History & Name

Chlorophyll serves as the predominant green pigment within plant organisms, functioning as the light-absorbing agent crucial for the process of photosynthesis. Variations in molecular structure give rise to several forms of chlorophyll, with chlorophyll a being the most prevalent and chlorophyll b constituting the second most common variant. The fundamental chlorophyll structure is characterized by a porphyrin ring system enveloping a singular magnesium atom, composed of four pyrrole subunits. A lengthy hydrocarbon chain is attached to this porphyrin ring, distinguishing the two primary forms of chlorophyll through variations in the side chain connected to one of the pyrrole groups. Chlorophyll a exhibits a methyl group, while chlorophyll b features a side chain consisting of CHO, as illustrated in the diagram highlighting these distinctions.

The evolution of chlorophyll knowledge is intertwined with advances in understanding the photosynthetic process and the emergence of modern chemistry. In the 1770s, Joseph Priestley observed that plants replenished oxygen when placed in an environment containing fixed air (carbon dioxide). Building upon Priestley's findings, Jean Senebier discovered the reciprocal relationship between oxygen replenishment and carbon dioxide consumption in plants. Jan Ingenhousz determined that the green portion of plants, requiring light, played a pivotal role in oxygen replenishment. In the early 19th century, Nicholas Theodore de Saussure identified water and carbon dioxide as the sources of hydrogen and carbon in plants, respectively.

The isolation and naming of chlorophyll were accomplished by Pierre Joseph Pelletier and Joseph Bienaimé Caventou in 1818, with the name derived from the Greek words "chloros," meaning yellow-green, and "phyllon," meaning leaf. René-Joachim-Henri Dutrochet recognized the necessity of chlorophyll for photosynthesis in 1837, while Julius Robert von Mayer proposed in 1845 that plants convert light into chemical energy. Chromatography, developed by Mikhail Semenovich Tsvett in the early 20th century, facilitated the separation of different chlorophylls. Richard Martin Willst?tter utilized chromatography to isolate plant pigments and identified the structural similarity between chlorophyll and hemoglobin, isolating chlorophyll a and chlorophyll b. Willst?tter was awarded the 1915 Nobel Prize in Chemistry for his contributions.

The comprehensive structure of chlorophyll was elucidated by Hans Fischer, who received the Nobel Prize in Chemistry in 1930. Fischer established the relationship between chlorophyll and hemin, further developing a synthesis for the latter.

Mechanism

The primary chlorophylls present in green plants are chlorophyll a and chlorophyll b, with chlorophyll a being predominant. All photosynthesizing organisms, including plants, green algae, and cyanobacteria, contain chlorophyll a, while chlorophyll b is specific to plants and green algae. In addition to chlorophylls a and b, various plants also possess chlorophylls c, d, and e. Chlorophyll, situated in the thylakoid membranes of chloroplasts, functions as the light-harvesting antennae in plants. It collects energy, initiating a sequence of biochemical reactions that ultimately transform radiant energy into chemical energy. Despite the numerous reactions involved in photosynthesis, the overall process is represented by the equation: 6CO2 + 6H2O → 6O2 + C6H12O6.

The structure of chlorophyll is pivotal to its role in energy transfer. The conjugated system of alternating single and double bonds results in delocalized electrons that can be excited by light into higher molecular orbitals. The release of energy when an excited electron returns to a lower molecular orbital produces visible light within the 400 nm to 700 nm range, which is the visible spectrum of light. Upon light absorption by chlorophyll, the energy is transferred to adjacent molecules. This process continues through the chlorophyll molecule system until reaching a location called the reaction center. At the reaction center, an electron is transferred to an electron acceptor. Together, the light-gathering chlorophyll antennae, the electron transfer chlorophyll, and the reaction center form a photosystem.

Two photosystems, Photosystem I (P700) and Photosystem II (P680), are associated with green plants. The numbers 700 and 680 denote the wavelengths of light, in nanometers, at which these systems are most efficient. In Photosystem II, chlorophyll absorbs light at a maximum wavelength of 680 nm, exciting an electron that moves through the chlorophyll to the reaction center. The photon's energy is then utilized by electron-transfer proteins to pump protons (H+) into the thylakoid, creating a proton gradient. Protons diffuse out through ATP synthase, generating adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphorus (Pi). The electron then moves to the Photosystem I reaction center, absorbing additional light energy at a maximum wavelength of 700 nm, and is used to produce NADPH.

The light reactions in photosynthesis, occurring in Photosystems I and II, require light to produce primary products such as ATP and NADPH. Dark reactions utilize ATP and NADPH to reduce carbon dioxide, facilitating carbohydrate synthesis. Chlorophyll a and b strongly absorb in the red and blue-green regions of the visible spectrum, resulting in the transmission and reflection of green wavelengths. This phenomenon imparts the characteristic green color to chlorophyll-containing plant tissues such as leaves and stems. Chlorophyll a is the principal pigment, but plants also contain accessory pigments such as other chlorophylls, carotenes, anthocyanins, and xanthophylls. The presence of a range of pigments, including chlorophyll b, enables plants to capture light across a broader spectrum. During summer, the abundance of chlorophyll a masks accessory pigment colors, while in autumn, changes in photoperiod and cooler temperatures signal the reduction and cessation of chlorophyll production, allowing other pigments to be displayed and resulting in the vibrant fall colors.

Reference

Richard L. Myers (2009). The 100 Most Important Chemical Compounds: A Reference Guide. Greenwood Publishing Group. October 1, 2009. https://doi.org/10.1021/ed086p1182