Chemistry 24.4

Lipids Before the invention of electric lighting, wax candles were a major source of lighting in homes. In this section, you will read more about waxes and similar compounds that make up the class of biomolecules known as lipids.

Triglycerides Triglycerides What physical property distinguishes lipids from other classes of biological molecules?

Fats, oils, and other water-insoluble compounds are called lipids. Triglycerides Fats, oils, and other water-insoluble compounds are called lipids. Moderate levels of dietary fats and oils are essential to health.

Triglycerides Natural fats and oils exist as triesters of glycerol with fatty acids, which are long-chain carboxylic acids (C12 through C24). This form of lipid is known as a triglyceride.

Triglycerides

Triglycerides Lipids tend to dissolve readily in organic solvents, such as ether and chloroform, rather than in highly polar solvents such as water. This property sets them apart from most biological substances such as carbohydrates and proteins.

Triglycerides The hydrolysis of oils or fats by boiling with an aqueous solution of an alkali-metal hydroxide is called saponification.

A typical saponification reaction is shown below. Triglycerides A typical saponification reaction is shown below.

Saponification is used to make soap. Triglycerides Saponification is used to make soap. These photographs illustrate soapmaking. Once the soap is formed, it is poured into molds. Later it may be milled, or shredded, with scent or color added, and then remolded to produce a finished product.

Phospholipids Phospholipids How do phospholipid molecules arrange themselves in water?

Phospholipids Phospholipids, or lipids that contain phosphate groups, are abundant in cells. A space-filling model of lecithin is shown here. In the simplified representation above, the hydrophilic head is shown as a sphere and the hydrophobic tails as wavy lines.

Phospholipids In water, phospholipids spontaneously form a spherical double layer, called a lipid bilayer, in which the hydrophobic tails of phospholipid molecules are sandwiched between two layers of hydrophilic heads.

A cell membrane has a lipid bilayer structure. Phospholipids A cell membrane has a lipid bilayer structure. A cell membrane has a lipid bilayer structure. a) The hydrophilic heads are in contact with water, but the hydrophobic tails are not. b) The lipid molecules move easily within their own layer but do not readily move to the other layer. Applying Concepts What prevents a lipid molecule from crossing to the opposite side of the bilayer?

Phospholipids A cell membrane has a lipid bilayer structure. a) The hydrophilic heads are in contact with water, but the hydrophobic tails are not. b) The lipid molecules move easily within their own layer but do not readily move to the other layer. Applying Concepts What prevents a lipid molecule from crossing to the opposite side of the bilayer?

Waxes Waxes Waxes are esters of long-chain fatty acids and long-chain alcohols. Waxes are also part of the lipid family. Waxes are low-melting, stable solids.

24.4 Section Quiz. 24.3.

24.4 Section Quiz 1. The class of substances called lipids includes proteins. hydrocarbons. soaps. fats, oils, and waxes.

24.4 Section Quiz 2. In water, phospholipids spontaneously form a solution in which the hydrophilic heads dissociate from the hydrophobic tails. a single layer in which hydrophobic tails are hydrogen-bonded to hydrophilic heads. a double layer in which the hydrophobic tails are sandwiched between the hydrophilic heads. a double layer in which the hydrophilic heads are sandwiched between the hydrophobic tails.

24.4 Section Quiz 3. The selective absorption of nutrients across a cell membrane occurs in channels formed by lipids. phospholipids. proteins. carbohydrates.

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