Advanced Placement (AP) Biology Practice Exam

What are carbohydrates primarily composed of?

1. C, H, O

2. N, O, H

3. C, N, O

4. H, O, S

The correct answer is: C, H, O

Carbohydrates are primarily composed of carbon (C), hydrogen (H), and oxygen (O), which is why the correct choice includes these elements. The general formula for many carbohydrates is represented as \( C_n(H_2O)_n \), emphasizing the relationship between carbon and water in their structure. This is significant because the arrangement of these elements allows carbohydrates to serve various roles in biological systems, including providing energy, serving as structural components in cells, and participating in cell signaling. The presence of hydroxyl groups (-OH) from the hydrogen and oxygen attached to the carbon skeleton contributes to the solubility of many carbohydrates in water, which is a key trait for their function in biological processes. In contrast, the other options contain elements that do not characterize carbohydrates. Nitrogen (N) is primarily associated with proteins and nucleic acids; thus, options including nitrogen and sulfur (S) do not pertain to the basic structure of carbohydrates. The fundamental makeup of carbohydrates is specific to the combination of carbon, hydrogen, and oxygen.