Membrane sterols structural elements

The formation of a complex with membrane sterols requires penetration of the glycoside into the membrane (corresponding blocks are connected by an arrow). Owing to the ability of an aglycon to form a complex [175] the corresponding block is connected with the structural elements related to an aglycon part of the molecule. The stereochemistry of lanostane nucleus plays a major role in the formation of this complex, besides, the presence of 18(20)-lactone is also important [9, 42] (these functional relations are depicted by bold arrows). 

One of the more tmusual characteristics common to different parts of Cuenof s Tree of Life is the presence of sessile animals (therefore benthic ), and this aspect of the animal world is specific to the marine environment (Cuenot, 1940,1952). Because animals are heterotrophic, those that are attached for life to their supporting structure must necessarily find their food in the water that sur-roimds them. The sessile animals are thus filterers or microphages, and the main part of their diet consists of bacteria and elements of the phytoplankton. One thus finds in their metabolites locatable elements, which are markers of this food chain, and in particular the membrane components fatty acids of the phosphohpids and sterols. 

In Chapter 1 we discussed the importance of the structure of the cell membrane in the classification of living organisms, and pointed out that there is always a double layer of phospholipids whose cohesion is provided by stabilizer elements, which may be long-chain glycerol ethers, carotenoids, hopanoids or sterols. The structural differences observed in sponge membranes are considerable, particularly with regard to phospholipid fatty acids and sterols. 

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