Photosynthesis is the process by which plants synthesize carbohydrates from carbon dioxide, water, and sunlight. It is significant as it maintains oxygen levels in the atmosphere, provides both direct and indirect food sources for humans and animals, and has created fossil fuel reserves like coal and oil. The key reaction of photosynthesis occurs in the chloroplasts of plant leaves where chlorophyll and other pigments absorb sunlight and convert it into chemical energy that is used to produce glucose and oxygen from carbon dioxide and water.