Answer: cholesterol synthesis. Cholesterol is mainly synthesized by the body itself, and only a small amount is taken in from food. Cholesterol can be synthesized in almost all tissues of the body except brain tissue and mature red blood cells, but 70% to 80% of the body's cholesterol is synthesized by the liver. Cholesterol synthesis is catalyzed by the cholesterol synthase system in the cytoplasm of tissue cells and on the membrane of the endoplasmic reticulum of the smooth surface of the tissue cells, with acetyl CoA, NADPH as the hydrogen donor and ATP as the energy supply to synthesize cholesterol. The synthesis process is roughly divided into three stages: ① synthesis of mevalonate. In the cytoplasm, 3 molecules of acetyl CoA are first condensed to HMGCoA, which is catalyzed by HMGCoA reductase in the endoplasmic reticulum membrane, and then reduced to mevalonate (MVA) by NADPH+H+, of which HMGCoA reductase is the rate-limiting enzyme for cholesterol synthesis. (ii) Synthesis of turtle alkene. MVA is supplied with energy by ATP and catalyzed by a series of enzymes in the cytoplasm, through the process of decarboxylation, phosphorylation, condensation, and reduction, i.e., it generates 30C squalene. ③ Cholesterol synthesis. Squalene is cyclized, oxidized, decarboxylated, reduced and so on, and 3 methyl groups are removed to produce 27C cholesterol.

Metabolic transformation: ① in the liver into bile acids. Cholesterol is converted to bile acids in the liver is the main way of cholesterol metabolism in the body. ② transformed into steroid hormones. Cholesterol can be synthesized into steroid hormones, including adrenocorticotropic hormone and sex hormone, in endocrine glands such as adrenal cortex, testis and ovary. ③Transformed into 7-dehydrocholesterol. Cholesterol is transformed into 7-dehydrocholesterol in liver, kidney and other tissues, transported with blood to subcutaneous storage, and transformed into VitD3 by ultraviolet light irradiation.