Biology STAAR Practice Test 2025 - Free State of Texas Assessments of Academic Readiness Practice Questions and Study Guide

The primary biomolecule involved in making up the cell membrane is lipids. The cell membrane, also known as the plasma membrane, is predominantly composed of a phospholipid bilayer. This structure consists of two layers of phospholipids, which have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. This arrangement creates a semi-permeable membrane, allowing selective entry and exit of substances, and provides fluidity and flexibility to the membrane.

In addition to lipids, proteins are embedded within the membrane and play critical roles in transport, signaling, and structural support. While carbohydrates are present on the extracellular surface of the membrane attached to proteins and lipids, they primarily function in cell recognition and signaling rather than forming the core structure. Nucleic acids, including DNA and RNA, are not involved in the composition of the cell membrane; instead, they serve functions related to genetic information and protein synthesis.