Light Reaction & Calvin Cycle

Objectives: •How do pigments like chlorophyll work to capture light energy?•What happens to water in the light reaction?•What are photosystems? What is their role in photosynthesis?•How are ATP and NADPH generated in the light reaction?•What are the reactants and products of the light reaction and the calvin cycle?•Where does each reaction take place?

PhotosynthesisPhotosynthesisHH22OO COCO22

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Light ReactionLight Reaction ““Dark” ReactionDark” Reaction

Light is AdsorbedLight is AdsorbedBy By

ChlorophyllChlorophyll

Which splitsWhich splitswaterwater

ChloroplastChloroplast

ATP andATP andNADPHNADPH22

ADPADPNADPNADP

Calvin CycleCalvin Cycle

EnergyEnergy

Used Energy and is Used Energy and is recycled.recycled.

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ATP = chemical energy

NADPH – carries Hydrogen(NADP + H)

Visible Light and the Electromagnetic Spectrum

• Electromagnetic Energy = Energy that travels as a WAVE.• Electromagnetic Spectrum= menu of energy that travels as a wave• The shorter the wavelength, the greater the energy….and vice-

versa.• VISIBLE LIGHT = Wavelength absorbed by plants to make food.

Light Used in Photosynthesis• Chlorophyll pigments can absorb only some wavelengths of the

visible light from the electromagnetic spectrum.• Visible light consists of the following colors or wavelengths in order of

increasing wavelengths / decreasing energy:– Violet Blue Green Yellow Orange Red.

• Chloroplasts absorb mainly blue-violet and red-orange lights.• Green light is reflected and transmitted by green plants – hence, they

appear green.

Chlorophyll absorbs blue-violets and red-orange for photosynthesis.

Green light is reflected; That’s why plants appear green.

Redox Rxns (Reducing / Oxidation Reactions)

Being “reduced” = accepting electrons (usually accompanied by H ions); electrons are negative; accepting them makes recipient “more” negative (hence reduced)

Example: NADP+ is reduced when it accepts electrons.NADP+ + 2e + H+ NADPH

Being Oxidized: Lose electrons (usually when reacting with oxygen); become more +Example: NADH is oxidized when it donates electrons.NADH NAD+ + 2 e +H+

Oxidation – Reduction Reactions (Redox) occur together.

6 CO2 + 6 H2O C6H12O6 + 6 O2

• CO2 is reduced as it accepts electrons and hydrogen ions.

• H2O is oxidized. It donates electrons and loses hydrogen ions.

Reduction

Oxidiation

Light ReactionWhere: Thylakoid membrane of chloroplast.What: Light energy is absorbed by chlorophyll.Energy is used to SPLIT H2O molecules into:

1. H+ ions and electrons (carried by NADPH)2. O atoms (released as O2 gas)3. electrons (go through electron

transport chain to make ATP and NADPH)

Light Reaction: 1. Making ATP & NADPH (used in Calvin cycle to make sugar)

2. Release O2 (from H2O)

Photosystems: Cluster of hundreds of chlorophyll molecules in thylakoid membrane.

Where Light Reaction starts.

Light-Dependent ReactionsLight-Dependent Reactions

Photon of light absorbed by pigments in photosystem II (P680); excited electrons pass to primary electron acceptor; Electrons of PS680 replaced by electrons from water (H2O H+,1/2 O2, and electrons)Photo excited electrons from P680 passed thru. Series of ETC proteins (exergonic; releases energy); energy fr. “falling” electrons used to PUMP H+ ions into thylakoid space (against gradient); Build up of H+ ions used to make ATP in ATP synthase; photo-excited electrons from photosystem II (P680) replaces excited electrons at photosystem I (P700); Excited electrons from photosystem I are used to reduce NADP to NADPH. NADPH is a high energy molecule that will bring electrons and hydrogens to CO2 to make sugar.

Chemiosmosis: Use H+ gradient across membrane to phosphorylate ADP ATP

High [H+] in thylakoid space.Active transport/ Used energy released from “excited” electrons “falling” to ground state in ETC to pump H+ ions against gradient.

H+ ions move down gradient to stroma through ATP synthase (transport membrane coupled w/ phosphorylation of ADP

Light-Dependent Reaction SummaryLight-Dependent Reaction Summary1. Light is absorbed by chlorophyll in PS II (P680)

2. An electron is excited and captured by the primary electron acceptor

3. Water is split to supply electrons to PSII

4. The leftover oxygen atoms pair up to form a molecule of O2

5. The electrons released from PSII are passed to PS I by an electron transport chain

6. This “fall” of electrons down the chain provides energy to pump H+ ions across the membrane which will then power the production of ATP via chemiosmosis. (photophosphorylation)

7. Meanwhile, light excites an electron of chlorophyll at PS I/ The primary electron acceptor captures this and an electron from the bottom of the ETC replaces the lost electron.

8. The excited electron of PS I is passed to NADP+, reducing it to NADPH

8.3: Calvin Cycle

Where: Stroma of chloroplast.What: Use ATP and NADPH from light rxns to MAKE GLUCOSE from CO2.Inputs: 1. CO2 (Stomate); 2. ATP fr. Light Rxn; 3. NADPH fr. Light Rxn

Outputs1.Glucose is made (used for energy; stored as starch; used to make cellulose)2.ADP + P (fr. ATP) back to light rxn3.NADP+ (fr. NADPH) back to light rxn

Calvin Cycle: Carbon fixation = making organic carbon molecules from inorganic carbon.

Light Reaction

• In Thylakoid Membrane

• Use light to split water into H+, O, and electrons.

• Make ATP and NADPH to be used in Calvin Cycle.

Calvin Cycle

• Stroma

• Use ATP and NADPH to make GLUCOSE from CO2

• Light Independent.