9 - Lipids
Describe the structure and basic function of steroids.
A steroid consists of a carbon skeleton composed of four fused rings. Steroids such as sex hormones and cholesterol are necessary for life functions. In particular, cholesterol is essential for the body to construct more complex steroid molecules. Cholesterol is produced by the liver or obtained from foods. Doctors monitor the level of certain cholesterol components in human blood. The lipoproteins HDL and LDL transport cholesterol through the bloodstream. HDL offers positive health benefits. Doctors check the levels of HDL and LDL in blood to monitor people for atherosclerosis.
Distinguish among triacylglycerols, saturated fatty acids, and unsaturated fatty acids.
Fats, also known as triacylglycerols, contain three fatty acids attached to a glycerol backbone. Each carboxyl functional group on the fatty acids is joined to one of the three hydroxyl functional groups on the glycerol by an ester linkage. In a saturated fatty acid, the carbons in the hydrocarbon tail are all bonded by single covalent bonds, which makes them "saturated" with hydrogen atoms. In an unsaturated fatty acid, the hydrocarbon tail contains one or more double covalent bonds between the carbons. Thus, the fatty acid's carbon skeleton is not "saturated" with hydrogen atoms. Both saturated and unsaturated fatty acids are found in triacylglycerols.
Describe the main characteristics of lipids.
Lipids include fats, phospholipids, and steroids. Lipids are composed primarily of carbon and hydrogen. They are generally nonpolar and hydrophobic, and they do not interact with water. However, some lipids contain functional groups that are polar in nature. These functional groups are hydrophilic and do interact with water.
Explain why phospholipids form bilayers and relate the structure of phospholipids to cell structure and function.
Phospholipids are composed of a glycerol bound to two fatty acids and one phosphate functional group. The hydrocarbon tails of fatty acids are hydrophobic and do not interact with water. The phosphate functional group is hydrophilic and interacts with water. When placed in water, phospholipids align with the phospholipid tails facing each other and the phospholipid head exposed to the outside environment. Phospholipids can form a micelle or a bilayer but because of the size of the hydrophobic tails, they tend to form a bilayer. The bilayer structure is important because it is the basis for cell membranes. A cell membrane controls what materials can enter or leave the cell; it separates and defines the inner cell from the outside environment.
Relate the structure of fat molecules to the function of energy storage.
Plants use polysaccharides, like starch, as energy reserves. These compounds are relatively bulky. In contrast, lipids contain twice as much energy as polysaccharides. Animals need lipids such as triacylglycerols to continue to maintain life functions. Animals store triacylglycerols in adipose tissues that shrink and swell as reserves are used or stored. Adipose tissue also provides insulation. Fat is synonymous with the term triacylglycerols.