increases the intracellular levels of certain enzymes in
fruit. Enzymes are proteins that make certain chemical reactions occur faster than they normally would.
The key enzymes involved in fruit ripening are amylase and pectinase. Amylase breaks down starch to
produce simple sugars, so is responsible for the increasing sweetness of a ripening fruit. Pectinase
breaks down pectin, a substance that keeps fruit hard, so is responsible for the increasing softness of
ripening fruit. Other enzymes cause the color of the fruit to change by breaking
down chlorophyll (which is green) and replacing it with pigments that are yellow, red, or other colors.
Procedure
1. Obtain a set of concentrations of glucose solutions. For each solution place a small amount into
a well plate.
2. Dip a test strip into each of the positive control solutions. Watch the test strip for 30 seconds (or
the time recommended in the test strip instructions) and match the color of the test strip to the
color on the bottle. Record.
Testing Bananas for Glucose Concentration
1. For each banana sample, make a 10% dilution. Add 1 gram of banana to 10 ml of distilled water.
Thoroughly mix the solution and test with the glucose test strip. If the strip changes immediately
add 1 ml of banana to 10 ml of distilled water to make a 100% dilution. Save a small sample of
each of your 10% banana dilutions to test for starch.
2. Start the stopwatch as soon as the test strip has been dipped.
3. Wait for the amount of time specified on the test strip directions, usually 30 seconds.
4. Compare the color on the test strip with the color on the side of the container to determine the
glucose concentration. Also, compare the colors with the dilution series you made.
a. If the color changes to the maximum range (2 percent) before 30 seconds, list it as greater
than 2 percent, "> 2%."
b. To determine the actual percent of glucose in samples with over 2 percent, dilute the
sample in water to bring the glucose level down within the range of the test strips.
c. Test the diluted sample. If it has 1 percent glucose, then the glucose in the sample is
really 10 percent, because you diluted it 1 part in 10.
5. Repeat steps 1–6, of this section, for all the banana samples.
6. Graph your results.
Testing Bananas for Starch
1. Add a small amount of the 10% banana dilution into a well plate. Add 1 drop of Iodine
indicator solution. Record results.
ANALYSIS
What is the role of ethylene in fruit ripening?
What is broken down to make sugar in fruit?
What enzyme is responsible for making more sugar in fruit?
Put these five food sugars in order, from the sweetest to least sweet: glucose, fructose, sucrose, lactose
and maltose.