Nicotiana tabacum was grown with a CO 2 -content in the atmosphereof 350 ppm and 700 ppm. After three weeksof growth in the respective atmospherewe were able to show that chloroplasts from plants grown under 700 ppm CO 2 exhibited an 18% higher photosystem I-mediatedelectron transport activity. Furthermore, we were able to show that in plants grown under the higher CO 2 -concentration the peptide composition of photosystem I had been quantitatively changed. It appeared that the CPI-peptides were increased, whereasthe LHCPI peptides were decreased. Corresponding to this change in the peptide composition the ratio chlorophyll/carotenoids/protein changes from 1:0.16:3.6 in the control plants to 1:0.23:5 in the “CO 2 -plants”. The chlorophyll a/b ratio changes from 1.9 in the control plants to 2.3 in the “CO 2 plants”. Furthermore, the carotenoid composition appears to be changed, β-carotene increases by 60% as expected from the increased CPI-content. The xanthophylls lutein, violaxanthin, neoxanthin and zeaxanthin increase also but to a lesser extent. Using specific antisera to lipids, we were able to show that photosystem I contains only the membrane lipids monogalactosyldiglyceride and phosphatidylglycerol. Both lipids are specifically bound in a restricted number onto peptides of the photosystem I complex. Both lipids differ with respect to their binding strength on the peptides of photosystem I. The changes in the peptide and lipid composition were established by chemical analyses and by immunological techniques (Western blot) using monospecific antisera to the photosystem I peptides, to the chloroplast lipids as well as to the chloroplast carotenoids. From the quantitative and qualitative analyses of the peptides, carotenoids and lipids, the molecular distribution of the carotenoids and lipids in the CPI and LHCPI complex of photosystem I was established.