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

Hemoglobin is a vital example of a protein, specifically a globular protein that plays a crucial role in biological systems. It is found in red blood cells and is responsible for transporting oxygen from the lungs to the tissues and returning carbon dioxide from the tissues to the lungs. The structure of hemoglobin consists of four polypeptide chains, each containing an iron-containing heme group that binds oxygen, demonstrating the complex nature and functional capabilities of proteins.

Proteins are made up of long chains of amino acids, which are linked together by peptide bonds. The sequence of these amino acids determines the protein's structure and function. Hemoglobin is a classic example of a protein due to its structure and role in oxygen transport.

In contrast, starch is a carbohydrate made up of glucose units; DNA is a nucleic acid that stores and transmits genetic information; and fatty acids are components of lipids. These substances have different structures and functions compared to proteins, emphasizing why hemoglobin is the most fitting choice in the context of proteins.