Illumination of euglenas grown in the dark induces the formation of chloroplasts characterized by the simultaneous appearance of chlorophyll and galactosyl diglycerides in a relatively fixed ratio. The fatty acyl chains of the galactosyl diglycerides are constructed so that they can provide a stable lock-and-key fit with the phytol chains of chlorophyll in such a way as to localize the porphyrin structures of chlorophyll and space them for efficient photoreception. Light-starved photobiotic euglenas show chloroplast shrinkage with a concurrent partial loss of galactosyl diglycerides.