Nearly all energy available to mankind is the result of photosynthesis. Coal, mineral oil, and natural gas are derived from the products of photosyntheses. During photosynthesis, first the light of the sun is absorbed by the antennae pigments of the light-harvesting complexes. Next, energy is transferred by excitonic interaction to the so-called photosynthetic reaction centre, where the primary change separation takes place and an electron is transferred across the photosynthetic membrane. Photosynthesis of the purple bacteria is well understood. This knowledge is based on the fact that the light-harvesting complexes and photosynthetic reaction centres of these bacteria could be isolated and crystallized, so that their structures could be determined by X-ray crystallography. The light-harvesting complexes of purple bacteria are highly symmetric ringlike oligomers of a basic unit that consists of two short protein chains, two or three bacteriochlorophylls and one carotenoid molecule. In the case of the purple bacterium Rhodospiriflum mo/ischionum eight of these basic units form the light-harvesting complex 2. The carotenoids possess a dual function: First, they serve as light-harvesting pigments covering spectral ranges different from that of the chlorophylls, and second they have a protective function against the damaging effects of light by quenching the triplet states of bacteriochlorophylls and preventing the formation of the very dangerous singlet oxygen. The ring-like arrangement of the pigments appears to optimal for energy storage and subsequent energy transfer of the reaction centre. There electrons are released from the "primary electron donor" which consists of a non-covalently linked dimer of two bacteriochlorophylls. The electron is transferred via a bacteriopheophytin and a first quinone molecule to a second one. Stable reduction of the second quinone requires two electrons, during or after the second electron transfer two protons are taken up from the cytoplasm. In subsequent reactions the hydroquinone is oxidized again, and adenosine -5'-triphosphate (ATP), the universal energy currency of life, is generated. ATP, and biologically fixed hydrogen are needed for the synthesis of sugar molecules from carbon dioxide. The sugar molecules are taken up and metabolized by other organisms. Carbon dioxide is formed again together with biologically fixed hydrogen. The fixed hydrogen is converted to water in the "respiratory chain" of the mitochondria or a aerobic bacteria. During this process protons are "pumped" across membranes. Therefore, an electric field across these membranes is formed, which drives electrons back through the ATP-synthase leading to ATP-synthesis. Cytochrome c oxidase is the terminal enzyme of the respiratory chains. It oxidizes cytochrome c and transfers the electrons to molecular oxygen. Water is formed. The protons needed for water formation originate from the cytoplasm of the bacteria or the interior of the mitochondria. Simultaneously the same number of protons are pumped across the mitochondrial (or bacterial) membrane. This fundamental enzyme could be crystallized from the soil bacterium Paracoccus denitrificans, using cocrystallization with an antibody Fv-fragment as a novel approach for membrane protein crystallization (Ostermeier et al., Nature struct. biol. 2, 842-846 (1995)). The structure could be determined at 2.8 A resolution (lwata et al., Nature 376, 660-669 (1995)), and then at 2.7 A resolution with a new crystal form (Ostermeier et al., Proc. Natl. Acad. Sci. USA 94, 10547-10553 (1997)). The arrangement of the prosthetic groups (three Cu-atoms, two haem A groups, one being called haem a and the other haem a3), their interaction with the protein surrounding and the structure of the four protein subunits are therefore known. Subunit I contains 12 membrane spanning helices in a rather regular arrangement and binds both haem groups and copper B. The oxygen is bound to the Fe-atom of haem a3, which is 5.2 A away from copper B. Subunit 11 possesses two membrane spanning helices and binds a binuclear copper A centre in a globular plastocyaninlike domain. Two possible proton transfer pathways could be identified in subunit I. Possible mechanisms of coupling water formation to proton pumping are discussed: Evidence for an involvement of a histidine residue undergoing protonation changes ("histidine cycle") is weak. The propionate side chains of the haem groups might be critical for proton pumping. They undergo structural and/or protonation changes upon reduction of the enzyme as indicated by Fourier Transform infrared spectroscopic experiments using cytochrome c oxidase, specifically 13C-labelled at the haem propionates (Behr et al., Biochemistry 37, 7 7400-7406 (1998)). A model for proton pumping, based on long range electrostatic interactions, will be presented.