GED Science Practice 2025 - Free GED Science Practice Questions and Study Guide

The correct choice is indeed the cell membrane. The cell membrane serves as a protective barrier that surrounds the cell, providing not only structure but also the ability to regulate the movement of substances in and out of the cell. This selective permeability is crucial for maintaining homeostasis, allowing essential nutrients to enter while keeping out harmful substances and waste products.

The other options do not fulfill this specific role of regulating entrance and exit at the cellular level. Cartilage is a flexible connective tissue that provides support and cushioning in joints but does not function as a cellular barrier. A cathode is an electrode through which electric current flows into a polarized electrical device, unrelated to cellular structure. Lastly, while a cell is a fundamental unit of life encompassing various components including the membrane, it does not specifically refer to the thin barrier that regulates transport. Hence, the cell membrane is the precise answer.