Photosynthesis Evolves

One billion years after the relatively simple photosynthetic systems of archaebacteria, the Great Oxidation Event took place 2.5 billion years ago. Many scientists think that this probable jump in oxidation rate was due to the appearance of cyanobacteria (a.k.a. blue-green algae) and its highly evolved photosynthetic system. In a process called endosymbiosis, a eukaryote cell incorporated a cyanobacterium. This became the energy-producing photosynthetic organelle, the chloroplast, in green algae and plants.

Cyanobacteria photosynthesis evolved during 1 to 1.5 billion years from halobacterium, photosynthesis. Cyanobacteria have chlorophyll molecules within protein complexes to harvest red and blue light energy in Photosystem II and Photosystem I. Chlorophyll molecules are excited once per second by photons.

In green algae and plants, the protein complexes are embedded in the thylakoid membrane and include Photosystem II, Cytochrome b6f, Photosystem I, and Ferredoxin NADP reductase (Figure 6‑20). In addition, there are also three mobile carriers that carry electrons from one position to another. For photosystem II, photons enter from the thylakoid membrane (Figure 6‑20) and provide energy for proteins to release hydrogen ions into the thylakoid lumen. The high concentration of proteins in the lumen drives ATP synthase. In photosystem I, proteins combine a hydrogen ion with NADP and form NADPH, an energy containing molecule.

In photosystem II, which is the first step in the electron transport chain, electrons are removed from water and are excited to a high energy level by a photon. The electrons then transfer energy to plastoquinone, which carries two electrons to the Cytochrome 6bf complex, which then pushes 4 protons behind the membrane into the lumen, using the energy from the excited electrons. This creates a high proton concentration in the lumen, which is then used to drive ATP synthase and make ATP. The electrons are then moved to the Photosystem I complex (Figure 6‑21) by the electron carrier plastocyanin, where they are again excited to higher energy states.

Red photons excite electrons in photosystem II to a higher energy level. The entire complex also absorbs blue light. The electron moves down another electron transport chain to ferredoxin, which adds electrons to the protein complex FNR, which then forms NADPH by adding two electrons and a hydrogen ion to NADP. NADPH is another energy molecule that can transfer energy to proteins. Cyanobacteria, green algae, and plants use the energy contained in ATP and NADPH to produce sugars in the Calvin Cycle. Evolution perfected the process of glucose sugar formation in the Calvin Cycle.

The Calvin cycle (Figure 6‑21) is initiated by the RuBisCo protein complex, which adds carbon dioxide from the atmosphere to carbon chains in order to form glucose sugar. RuBisCo is by far the most abundant protein in the leaves of plants. In the Calvin Cycle, ATP provides the energy to sequentially add carbon atoms to sugars and eventually form a 5 carbon sugar called Ribulose 1,5-biphosphate.

Photosynthesis in plants is like cyanobacteria. Plants use the same PSI, PSII, and Calvin Cycle as in cyanobacteria (blue-green algae), but structures evolved to maximize the rate of photosynthesis. The cells in which photosynthesis takes place in plants are called palisade cells (Figure 6‑22), which contain many chloroplasts for energy production (formation of sugars) from photosynthesis and many mitochondria for energy processing of the sugars during respiration. Many photosystem I and II complexes and ATP synthase are embedded in the thylakoid membrane of the chloroplast.

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Cyanobacteria in Lake Koylio. Credit: kallerna. Used here per CC BY-SA 3.0.