4 Chapter 4 – Analysis of Carbohydrates
Analysis of Carbohydrates
BACKGROUND
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
Objectives
- 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.
Materials
PER GROUP:
- 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.
Exercise
PROCEDURE
- Set up two test tube racks with seven test tubes each.
- Label one rack Simple and the other rack Complex.
- 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.
- In each rack, label one tube for each of the five samples provided.
- Dispense 2 mL of each sample, including the starch and glucose, into the corresponding tubes.
- 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.
- 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.
- 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.
DATA
|
Sample |
Benedict Testing |
Iodine Testing |
Sugar Type: Simple, Complex |
| Glucose | |||
| Starch | |||
| 1 | |||
| 2 | |||
| 3 | |||
| 4 | |||
| 5 |
DATA ANALYSIS
- Name the three elements that bond to form carbohydrates.
- What are the two major classes of carbohydrates?
- What distinguishes a monosaccharide from a disaccharide?
- What is the name and structure of the functional group that characterizes a simple sugar?
- What color changes indicate a simple sugar when Benedict’s solution is added to a sample and heated?
- What color change indicates the presence of starch when iodine solution is added?
QUESTIONS
- Why can the polysaccharide starch be broken down by human digestive enzymes but the polysaccharide cellulose fiber cannot? Explain the difference in structure.
- What is the function of the liver with respect to carbohydrate metabolism?
- Name the two hormones associated with maintaining the optimal blood glucose level in the human body.
- Name two diseases associated with carbohydrate management.
Licenses and Attributions
One of the major groups of organic molecules that make up the cell. 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.
The most basic unit of carbohydrates. They are the simplest of the sugars and consist of a single sugar unit.
a molecule that is the basic building block for larger molecules of the same type; The simplest and smallest functional unit of a type of molecule
Loss of an electron through donation of a hydrogen atom.
Gaining an electron (reduction of charge) by adding a hydrogen atom.
two monosaccharides chemically bonded together
A chemical reaction that combines two molecules by removing a hydrogen atom from one molecule and a hydroxide from the other molecule, forming a water molecule that is released during the reaction.
Large sugar chains composed of many monosaccharides chemically bonded together
monomers that are chemically bonded together
A polysaccharide that is found in plants.
A polysaccharide used for energy storage in bacteria, fungi, and animals
A polysaccharide that is found in some bacteria and as a structural unit of cell walls in oomycetes, plants, and many forms of algae.
An enzyme that breaks down starch into simple sugars
An enzyme that specifically breaks down maltose into two subunits of glucose
An enzyme that specifically breaks down lactose into glucose and galactose
a common term for monosaccharides and disaccharides
an aldehyde that is unbound or unreacted and is present in its original form. It signifies that the aldehyde group is not involved in any chemical bonding or modification at that particular site within the molecule.
In the context of redox (reduction-oxidation) reactions, to "reduce" refers to the process of gaining electrons or decreasing the oxidation state of an atom, ion, or molecule. Reduction occurs simultaneously with oxidation in a redox reaction, where one species undergoes reduction (gains electrons) while another species undergoes oxidation (loses electrons).
Reduction can be understood in terms of electron transfer. When a species accepts one or more electrons from another species, it is considered to be reduced. The species that donates electrons is the reducing agent, as it facilitates the reduction of the other species.
During a reduction process, the oxidation state of the reduced species decreases. The oxidation state is a measure of the degree of electron loss or gain by an atom in a compound. For example, if a species changes from an oxidation state of +2 to 0, it has been reduced by gaining two electrons.
Reduction reactions are fundamental in many chemical and biological processes. They are involved in energy production, such as in cellular respiration, where oxygen is reduced to form water. Reduction reactions are also essential in various industrial processes, such as the production of metals through the reduction of metal oxides.
In summary, in the context of redox reactions, "reduce" refers to the process of gaining electrons or decreasing the oxidation state of a species. It involves the transfer of electrons from a reducing agent to the species being reduced.
A category of carbohydrates that possess certain chemical properties allowing them to act as reducing agents. They are capable of reducing other substances by donating electrons during chemical reactions. The key characteristic of reducing sugars is the presence of a free or potentially free aldehyde group (in the open-chain form) or a ketone group (in the cyclic form).
The most common reducing sugar is glucose, but other examples include fructose, lactose, maltose, and galactose. These sugars have the ability to reduce certain oxidizing agents, such as Benedict's solution or Fehling's solution, by undergoing oxidation themselves.
When a reducing sugar reacts with an oxidizing agent, the aldehyde or ketone group undergoes oxidation, resulting in the formation of an acid or a carboxylate ion. This process leads to the reduction of the oxidizing agent. The reduction can be observed visually through color changes or measured quantitatively using specific assays.
It's worth noting that not all carbohydrates are reducing sugars. Some carbohydrates, such as sucrose (table sugar), do not have a free aldehyde or ketone group and therefore do not possess reducing properties. However, if sucrose is hydrolyzed (broken down) into its component monosaccharides (glucose and fructose), the resulting mixture will contain reducing sugars.
In summary, reducing sugars are a subset of carbohydrates that have a free or potentially free aldehyde or ketone group, enabling them to act as reducing agents in certain chemical reactions.
A reagent used to test for the presence of reducing sugars, particularly glucose, in a solution. It is named after its developer, Stanley Rossiter Benedict. The solution is composed of copper(II) sulfate (CuSO4), sodium citrate (Na3C6H5O7), and sodium carbonate (Na2CO3) dissolved in water.
The main component responsible for the reaction is the copper(II) sulfate, which acts as an oxidizing agent. When Benedict's solution is heated in the presence of a reducing sugar, such as glucose or fructose, a redox reaction occurs. The reducing sugar reduces the copper(II) ions in the Benedict's solution to copper(I) oxide, resulting in a color change from blue to various shades of green, yellow, orange, or red. The intensity of the color change can indicate the concentration of reducing sugars present.
Benedict's solution is commonly used in qualitative and semi-quantitative tests to detect the presence of reducing sugars in various applications, including clinical diagnostics (e.g., urine glucose testing for diabetes), food analysis (e.g., determining sugar content in fruits or beverages), and biochemical experiments. It provides a visual indication of the presence or absence of reducing sugars based on the color change observed upon heating the solution with the sample under test.
Iodine is an element with the symbol "I" on the periodic table. It is a nonmetal and belongs to the halogen group. In the context of identifying chemical compounds, iodine is often used as a reagent or indicator due to its unique properties.
One common use of iodine is in the iodine test, which helps identify the presence of starch. Iodine solution, typically prepared by dissolving iodine in water along with a solubilizing agent like potassium iodide, is used in this test. When iodine comes into contact with starch, a complex is formed that results in a characteristic deep blue or black color. This reaction is often employed to determine the presence of starch in food samples or other substances.
Another application of iodine is in the iodoform test, which helps identify compounds containing the CH3CO- group. In this test, iodine reacts with compounds like ethanol or acetone in the presence of a base to produce iodoform (CHI3). Iodoform has a distinct yellow color and a distinctive smell, allowing for the identification of compounds that yield this reaction.
Iodine can also be used in various other tests and reactions to identify specific functional groups or chemical compounds. For instance, it can be used to test for the presence of unsaturated compounds (double or triple bonds) by forming a brown or purple color upon reaction.
Overall, iodine's ability to react with certain compounds and produce characteristic colors or precipitates makes it valuable in identifying and distinguishing various chemical compounds in qualitative analysis.
