The structural-functional characteristics of the cells of wild type CC-124 and Brc-1 mutant of the unicellular green algae Chlamydomonas reinhardtii grown in the dark and in the light were studied. The cells of the wild type in heterotrophic and mixotrophic conditions had a well developed structure and high functional activity due to the ability of the cells to synthesize chlorophyll both in the light and in the dark. The cells of Brc-1 mutant lost their ability to synthesize chlorophyll in the dark and the cell color was orange due to brc-1 mutation in the nuclear gene LTS3 that regulated the activity of Mg-chelatase enzyme. In the dark the mutant cells accumulated protoporphyrin IX and had weakly developed structure with low functional activity. Because of the high content of protoporphyrin IX, even a short-term exposure of the Brc-1 mutant cells to the light was accompanied by very strong destructive changes in all the membranes in a cell: plasmalemma, chloroplast, mitochondrion, envelopes of the nucleus a