26.3. Lipids http://www.ck12.org
Digestion of Lipids
Lipids are consumed in the diet from a variety of foods. Animal products contain several different types of lipids,
including saturated fats, unsaturated fats, and cholesterol. The degree of unsaturation in the fatty acids found in food
(usually as part of a triglyceride) is thought be associated with lowering the body’s stores of cholesterol.
Triglycerides in the diet are digested in the small intestine. Since lipids are not water-soluble, they are emulsified
with the help of bile acids, which are produced by the liver and stored in the gall bladder. Bile acids are cholesterol-
derived steroids that also contain a water-soluble component. The nonpolar region of these acids can interact with
triglycerides, and the water-soluble portion allows the entire complex to dissolve in the aqueous environment within
the body.
The pancreas releases the enzyme lipase to hydrolyze at least two of the ester linkages of each triglyceride. The
fatty acids and glycerol esters then pass across the intestinal membrane and are resynthesized into triglycerides on
the other side. The re-formed triglycerides (along with cholesterol and other lipids) are not water-soluble, so they
will not dissolve in the aqueous environment of the bloodstream. Instead, the newly digested lipids will reassemble
as lipoproteins known aschylomicronsin order to move throughout the body.
FIGURE 26.12
The outer shell of the chylomicron consists primarily of phospholipids (green), in which the polar phosphorus-
containing groups point outward towards the surrounding water, allowing these structures to be relatively water-
soluble. Also in the outer shell are various proteins, labeled in theFigure26.12 as apo A, B, C, and E. Triglycerides
(T) and cholesterol (C) are sequestered on the inside of the chylomicron, along with the nonpolar portions of the
phospholipids. As the chylomicrons circulate, the inner triglycerides are transported to certain regions of the body,
such as the liver and adipose tissue, where they are unloaded.
Other lipoproteins also transport lipids throughout the body, but different types of lipoproteins tend to serve dif-
ferent functions. We will discuss some of these differences in the section below on cholesterol and heart disease.
Lipoproteins are often classified into five different groups based on their densities. In increasing order of density,
these groups include chylomicrons, very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL),
low-density lipoproteins (LDL), and high-density lipoproteins (HDL).