Summary
Highlights
Life on Earth has existed for billions of years due to the recycling of molecules like water and carbon dioxide, and individual atoms such as nitrogen, phosphorus, and carbon. These recycled atoms form new organisms over millions of years, meaning many of the atoms within us have been part of oceans, volcanoes, other animals, and even other humans.
The water cycle begins with evaporation, where solar energy converts liquid water from various sources (lakes, oceans, rivers, puddles, soil, and transpiration from plants) into water vapor. This vapor accumulates and condenses into clouds, which then move and release water back to Earth as precipitation (rain). Once back on Earth, the water can seep into the soil, flow into rivers, or be absorbed by plants, restarting the cycle.
The carbon cycle is more complex and involves many living organisms. Carbon is stored in five main places: as carbon dioxide in the air, in plants as biological molecules, in the soil with microorganisms, in fossil fuels underground, and in animals as biological molecules. Understanding these stores is key to understanding carbon movement.
Carbon moves between these stores through several processes. Photosynthesis is crucial, where green plants and algae absorb atmospheric carbon dioxide and convert it into glucose. This carbon can then be released back into the atmosphere by respiration from plants or animals that consume plants. When plants and animals die, their carbon can be returned to the atmosphere as carbon dioxide through microbial respiration in the soil. Alternatively, if decayed in anaerobic conditions, dead organisms can form fossil fuels like oil, natural gas, or coal, which humans burn to release carbon dioxide.