Cellular Energy

Chloroplasts

Chloroplast-cyanobacterium comparison

Chloroplasts arose through a second endosymbiotic event in plants and various protists. These light harvesting organelles share similarity in structure and genome to photoautotrophic cynaboacteria.

Light Harvesting

Thylakoid membrane 3

The thylakoid membranes of chloroplasts and cyanobacteria provide additional surface area for energy capture of light to occur. The light-dependent reactions in chlorplasts utilize two protein complexes referred to as Photosystem I (PSI) and Photosystem II (PSII)located on the thylakoid membranes. At the center of each photosystem complexes are photopigments optimized to absorb specific wavelengths of light. When light is absorbed in a photosystem, an electron is excited and transferred to the electron transport chain. In PSII, the electron is regenerated by splitting of two water molecules into 4H+ + 4e + O2. As the electrons move through the ETC, protons are pumped into the thylakoid space. The ETC leads to the reduction of a high energy electron carrier NADP+ to NADPH. Since this pathway uses consumes water in a chemical reaction, the apparent loss of water in the thylakoid space is referred to as chemiosmosis.

PSI is also known as the cyclic pathway since the excited electron runs through a closed circuit of the ETC to regenerate the lost electron. This closed circuit also generates a proton gradient through powering of a proton pump but does not lead to the reduction of NADPH. As with the ETC-powered proton pump in mitochondria, the proton gradient is used to power ATP-synthase in producing ATP molecules.

Light Independent Reactions

Calvin-cycle4

The light independent reactions are also known as the dark reactions or Calvin Cycle and utilize the ATP and NADPH from the light-dependent reactions to fix gaseous CO2 into carbohydrate backbones. Photosynthesis is often simplified into 6CO2 + 6H2O + light –> C6H12O6 + 6O2 . However, the true product is 3-phosphoglycerate that can be used to generate longer carbohydrates like glucose. The starting point of carbon fixation is the carbohydrate Ribulose 1,5-bisphosphate. The enzyme Ribulose Bisphospate Carboxylase (RuBisCO)  captures a CO2 molecule onto Ribulose 1,5-bisphosphate to generate 2 molecules of 3-phosphoglycerate which can enter the process of gluconeogenesis to generate glucose. ATP from the light reactions can then facilitate the conversion of 3-phosphoglycerate to 1,3 bisphosphoglycerate which can be reduced by NADPH to glyceraldehyde-3-phosphate (G3P). G3P can then be used to regenerate  Ribulose 1,5-bisphosphate.

Calvin cycle step 1
1: Carbon fixation by RuBisCO
Calvin cycle step 2 (doubled)
2: Reduction by NADPH
Calvin cycle step 3
3: Ribulose ,5-bisphosphate regeneration

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