Photosystem Discovery (Emerson & Arnold, 1932) Q: What determines the saturation point of photosynthesis? H: If photosynthesis is limited by the number of chlorophyll molecules P: Then the rate of photosynthesis will increase with light intensity until all chlorophyll molecules are in use. E: O 2 yield measured in Chlorella when exposed to brief flashes of light. Also determined the number of chlorophyll molecules.
Photosystem Discovery (Cntd.) Results: Only 1 molecule of O 2 produced per 2500 chlorophyll molecules. Conclusions: Light is absorbed by photosystems (clusters of chlorophyll and accessory pigments).
Photosystems (Cntd.) Organization An antenna complex and a reaction center that are held within a protein matrix in the thylakoid membrane. Antenna Complex Composition Many chlorophyll molecules and carotenoid accessory pigments. How it Works A pigment molecule absorbs a photon. Excitation energy is transferred from pigment molecule to pigment molecule. This energy is passed to the reaction center.
Photosystems (Cntd.) Reaction Center Composition Transmembrane protein-pigment (including chlorophyll a) complex. How it Works Chlorophyll a absorbs excitation energy and uses it to transfer an excited electron to an electron acceptor (quinone). *Light energy Chemical energy Chlorophyll a replaces the lost electron with a low energy electron, transferred by a “weak” electron donor (water in plants).
Structure of Photosystem II http://www.bio.ic.ac.uk/research/barber/psIIimages/PSII.html
Photosystem Types in Plants and Algae Photosystem I (P 700 ) Absorption peak = 700 nm Produces NADPH Reaction center - transmembrane complex of ≥ 13 protein subunits. Antenna complex of 130 chlorophyll a and accessory pigments. Photosystem II (P 680 ) Absorption peak = 680 nm Synthesizes ATP and passes electrons to PSI Reaction center - > 10 transmembrane subunuts. Antenna complex - 250 molecules of chlorophyll a and accessory pigments bound to protein chains.
When both photosystems are used in series to produce both ATP and NADPH. Photosystem II 1. PS II absorbs a photon & transfers an electron electron acceptor. 2. Electron acceptor transfers the electron down the electron transport chain (loses potential energy as it goes). 3. Electron passed PSI. 4. As the electron passes from PSII to PSI H + is pumped across the thylakoid membrane to make ATP (photophosphorylation). 5. An enzyme associated with PSII reaction center splits 2 H 2 O to replace the lost electron. O 2 is released.
Noncyclic Photophosphorylation (Cntd.) Photosystem I 1. PSI absorbs a photon to eject an electron from the reaction center electron acceptor. 2. PSI accepts an electron from the electron transport chain. These electrons still carry much of their excitation energy, so on ejection these electrons carry very high energy. 3. Electron is passed down electron transport chain (losing potential energy). 4. 2 electrons are donated to NADP + NADPH on the stromal side of the membrane.
Proton Gradient Across Thylakoid Membrane What factors contribute to the proton gradient?
Comparison of Electron Transport in Photosynthesis and Respiration PhotosynthesisRespiration Location Electron Source Function H + Pumping Location of ATP Generation
Comparison of Electron Transport in Photosynthesis and Respiration PhotosynthesisRespiration Location Thylakoid Membrane Inner Mitochondrial Membrane Electron Source WaterNADH & FADH 2 Function Generate NADPH & ATP Generate ATP H + Pumping Stroma Thylakoid space Matrix Intermembrane space Location of ATP Generation StromaMatrix
Cyclic Photophosphorylation What is cyclic Photophosphorylation? Energetic electron is passed back to the same photosystem in a circle. Function Sulfur Bacteria Only have 1 photosystem - used to make ATP only. Plants Glucose synthesis requires 1.5 ATP for every NADPH. To make up for the ATP shortage plants can bypass PSI.
The Calvin Cycle Location Stroma Use of the Products of the Light Reactions 1. ATP Drives the endergonic reactions. 2. NADPH Provides the H atoms and electrons (high potential energy) needed to make the sugar (energy-rich C-H bonds).
The Calvin Cycle (Cntd.) Principle stages 1. Carbon Fixation (atmospheric C organic C) A C from CO 2 is added to RuBP (5C) 6C sugar splits to form PGA (2 X 3C). 2. Reduction of Sugar ATP, H, & electrons are used to reduce the sugar (storage of potential energy). 3. Sugar Production & Regeneration of Starting Material Some 3C molecules are removed from the cycle to make sugars (G3P). The rest are recycled for re-use in the Calvin cycle.