There are a number of bonds in The Merchant of Venice, as many characters are bound in loyalty, duty or through financial connection to each other.
Bassanio is bound to Antonio. Though they are friends, Bassanio owes money to Antonio and extends this bondage even further in Act I, scene i, when he appeals to Antonio for more money so that he may travel to try to win Portia's hand.
Portia is bound to her father's plan to test any potential suitors for her hand. She must marry the man who solves the riddle set up by her deceased father.
Launcelot Gobo is bound to Shylock, his master, and, in Act II, scene ii, goes through quite a struggle between his conscience and "the fiend" regarding whether he should simply run from his master or not. Bassanio helps him to decide to return to Shylock and be quit of service to him in an honorable fashion.
Jessica is bound to her father's choice of husband, yet she defies tradition and elopes with Lorenzo, stealing gold and precious items from her father.
All of Portia's suitors (including Bassanio) are bound to follow the decree of her father, which is, if they do not choose correctly and win Portia's hand in marriage, they must "never speak to a lady afterward/In way of marriage." (II, i, 41-2)
Lorenzo and Jessica (Act III, scene iv) are bound by Portia to manage and guard her estate in the absence of herself and Bassanio.
And finally, Shylock is bound to follow the law of Venice, which ultimately, loses him all his property for bringing charges against a citizen, Antonio, ( which, being a Jew, Shylock cannot be) that are decided to be false.
The examination of differing bonds, bondage and service is a very rich topic to consider in this play. For more on bonds and the relationships that breed them in The Merchant of Venice, please follow the links below.