Chapter 10

Protein complex consisting of an outer protein shell and a reaction center that captures light energy, facilitating the splitting of water molecules to release oxygen.

Light strikes one of the pigment molecules in Photosystem II outer layer, exciting an electron. Released energy is transferred from pigment to pigment.
P680 Pair (Chlorophyll A molecules) is excited by the traveling released energy and the primary electron acceptor captures the electrons in a redox reaction.
P680 becomes P680+, becoming an oxidizing agent, meaning the electron must be replaced. This is achieved by step 3, an enzyme causes H2O release 2 electrons, which are then used to replenish the lost electron in P680, ultimately leading to the production of O2 as a byproduct.

Electron Transport Chain: Electrons from Photosystem II pass through Pq, Cytochrome Complex, and plastocyanin before they reach Photosystem I, where they are re-energized by light absorption.
This process releases free energy (due to a proton gradient) through it, forming ATP (chemical energy)

Light strikes one of the pigment molecules in Photosystem 1 outer layer, exciting an electron, which gives up it’s energy to another molecule before hitting P700 and exciting it.
P700 transfers energy to primary electron acceptor, becoming P700+, and accepting the remaining electrons from the electron transport chain. The transferred energy undergoes redox reactions through Fd.
1. This second transport chain does not produce a proton gradient, so no ATP is produced.
NADP+ reductase (enzyme) takes two electrons and turn NADP+ into NADHP (reducing power)

Photosynthesis

Requires Carbon Dioxide, which enters through pores, and into cells where it diffuses into chloroplasts.
Thylakoids: membrane-bound structures within chloroplasts that house the components for the light-dependent reactions of photosynthesis.
Inside the thylakoid, light reactions occur that capture sunlight and convert it into chemical energy in the form of ATP and NADPH, which are then utilized in the Calvin cycle.
Large amount of hydrogen inside thylakoid causes ATP Synthase to produce ATP as hydrogen ions flow back into the stroma, driving the process of phosphorylation.

C3 Cycle: Takes place in the stroma, carbon dioxide combines with molecules called RUBP to form 3-phosphoglycerate (3-GPs), which is then converted into glucose through a series of enzymatic reactions powered by ATP and NADPH.
Some 3GPs end up being re-arranged back into RUBP to continue the cycle, ensuring that the process of carbon fixation can sustain continuously with the help of regenerated carbon molecules.
The rest become sucrose, glucose and broken down in cellular respiration, providing energy for plant growth and metabolism.

Note:

PS II: Oxidation of water and reduction of transport chain.
PS I: Reduction of NADP+ and oxidation electron transport to create NADPH for the Calvin cycle.
Both involve light absorption and reduction of primary electron accepter
PS II and I light energy used to drive redox reaction that would not otherwise occur. Redox reaction moves an electron from special chlorophyll pair (P680 and P700) to primary electron accepter. Resulting in each reduction of the primary electron acceptor and an oxidant (P680+ and P700+) which power the rest of transfer reactions without energy.
Photosynethic electron transport contributes to formation of a proton (H+) gradient across the thylakoid membrane in two places.
- PS II: oxidation of water releases protons into thykaloid space.
- Electron transport between PS II and cytochrome complex (through Pq) pumps protons from stroma into thyalkoid space.
- The resulting proton gradient (because now the inside of thylakoid space is high H+ AND stroma is lower H+) forces the H+ through ATP synthase, producing ATP as it flows back into the stroma, powering the conversion of ADP and inorganic phosphate into ATP.