Hemoglobin is the protein in red blood cells that carries oxygen from the lungs to tissues and organs in the body. When oxygenated, hemoglobin appears bright red. This is what gives blood its red color. However, when hemoglobin is not carrying oxygen, it changes to a dark reddish-purple color. So in its deoxygenated state, hemoglobin does appear more purple than red. But calling it simply “purple” would be inaccurate.
What causes the color change in hemoglobin?
Hemoglobin contains four iron-containing heme groups that bind to oxygen. It is the iron atom at the center of these heme groups that changes color depending on its oxidation state.
In its oxygenated form (oxyhemoglobin), the iron atom is in the Fe2+ (ferrous) state. This allows electron transfer and gives oxyhemoglobin its bright red color.
When hemoglobin releases oxygen to tissues, the iron oxidizes to the Fe3+ (ferric) state, forming deoxyhemoglobin. In this form, the iron atom strongly absorbs green and blue light, giving deoxyhemoglobin more of a purple-red hue.
Does blood ever appear purple?
Blood does not actually appear purple when directly observed under normal physiological conditions. The deep red color of venous blood results from its deoxygenated hemoglobin. But the plasma, white blood cells, platelets, and other components of blood also contribute to its overall red appearance.
However, under certain situations, the purple color of deoxygenated hemoglobin becomes more apparent:
- Veins sometimes appear blue/purple through the skin. This is due to the combination of deoxygenated blood and how light scatters when passing through tissue.
- Blood removed from the body rapidly deoxygenates and darkens. In a test tube, this blood takes on a distinctly purple hue.
- Spectral measurements of deoxygenated hemoglobin isolate the purplish color from the rest of blood’s components.
Summary of hemoglobin’s color
Here is a summary of the different colors hemoglobin displays depending on its oxygenation state:
| Hemoglobin form | Oxygenation state | Iron oxidation state | Color |
|---|---|---|---|
| Oxyhemoglobin | Oxygenated | Fe2+ (ferrous) | Bright red |
| Deoxyhemoglobin | Deoxygenated | Fe3+ (ferric) | Dark reddish-purple |
Biological role of hemoglobin’s color change
The color change of hemoglobin plays an important biological role. Oxyhemoglobin’s bright red color allows larger quantities of oxygenated blood to absorb more light. Deoxyhemoglobin absorbs less light, appearing darker, which helps distinguish oxygenated blood from deoxygenated blood.
This distinction allows easy visualization of oxygen binding at the tissues, enabling scientists to determine oxygen saturation levels. It also enables doctors to identify potential problems with a patient’s oxygen circulation.
So in summary, while hemoglobin does appear purplish when deoxygenated, calling it simply “purple” is inaccurate. The color change from red to dark reddish-purple does serve an important biological function though.
Other colorful proteins in the body
While hemoglobin is responsible for the red color of blood, there are other brightly colored proteins in the human body:
- Myoglobin – This protein stores oxygen in muscle tissue. It contains a single heme group and appears dark red.
- Cytochromes – These are electron transport proteins involved in cellular respiration. They contain heme groups and range in color from orange to deep red.
- Rhodopsin – The main light-sensitive protein in the eye’s rods. It appears reddish-purple.
- Anthocyanins – Water-soluble pigments in plants that appear red, purple, or blue according to pH.
- Chlorophyll – The green pigment that enables photosynthesis in plants.
- Carotenoids – Organic pigments produced by plants and algae that appear yellow, orange, and red.
While not found in the human body, many colorful plant pigments play important biological roles in nature. The variety of colors in living organisms contributes to the visual diversity we observe in the natural world.
Conclusion
In conclusion, hemoglobin does take on a dark reddish-purple color when deoxygenated. But while this color may appear somewhat purple, hemoglobin is not considered simply “purple”. Its color results from complex factors involving the oxidation state of its iron-containing heme groups and how it absorbs and reflects light. The color change serves an important biological function, enabling easy distinction between oxygenated and deoxygenated blood. So in summary, saying hemoglobin is purple would be an oversimplification of its true color and properties.
