什么是 Steroids, isoprenoids？

Steroids share cholesterol’s four fused rings: cholesterol itself (membranes, and the precursor of the rest), the bile salts, vitamin D, and the steroid hormones — cortisol, aldosterone, the sex hormones — small, hydrophobic molecules that cross membranes freely and act on receptors inside the cell. Isoprenoids (terpenes) are chains and rings of five-carbon isoprene units: the carotenoids that colour carrots and protect chloroplasts (Chapter 14), the side chain of chlorophyll, the quinones of the electron-transport chains, rubber, and the fat-soluble vitamins A, E and K.