Photosynthesis Divided into two steps: The Light Reactions Noncyclic electron flow 2. The Calvin Cycle Cyclic electron flow

Photosynthesis: The Details

The Light Reactions Photosystems are embedded in the thylakoid membrane. They contain chlorophyll and accessory pigments that are associated with proteins. A photosystem consists of an antenna complex and a reaction centre.

The Light Reactions The antenna complex absorbs a photon and transfers energy to the reaction centre. The reaction centre contains chlorophyll a, whose electrons absorb energy and begin photosynthesis.

The Light Reactions Photosystem II (P680) Two photons strike photosystem II and excite 2 electrons from chlorophyll P680. The excited electrons are captured by a primary electron acceptor and are transferred to plastoquinone (PQ) and the ETC.

The Light Reactions Photosystem II (P680) In the ETC, the 2 electrons pass through a proton pump (Q cycle). The Q cycle transports 4 protons from the stroma into the thylakoid lumen to create a proton gradient.

The Light Reactions Photosystem II (P680) The electrochemical gradient drives the photophosphorylation of ADP to ATP. 1 ATP forms for every 4 protons that pass through ATPase from the thylakoid lumen into the stroma.

The Light Reactions Photosystem II (P680) Z protein splits water into 2 protons, 2 electrons and 1 oxygen atom. The electrons replace those lost from chlorophyll P680. The protons remain in the thylakoid lumen to add to the proton gradient. Oxygen leaves as a byproduct.

The Light Reactions Photosystem I (P700) Two photons strike photosystem I and excite 2 electrons from chlorophyll P700 (replaced by electrons from P680). These electrons pass through another ETC. The enzyme NADP reductase uses the 2 electrons and a proton from the stroma to reduce 1 NADP+ to 1 NADPH.

The Calvin Cycle Occurs in the stroma of chloroplasts. Cyclical reactions similar to the Krebs Cycle. Divided into three phases: Carbon Fixation Reduction Reactions Regeneration of RuBP

The Calvin Cycle Phase 1: Carbon Fixation 3 CO2 are added to RuBP to form 3 unstable 6-carbon intermediates. The intermediates split into six 3-carbon molecules called PGA. These reactions are catalyzed by rubisco.

The Calvin Cycle Phase 2: Reduction Reactions 6 PGAs are phosphorylated by 6 ATPs to form 6 molecules of 1, 3-BPG. 6 NADPH molecules reduce the six 1,3-BPG to 6 G3P or PGAL. One molecule of G3P exits the cycle as a final product.

The Calvin Cycle Phase 3: Regeneration of RuBP 3 ATP are used to rearrange the remaining 5 G3P into 3 molecules of RuBP. The cycle continues with the RuBP fixing more CO2.

To Produce One G3P… 3 RuBP + 3 CO2 + 9 ATP + 6 NADPH + 5 H2O  9 ADP + 8 Pi + 6 NADP+ + G3P + 3 RuBP