Plants usually use photosynthesis to make sugar from carbon dioxide. However, sugar contains more energy than carbon dioxide and this chemical reaction does not occur spontaneously. Extra energy is needed to make this reaction to happen. In photosynthetic plants, this energy comes in the form of light. The energy is used to split the hydrogens in water from oxygen. The hydrogens then are used to reduce carbon dioxide to sugar.

However, plants do not necessarily need sunlight to survive. If nutrients can be obtained elsewhere, a plant can live without sunlight. For example, there is a type of plant that forms a symbiotic relationship with some fungi and cannot photosynthesize at all.