Hans Fischer: Why blood is red and leaves are green

Hans Fischer was born on July 27, 1881. He became the chemist who helped explain Why blood is red and why plant leaves are green. Behind these contrasting colors lie related molecular frameworks: one is involved in oxygen transport and cellular respiration, while the other allows plants to capture energy from light.

Fischer studied the substances that give color to blood, bile, and plants. Today, their formulas, three-dimensional structures, and biological functions are well characterized. In the early twentieth century, however, complex natural molecules had to be broken apart through chemical reactions, their fragments analyzed, and the original compounds reconstructed step by step.

This was the approach Fischer used to investigate haemin and other porphyrin-related compounds. His work helped establish the molecular framework of heme, the non-protein component of hemoglobin that directly participates in oxygen transport.

Turning a blood pigment into a chemical structure

Hemoglobin contains a protein component and four heme groups. Each heme consists of a large ring built from smaller connected units. Ring systems of this type are known as porphyrins. An iron atom is held at the center of the heme.

In functional hemoglobin, the iron is mainly in the ferrous state and can bind oxygen reversibly. Oxygen attaches to the iron in the lungs and is released in tissues, where oxygen availability is lower. The surrounding protein controls this process and allows the four parts of hemoglobin to influence one another.

Fischer himself worked mainly with haemin. Haemin is not identical to the heme found in normal functioning hemoglobin. It contains oxidized ferric iron and is commonly associated with a chloride ion. This form is more stable and was easier to isolate and analyze chemically.

Fischer and his colleagues broke natural pigments into simpler components, studied pyrrole derivatives, determined the positions of chemical side groups, and synthesized the structures they believed were correct. When a synthetic product matched the natural material in its chemical properties, it provided strong evidence that the proposed structure was right.

By the end of the 1920s, Fischer had achieved the total synthesis of haemin. In 1930, he received the Nobel Prize in Chemistry for his research on the constitution of haemin and chlorophyll, particularly for the synthesis of haemin. At the time, assembling such a complex natural pigment from simpler starting materials was an exceptional achievement.

What actually makes blood red

A common simplified explanation says that blood is red because it contains iron. But a piece of iron does not have the color of blood. The red appearance comes from the complete electronic system of heme: a large ring with alternating chemical bonds and an iron atom at its center.

Electrons distributed across this ring interact with visible light. The molecule absorbs some wavelengths more strongly than others, and the light that remains is perceived as red. Iron affects the electronic structure of heme, but the color cannot be attributed to the metal atom alone.

Oxygen binding changes the interactions among the iron, the porphyrin ring, and the surrounding protein, which alters the shade. Oxygen-rich arterial blood is bright red, while blood carrying less oxygen is darker. Venous blood is still red, however. Veins can appear blue through the skin because of the way light travels through and is reflected by tissues, not because the blood itself is blue.

Heme is also used far beyond hemoglobin. It is found in myoglobin, which helps store oxygen in muscle, and in cytochromes, catalase, and several other enzymes. These proteins contribute to cellular energy production, protection against reactive oxygen species, and the processing of many different compounds.

Fischer’s research therefore concerned much more than the color of blood. It helped reveal the structure of a molecular component connecting oxygen transport with cellular metabolism.

The chemical relationship between blood and leaves

After his work on blood pigments, Fischer devoted substantial effort to the chemistry of chlorophyll. Chlorophyll absorbs light and initiates photosynthesis, the process by which plants use carbon dioxide and water to produce organic molecules.

Heme and chlorophyll belong to the broader family of tetrapyrrole compounds. Their basic frameworks contain four connected nitrogen-containing rings. Heme holds iron at its center, whereas chlorophyll contains magnesium.

The difference is often described as a simple exchange of iron for magnesium, but that is too crude. The ring structures, side groups, and other parts of the molecules also differ. Chlorophyll, for example, carries a long hydrophobic tail that helps anchor it within a membrane, and its central ring system differs from the classical porphyrin structure of heme.

Their chemical relationship is nevertheless fundamental. Nature uses related molecular frameworks for two essential processes. Chlorophyll captures energy from sunlight, while heme helps cells use oxygen and extract energy from nutrients.

Fischer did not single-handedly explain every detail of hemoglobin function or photosynthesis. Later work using X-ray crystallography, spectroscopy, and molecular biology showed how heme fits inside hemoglobin, why oxygen binding is cooperative, and how chlorophyll functions within large photosynthetic complexes.

Those later discoveries depended on knowing the underlying chemistry. Fischer helped transform the colors of blood and leaves from visible traits into molecular structures that could be tested experimentally.

His story shows what fundamental chemistry can accomplish. Red blood and green leaves look completely different, yet at the molecular level they speak related chemical languages.