C. Lipids

Chemically, fats are carboxylic esters from glycerol, and are known as glycerides. Fatty acids are generally straight-chained compounds from three to eighteen carbons long. As a rule, living organisms tend to produce fatty acids containing an even number of carbons. Were these fatty acids not associated with triglycerols, they would be considered carboxyllic acids. An example of a fatty acid would be linoleic acid.

Cholesterol is a precursor to the five major classes of steroid hormones; progestagens, glucocorticoids, mineralocorticoids, androgens, and estrogens.

Phospholipids are triglycerides with ester linkages to fatty acid chains having two fatty acids and a phosphate group as a part of the third acid. These are important since phospholipids make up cell membranes, adenosine triphosphate is involved in energy exchange in all known life forms, and nucleic acids, which contain the blueprints of life, contain organic phosphates.

1. A diverse group of molecules that includes fatty and oily substances, lipids serve as energy storage units and waterproof coverings around cells

2. All lipids are hydrophobic (water fearing) and insoluble in water (and therefore often water repellent)

3. Lipids yield more energy per gram (9.3 kcal/g) than do carbohydrates (3.8 kcal/g) when used as an energy source by cells

4. Lipids include fats, cutin, suberin, waxes, and phospholipids

a. Fats (saturated and unsaturated), also called triglycerides, consist of three fatty acids joined to a glycerol molecule

(1) Saturated fats consist of carbon atoms that hold (are bonded to) as many hydrogen atoms as possible

a) They generally exist as animal fats, such as lard and butter

b) They melt at temperatures higher than those required to melt unsaturated and polyunsaturated fats

(2) Unsaturated and polyunsaturated fats contain carbon atoms joined together by double bonds; the carbons can form additional bonds with various other atoms

a) Commonly produced by plants, unsaturated fats tend to be oils

b) Examples include safflower oil, peanut oil, and corn oil

b. Cutin and suberin are lipid polymers that are found in the cell walls of certain plant tissues

(1) Cutin and suberin form a matrix in which waxes (long-chain lipids) are imbedded; they differ from one another in the types of imbedded waxes and their chemical composition

(2) Cuticle (cutin with embedded waxes) forms the outermost covering of the epidermal cells of leaves, flowers, and fruits and prevents excess water loss from these above-ground plant parts

(3) Suberin is found in the cork cells of bark and the Casparian strip of root endodermal cells

c. Phospholipids are closely related to fats

(1) Each phospholipid consists of a glycerol molecule attached to two fatty acids and a phosphorus-containing molecule

(2) Phospholipids have a "split personality"

a) The fatty acid portion contains hydrocarbons, making the molecule hydrophobic (water repellent)

b) The phosphate portion is hydrophilic (water attracting), making the molecule water soluble

c) In water, phospholipids tend to form a film; the hydrophobic portion faces away from water and the hydrophilic portion dissolves in water

(3) They are the predominant component of cellular membranes

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