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Lipid Classification

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Lipid Classification

You’ve probably seen dozens of nutrition facts labels like the one in the opening image. The labels show the nutrients that foods contain. Many people read nutrition facts labels to see how much fat there is in particular foods. That’s because eating too much fat, especially saturated fat, can be unhealthy and contribute to weight gain. Fats are a type of biochemical compound called lipids.

What Are Lipids?

Lipids are one of four classes of biochemical compounds, which are compounds that make up living things and carry out life processes. (The other three classes of biochemical compounds are carbohydrates, proteins, and nucleic acids.) Living things use lipids to store energy. Lipids are also the major components of cell membranes in living things. Types of lipids include fats and oils.

  • Fats are solid lipids that animals use to store energy.
  • Oils are liquid lipids that plants use to store energy.

Q: Can you name some lipids that are fats? What are some lipids that are oils?

A: Lipids that are fats include butter and the fats in meats. Lipids that are oils include olive oil and vegetable oil. Examples of both types of lipids are pictured in the Figure below .

(A) The white bands on these lamb chops are fat. (B) The yellow liquid in this bottle is olive oil.

Fatty Acids

Lipids consist only or mainly of carbon, hydrogen, and oxygen. Both fats and oils are made up of long chains of carbon atoms that are bonded together. These chains are called fatty acids. Fatty acids may be saturated or unsaturated. In the Figure below you can see structural formulas for two small fatty acids, one saturated and one unsaturated.

  • Saturated fatty acids have only single bonds between carbon atoms. As a result, the carbon atoms are bonded to as many hydrogen atoms as possible. In other words, the carbon atoms are saturated with hydrogens. Saturated fatty acids are found in fats.
  • Unsaturated fatty acids have at least one double bond between carbon atoms. As a result, some carbon atoms are not bonded to as many hydrogen atoms as possible. They are unsaturated with hydrogens. Unsaturated fatty acids are found in oils.

Q: Both of these fatty acid molecules have six carbon atoms and two oxygen atoms. How many hydrogen atoms does each fatty acid molecule contain? What else is different about the two molecules?

A: The saturated fatty acid molecule has 12 hydrogen atoms. This is as many hydrogen atoms as can possibly be bonded to carbon atoms in this molecule. The unsaturated fatty acid molecule has 10 hydrogen atoms, or two less than the maximum possible number. The saturated fatty acid has only single bonds between its carbon atoms. The unsaturated fatty acid has a double bond between two of its carbon atoms.

Phospholipids

Some lipids contain the element phosphorus as well as carbon, hydrogen, and oxygen. These lipids are called phospholipids. Two layers of phospholipid molecules make up the cell membranes of living things. In the Figure below , you can see how phospholipid molecules are arranged in a cell membrane.

  • One end of each phospholipid molecule is polar, so it has a partial electric charge. Water is also polar and has electrically charged ends, so it is attracted to the oppositely charged end of a phospholipid molecule. This end of the phospholipid molecule is described as hydrophilic, which means “water loving.”
  • The other end of each phospholipid molecule is nonpolar and has no electric charge. This end of the phospholipid molecule repels polar water and is described as hydrophobic, or “water hating.”

In the Figure above , the hydrophilic ends of the phospholipid molecules are on the outsides of the cell membrane, and the hydrophobic ends are on the inside of the cell membrane. This arrangement of phospholipids allows some substances to pass through the cell membrane while keeping other substances out. You can see how this works in the video at the URL below.

http://www.beyondbooks.com/lif71/4b.asp

Summary

  • Lipids are a class of biochemical compounds that living things use to store energy. Types of lipids include fats and oils.
  • Both fats and oils consist of long chains called fatty acids, which may be saturated or unsaturated.
  • Some lipids, called phospholipids, contain phosphorus as well as carbon, hydrogen, and oxygen. Two layers of phospholipid molecules make up the cell membranes of living things.

Vocabulary

  • lipid : Biochemical compound such as fat or oil that contains oxygen in addition to carbon and hydrogen and is made up of long carbon chains called fatty acids.

Practice

At the following URL, watch the animation about lipid structure and function. Then answer the questions below.

http://www.youtube.com/watch?v=ESPNqKUluRs

  1. Give an example of a saturated fatty acid and an example of an unsaturated fatty acid.
  2. Contrast the shapes of saturated and unsaturated fatty acid molecules.
  3. Relate the shape of saturated and unsaturated fatty acid molecules to their melting point and state at room temperature.

Review

  1. What are lipids? Name two types of lipids.
  2. Describe fatty acids.
  3. Relate the structure of phospholipids to their function in cell membranes.

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