Do plant cells contain mitochondria?
Plant cells DO contain mitochondria.
While plant cells have chloroplasts to photosynthesize, they also require ATP for cellular functions, and do use oxygen to break down some of the sugar they produce in order to generate that ATP. They need mitochondria for this.
In particular, at night when there is no light, plants undergo cellular respiration since there is no sunlight to photosynthesize.
They do, however, produce far more sugar and oxygen through fotosintesi than they use up in respiration.