双半乳糖甘油二酯，Digalactosyl diglyceride 

Monogalactosyldiglyceride (MGDG) and digalactosyldiglyceride (DGDG) are two important biomolecules that are especially prevalent in chloroplast membranes and photosynthetic tissues, including those of algae and some bacteria. MGDG is synthesized by MGDG synthase localized in chloroplast envelope membranes.1 DGDG is formed by the de-phosphorylation of phosphatidic acid or phosphatidylcholine and the subsequent reaction with two 5-diphosphate(UDP)-galactoses. A proper ratio of MGDG and DGDG is critical for membrane function. MGDG plays an important role in the organization and activity of the embedded protein supercomplexes of the thylalkoid membrane of photosystem II and it helps to maintain an optimum lipid to protein ratio.2 DGDG forms a lipid bilayer in the lipid membrane whereas MGDG does not.

References:

Yoshiki Yamaryo et al. "In vitro reconstitution of monogalactosyldiacylglycerol (MGDG) synthase regulation by thioredoxin" FEBS Letters, Vol. 580:17 pp. 4086-4090, 2006

Feng Zhou et al. "Effect of monogalactosyldiacylglycerol on the interaction between photosystem II core complex and its antenna complexes in liposomes of thylakoid lipids" Photosynthesis Research, Volume 99:3 pp. 185-193, 2009

Monogalactosyldiglyceride (MGDG) is one of the main lipids of chloroplast membranes and is one of the most abundant lipids in all photosynthetic tissues, including those of algae and some bacteria. In plants it is much less prevalent in nonphotosynthetic tissues and is synthesized by MGDG synthase localized in chloroplast envelope membranes.1 MGDG is formed by the de-phosphorylation of phosphatidic acid or phosphatidylcholine and the subsequent reaction with 5- diphosphate(UDP)-galactose. Monogalactosyldiglycerides do not form a lipid bilayer in the lipid membrane whereas digalactosyldiglycerides (DGDG) do and a proper ratio of these two diglycerides is critical to membrane function. MGDG is an important lipid in photosynthesis although its role is not fully understood and is undergoing much active research.2 In cells undergoing photosynthesis the photosystem I complex contains three moles of MGDG and one mole of phosphatidylglycerol while photosystem II contains four moles of DGDG. The proper ratio of lipids is needed for the effective crystallization of the light-harvesting complex II. MGDG plays an important role in the organization and activity of the embedded protein supercomplexes of the thylalkoid membrane of photosystem II and it helps to maintain an optimum lipid:protein ratio.3

References:
1. Yoshiki Yamaryo et al., FEBS Letters (2006) Vol. 580:17 pp.4086-4090
2. Henrik Aronsson, Plant Signaling and Behavior (2008) Vol. 3:12 pp. 1093-1095
3. Feng Zhou et al., Photosynthesis Research (2009) Vol. 99:3 pp. 185-193