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or fruits that serve as energy reserves for the next generation. This energy can be tapped by people who eat these plant oils or use them for fuel. Indeed, the famous German engineer Rudolf Diesel used peanut oil to power one of his early automobile engines in 1900.

Phospholipids form the core of biological membranes

Because lipids and water do not interact, a mixture of water and lipids forms two distinct layers. Many biologically important substances—such as ions, sugars, and free amino acids—that are soluble in water are insoluble in lipids.

Like triglycerides, phospholipids contain fatty acids bound to glycerol by ester linkages. In phospholipids, however, any one of several phosphate-containing compounds

Hydrophilic "heads"

Hydrophobic fatty acid "tails"

Hydrophilic "heads"

Hydrophilic "heads"

Hydrophobic fatty acid "tails"

Hydrophilic "heads"

^ Phospholipid bilayer

3.21 Phospholipids Form a Bilayer In an aqueous environment, hydrophobic interactions bring the "tails" of phospholipids together in the interior of a phospholipid bilayer. The hydrophilic "heads" face outward on both sides of the bilayer, where they interact with the surrounding water molecules.

(a) Phosphatidylcholine

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Essentials of Human Physiology

Essentials of Human Physiology

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