Lipid (noun, “LIP-id”)
Lipids are a class of molecules that contain carbon and hydrogen. They make up one of the four types of molecules in living things. Besides lipids, these include proteins, nucleic acids and carbohydrates. Living things make each type of molecule from carbon and other elements. As a group, life’s molecules are also called biomolecules.
There are four groups of lipids: oils and fats, waxes, phospholipids and sterols. Two general traits set all these lipids apart from other biomolecules. First, lipids are hydrophobic. This means lipids do not dissolve in water. Instead, they dissolve in other lipids. Second, lipids contain long chains made of hydrogen and carbon, called hydrocarbon chains. (Biologists tend to call hydrocarbon chains “fatty acids.”)
There are exceptions to both rules. For example, phospholipids and some sterols partially dissolve in water. But generally, lipids are hydrophobic and contain hydrocarbon chains.
Lipids carry out many important jobs in the body. One of them is to store energy. Much of the fatty tissue of our bodies is made of lipids. This is true for other animals, too. Hibernating bears, for example, eat lots of extra food in the fall. Their bodies convert this extra food into fatty tissues, which they use for energy while hibernating — and not eating — in the winter.
Some lipids also serve as chemical signals in our bodies called hormones. Examples include cortisol, estrogen and testosterone. Lipids also help our nerves send signals faster. And they help the body absorb and process vitamins.
Lipids called phospholipids make up the membranes, or outer layers, that surround cells and their structures. This includes the cell’s outermost layer, called the cell membrane. Lipids also make up the membranes around a cell’s organelles. A lot of cellular work occurs across membranes. So, the lipid’s role in membrane structure is a big deal to biologists.
In a sentence
Researchers investigate the mysterious conditions under which soft, lipid-rich human brains have resisted decay for many thousands of years.