Text: Plants are eukaryotes with cell walls composed of cellulose. Most obtain energy predominantly from sunlight through photosynthesis using chlorophyll. The process of photosynthesis takes carbon dioxide and water and converts it into energy-containing sugars and other substances that provide the fundamental nutritional needs of animals and humans, either directly or indirectly. In addition, oxygen is a by product of photosynthesis. Plants can also have negative effects on human health. Plant pollen can trigger allergic responses, e.g., hay fever. There are also many poisonous plants which can be fatal. Others cause skin irritations when touched, such as poison ivy. Some plants also synthesize substances that have drug-like effects, such as foxglove (digitalis), tobacco, cannabis (marijuana), cocaine and opium. Some plants cause allergic reactions when ingested. Still other plants cause food intolerances that negatively affect health, such as celiac disease.