Light Reaction

2 Halves of Photosynthesis: Light Reaction & Calvin Cycle

Light Reaction:

  • occurs in the thylakoid

  • utilizes photosynthetic pigments to absorb light

  • light energy splits H2O & produces O2 as a byproduct

    • point of light reaction is NOT to make O2

  • creates ATP & NADPH (high energy compounds) to be used by the Calvin Cycle

Calvin Cycle:

  • occurs in stroma

  • uses ATP & NADPH from the light reaction

  • major first step: Carbon fixation of CO2 from the atmosphere

    • extracted from the atmosphere and attached to other carbon compounds

  • produces sugars: glucose

Key Concept: how are the two connected?

  • thylakoids and light reaction are within the stroma —> stroma is where CC occurs

  • Light Reaction sends ATP & NADPH to CC

  • Calvin Cycle sends ADP, a phosphate, & NADP+ back to the Light Reaction

  • Water goes into light reaction and O2 comes out

  • CO2 goes into the Calvin Cycle & sugars come out (glucose)

Why is it incorrect to say that photosynthesis converts CO2 into O2?

  • Oxygen gas comes from the water molecules

  • 6CO2 + 12 H2O —> C6H12O6 + 6H2O + 6O2

Photosynthesis is a series of REDOX Reactions (transfer of electrons):

  • Reduction: gaining electrons & the charge is “reduced” bc it becomes more negative

  • Oxidation: losing electrons & the charge increases bc it becomes more positive

  • OIL RIG

  • LEO says GER

  • if one thing is reduced the other is oxidized

The Light Reaction:

Photosystems: integral protein complexes (multiple proteins) located w/in the phospholipid bilayer

  • must be amphipathic

  • In chloroplasts —> in the thylakoid membrane

  • In cyanobacteria —> the cell membrane

  • Photosystems are embedded w/ chlorophyll & other accessory pigments that will absorb light energy

Photoactivation in Photosystems:

  • Photons of light strike the pigment molecule w/in the photosystem

  • that light excites electrons in the pigments contained in the photosystem

  • excited electrons are transferred between the array of pigments w/in the photosystem

    • they jump around like a hyper child

  • excited electrons eventually end up in the reaction center —> a special chlorophyll a molecule

  • At the reaction center, the excited electrons will be emitted from the photosystem w/ the extra energy

    • like a trampoline, they are bounced off and ejected

  • The photosystem has become oxidized (lost an electron)

Photosystem II v. Photosystem I

  • the difference is that they are most sensitive to different wavelengths of light

    • PS II = 680nm

    • PS I = 700nm

  • maximizes the absorption & sensitivity of light to different wavelengths —> increases the efficiency of photosynthesis overall

    • “diversifying your assets”

  • PS II COMES FIRST in the light reaction

  • The light reaction is all about the flow of electrons

Photosystem II:

  • PSII is the first photosystem to undergo photoactivation

  • after the electron is emitted from PSII, it is transferred from the reaction center to the Electron Transport Chain (ETC)

  • PSII is now missing an electron (oxidized) —> this is very unstable

  • electrons are replaced during the process of Photolysis

Photolysis in PSII:

  • process of using light energy to break apart water molecules in order to replace missing electrons in PSII

  • Equation: 2H2O —> 4H+ + O2 + 4e-

  • Photolysis occurs in the thylakoid space by PSII

  • H+ (protons) remain in the thylakoid space, beginning to build a concentration gradient

  • O2 diffuses out of the chloroplast → cell → leaf → atmosphere

  • e- (electrons) from H2O are transferred to PSII

First Electron Transport Chain (ETC):

  • A series of integral protein complexes within the thylakoid membrane

  • The first ETC receives excited electrons from PSII

2 functions of the first ETC:

  1. Transfer electrons from PSII to PSI

  2. Harness the extra energy from excited electrons & use it to pump H+ (protons) INTO the thylakoid space

    -THIS ESTABLISHES A PROTON CONCENTRATION GRADIENT: HIGH [H+] IN THYLAKOID

Proton Concentration Gradient:

High concentration of H+ in the thylakoid space because of 3 reasons:

  • H+ produced in the thylakoid during photolysis

  • H+ pumped into the thylakoid by the first ETC

  • Thylakoids are small spaces so H+ accumulates quickly

Chemiosmosis:

  • The proton concentration gradient allows for passive transport of protons OUT of the thylakoid (down its concentration gradient)

  • Can the protons (H+) pass through the membrane unassisted via simple diffusion?

    • NOOOOOOO

  • Because of their charge, protons (H+) can only exit the thylakoid via a transmembrane integral protein

  • Chemiosmosis is the diffusion of H+ down its concentration gradient through ATP Synthase

ATP Synthase: transmembrane integral protein that is also an enzyme (-ase)

  • ATP Synthase performs ADP phosphorylation to create (synthesize) ATP

  • This process requires energy —> chemiosmosis drives ATP Synthesis

  • As the H+ diffuses through ATP synthase, it causes the enzyme to turn - much like a water wheel creating power

  • This provides the energy needed to phosphorylate ADP into ATP

  • This process is ultimately driven by light: “photophosphorylation”

Key Concept: The ATP made during photophosphorylation will go to power the Calvin Cycle

Photoactivation in PSI:

  • Photoactivation occurs in PSI

  • Excited electrons are:

    • Transferred between the pigments & end up in the reaction centre

    • Emitted from the reaction centre and are transferred to an enzyme called NADP+ reductase

  • After the excited electrons are emitted from PSI, they need to be replaced

  • Electrons traveling from PSII via the 1st ETC will replace the missing electrons from PSI

NADPH/NADP+

  • NADP+ (Nicotinamide adenine dinucleotide phosphate) is an electron carrier

  • NADP+ is the oxidized form (“empty” of electrons)

  • When NADP+ picks up 2 electrons it becomes reduced (NADPH, “full” of electrons)

  • Electrons leave PSI and are transferred to NADP+ Reductase

  • NADP+ Reductase is an enzyme that combines the electrons with NADP+ to form NADPH

  • This process reduces NADP+ into NADPH

  • Occurs on the stroma side of the thylakoid membrane

Key Concept: The NADPH produced (“filled”) during the light reaction will go to the Calvin Cycle to drop off the electrons

Non-cyclic Photophosphorylation:

  • The process of photophosphorylation that was described earlier is non-cyclic photophosphorylation

  • Electrons flow from:

    • Water → PSII → 1st ETC → PSI → NADPH

  • ATP is generated as a result of the 1st ETC’s function

Cyclic Photophosphorylation:

  • The thylakoid membranes contain thousands of PSIIs, PSIs, ETCs, and ATP Synthases

  • Sometimes electrons that are emitted from PSI are transferred back to the 1st ETC (instead of NADP+ reductase)

  • The electrons travel from: PSII → 1st ETC → PSI

  • When this happens ATP is made like normal because the ETC still creates the proton gradient

  • What makes this “cyclic” is the pathway that the electrons take: electrons are lost from and return to the same photosystem

Summary:

  • Pigments harness light energy to excite electrons that will eventually reduce NADP+ into NADPH

  • ATP is synthesized using photophosphorylation

  • O2 is produced as a byproduct of the photolysis of H2O

  • ATP and NADPH will go to the Calvin Cycle