Lipids Are Absorbed Through the Apical Membrane by Carrier Proteins and Simple Diffusion

The process of lipid absorption into the enterocytes is incompletely understood. As the micelles come close to the surface of the enterocytes, the various lipid components dif­fuse the short distance through the glycocalyx to the apical membrane by means of special fatty acid-binding proteins (not shown in Figure 30-27).

Nonesterified fatty acid

FIGURE 30-24 Chemical structures and their schematic representations for lipid molecules involved in fat digestion and absorption, n, Number of carbon atoms in fatty-acid chains; X, phospholipid head group, most often choline.

to be taken up and transported across the apical membrane by special fatty acid-binding proteins in the apical membrane. Other micellar components appear to simply diffuse into the apical membrane; they include such lipids as monoglycerides, cholesterol, and vitamin A. The apical membrane, as with other cellular membranes, is composed primarily of phospho­lipids (see Chapter 1). The highly hydrophobic products of lipid digestion are soluble in the phospholipid matrix of the

FIGURE 30-25 Sites and reactions involved in fat digestion and absorption (A,

angstroms).

FIGURE 30-26 Portion of the surface of a bile- coated, emulsified fat droplet. Bile components reach the surface of the droplet through micelles (A) coming from the gallbladder. Co-Iipase clears bile constituents from an area of the surface of the droplet, allowing the attachment of lipase. Lipase catalyzes the formation of fatty acids and monoglycerides from triglycerides.The surface components and products of lipase action combine to form micelles (B) containing fatty acids and monoglycerides as well as bile constituents.

membrane and thus may diffuse freely through the apical membrane and into the cell. Figure 30-27 illustrates lipid absorption from micelles.