After you make your way through the back offices and crawlspaces of Aperture Science, and you find yourself in a new area.
Beyond the Particle Field
Step through the particle field and there will be some dialogue, during which something will fall to the floor. It is a sphere with a purple eye. Pick it up.
Then look at the back wall (across from where you entered the room) and you will see an incinerator, much like the one from a previous test chamber. Put a portal on the ceiling above the incinerator, then look for the stairs at the side of the room that lead up to a small ledge. Walk to the end of that ledge to find an enclosed area with a red button. Make a portal of the other color on the wall nearby, then push the red button to open the incinerator, then drop the purple sphere through the portal to incinerate it.
After some more dialogue, the room begins to fill with a deadly neurotoxin, and you have six minutes to stop it. Since you destroyed the purple sphere (the morality core), the turret defense system has been activated. Make a portal on the floor beneath the robot lady (whose name is ), then stand in the path of the turret's laser sight (that way you don't need to wait for it to aim at a specific spot), quickly make a portal of the other color on the wall or floor where the rocket will shoot, and get out of the way. If the rocket went in at a mostly straight angle, it will hit the robot lady.
This will knock off another sphere. It will land on a light fixture. You can make a portal behind where it landed, and make a portal of the other color to pull the sphere through. Put a portal above the incinerator, and a portal of the other color near the red button, then press the red button and drop this yellow sphere into the incinerator.
After a bit more dialogue, the turret will reactivate. As before, make a portal on the floor beneath the robot lady, then stand in the path of the turret and make a portal where its rocket will hit, and get out of the way.
This time, a blue sphere falls off. It lands on a ledge up above. Make a portal high up on the wall that faces that ledge, then fall through a portal of the other color, and as you fall again, make another portal on the floor to fall through to give yourself enough momentum to reach the ledge. Take the blue sphere and incinerate it the same way as you did the other two.
After that, make a portal on the floor beneath the robot lady, and make the turret shoot through a wall portal to hit the robot lady again. This time, the sphere is held in mid-air. Make a portal high on the wall that faces the floating sphere, then fall through a floor portal of the other color, and take the sphere as you fall. Incinerate it as before. Congratulations! You won! Enjoy the ending!