Bonds of the city shall be prepared under the direction of the city council, shall be signed by the mayor and countersigned and registered by the comptroller, and shall be sold and disposed of by and under the direction of the city council. They shall be delivered by the superintendent of the department of accounts and finances, who shall report the proceeds therefrom 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 therein 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 the same, 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 the same is to issue. Bonds of the city shall not be sold or exchanged for less than par value thereof and shall bear interest payable semiannually. Interest coupons at the rate of interest specified may be annexed thereto. Interest coupons may be signed by the lithographed signatures of 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.