the chloroplast. Once detached from the porphyrin ring, phytol becomes the precursor of two biomarkers, pristane and phytane, which are important in the study of geochemistry and the determination of petroleum sources. Biosynthesis The Chlorophyll a biosynthetic pathway utilizes a variety of enzymes. In most plants, chlorophyll is derived from glutamate and is synthesised along a branched pathway that is shared with heme and siroheme. The initial steps incorporate glutamic acid into 5-aminolevulinic acid (ALA); two molecules of ALA are then reduced to porphobilinogen (PBG), and four molecules of PBG are coupled, forming protoporphyrin IX. Chlorophyll synthase is the enzyme that completes the biosynthesis of chlorophyll a by catalysing the reaction chlorophyllide a + phytyl diphosphate chlorophyll a + diphosphate This forms an ester of the carboxylic acid group in chlorophyllide a with the 20-carbon diterpene alcohol phytol. Reactions of photosynthesis Absorbance of light Light spectrum Chlorophyll a absorbs light within the violet, blue and red wavelengths. Accessory photosynthetic pigments broaden the spectrum of light absorbed, increasing the range of wavelengths that can be used in photosynthesis. The addition of chlorophyll b next to chlorophyll a extends the absorption spectrum. In low light conditions, plants produce a greater ratio of chlorophyll b to chlorophyll a molecules, increasing photosynthetic yield. Light gathering Absorption of light by photosynthetic pigments converts photons into chemical energy. Light energy radiating onto the chloroplast strikes the pigments in the thylakoid membrane and excites their electrons. Since the chlorophyll a molecules only capture certain wavelengths, organisms may use accessory pigments to capture a wider range of light energy shown as the yellow circles. It then transfers captured light from one pigment to the next as resonance energy, passing energy one pigment to the other until reaching the special chlorophyll a molecules in the reaction center. These special chlorophyll a molecules are located in both photosystem II and photosystem I. They are known as P680 for Photosystem II and P700 for Photosystem I. P680 and P700 are the primary electron donors to the electron transport chain. These two systems are different in their redox potentials for one-electron oxidation. The Em for P700 is approximately 500mV, while the Em for P680 is approximately 1,100-1,200 mV. Primary electron donation Chlorophyll a is very important in the energy phase of photosynthesis. Two electrons need to be passed to an electron acceptor for the process of photosynthesis to proceed. Within the reaction