BIO - Carbohydrates and Lipids [LESSON]
Carbohydrates and Lipids
Carbohydrates
Foods that you eat all have the four biomolecules contained within them at different ratios. The primary biomolecule that provides energy to the body is the carbohydrate. Your body breaks down carbohydrates during cellular respiration to make lots of ATP (a molecule that carries energy in the body). Carbohydrates also have other functions ranging from cellular identification to building strong supports in plant cell walls. Carbohydrates are biomolecules (macromolecules) composed of carbon, hydrogen, and oxygen in an approximate ratio of 1:2:1. For example, glucose, galactose, and fructose all have the molecular formula of C6H12O6. Note that there are twice as many hydrogens as there are carbon and oxygen. Carbohydrates are typically depicted as a ring structure but can also take the shape of a linear carbon chain. See the images below for both types of structures.
A single building block (monomer) of carbohydrates is called a monosaccharide. Monosaccharides (mono- = “one”; sacchar- = “sugar”) are simple sugars, the most common of which is glucose. Fructose and galactose are also monosaccharides. They all have a single carbon ring.
When two monosaccharides join together in a dehydration synthesis reaction (see the previous lesson), a disaccharide is formed. A disaccharide, such as lactose or maltose, has two carbon rings. When even more monosaccharides join, a polysaccharide is formed.
Watch the Introduction to Carbohydrates video below.
There are a few polysaccharides that are very important to know. First, animals link together chains of glucose into a polysaccharide called glycogen. Why? Well, when you eat a sugary meal, your body doesn’t need to immediately turn all of that sugar into energy. It’s stored very temporarily in a glycogen molecule in the liver. When your blood sugar drops because you haven’t eaten in a while, the liver is signaled to chop up glycogen and release glucose into the blood for the body to use. This same glucose storage system is used by plants and their version is called starch. The starch in the seeds provides food for the embryo as it germinates while the starch that is consumed by humans is broken down by enzymes into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose. Another important polysaccharide is cellulose. Cellulose is a major component of plant cell walls, which are rigid structures that enclose the cells (and help make lettuce and other veggies crunchy). Wood and paper are mostly made of cellulose, and cellulose itself is made up of unbranched chains of glucose monomers. You may find it interesting that animals cannot digest cellulose which is why parts of plants that contain large amounts of cellulose (such as corn kernels) remain undigested in feces. Additionally, there is a strong polysaccharide called chitin that forms the fungal cell wall and the arthropod exoskeleton. This is why insects crunch when they are stepped on. Each of these polysaccharides is formed from different arrangements of monosaccharides.
Remember that one of the core themes of biology is the relationship between structure and function. Polysaccharide structure is a great example of that. Even such a small change as the -OH (hydroxyl) group moving to another area in space around the carbon atom is enough to render the molecule so different that it cannot serve the same function. This is why animals cannot digest cellulose but they can digest starch even though they are very chemically similar. When the structure changes, even in the slightest, the function changes as well.
Lipids
Lipids are biological molecules known as fats, waxes, oils, and steroids. They are primarily composed of carbon and hydrogen with very little oxygen and are used by the body for long-term energy storage. You may recall that C-H bonds are nonpolar and this is the integral piece of lipid structure and determines interactions with water. Although they are grouped with the other macromolecules, they are generally not big enough to be considered a polymer and have no true monomers. Long molecules of fatty acids attached to a glycerol molecule are the subunits of lipids. The two big examples to know are triglycerides and phospholipids. See the video below for more information about lipids.
Watch the Introduction to Lipids video below.
Note that fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is said to be saturated. Saturated fatty acids are saturated with hydrogen since single bonds increase the number of hydrogens on each carbon. This is an important concept that we will revisit during the cell membrane structure lesson in the next unit.
Use the information from this lesson in the Carbohydrate and Lipid Classification Activity below. Think about the correct answer, and then check yourself by selecting the "show answer" button. Select Next to move through each example.
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