Light Cycle Reactions

1
Light Cycle Reactions

• Takes place in the Thylakoids of chloroplasts in
  eukaryotes
• Captures solar energy and converts it to Energy
  storage molecules ATP and NADPH along with O2
• NADPnicotinamide adenine dinucleotide phosphate
• NADP H 2e- ---gtNADPH
• Method of storing high energy electrons

2
Chlorophyll

• Contained in Thylakoids
• Similar to Heme in structure
• Two main types chlorophyll a and chlorophyll b
• Structurally similar, but slight differences
• Absorbs light at red and blue wavelengths
• Reflects light at green wavelenghts

3
Spectral absorption of light

• Absorbed light is used as energy
• Chlorophyll a and b have slight differences in
  absorption
• Carotenoids can absorb light energy
• Red, yellow and orange pigments
• Sometimes function in photosynthesis but are
  usually for sunscreen
• In fall, chlorophyll production drops and
  carotnoids remain

4
How solar power works

• Excites electrons to a higher energy state
• Electrons remain at excited state for very short
  periods of time
• When electron returns to ground state a photon of
  light is given off
• This photon may excite an electron on another
  chlorophyll molecule
• Process continues

5
Photosystems

• Act as dish antennae
• Many photon collectors
• Only one reaction center
• Reaction center is adjacent to Primary electron
  acceptor
• Electron acceptor captures excited electron from
  reaction center before it is able to return to
  ground state

6
Photsystems I and II

• Photosystem I
• Discovered first
• Acts second
• Has P700 chlorophyll at reaction center
• Yields NADPH

• Photosystem II
• Discovered second
• Acts first in Light Cycle
• Has P680 chlorophyll at reaction center
• Yields ATP O2

7
Noncyclic electron flow
8
Noncyclic electron flow 2

• Photosystem II collects photon and activated
  electron is harvested by Primary e acceptor
• Reaction center chloropyll requires electron
• Enzyme degrades H2O to obtain 2e-, 2H and O
• Electron is transferred to reaction center
• Electron is transferred through electron
  transport chain
• Energy levels decrease while released energy is
  used to make ATP
• Plastoquinone, Cytochrome complex and plastocyanin

9
Noncyclic electron flow 3

• Newly grounded electron is used to fill void in
  P700 following electron loss
• Electron is excited and captured by Primary
  acceptor
• Transferred through another electron transport
  chain
• Ferredoxin, then NADP reductase
• NADP reductase takes 2H produced earlier plus 2
  high energy electrons NADP and produces NADPH
  H
• NADPH stores activated electrons for later use

10
Noncyclic electron flow summary
11
Cyclic electron flow

• Ferredoxin transfers electron back to Cytochrome
  complex
• Electron is recycled and ATP is produced instead
  of NADPH
• Results in more ATP than NADPH (required in
  Calvin)