什么是脂质？

Lipids are grouped together by a single defining factor. They are hydrophobic, insoluble in water, due to uncharged hydrocarbons.

Based on the number and type of hydrocarbons, lipids are divided into three classes, fats, phospholipids, and steroids.

Fats, also known as triglycerides, are made up of a glycerol and three fatty acids. They can be considered saturated or unsaturated, based on the presence or absence of double bonds in each fatty acid, and thus potential saturation with the maximum number of hydrogens.

These saturated and unsaturated fatty acids also exist in phospholipids, but there are only two fatty acids attached to glycerol, and a charged phosphate group in place of the third fatty acid.

This causes the unique feature of phospholipids, an hydrophilic phosphate head and hydrocarbon tails, which form a bilayer, the cell membrane. The hydrophilic phosphate heads are in contact with the aqueous solution inside and outside of the cell, whereas the hydrophobic tails face each other, shielded and away from contact with water. The third type, steroids, like cholesterol, have a unique fused structure with a carbon skeleton of four hydrocarbon rings, so their chemical structures look very different from fats and phospholipids.