Lipids primarily make up the cell membrane but also function as ________ and can be used for energy.

• A. Enzymes
• B. Nucleic acids
• C. Signaling molecules
• D. Structural proteins

Answer: (C)

Lipids have signaling roles in addition to forming membranes and serving as an energy reserve. They act as signaling molecules that help cells communicate and regulate processes. For example, steroid hormones derived from cholesterol travel through the body to affect gene expression and physiology, while lipid-derived second messengers like diacylglycerol (DAG) and inositol trisphosphate (IP3) transmit signals inside the cell after a receptor is activated. Other lipids, such as prostaglandins, also function to regulate inflammation and other signaling pathways.

Because signaling is a distinct and essential function of many lipid molecules, this option best fits the idea that lipids do more than just structure and store energy. Enzymes are typically proteins (or RNA in some cases), nucleic acids are genetic material, and structural proteins are proteins, so those roles don’t align with what lipids commonly do.