The cells of some organisms contain both chloroplast and mitochondria, what would happen if these cells would move from a light environment to a dark one?

Both chloroplasts and mitochondria

If these cells would move from a light environment to a dark one, the chloroplasts would no longer function as the photosynthesis sites due to absence of light. The chlorophyll will break down, the green pigment in the chloroplast would change color to first yellow and then brown. If these are plant cells, the plant will continue to respire due to the presence of mitochondria but as photosynthesis no longer takes place, it would use up its energy reserves and no more energy will be available from food molecules that were previously synthesized by photosynthesis. As a result, the cells will get dehydrated as the transpiration stream will slow down, and the plant will wilt and eventually die.