Flashcards Cell Bio Lab #2 - Chemical Composition of Biological Materials - Review

Iodine test

This test for carbohydrates is used to detect starch with I2KI solution. In the presence of starch, an iodine-starch complex is formed, causing a color change from reddish/yellow/brown to violet-black

Benedicts test

This test for carbohydrates is used to detect reducing sugars which are small sugar subunits and individual sugar monomers. The reagent used is a solution of sodium citrate and CuSO4, and it produces an insoluble orangish CU2O precipitate in the presence of reducing sugars (aldoses and some ketones) and heat

Sudan IV dye technique

The reagent used for this technique will only dissolve in hydrophobic compounds such as lipids. When in contact with lipids, the dye will lend a red-pink color to the solution.

Biuret test

In this test for proteins, NaOH and CuSO4 are added to test solutions. In alkali environments, the Cu^2+ ions will oxidize peptide bonds in protein. In the process, Cu^2+ ions are reduced to Cu^+1 producing a purple color whose intensity directly correlates with the amount of protein present

Principle

This term describes the chemical reaction that is occurring

Reducing sugars can be either a mono- or disaccharide which contains a hemiacetal group with one OH group and one OR group attached to the same carbon. Simple (reducing) sugars have free aldehyde and ketone groups. The OH group is able to reduce other compounds.

Starch is a polysaccharide with many glucose monomers joined together by glycosidic bonds.

What is the difference between starch and reducing sugars?

goes from yellow/brown (negative) to a violet/black color

What is the observable change in a positive starch test?

If starch is present, the iodine in the reagent reacts with starch to create an iodide complex

What is the chemical basis of the change in a positive iodine test?

The color changes from a light blue (seen in negative test and the reagent itself) to a green/orange/brick red color when placed in heat and if reducing sugars are present.

What is the observable change in a positive Benedict's test?

If reducing sugars are present, Cu^2+ in the form of sodium citrate and CuSO4 (Benedict's Reagent) is reduced to Cu^1+ as an insoluble Cu2O precipitate is formed

What is the chemical basis of a positive Benedict's test?

Reducing sugars react because the simple carbohydrates and their OH groups have the ability to reduce compounds while starch cannot. Benedict's test only tests for simple sugars which have free aldehyde and ketone groups

Why do reducing sugars react to the Benedict's test while starch does not?

Red sudan IV crystals dissolve and create a red solution

What is the observable change in a positive Sudan IV test?

Sudan IV dissolves in a hydrophobic/lipophilic solution due to its solubility in lipids. Once dissolved it stains the lipids with a red color

What is the chemical basis of a positive Sudan IV test?

The other tests form a complex or undergo a redox reaction, so a chemical reaction and change is occurring. The Sudan IV test, however, only stains lipids if they are present due to its solubility in them

How is the Sudan IV test different from the other tests mentioned?

The color changes from blue (negative result and reagent color) to purple

What is the observable change in a positive Biuret test?

The change in color occurs due to the reduction of Cu^2+ ions to Cu^1+. In alkali environments, Cu^2+ ions oxidize peptide bonds in proteins and the Cu^2+ ions are reduced to Cu^1+

What is the chemical basis of a positive Biuret test?

The negative controls allow for the visual of the original color and tests the reagents and makes sure the solvent is pure

What is the significance of the use of negative control and positive control in all of the tests mentioned?