chlorophyll - absorbs photons in a way similar to photoelectric effect
- porphyrin ring - ring structure w/ alternating single/double bonds w/ Mg atom in middle
- energy channeled through carbon-bond system
- side groups on outside of ring change absorption characteristics
- action spectrum - relative effectiveness of different light wavelengths on photosynthesis
- T. W. Englemann - found that chlorophyll work best under red/violet light
- photoefficiency - high absorption efficiency leads to ability to absorb only a narrow bands of light
- retinal absorbs large range of wavelengths but at low efficiency
carotenoids - made of carbon rings linked to chains w/ alternating single/double bonds
- responsible for change in leaf color in fall
- not very efficient in transferring energy, but absorbs a wide range of energies
- beta-carotene - typical carotenoid; 2 carbon ring connected by 18-carbon chain
- halves same as vitamin A
- oxidation of vitamin A >> creates retinal, pigment used for vertebrate vision
light-reactions - 4 stages
- primary photoevent - light photon captured by pigment, exciting the electrons in the pigment
- charge separation - energy transferred to reaction center (special chlorophyll pigment)
- transfers energetic electron to acceptor molecule, starts electron transport
- electron transport - electrons go through multiple electron carriers in the membrane
- pumps induce mov’t of proton across the membrane
- electron passed to an acceptor in the end
- chemiosmosis - protons flow down gradient to power ATP synthase
photosystems - light absorbed by clusters of pigments, not single pigments
- discovered after saturation was reached much faster than expected in experiments
- contains network of chlorophyll a molecules, accessory pigments, proteins held in protein matrix on photosynthetic membrane
- antenna complex - captures photons from sunlight
- web of chlorophyll held together by protein matrix
- protein matrix holds the chlorophyll in the most efficient shape for absorbing energy
- energy moves towards reaction center (electrons don’t move)
- reaction center chlorophyll - transmembrane protein-pigment complex
- passes energy out of the photosystem so it can be used elsewhere
- transfers energized electron to primary electron acceptor (quinone)
- water serves as weak electron donor in plants
bacteria photosystem - 2-stage process w/ just 1 photosystem
- excited electron combines w/ proton to form hydrogen atom
- H2S becomes sulfur and protons
- H2O becomes oxygen and protons
- electron recycled back to chlorophyll through an electron transport system
- 1 ATP produced per 3 electrons that move through the path
- cyclic photophosphorylation - name for electron transfer process
- only produces energy, no biosynthesis
- doesn’t have good source of reducing power
plant photosystem - plants use 2 photosystems
- additional photosystem using different chlorophyll a arrangement added on to bacteria photosystem
- enhancement effect - where use of 2 different light beams leads to faster rate of photosynthesis
- due to fact that photosystems have different optimum wavelengths
- electron moves from H2O to NADPH
- noncyclic photophosphorylation - name for 2-stage process
- electrons not recycled
- 1 NADPH, more than 1 ATP created w/ every 2 electrons from H2O
photosystem II - absorbs shorter wavelength, higher energy photons
- absorption peak = 680 nanometers
- reaction center called P680
- H2O binds to manganese atoms on enzyme bound to reaction center
- enzyme splits H2O
- O2 leaves after 4 electrons removed
- quinone - main electron acceptor for energized electrons leaving photosystem II
- becomes plastoquinone, strong electron donor after being reduced
- b6-f complex - proton pump in the thylakoid membrane; pumps a proton into the thylakoid when energetic electron arrives
- plastocyanin (pC) - copper-containing protein that carries electron to photosystem I
- ATP produced by ATP synthases like w/ aerobic respiration
photosystem I - older, ancestral photosystem
- absorption peak = 700 nanometers
- reaction center called P700
- receives electrons from plastocyanin
- incoming electrons have only lost 1/2 of energy, boosted to a very high energy level once photons strike the chlorophyll
- ferredoxin (Fd) - iron-sulfur protein; acts as main electron acceptor for photosystem I
- NADP reductase - uses 2 electrons from ferredoxin proteins to make NADPH from NADP+
- uses up a proton outside the thylakoid in stroma, contributing to proton gradient
- electrons might get passed back to b6-f complex instead of being used for NADPH (in cyclic photophosphorylation)
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