Table of Contents
Light Dependent Reactions
The thylakoid membranes of chloroplasts and cyanobacteria provide additional surface area for energy capture of light to occur. The light-dependent reactions in chloroplasts 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.
The reactions that occur on thylakoid membrane include the following:
- the splitting of water
- the release of oxygen
- the reduction of NADP+ to NADPH
- the production of ATP by photophosphorylation
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. This splitting of water is called photolysis. As the electrons move through the electron transport chain (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
The light independent reactions are also known as the dark reactions or the Calvin Cycle. These reactions 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 glyceraldehyde-3-phosphate (G3P) that can be used to generate longer carbohydrates like glucose.
The Calvin cycle has three phases:
- carbon fixation
- reduction
- regeneration of the CO2 acceptor
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 two 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). Note that although six molecules of G3P are eventually made, only one will be used to make carbohydrates. The other five molecules are used in the regeneration of ribulose 1,5-bisphosphate.
