Chemistry 24.4

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Chemistry 24.4

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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.


Lipids >

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24.4 Triglycerides

Triglycerides

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

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24.4 Lipids > Triglycerides

Fats, oils, and other water-insoluble compounds are called lipids.

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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.

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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.

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

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A typical saponification reaction is shown below.

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Saponification is used to make soap.


Lipids >

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24.4 Phospholipids

Phospholipids

How do phospholipid molecules arrange themselves in water?

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24.4 Lipids > Phospholipids

Phospholipids, or lipids that contain phosphate groups, are abundant in cells.


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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.

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A cell membrane has a lipid bilayer structure.

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24.4 Lipids > 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.


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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

24.4 Section Quiz.

24.3.


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24.4 Section Quiz

1. The class of substances called lipids includes

a. proteins.

b. hydrocarbons.

c. soaps.

d. fats, oils, and waxes.


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24.4 Section Quiz

2. In water, phospholipids spontaneously form

a. a solution in which the hydrophilic heads dissociate from the hydrophobic tails.

b. a single layer in which hydrophobic tails are hydrogen-bonded to hydrophilic heads.

c. a double layer in which the hydrophobic tails are sandwiched between the hydrophilic heads.

d. a double layer in which the hydrophilic heads are sandwiched between the hydrophobic tails.


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24.4 Section Quiz

3. The selective absorption of nutrients across a cell membrane occurs in channels formed by

a. lipids.

b. phospholipids.

c. proteins.

d. carbohydrates.

END OF SHOW

Chemistry 24.4

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