Final answer:
O2 (oxygen) is transported across the lung membrane by diffusion, moving from alveoli to the bloodstream and binding to hemoglobin in red blood cells. Hemoglobin can carry up to four oxygen molecules, forming oxyhemoglobin which gives blood its red color.
Step-by-step explanation:
Oxygen is transported across the lung membrane through a process of simple diffusion. In the lungs, the oxygen enters the bloodstream from tiny sacs called alveoli, where the exchange of gases takes place. Gas exchange occurs when oxygen molecules move down their concentration gradient from an area of higher concentration in the alveoli to an area of lower concentration in the blood of the pulmonary capillaries.
This process is facilitated by the large surface area of the alveoli and a steep concentration gradient, which is maintained by continuous blood flow and breathing. The oxygen-rich blood then returns to the heart, where it is circulated to provide oxygen to the body’s tissues. The majority of oxygen molecules are transported bound to hemoglobin in red blood cells, forming oxyhemoglobin, a bright red molecule responsible for the color of oxygenated blood.
In the red blood cells, hemoglobin, a metalloprotein containing iron in its heme groups, binds oxygen. Each hemoglobin molecule can carry up to four oxygen molecules. The affinity of iron for oxygen is illustrated by iron's tendency to rust when exposed to air.