Lipids

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:

You’ve probably heard about polyunsaturated fats, trans fats, monounsaturated fats, and saturated fats. Saturation, when it comes to fatty acids, involves the number of hydrogen atoms that are attached to the fatty acid. Saturated means the carbon atoms are all saturated with hydrogen atoms and that it is a straight molecule. Monounsaturated and polyunsaturated fats have some hydrogen atoms missing, making the molecule “kinky”. Trans fats have a bend in the fatty acid but the “kink” goes in the wrong way, making it more likely to block arteries in a more intense way than even saturated fats.

• Phospholipids—these look like triglycerides because they have fatty acid chains; however, one of the fatty acids has been replaced with a phosphate group, with the chemical symbol of PO4. These are highly important molecules in living things because they have hydrophobic tails and a hydrophilic phosphate group.

This makes them great for the making of membranes because the phosphate groups are on the outside of the membrane and the fatty acid (hydrophobic) groups are on the inside of the membrane. It takes two sheets of phospholipids sandwiching the hydrophobic tails in between to make a membrane. This is how membranes are made all over cells and cell structures. Figure 3 illustrates what a lipid bilayer looks like: