Advanced Placement (AP) Biology Practice Exam

From where does a plant obtain most of its inorganic mass?

1. Atmospheric nitrogen

2. Soil minerals

3. Water absorbed through the roots

4. Carbon dioxide from the air

The correct answer is: Soil minerals

Plants primarily obtain most of their inorganic mass through carbon dioxide from the air, which contributes significantly to their structure and growth. During photosynthesis, plants take in carbon dioxide and, through a series of chemical reactions, convert it into glucose and other organic compounds. This process not only produces energy but also forms the basic building blocks of plant cells. Additionally, water absorbed through the roots plays a crucial role in maintaining turgor pressure and is necessary for photosynthesis, but it does not directly contribute to the bulk of the plant’s inorganic mass in the way that carbon does. Atmospheric nitrogen and soil minerals also provide essential nutrients, such as nitrogen, phosphorus, and potassium, but these are incorporated in much smaller quantities compared to carbon obtained from the air. In summary, while all the options are relevant to plant growth, carbon dioxide from the air is the primary source that plants utilize to build their biomass, making it the most significant contributor to their overall inorganic mass.