Laboratory testing for Carbohydrates:
The body uses carbohydrates as “fast fuel.” It is the first macromolecule used to obtain energy for the body because very little energy is required to break down carbohydrates. Carbohydrates are sugar molecules. They are made up of the base elements C, H and O in a 1:2:1 ratio. The simplest carbohydrate is a monosaccharide (a simple sugar). An example of a simple sugar is glucose, which is created during photosynthesis. Monosaccharides are covalently bonded together to create more complex sugars. A disaccharide is two covalently bonded simple sugars or monosaccharides. A polysaccharide is the carbohydrate polymer and consists of several monosaccharides bonded together. A common polysaccharide is starch. Starch is a storage polysaccharide found in plants. Another plant polysaccharide is cellulose, a major component of a plant’s cell wall.
Scientists test for the presence of glucose using Benedict’s Test for Reducing Sugars
(monosaccharides). A positive reaction changes the clear light blue solution to an opaque orange-brown solution. This color change indicates the presence of Glucose in a given solution.
Can you identify the positive Benedict’s Test versus the negative test?
A reaction occurs when iodine is added to a solution of starch, turning the solution a blue-black. If the solution remains the color of iodine, reddish-orange, there is no starch present, a negative test.
Test the samples you have been given to identify which contains carbohydrates. Be sure to use only a portion of your sample, as a single sample may contain more than one type of macroolecule.
Examples of Positive and negative carbohydrate test
Laboratory testing for Lipids:
The body stores lipids as reserve energy. Lipids are hydrophobic (“water-hating”) and thus much harder to break down for energy than carbohydrates. Lipids, however, contain more energy per unit weight then carbohydrates. Therefore it is more efficient for the body to use lipids as stored energy. The body will use its carbohydrate source for initial fuel, but if the “fast fuel” runs out, the body will turn to breaking down lipids for a rich energy source. Lipids are fat molecules and there are many different kinds. Triglyceride molecules are used by organisms for energy storage. Triglycerides are composed of three fatty acid molecules and one glycerol molecule bonded in an ester linkage. An easy test for us to use to identify the presence of lipids is the "Brown Paper" test. A substance is rubbed on a piece of brown paper, and lipids indicate their presence by turning the paper translucent.
You can see the fat has made the brown paper translucent.
Laboratory testing for Proteins:
Proteins are the most complex and functionally diverse molecules of living organisms. Proteins compose enzymes, blood cells and muscle tissue just to name a few and are therefore associated with meat products. Proteins are created by RNA during DNA Transcription and Translation, a process we will be discussing later this year. The base elements of proteins are C, H, O and N. The monomers of proteins are 20 different amino acids. The amino acids are bonded together in unique combinations to create a polypeptide chain, the protein polymer. This chain is then folded into a unique, functional protein.
Image of a complex protein
Testing for proteins can be accomplished with a pH test after mixing Calcium Oxide to a liquid sample. When heated, Protein will release Ammonia, changing the color of the litmus paper due to a change in pH. No change in pH indicates the lack of protein present. This manipulation of a protein in these ways denatures (changes the shape, and damages or destroys the functionality of a protein ) the protein.
The first step is to add equal volumes (dropwise) of 1% copper sulphate (sometimes called Biurets A) and either 1% potassium hydroxide or sodium hydroxide (Biurets B) to a test tube.
After adding Biurets A and B, you'll have a lump of blue precipitate at the bottom of the test tube.
Hand-warm the precipitate until it becomes liquid (you can do this in a water bath set at 40).
The easiest test to do to see the positive result is to test egg white or milk. Add some of the egg white to the Biurets solution in the test tube and mix thoroughly while still hand warming. The blue colour will change to violet if protein is present. If protein is not present, the blue colour will remain.
We can use the color gradient shown to the right to identify the amount of starch in a sample. The addition of Benedicts solution (or iodine starch) can determine the starch concentration in a sample.
The test tube on the right shows a negative test for starch. Each of the tubes to the left show higher concentrations of starch, until we see the far left pure starch solution.
An example of positive and negative tests for protein.
This video demonstrates how to perform the activity safely.