Lipids (Fats)

A diverse group of water-repelling molecules — the body’s dense energy store and the material of its membranes.

Main types

• Triglycerides — the storage fat held in adipose tissue; the body’s largest energy reserve.
• Phospholipids — the two-layered building material of every cell membrane.
• Steroids — including cholesterol and the steroid hormones (cortisol, estrogen, testosterone).
• Fatty acids — the chains that make up most other lipids; saturated or unsaturated.

What they do

• Energy storage — fat holds more than twice the energy of carbohydrate per gram.
• Membranes — phospholipids wrap every cell and organelle.
• Insulation and protection — body fat conserves heat and cushions organs.
• Signaling — the raw material for steroid hormones and other messengers.

Key idea

“Fat” is not one substance but a family defined by a shared trait — insolubility in water — which suits its members equally for storing energy and for forming barriers.