Photosynthesis: Light Reactions

Flashcards reviewing key vocabulary and concepts related to the light reactions of photosynthesis.

Electromagnetic Radiation

Energy of photons; E = hν = hc/λ; h is Planck's coefficient

Photosynthetically Active Radiation (PAR)

400-700 nm; light range used in photosynthesis

Significance of Photosynthesis

Photosynthesis provides organic molecules and oxygen for autotrophs and heterotrophs.

Chloroplast Anatomy

Outer membrane, Inner membrane, Intermembrane space, Stroma, Granum, Thylakoid

Glycosil-glycerides

Monogalactosyldiacylglycerol, Digalactosyldiacylglycerol, Sulfoquinovosyldiacylglycerol

Photosynthesis Reactions

Light reactions and dark reactions (CO2 fixation)

Hill reaction

In light O2 evolution in presence of Fe3+ (oxidant, electron acceptor); Fe3+ substitutes the CO2

Photosynthesizing Pigments

Chlorophylls and Carotenoids

Phycobilins

algal pigments

Carotenoids

carotenes and xanthophylls

Photochemical reactions

light (photon) absorption, energy transfer, electron transfer

Pigment* return to the ground state through four processes

loss of the excitation energy as heat, loss of the excitation energy by photon emission (fluorescence), energy transfer, electron transfer

Photosynthesizing Pigments

bound to proteins; Localised in two large protein complexes called as photosystems: Photosystem II (PSII) and photosystem I (PSI)

Functionally two different parts of photosystems

Reaction center and light harvesting antenna

Four protein complexes

PSII, PSI, cytochrome b6f, ATP-synthase

Oxygenic Photosynthesis

P680* is a very strong oxidant – strong enough to pull electrons from H2O; P700* is a very strong reductant – strong enough to donate electrons to NADP+

Electron transport chain

PSI and PSII are connected by an electron transport chain

LHC

Light harvesting complexes (LHC) or antenna complexes

PSII

Core complex II (CC II): containing the reaction centre of PSII (P680) where two large key proteins D1 and D2 + inner antenna proteins; LHC II: light harvesting complex

PSI

Core complex I (CC I): containing the reaction centre (P700) with two large key proteins PsaA és PsaB; LHC I: light harvesting complex

PSII LHCII

Light harvesting proteins containing 50 % of the total chlorophyll content; Mobile LHC II: can dissociate from PSII and participates in excitation energy distribution between PSII and PSI.

Mobile LHC II

Can dissociate from PSII and participates in excitation energy distribution between PSII and PSI. chl a/b ratio = 1,1

Plastocyanin

Water soluble metalloprotein, containing Cu. Transfers electron from cyt b6 f complex to P700 of PSI.

Electron transfer in PSI

from P700 to ferredoxin through phylloquinone and three different Fe-S centers

Ferredoxin

small iron-containing Fe2S2 protein; Reduced Fd can also pass electrons to other enzymes !!!!!!

FNR

Ferredoxin transfers electrons via ferredoxin:NADP+ reductase (FNR)

Cyclic electron transport

• Involves PSI • Does not involve PSII • Involves the electron transport chain • Results in ATP production • Does not liberate O2 • Does not produce NADPH

CFo

hydrophobic, bond as integral portion to the membrane; polypeptide a, b, b’, c; forms a channel for proton

CF1

attached to stromal side of the thylakoid membrane; polypeptide  and  in 3 copies situated in alternate positions, three additional peptides (gamma, delta, epszilon); contains the catalytic sites on beta polypetide.

Chemicals acting as electron acceptors

DCMU can bind to the PQB binding site and acts as electron acceptors in PSII; Paraquat: competes for electrons with the early electron acceptors in PSI

Summary of light reactions

production of O2, ATP and NADPH