Chamber 07 uses pellets for the second time, with the pellet receiver powering a platform to take you to the door this time.
The first portal simply needs to be placed where the pellet is making a burn mark in the wall opposite the pellet launcher; the pellet will go through the portal and come out the fixed orange end of the portal on the floor, heading into the pellet receiver. This will start the platform moving.
At first it might seem impossible to get onto the platform, but if you notice, there's a space on the ceiling above the platform at one end where a portal can be made. Fire the portal gun at the ceiling, wait until the platform's beneath the portal, then fall through the fixed orange end of the portal to land on the platform, and proceed to the chamberlock.