Chlorophyll, Light Reactions

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)