A double bond between adjacent carbons in a fatty acid tail creates a bend of about:

1. Saturated tails are straight because every carbon carries its full hydrogen count. 2. Unsaturated tails contain some double bonds between adjacent carbon atoms. 3. A double bond results in a bend in the string of carbons of approximately 30 degrees. 4. That bend is what stops the tails packing tightly and keeps the membrane fluid. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 5 'Structure and Function of Plasma Membranes', sections 5.1-5.4_