The processes of photosynthesis and respiration will both occur in the terrarium provided it has the right conditions to start with. You will include good soil--a combination of sand and loam and humus. You must add water to the soil. The plants will be planted and the terrarium will be sealed. It must be placed in the window to obtain light energy necessary for the process of photosynthesis. The plants will produce glucose(to use as a food source)as well as oxygen and water vapor. The water vapor will condense on the sides of the terrarium and eventually help to keep your plants watered. The oxygen produced by photosynthesis, along with the glucose, can be used for respiration and the release of ATP, to give the plant cells the energy they need to live. As a result of respiration, water vapor and carbon dioxide will be released and these can be used during photosynthesis. This should continue indefinitely, unless the soil runs out of nutrients or if the plant gets a disease.