A Level Biology Practice Exam 2026 - Free A Level Biology Practice Questions and Study Guide

The characteristic of blood that allows it to carry oxygen effectively is the structure of hemoglobin. Hemoglobin is a protein found in red blood cells that has a unique quaternary structure, consisting of four polypeptide chains, each capable of binding to an oxygen molecule. This structure allows hemoglobin to pick up oxygen in the lungs and release it in the tissues where it is needed.

The high affinity of hemoglobin for oxygen is influenced by factors such as partial pressure of oxygen and the presence of carbon dioxide, which can affect its saturation. Additionally, hemoglobin can undergo conformational changes, enhancing its ability to pick up and release oxygen as required by the body’s metabolic needs.

While other components and characteristics such as plasma proteins or the small size of red blood cells contribute to overall blood function, it is the specific structure and function of hemoglobin that is crucial for effective oxygen transport.