Macromolecules

Knowing what macromolecules are is an important step to understanding how nutrients are absorbed and used by cells. The term macromolecules simply means large molecules and refers to four different compounds, carbohydrates, lipids, proteins, and nucleic acids. Each of these has a unique structure and function. Let's talk about carbohydrates first.

All carbohydrates are composed of carbon, hydrogen, and oxygen. Those with simple structures are called monosaccharides or sugars. Monosaccharides can join to form more complex carbohydrate chains called polysaccharides or starch. Starch's normal form resembles a coiled spring because of the bond angles in the polymer chain.

The next type of macromolecule we'll discuss is protein. Proteins are built from chains of amino acids, organic compounds that each have an amino group or N-terminal at one end and a carboxyl group, or C-terminal at the other. There are 20 standard amino acids and while they all have a similar base structure, each has a unique side chain known as the R-group. Different amino acids can link together to form a chain connected by peptide bonds and create a protein.

The final class of macromolecules we will examine is lipids. The most common types of lipids are fats, also called triglycerides because they are composed of a glycerol molecule connected to three fatty acid chains.

Now, let's talk about detecting these macromolecules in a lab setting. Benedict's Reagent is commonly used for detecting monosaccharides. Benedict's Reagent is normally blue because it contains copper ions in the form of copper sulfate. This metal salt is an example of an oxidizing agent. Monosaccharides that contain a hemiacetal group, such as glucose, are considered reducing sugars. In an aqueous solution, monosaccharides with hemiacetal groups exist in both the cyclic and open chain form which has a reactive aldehyde. From this aldehyde, an electron is transferred to the copper II ion. This reduces the copper II ion to copper I ion and oxidizes the reducing sugar. Because copper I ions are seen to be rust red, the solution will begin to change color. In the presence of small amounts of monosaccharide, the solution will change to green. Higher concentrations of the monosaccharide will lead to more copper being reduced and the solution's color will change more dramatically, eventually becoming a reddish orange.

Starches can be detected with the normally yellow-brown iodine indicator. This indicator contains various polyiodide anion species such as I-3 negative, I-5 negative, and I-7 negative. The negatively charged iodines in each anion act as charge donators while the neutral iodines act as charge acceptors. This is a charge transfer complex and electrons in these complexes are easily excited with light to a higher energy level. The light is absorbed in the process and its complementary color can be seen by the human eye. The absorptions of the different species in this solution are what give it the yellow-brown color. When a starch is added, it forms a new charge transfer complex with the polyiodide anions. With the starch acting as a charge donator and the polyiodide as an acceptor. While the specifics of the reaction are not known, it is thought that the iodine and starch combine to form infinite polyiodide homopolymer. This complex absorbs light of a different wavelength, turning the color of the solution dark blue.

Proteins can be detected using Biuret's Reagent, a blue solution that contains copper ions but bind to the nitrogen found in peptide bonds, forming chelate complexes. Chelate complexes involving three to four peptide bonds produce a violet color, which changes the color of the solution. The intensity of the color is directly related to the peptide bond concentration. The greater the concentration, the more intense the violet color will be.

Finally, lipids can be detected in a solution using the Sudan IV test. Sudan IV is a dye that is not soluble in water, but it is soluble in lipids, so when it is added to a solution containing lipids it will stain them red.

In this laboratory, you will determine which macromolecules are in the provided mystery solutions using four different indicator reagents.