Unit 5 IB HL Biology Notes - Light Reaction

Light reaction vs. Calvin Cycle: Location

Light reaction: occurs in the thylakoids

Calvin Cycle: occurs in the stroma

Light reaction vs. Calvin Cycle: Description

Light reaction: utilizes photosynthetic pigments to absorb light

Calvin Cycle: uses ATP and NADPH to convert carbon dioxide into glucose

Light reaction vs. Calvin Cycle: Inputs and Outputs

Light reaction:

• light energy splits H2O and produces O2 as a byproduct

• Creates ATP and NADPH to be used by the Calvin Cycle

Calvin Cycle:

• Carbon fixation of CO2 from the atmosphere

• Produces sugars, glucose

Redox Reactions: Reduction vs. oxidation

Reduction: gain of electrons

Oxidation: loss of electrons

Photosystems: Definition

Integral protein complexes located within the phospholipid bilayer

Photosystems: Chloroplasts + Cyanobacteria (location)

Chloroplasts: the thylakoid membrane

Cyanobacteria: the cell membrane

Photosystems: why they can absorb light

Photosystems contain chlorophylls and other accessory pigments that will absorb light energy

Photoactivation: the process

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

• Excites elections in the pigments contained in the photosystems

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

• Excited electronss finally reach the reaction centre- a special “chlorophyll a” molecule

• At the reaction centre, the excited electron will be emitted from the photosystem

Photosystems: reduced or oxidized

The photosystem has become oxidized (lost an electron)

Photosystem 1 (PSI) vs. Photosystem 2 (PSII)

They are the most sensitive to different wavelengths of light

• PSII = 680nm

• PSI = 700nm

Photosystems: which is first?

PSII

The Light Reaction: the particle we’re most concerned with

Electrons

Photosystem II: which photosystem undergoes photoactivation first

PSII is the first photosystem to undergo photoactivation

Photosystem II: where the electron goes after it leaves PSII

It is transferred from the reaction centre to the first Electron Transport Chain (ETC)

Photosystem II: Define photolysis

• Electrons are replaced during the process of photolysis

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

Photosystem II: Equation for photolysis

2 H2O —> 4 H+ + 4 e-

Photosystem II: Where photolysis occurs

Photolysis occurs in the thylakoid space by PSII

Photosystem II: H+ during photolysis

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

Photosystem II: O2 during photolysis

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

Photosystem II: e- during photolysis

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

First ETC: Structure of 1st ETC

• A series of integral protein complexes w/in the thylakoid membrane

• The first ETC receives excited electrons from PSII

First ETC: Two 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 Gradient: high vs. low concentration of protons (location)

High concentration: in the thylakoid space

Low concentration: outside of the thylakoid space

Proton Gradient: 3 ways protons are concentrated inside the thylakoid

1. H+ produced in the thylakoid during photolysis

2. H+ pumped into the thylakoid by the first ETC

3. Thylakoid are small spaces so H+ accumulates quickly

Proton Gradient: what is able to occur once the proton gradient has been established

• Allows for passive transport of protons OUT of the thylakoid (down its concentration gradient)

  →H+ can only exit the thylakoid via a transmembrane integral protein

Photophosphorylation: Define chemiosmosis

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

Photophosphorylation: Function of ATP Synthase

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

  →Transmembrane integral protein that is also an enzyme (-ase)

Photophosphorylation: how chemiosmosis drives ATP synthase

• 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

Photophosphorylation: define photophosphorylation

The previous process is driven by light

Photophosphorylation: what happens to ATP that is made during light reaction

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

Photosystem I: What process occurs in PSI

Photoactivation

Photosystem I: Where excited electrons go after leaving PSI

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

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

Photosystem I: How missing electrons are replaced at PSI

Electrons travelling from PSII via the 1st ETC will replace the missing elections from PSI

NADP+/NADPH: State the function

It is an electron carrier

NADP+/NADPH: Distinguish

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

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

NADP+/NADPH: What happens to the NADPH that is made during the light reaction

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

NADP+/NADPH: How is NADP+ is reduced (+ location)

• Electrons leave PSI and are transferred to NADP+ Reductase*

• This process reduces NADP+ into NADPH

• Occurs in the stroma side of the thylakoid membrane

*an enzyme that combines the electrons w/NADP+ to form NADPH

Cyclic v. Non-Cyclic Photophosphorylation: flow of electrons during non-cyclic photophosphorylation

Electrons flow from:

Water→PSII→1st ETC→PSI→NADPH

Cyclic v. Non-Cyclic Photophosphorylation: What “step” allows for the generation of ATP during non-cyclic photophosphorylation

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

Cyclic v. Non-Cyclic Photophosphorylation: describe the flow of electrons during cyclic photophosphorylation

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

• Electrons travel from: PSII→1st ETC→PSI

Cyclic v. Non-Cyclic Photophosphorylation: What “step” generates ATP during cyclic photophosphorylation

ATP is made like normal b/c the ETC still creates the proton gradient

Cyclic v. Non-Cyclic Photophosphorylation: What makes cyclic photophosphorylation “cyclic”

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

Summarize the major highlights of the light reaction

• 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