Galactolipids occur in all cells capable of the photosynthetic evolution of oxygen. The main galactolipids are monogalactosyl diglyceride (MGDG), digalactosyldiglyceride (DGDG) and sulfolipid (SL), MGDG being the most abundant. These three together form the major nonpigment lipids in both leaves and green algae, accounting for about 40% of the total lipids. Nearly all of the cellular MGDG of plants is present in the chloroplasts and is concentrated in the lamellae. Galactolipids contain unusually high percentages of polyunsaturated acids (MGDG contains greater than 90% in some plants), the major one being α-linolenic acid (9,12,15-octadecatrienoic acid; 18:3). Thus in higher plants most of the MGDG is monogalactosyl dilinolenoyl glycerol. MGDG may be the actual substrate of the desaturation reactions of α-linolenic biosynthesis.