5.2 - The Chemist’s View of Phospholipids and Sterols

Describe the chemistry, food sources, and roles of phospholipids and sterols.

What is the best-known phospholipid?

Lecithin.

How is lecithin structurally similar to a triglyceride?

Both contain glycerol and fatty acids.

How does lecithin differ from a triglyceride?

Lecithin has two fatty acids instead of three.

What occupies the third attachment site of glycerol in lecithin?

A phosphate group and a molecule of choline.

What are phospholipids composed of?

Glycerol, two fatty acids, a phosphate group, and an attached molecule (such as choline).

Which part of a phospholipid is hydrophobic?

The fatty acid tails.

Which part of a phospholipid is hydrophilic?

The phosphate-containing head.

Why can phospholipids dissolve in both fat and water?

They contain both hydrophobic and hydrophilic components.

What is an emulsifier?

A substance that helps mix fat and water.

Why are phospholipids effective emulsifiers?

Their hydrophobic tails interact with fat while their hydrophilic heads interact with water.

What food products commonly use phospholipids as emulsifiers?

Mayonnaise, salad dressings, and candy bars.

Which foods are rich sources of lecithin?

Eggs, liver, soybeans, wheat germ, and peanuts.

What major structural role do phospholipids play in the body?

They are major components of cell membranes.

How are phospholipids arranged in cell membranes?

In a bilayer.

How are phospholipids oriented in a cell membrane?

Hydrophilic heads face the watery fluids; hydrophobic tails face inward away from water.

Why does the phospholipid bilayer form spontaneously?

Because hydrophilic heads are attracted to water and hydrophobic tails repel water.

How do phospholipids help fat-soluble substances move in the body?

They help fat-soluble vitamins and hormones pass in and out of cells.

What role do phospholipids play in the bloodstream?

They help keep fats suspended in blood and body fluids.

Why are phospholipids essential to life?

They form cell membranes and aid in fat transport and emulsification.

What key property allows phospholipids to function in both food systems and the body?

Their dual solubility in water and fat.

What are sterols?

Lipids with a multiple-ring chemical structure.

How do sterols differ structurally from triglycerides and phospholipids?

Sterols have a multiple-ring structure instead of fatty acid chains.

What is the most well-known sterol?

Cholesterol.

How is cholesterol structurally related to vitamin D?

Vitamin D is synthesized from cholesterol and has a similar structure.

What key structural difference gives vitamin D its vitamin activity?

Vitamin D has an open ring, whereas cholesterol has a closed ring.

Do both plant and animal foods contain sterols?

Yes, but only animal foods contain significant cholesterol.

Which foods are major sources of dietary cholesterol?

Meats, eggs, seafood, poultry, and dairy products.

Is “good cholesterol” found in foods?

No. “Good cholesterol” refers to how cholesterol is transported in the blood.

What sterols are naturally found in plant foods?

Plant sterols (phytosterols).

How do plant sterols affect cholesterol absorption?

They interfere with cholesterol absorption in the intestine.

How does a diet rich in plant sterols affect blood cholesterol?

It lowers blood cholesterol levels.

Why are some margarines fortified with plant sterols?

To create functional foods that help reduce blood cholesterol.

What important body compounds are sterols?

Cholesterol, bile acids, vitamin D, and certain hormones.

Which hormones are synthesized from cholesterol?

Sex hormones (testosterone, estrogen, androgens) and adrenal hormones (cortisol, cortisone, aldosterone).

What structural role does cholesterol play in the body?

It is a component of cell membranes.

Where is most of the body’s cholesterol found?

In cells (more than 90%).

Can cholesterol be used for energy?

No.

What is endogenous cholesterol?

Cholesterol made by the body.

What is exogenous cholesterol?

Cholesterol obtained from food.

Which organ produces most of the body’s cholesterol?

The liver.

Approximately how much cholesterol does the liver produce daily?

About 800–1500 mg per day.

How does liver cholesterol production compare to dietary intake?

The liver produces much more cholesterol than is consumed in the diet.

What is the average dietary cholesterol intake in the U.S.?

About 300 mg per day.

When does cholesterol become harmful in the body?

When it accumulates in artery walls.

What disease results from cholesterol plaque buildup in arteries?

Atherosclerosis.

What health events can atherosclerosis cause?

Heart attacks and strokes.

What are the three major lipid classes?

Triglycerides, phospholipids, and sterols.

What distinguishes sterols from other lipids?

Their multiple-ring structure.

Which lipid class includes cholesterol?

Sterols.

Which lipid class forms cell membranes along with cholesterol?

Phospholipids.