Of Cells

In multicellular organisms, particularly mammals, lipids are often imported and exported from cells and transported

(a) Transport protein-mediated export and import of lipid

(b) Lipoprotein- and receptor-mediated export and import

Exterior

Cytosol

Lipoprotein 4

Receptor

Exterior

Import

Cytosol

Lipid

Transport proteins Extracellular binding protein ^^ Intracellular binding protein

Lipoprotein 4

Receptor

Exterior

Cytosol

-Microsomal transfer protein

Lysosome

-Microsomal transfer protein

Lysosome among different tissues by the circulation. Such lipid movements help maintain appropriate intracellular and whole-body lipid levels and have other advantages for an organism. For instance, lipids absorbed from the diet in the intestines or stored in adipose tissue can be distributed to cells throughout the body. In this way, cells can obtain essential dietary lipids (e.g., linoleate) and can avoid wasting energy on the synthesis of lipids (e.g., cholesterol) otherwise available from the diet. The ability of some cells to export lipids permits the excretion of excess lipids from the body or their secretion into certain body fluids (e.g., milk in mammary glands). In addition, the coordination of lipid and energy metabolism throughout the organism depends on intercellular lipid transport.

Not surprisingly, intracellular and intercellular lipid transport and metabolism are coordinately regulated, as discussed in Section 18.4. Here, we review the primary ways in which cells import and export lipids. The two main mechanisms of cellular lipid export and import are summarized in Figure 18-10. In the first mechanism, the import or export of individual lipid molecules is mediated by cell-surface transport proteins. The principles underlying this mechanism are similar to those for small water-soluble molecules such as glucose, discussed in Chapter 7. A noteworthy difference is that the hydrophobic lipids, which are poorly soluble in aqueous solution, often associate with lipid-binding proteins in the extracellular space or the cy-tosol, rather than remaining free in solution. In the second mechanism of lipid import and export, collections of lipids are packaged with proteins into transport particles called lipoproteins. These lipoproteins are exported from cells through the classic secretory pathway (Chapter 17). Lipids carried by extracellular lipoproteins are taken up by cells through surface receptors.

M FIGURE 18-10 The two major pathways for importing and exporting cellular lipids. (a) Individual lipid molecules cross the plasma membrane with the assistance of transmembrane transport proteins. Intracellular and extracellular lipid-binding proteins (shown here), lipid micelles, or membranes normally participate as the donors and acceptors of the lipids. In some cases, the concentration gradients of the transported substances are sufficient to drive transport (e.g., import of bile acids in the liver and intestines by Na+-linked symporters). In others, coupled ATP hydrolysis helps drive lipid transport (e.g., secretion of bile lipids from hepatocytes by ABC proteins). (b) Lipids are also transported as components of lipoproteins. These large assemblies of protein and lipids are put together in the ER with the assistance of microsomal transfer protein (1), are exported ( 2|) through the secretory pathway ( 3) as water-soluble particles, and then circulate in the blood. After circulating lipoproteins bind to certain cell-surface receptors (4|), the intact particles can be internalized by endocytosis (5) and the lipids are subsequently hydrolyzed in lysosomes (6), as depicted here. Other receptors mediate the uptake of individual lipid components from lipoproteins, releasing a lipid-depleted particle into the extracellular space.

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