Photosynthesis The Details Photosynthesis Divided into two steps

Photosynthesis: The Details

Photosynthesis • Divided into two steps: 1. The Light Reactions • Noncyclic electron flow 2. The Calvin Cycle • Cyclic electron flow

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 (P 680) • Two photons strike photosystem II and excite 2 electrons from chlorophyll P 680. • The excited electrons are captured by a primary electron acceptor and are transferred to plastoquinone (PQ) and the ETC.

The Light Reactions Photosystem II (P 680) • 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 (P 680) • 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 (P 680) • Z protein splits water into 2 protons, 2 electrons and 1 oxygen atom. – The electrons replace those lost from chlorophyll P 680. – The protons remain in the thylakoid lumen to add to the proton gradient. – Oxygen leaves as a byproduct.

The Light Reactions Photosystem I (P 700) • Two photons strike photosystem I and excite 2 electrons from chlorophyll P 700 (replaced by electrons from P 680). • 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: 1. Carbon Fixation 2. Reduction Reactions 3. Regeneration of Ru. BP

The Calvin Cycle Phase 1: Carbon Fixation • 3 CO 2 are added to Ru. BP 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 G 3 P or PGAL. • One molecule of G 3 P exits the cycle as a final product.

The Calvin Cycle Phase 3: Regeneration of Ru. BP • 3 ATP are used to rearrange the remaining 5 G 3 P into 3 molecules of Ru. BP. • The cycle continues with the Ru. BP fixing more CO 2.

To Produce One G 3 P… 3 Ru. BP + 3 CO 2 + 9 ATP + 6 NADPH + 5 H 2 O 9 ADP + 8 Pi + 6 NADP+ + G 3 P + 3 Ru. BP