Final answer:
Carbon enters the biotic portion of the carbon cycle through photosynthesis, where plant leaves absorb carbon dioxide from the atmosphere and convert it into organic compounds.
Step-by-step explanation:
Carbon enters the biotic part of the carbon cycle primarily through the process of photosynthesis, where it is absorbed from the atmosphere by plant leaves. During photosynthesis, plants convert carbon dioxide (CO2) and water (H2O) into organic carbon compounds like glucose, which they use for energy and growth. The carbon cycle is closely connected to other biogeochemical cycles, such as the nitrogen cycle, but it is distinct in the way carbon enters the living systems. While the nitrogen cycle involves nitrogen fixation performed by bacteria to make nitrogen accessible to plants, the carbon cycle does not rely on bacteria for its initial step into the biotic world. Instead, producers, such as land plants and photosynthetic bacteria, play a critical role in fixing atmospheric carbon and incorporating it into the biosphere.