4 Chapter 4 – Analysis of Carbohydrates

Analysis of Carbohydrates


are one of the major groups of organic molecules that make up the cell, along with proteins, lipids and nucleic acids. They are primarily responsible for the storage of energy within the cell, but they also aid in the structural support of tissues. They are commonly referred to as sugars, starches, or fibers. Carbohydrates include monosaccharides, disaccharides, and polysaccharides.

are the most basic unit of carbohydrates. They are the simplest of the sugars and consist of a single sugar unit called a . The functional group characteristic of monosaccharides is the aldehyde group (H-C=O). Aldehydes tend to become (lose an electron through donation of a hydrogen) and cause the of another molecule. Monosaccharides are usually colorless, water-soluble, crystalline solids. Some have a sweet taste. Examples include glucose (dextrose), fructose, and galactose. Monosaccharides are the building blocks of disaccharides (sucrose) and polysaccharides (cellulose and starch).

are two monosaccharides chemically bonded together. They are formed when two monomers are joined together by a (also called a condensation reaction) and a molecule of water is removed. Examples include lactose (made from glucose and galactose) and sucrose (made from glucose and fructose).

are composed of many monosaccharides chemically bonded together. and are polysaccharides that usually consist of several hundred to several thousand glucose units. Glycogen is the storage form of energy in the animal body. Plants and other organisms store their energy in the form of starch. The chains of glucose units are highly branched in glycogen compared to starch. Both starch and glycogen must be broken down into individua* glucose molecule to become available as an energy source in the cells. is also a polysaccharide composed of glucose subunits and is classified as a fiber. It is the primary constituent of plant cells, present in vegetables, fruits, and legumes. Cellulose differs from starch in the orientation of the bonds between the subunits of glucose.


Digestion of carbohydrates begins in the mouth with the secretion of the enzyme amylase from the serous cells of the salivary gland. Amylase breaks starch and glycogen into disaccharides. The salivary glands are grouped into three categories: parotid, submandibular, and sublingual. The stomach works to mix and churn the food, which aids in further breakdown, but has no enzyme for the digestion of carbohydrates. The majority of the digestion of carbohydrates takes place in the small intestine. As the food moves into the duodenum, an enzyme called pancreatic is released through the pancreatic duct which reduces starch and glycogen into disaccharides. Specific enzymes then work to finish the job. breaks maltose into two units of glucose, sucrase breaks sucrose into glucose and fructose, and breaks lactose into glucose and galactose. Only fibers remain to move into the large intestine. Fibers attract water which softens stool. Some bacteria ferment some fibers, releasing water, gas, and short-chain fatty acids. The liver can change glycogen to glucose to increase blood glucose levels or convert glucose to glycogen to decrease blood glucose. The liver also converts other non-carbohydrates into glucose when needed.


exist in equilibrium between their ring structure form and their straight chain form. Solutions of monosaccharides and certain disaccharides, such as maltose and lactose, contain functional groups. The term ‘free’ means that the aldehyde is exposed at the end of the straight chain form rather than being bonded to other parts of the molecule on both ends in ring form. The free aldehyde functional group can other molecules by donating an electron to another molecule via loss of a hydrogen. Due to this ability, simple sugars are calked . One of the molecules that can be reduced by the free aldehyde is Benedict’s solution. When the is reduced, it changes color. A very small amount of reducing sugar will turn the Benedict’s solution a greenish yellow; greater amounts of reducing sugar will be indicated by color changes from yellow through orange to brick red. A rust color indicates that a large number of reducing sugars are present.

Both starch and cellulose are polysaccharide carbohydrates composed of many subunits of the monosaccharide glucose joined in long chains. However, the orientation of the bond between the glucose monomers is different in the two polysaccharides. Cellulose bonds are oriented so that the bonds are resistant to human digestive enzymes. Starch bonds are oriented so that human digestive enzymes can hydrolyze the bond and break down the starch molecule. The bond orientation in starch also causes starch to react with solution to display a dark blue/black color.

Key Terms

  • Monosaccharide                  •  Monomer
  • Disaccharide                        •  Polymer
  • Polysaccharide                    •  Amylase
  • Reducing Sugar                   •  Simple Sugar
  • Oxidize                                 •  Reduce
  • Free Aldehyde


  • Test for different types of carbohydrates based on bond types and functional groups contained within the molecule.
  • Understand the classification of carbohydrates and discuss structural characteristics of carbohydrate molecules.
  • Learn the nutritional differences between simple and complex carbohydrates.




  • Carbohydrate samples* (5)            •  Benedict’s solution
  • 2% starch solution                          •  Iodine solution
  • 2% glucose solution                       •  60° water bath
  • 10x100mm test tubes (14)              •  Medicine Cups
  • Test tube rack (2)
  • Marker/wax pencil
  • Test tube clamp
  • Disposable pipettes OR p-1000 Micropipettes with tips

*Examples include 2% solutions of glucose, starch, sucrose, lactose, fructose, albumin, etc.




  1. Set up two test tube racks with seven test tubes each.
  2. Label one rack Simple and the other rack Complex.
  3. In each rack, label one tube starch and one tube glucose. These are your controls. For the simple sugars, glucose is the positive control and starch is the negative control. For complex sugars, glucose is the negative control and starch is the positive control.
  4. In each rack, label one tube for each of the five samples provided.
  5. Dispense 2 mL of each sample, including the starch and glucose, into the corresponding tubes.
  6. For the rack labeled Simple:
    • Add 0.5 mL of Benedict’s solution to each tube.
    • Place the rack in the 60 0 C water bath for 10 minutes.
    • After 10 minutes, remove each tube from the water bath using a test tube clamp and record the color change in Table 1.
  7. For the rack labeled Complex:
    • Add 0.5 mL of iodine solution to each tube.
    • The iodine reaction will be immediate. Record the results in Table 1.
  1. After all samples have been tested and results recorded:
    • Dispose of all samples in a waste container as instructed by the professor.
    • Wash all test tubes, using the test tube brush to remove any residue.
    • Return cleaned test tubes to their rack, placing them upside down to dry.
    • Return any unused samples and sample bottles to the tray/basket.


Table 1: Testing for Simple Sugars


Benedict Testing

Iodine Testing

Sugar Type: Simple, Complex



  1. Name the three elements that bond to form carbohydrates.
  2. What are the two major classes of carbohydrates?
  3. What distinguishes a monosaccharide from a disaccharide?
  4. What is the name and structure of the functional group that characterizes a simple sugar?
  5. What color changes indicate a simple sugar when Benedict’s solution is added to a sample and heated?
  6. What color change indicates the presence of starch when iodine solution is added?


  1. Why can the polysaccharide starch be broken down by human digestive enzymes but the polysaccharide cellulose fiber cannot? Explain the difference in structure.
  2. What is the function of the liver with respect to carbohydrate metabolism?
  3. Name the two hormones associated with maintaining the optimal blood glucose level in the human body.
  4. Name two diseases associated with carbohydrate management.

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Biology I Cellular Processes Laboratory Manual by The authors & Hillsborough Community College is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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