General Biochemistry 2: Lecture 10,11 (Photosynthesis, Photosystems, Cyclic)

LECTURE 10,11: PHOTOSYNTHESIS, REACTION CENTERS, PHOTOSYSTEMS

A. Photosynthesis- light driven synthesis of CH2O from CO₂; also produce oxygen by oxidating water; overall: CO₂ + 2 H₂O à CH₂O + O₂ + H₂O

Chloroplasts-site of photosynthesis; 1-1k chloroplast per cell, very in size/shape, usualy 5 um long ellipsoids.

-3 membranes: inner, outer, thylakoid membranes (where light driven reactions take place)

-3 regions: intermembrane space, stroma (dark reactions), thylakoid lumen

-thylakoid membrane- light reactions; invaginations of inner membrane cause these, resemble cristae

-thylakoid vesicle- arranged to have disk-like sacs (grana), stacked in piles, connected to each other by stroma lamellae.

-1 chloroplast=10-100 grana

-stroma-concentrated solution w enzymes, dna, rna, ribosomes

B. Reaction Centers

I. Light reactions (thylakoid membrane)- use light to generate nadph, atp and O₂. Resembles ET and OP in mitochondria.

1. Photosynthetic Reaction Center(RC)-primary photochemical rxns take place in RC.

-chlorophyll- green pigments that capture sunlight, located in and around photosystems embedded in thylakoid membrane of chloroplasts. Structure- porphyrin with magnesium at the center.

-Bacteriopheophytin- a bacteriochlorophyll where Mg²⁺ ion is replaced by 2 H⁺; Reaction center from a purple bacteria (rhodopseudomonas viridis) is a transmembrane protein with several chromophores (BchI a and b, nonheme Fe(II), Bpheo b, ubiquinone, menaquinone)

-antenna chlorophyll- chlorophylls not in RC;  don’t participate directly in photochemical reaction, but capture photons from sunlight.

2.Light Harvesting Complex B (antenna complex)- has several membrane-bound hydrophobic proteins, each with many chlorophylls and other pigments.

-carotenoid- accesory pigments; non-chlorophyll pigments that absorb wavelengths chlorophylls don’t strongly absorb.

-number of LHCs- many more LHCs than PRCs.

-lower energy state- light captured by antenna complex, transferred to series of antenna pigment molecules, eventually trapped by PRC (10E-10 seconds, >90% efficiency); PRC has lower excited energy state than antenna pigments.

-all O₂ cells- have psysI and II; non-O₂ cells only have psys I.

C.Photosystems

1.Photosystem 1-forms nadph

-max efficency at 700nm; absorbs 4 quanta, makes strong reductant, weak oxidant

-4[ P₇₀₀ + q à P₇₀₀* (strong reductant) à P₇₀₀⁺ (weak oxidizer)+ e]

-4[ P₇₀₀⁺ + e(from P₆₈₀*) à P₇₀₀]

-P700*- strong reductant provides 4e to reduce 2 NADP+ to 2 NADPH

-P700+ - weak oxidant recieves 4 e from weak reductant made by psys II to regenerate ground state psys I P₇₀₀

- ferridoxin-nadp+ reductase-forms nadph in stroma

2.Photosystem II-forms O₂ by oxygen-evolving center (OEC)

-max efficiency drops at >680nm; absorbs 4 q, makes weak reductant and strong oxidant

-4 [P₆₈₀ + q à P₆₈₀* (weak reducer) à P₆₈₀⁺ (strong oxidant) + e]

-4 [P₆₈₀⁺ + e (from water) à P₆₈₀]

-P680+ -strong oxidant takes 4 e from 2water to make 4H+ and O2

-P680*- weak reductant provides 4 e to weak oxidant formed by psysI

3.H⁺ gradient- drives atp synthesis by atp synthase; similar to OP

-thylakoid lumen- 4e goes from weak reducer P680* to weak oxidant P700+, coupled to pumping of 8 H⁺ into thylakoud lumen.

-Evolution of one O₂- from 2 waters produces 4H+ in thylakoid lumen.

-redox-active factors- chlorophylls, cytochromes, Q, FeS clusters, plastocyanin help transfer 4e between photosystems.

-plastocyanin- peripheral membrane protein on thylakoid luminal surface, cycles between CuI and CuII, a redox-active factor.

4.cyclic transport- produces atp, but no nadph or O₂

-involves only photosystem I;

-allows cells to adjust ATP:NADPH ratio

-regulation unknown

5.efficiency of non-cyclic ET: 1.25 ATP/quantum (10 atp/8 quanta)

-for every O₂ made or 8 quanta absorbed

-water evolution: 4H+ made in lumen from 2H2Oà4H+ + O₂ 

-4e from PSII to PSI: 8H+ pumped into lumen

-H⁺ gradient: 12 H⁺/3 H⁺ to power synthase= 4 ATP

-PSI’s p700*: makes 2 NADPH, 3 atp/nadph, equals 6 ATP

6.efficency of cyclic ET: 0.67 ATP/quanta

-for every 4 q absorbed by PSI, 8H+ pumped into lumen, (8H+/3H+ to power synthase)/4 quanta

Dark reactions- use nadph and atp to make ch2o from water and CO₂. in eukaryotes, happens in stroma.