I was just reviewing some organic molecule structures and came across cycloalkenes. Reading through some material, it states that the planar structure of cyclopentane is more stable structurally than cyclohexane. Yet cyclohexane is more common in biological systems as opposed to any other type of structure. It doesn't offer any comparison between the chair or boat structure of cyclohexanes and the regular structure of cyclopentanes, but if there was an evolutionary reason cyclohexane is more predominant than any other cyclic molecule, cyclopentane particularly. Also, what is the exact reason that only cis bonds are allowed in cyclic molecules with 7 or less central atoms? I am trying to come up with the structure but why 7 exactly? What exactly about the trans orientations in other types of cyclic molecules makes it unstable?