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Red vs. Green - The Similarities Between Hemoglobin and Chlorophyll

(Top left: Red blood cells under an electron miroscope, top right: chloroplasts under an electron microscope, bottom: the oxygen binding heme group of hemoglobin with Iron (Fe), and the central molecular structure of chlorophyll a with Magnesium (Mg). Side chains are not shown)

Hemoglobin and Chlorophyll are both fundamental molecules of life on Earth. Both contribute to the color of their respective cells and both are necessary for the production and transport of essential molecules and nutrients. Despite the complex differences between plants and animals, both of these molecules are more similar than you’d think. 

Hemoglobin is one of the most thoroughly studied and understood proteins. It was one of the first for which three-dimensional structures were determined. Hemoglobin demonstrates one of the most central aspects of biochemical processes: reversible binding of a ligand to a protein. Oxygen’s nature makes it poorly soluble in the aqueous environment of the body, and cannot be carried to tissues in sufficient quantities by the blood. Higher multicellular organisms needed to evolve in building specific proteins to transport and store oxygen through acceptance from a surface in contact with air (lungs) or water (gills). This role is fulfilled by certain transition metals, in hemoglobin’s case, iron (Fe). Transition metals have a higher tendency to bind oxygen, allowing carrier proteins to reversibly bind and circulate needed oxygen to tissues. However, free transition metals promote the formation of harmful oxygen radicals that can damage DNA and other biological macromolecules. As a result, iron is sequestered and stabilized by protein bound prosthetic groups called hemes, as shown above. (Video: Oxygen Transport by Hemoglobin)

At the other side of the spectrum (literally), we have chlorophyll. Chlorophyll is the most important light absorbing pigment in plant membranes and is critical in photosynthesis, allowing plants to absorb photons of energy from light. As hemoglobin is packed to red blood cells, chlorophyll is packed in chloroplasts. From the electromagnetic spectrum, chlorophyll absorbs blue light best, followed by red light. However, it poorly absorbs green light, explaining the green color of plant tissue (Green is the only color not absorbed and is transmitted to your eyes). The structure of chlorophyll resembles the porphyrin ring of hemoglobin, except a magnesium occupies the heart of its ring. This magnesium is constantly bombarded by photons; for each photon it absorbs, one of its electrons is excited to a higher state. With energy from the absorbed photons, chlorophyll molecules pass electrons to a chain of proteins that use the energy for making chemical bonds. Some of the energy is used to generate reduced NADPH and ATP, its main energy sources. And other energy is used to strip electrons from compounds such as water, producing oxygen gas. 

These two molecules do not play analogous roles in plants and animals, but they certainly complement each other in several ways. The survival of both plants and animals seems uniquely dependent on the other. Humans inhale oxygen and exhale carbon dioxide; plants take in carbon dioxide and produce oxygen. Even further, hemoglobin binds and transports oxygen to tissues around the body; chlorophyll channels energy from light to transform carbon dioxide and water into carbohydrates and water, producing oxygen as a byproduct. 

Interestingly enough, when ingested, chlorophyll promotes circulation, cleanses the body, increases the number of red blood cells and therefore increasing oxygen content in the body. This makes it easier for the body to heal and repair tissues and get rid of toxins. The near identical structure of hemoglobin and chlorophyll also makes it very easy for the body to develop new red blood cells from green vegetables.

Sources:

Chlorophyll and Hemoglobin Regeneration after Hemorrhage

Lehninger, Principles of Biochemistry, Fifth Edition

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