These two forms of unsaturated fatty acids have different steric structure. Thecis configuration of the acyl-chain has a nonmovable bend of 30°, which causes steric hindrance and disturbs the highly ordered fatty acid package. In contrast, the steric behaviour of trans fatty acids and saturated fatty acids is very similar. Nonmovable bends of trans fatty acids have 6°. Both trans and long chain saturated fatty acids possess a long-extended conformation. It enables them to adopt a denser packing in the cytoplasmic membrane and allows protecting membrane against the fluidizing molecules. That is the reason why the transformation of cis to trans fatty acid leads to the decrease of membrane fluidity. Another reason for an ordered packing of trans fatty acids compared to cis isomers is their higher TM (transition temperature). This mechanism was monitored in growing as well as in growth-inhibiting conditions. Cis-trans isomerase is constitutively present, does not require ATP or other cofactors including NAD(P)H and glutathione, and works in the absence of de novo synthesis of lipids. The trans fatty acids are formed by direct isomerization of the complementary cis configuration of the double bond without a shift position. Because of the steric differences between cis and trans configurations, this conversion reduces membrane fluidity.