What's the least amount of data needed to update a cart object

A cart object with 1 item in it is 1154 lines long (give or take, mileage may vary). From what I can tell there are basically 4 data points I need when "putting an object in a cart":

ID
Product code
Price (if overriding)
Quantity

Right? In theory that's all I really need to start the process. That's a lot less than 1154 lines. What's the smallest/least amount of data necessary to update a cart, given a cart ID and the 4 data points above? Do I honestly have to fill out the BillToCustomerParty/Party section, the Delivery section, discount totals, etc?

Thanks.