Cholesterol is an enhancer of membrane fluidity Why would ch

Cholesterol is an enhancer of membrane fluidity. Why would cholesterol be particularly common in animal cell membranes and less common in plant cell membranes?

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Cholesterol is an organic molecule and a sterol (or modified steroid) a type of lipid molecule, and is biosynthesized by all animal cells, because it is an essential structural component of all animal cell membranes. It composes about 30% of all animal cell membranes, is required to build and maintain membranes and modulates membrane fluidity over the range of physiological temperatures. The hydroxyl group on cholesterol interacts with the polar heads of the membrane phospholipids and sphingolipids, while the bulky steroid and the hydrocarbon chain are embedded in the membrane, alongside the nonpolar fatty-acid chain of the other lipids. Through the interaction with the phospholipid fatty-acid chains, cholesterol increases membrane packing, which both alters membrane fluidity and maintains membrane integrity so that animal cells do not need to build cell walls (like plants and most bacteria). The membrane remains stable and durable without being rigid, allowing animal cells to change shape and animals to move.

In addition to its importance for animal cell structure, cholesterol also serves as a precursor for the biosynthesis of steroid hormones, bile acid and vitamin D. Cholesterol is the principal sterol synthesized by all animals. But plants make cholesterol in very small amounts. Plants manufacture phytosterols (substances chemically similar to cholesterol), which can compete with cholesterol for reabsorption in the intestinal tract, thus potentially reducing cholesterol reabsorption. When intestinal lining cells absorb phytosterols, in place of cholesterol, they usually excrete the phytosterol molecules back into the GI tract, an important protective mechanism.