Final answer:
Symporters are membrane carrier proteins that allow one substance to move downhill into the cell while another substance is being moved uphill out of the cell.
Step-by-step explanation:
The type of membrane carrier that allows one substance to move "downhill" into the cell while another substance is being moved "uphill" out of the cell is called a symporter.
Symporters are a type of carrier protein that transports two or more substances in the same direction across the cell membrane.
They utilize concentration gradients to move one substance down its concentration gradient while simultaneously moving another substance against its concentration gradient. This process is known as co-transport or secondary active transport.