There are three monosaccharides in living things, each of which follows the hexose ring structure but has different placements of hydrogen atoms and oxygen atoms. These are glucose, galactose, and fructose. Disaccharides are two sugar molecules linked together. Three major disaccharides are lactose (a combination of glucose and galactose), sucrose (a combination of glucose and fructose), and maltose (a combination of 2 glucose molecules). A polysaccharide is any grouping of monosaccharides that is greater than 2 in a row. Cellulose is a plant-based polysaccharide that is not digested by humans but is digested by other organisms. Starch is another polysaccharide that is used in plants for their cellular energy. The same molecule comparable to starch in animals is glycogen, the energy storage molecule in animals.
LIPIDS Lipids are a little bit more complex in their diversity when compared to carbohydrates. What they share in common is that they are insoluble in water and in other polar solvents. You’ll hear more about polarity when it comes to water in the next major section. There are three major types of lipids in living systems: •
Triglycerides—these are basically fatty acid chains (three of them) attached to a glycerol molecule. A fatty acid is just chains of carbon and hydrogen atoms with a carboxyl group attached at the end. A carboxyl group is basically a -COOH grouping. Figure 2 is an illustration of a triglyceride:
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