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

The plasma membrane, which surrounds and protects the cell, is primarily made up of phospholipids. These molecules have a unique structure that includes a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails." This arrangement allows them to form a bilayer, where the hydrophilic heads face outward toward the watery environments both inside and outside the cell, while the hydrophobic tails face each other, away from water.

This phospholipid bilayer is crucial for the membrane's functions, providing a barrier to most water-soluble substances and maintaining the cell's internal environment. While proteins, carbohydrates, and nucleic acids play important roles in cellular functions and processes associated with the plasma membrane, such as signaling and structural support, it is the phospholipids that create the fundamental structure of the membrane itself. Thus, phospholipids are essential for forming the basic framework that enables the membrane's selective permeability and overall integrity.