Bonds of a city of the metropolitan class shall be prepared under the direction of the city council, shall be signed by the mayor and countersigned and registered by the city comptroller, and shall be sold and disposed of by and under the direction of the city council. Such bonds shall be delivered by the city finance director who shall report the proceeds from such bonds to the city treasurer in all cases except where an exchange of bonds is directed. The purpose of the issue of bonds shall be stated in such bonds and the proceeds received from the sale shall be used for no other purpose. Whenever an issue of bonds is required to be submitted to the electors for authority to issue such bonds, the proposition submitting such question shall contain but a single issue and but one subject, shall specify the maximum amount proposed for issue and state distinctly the purpose for which such bonds are to be issued. Bonds of the city shall not be sold or exchanged for less than par value of such bonds and shall bear interest payable semiannually. Interest coupons at the rate of interest specified may be attached to such bonds. Interest coupons may be signed by the mayor and city clerk. Bonds shall be made payable at the office or place provided by general law for the payment of bonds of the city. Where this section, in its application to water bonds or bonds issued for the extension or improvement of a gas plant or other public utility, is in conflict with any provision which has been or may be made by statute respecting such bonds, the latter shall control.