Chapter 5 The Structure and Function of Macromolecules Lipids

Lipids Little to no affinity for water Consist mostly of hydrocarbons although they may have some polar bonds due to the presence of oxygen Major classes: Fats Phospholipids Steroids

Fats Functions Energy storage Cushioning vital organs Insulation Major function is energy storage / A gram of fat stores twice as much energy as a gram of protein or carbohydrates

Fats, cont’d Triglycerides or triacylglycerols Consist of 1 glycerol + 3 fatty acids joined in a series of condensation reactions that lead to the formation of an ester bond between the glycerol molecule and each fatty acid Glycerol is an alcohol with 3 carbons each bearing a hydroxyl group Fatty acids have a carboxylic acid group with a long hydrocarbon tail

Figure 5.10 The synthesis and structure of a fat, or triacylglycerol

Fats, cont’d Saturated Fats Unsaturated Fats The fatty acids in a fat can be the same or they can be of two or three different kinds Fatty acids vary in length and in number and locations of double bonds Saturated Fats No double bonds in the fatty acid tails, hydrocarbon is “saturated” with hydrogen / tails pack closely / typically solids at room temperature / Ex: butter, lard Unsaturated Fats Fatty acids contain double bonds that generate kinks in the tails / tails cannot pack tightly / typically oils at room temperature / Ex: corn oil, cod liver oil

Figure 5.11 Examples of saturated and unsaturated fats and fatty acids

Phospholipids Major components of cell membranes Composed of glycerol, two fatty acids, and a phosphate group Additional small molecules, usually charged and polar, can be attached to the phosphate group (Ex: choline, serine, etc) Have a hydrophilic head and a hydrophobic tail Will self-assemble into aggregates when placed in an aqueous solution Micelle Phospholipid bilayer

Figure 5.12 The structure of a phospholipid

Figure 5.13 Two structures formed by self-assembly of phospholipids in aqueous environments   

Steroids A class of lipid hormones that consist of four fused rings Steroid activity depends on the functional groups that are attached to the four fused rings (Ex: testosterone vs estradiol) The steroid cholesterol is a common component of animal cell membranes and is a precursor from which other steroids are synthesized

Figure 5.14 Cholesterol, a steroid

Figure 4.8 A comparison of functional groups of female (estradiol) and male (testosterone) sex hormones