LIPIDS are molecules grouped together because they share one important trait:
they mix poorly, if at all, with water. They are considered hydrophobic, which
literally means “scared of water” (hydro = water, phobic = scared).
There are three main categories of lipids:
FATS – the major function of fats is energy storage. Fat molecules contain
hydrocarbon tails (long chains made of hydrogen and carbon atoms) that are rich
in energy. A gram of fat stores more than twice as much energy as a gram of
carbohydrates. Humans and other mammals store their fat for long-term use in
adipose cells, which swell and shrink as fat is deposited and withdrawn from
storage (hence how we lose and gain weight!). In addition to storing energy, fats
also cushion our vital organs and insulate our bodies. Many marine mammals,
such as whales and seals, have large amounts of fat to protect them from the
cold ocean water.
Fats can be saturated or unsaturated. Saturated fats are more compact and thus
are solid at room temperature (think about a block of butter). Unsaturated fats
contain “kinks” in their hydrocarbon tails, and are thus more fluid. They are liquid
at room temperature (think of vegetable oil).
Saturated Fats
A mixture of Saturated and Unsaturated Fats
PHOSPHOLIPIDS – are vital lipids that make up a cell’s membrane. The two
main parts of a phospholipid are its hydrophilic (water-loving) head and its
hydrophobic (water-fearing) tails (similar to the tails of a fat). As a result, when
phospholipids are added to water, they form a double-layered membrane that
serves to protect a cell’s contents. We will learn more about phospholipids when
we talk about cell membranes next week.
Structure of a phospholipid:
The phospholipid bilayer of a cell:
CHOLESTEROL – is another common lipid in animal cell membranes. It helps
the membrane remain fluid and permeable. Cholesterol is also crucial to animals
because it can be made into important hormones, such as vertebrate sex
hormones. However, high levels of cholesterol in the blood have been shown to
contribute to heart disease.
SOME IMPORTANT SOURCES OF LIPIDS ARE:
FATS – butter, milk, yogurt, vegetable oil, olive oil
CHOLESTEROL – cheese, egg yolks