if the atmosphere is like a greenhouse, what parts of its function as the glass

Well the earth's atmosphere is kept around the earth by gravity its a little different than a green house but its the same idea. the gravity keeps a barrier between space and the earth and it acts like a green house the sun goes threw the atmosphere and is magnified by the humidity and t reflections which are the clouds. it all kept around the earth by the pressure the gravity causes and it also contains any and all pollutants we release into the air. the barrier the gravity makes is like the glass in the green house. the plants and trees on the earth produce the oxygen we breath and is kept down around the earth because the earth spins and causes centrifugal force

The carbon dioxide in the atmosphere primarily traps and bounces back heat similar to glass in a greenhouse