OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What is the first step in the carbon cycle?

1. Animals die and release carbon dioxide

2. Decomposers respire and release carbon dioxide

3. Plants die and form Dissolved Organic Matter (DOM)

4. Photosynthesis takes in carbon dioxide from the atmosphere

The correct answer is: Plants die and form Dissolved Organic Matter (DOM)

The process that initiates the carbon cycle is photosynthesis, where plants absorb carbon dioxide from the atmosphere and convert it into organic matter. This reaction is fundamental, as it forms the basis for the entire cycle by capturing carbon in a form that can be used by living organisms. During photosynthesis, sunlight is harnessed by chlorophyll in plants, leading to the production of glucose and oxygen. This not only contributes to the biomass of plants but also provides energy to other organisms that consume them, thus continuing the flow of carbon through ecosystems. While the decomposition of dead organisms and the respiration of both plants and animals are important processes in the carbon cycle that recycle carbon back into the atmosphere, they rely on the initial capture of carbon through photosynthesis. Without this crucial step, there wouldn't be a sustainable source of organic carbon for other life forms, and the cycle would not function as it does.