Photosynthesis: The Light Reactions & The Calvin Cycle

Photosynthesis:Photosynthesis:

The Light Reactions & The Light Reactions & The Calvin CycleThe Calvin Cycle

Photosynthesis: OverviewPhotosynthesis: Overview

4.3 The Light Reactions4.3 The Light Reactions

General Summary InfoGeneral Summary Info Conversion of visible light into chemical Conversion of visible light into chemical

energy needed to produce sugars in the Calvin energy needed to produce sugars in the Calvin cyclecycle

Chlorophyll in the thylakoid absorb the light, Chlorophyll in the thylakoid absorb the light, water is split into Hydrogen and Oxygen, & water is split into Hydrogen and Oxygen, & light energy is converted into chemical energy.light energy is converted into chemical energy.

4.3 The Light Reactions4.3 The Light Reactions Light absorbing pigments form 1 of 2 kinds of Light absorbing pigments form 1 of 2 kinds of

clusters: Photosystem (PS) I and II.clusters: Photosystem (PS) I and II. The PS absorbs light energy & this energy gets The PS absorbs light energy & this energy gets

transferred to different molecules until it ends transferred to different molecules until it ends up at a reaction centerup at a reaction center

Reaction center – a specific chlorophyll Reaction center – a specific chlorophyll aa moleculemolecule

The reaction center accumulates so much The reaction center accumulates so much energy that some of its eenergy that some of its e-- jump to electron jump to electron carrierscarriers


4.3 The Light Reactions4.3 The Light Reactions These electron carriers form an electron These electron carriers form an electron

transport system between the 2 photosystems.transport system between the 2 photosystems. Electrons from PS II replace electrons lost from Electrons from PS II replace electrons lost from

PS I.PS I. PS II receives replacements from an enzyme PS II receives replacements from an enzyme

near its reaction center that splits water into near its reaction center that splits water into protons, electrons, & oxygen.protons, electrons, & oxygen.

2 H2 H22O O 4 H 4 H++ + 4 e + 4 e-- + O + O22

4.3 The Light Reactions4.3 The Light Reactions When the enzyme oxidizes the water, oxygen When the enzyme oxidizes the water, oxygen

is released as a gas & protons build up in the is released as a gas & protons build up in the thylakoid.thylakoid.

The eThe e-- replace the lost e replace the lost e-- in PS II. in PS II.

4.3 The Light Reactions4.3 The Light Reactions When eWhen e-- from H from H22O reach PS I, they receive an O reach PS I, they receive an

energy boostenergy boost from the reaction center & this from the reaction center & this energy is used to reduce NADPenergy is used to reduce NADP++ into NADPH. into NADPH.

BothBoth the protons and electrons from water are the protons and electrons from water are used to convert NADPused to convert NADP++

The protons & electrons of NADPH is then The protons & electrons of NADPH is then used later to reduce COused later to reduce CO22 in the Calvin Cycle. in the Calvin Cycle.


4.3 The Light Reactions4.3 The Light Reactions As electrons are flowing, some of the solar As electrons are flowing, some of the solar

energy powers the active transport of H+ energy powers the active transport of H+ across the thylakoid membrane.across the thylakoid membrane.

So a large number of H+ build up inside the So a large number of H+ build up inside the thylakoid causing a difference in charge, thylakoid causing a difference in charge, which creates a difference in potential energy which creates a difference in potential energy (think of a battery).(think of a battery).

Just like a battery, this potential energy can do Just like a battery, this potential energy can do work.work.

4.3 The Light Reactions4.3 The Light Reactions The protons diffuse out of the thylakoid The protons diffuse out of the thylakoid

through an enzyme complex (ATP synthetase) through an enzyme complex (ATP synthetase) & as they pass through, they transfer energy to & as they pass through, they transfer energy to the ATP synthetase.the ATP synthetase.

ATP synthetase uses the energy to synthesize ATP synthetase uses the energy to synthesize ATP from ADP & a phosphate.ATP from ADP & a phosphate.


A SummaryA Summary Energy from light forces electrons to flow Energy from light forces electrons to flow

from water to NADP+.from water to NADP+. The electrons retain the energy in NADPH, The electrons retain the energy in NADPH,

which is then used to synthesize ATP.which is then used to synthesize ATP. So the light reactions, convert light energy into So the light reactions, convert light energy into

the chemical energy found in ATP and the chemical energy found in ATP and NADPH, with the overall products being ONADPH, with the overall products being O22, , ATP, & NADPH.ATP, & NADPH.

Light Reactions AnimationLight Reactions Animation

http://www.stolaf.edu/people/giannini/http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swfflashanimat/metabolism/photosynthesis.swf

4.4 The Calvin Cycle4.4 The Calvin Cycle

General Summary IntroGeneral Summary Intro

Saves the chemical energy produced in the Saves the chemical energy produced in the light reactions in the form of sugarslight reactions in the form of sugars

The Calvin Cycle occurs in the stroma of the The Calvin Cycle occurs in the stroma of the chloroplasts.chloroplasts.

4.4 The Calvin Cycle4.4 The Calvin Cycle1. CO2 combines with a 5 carbon phosphate-1. CO2 combines with a 5 carbon phosphate-

sugar, called ribulose biphosphate (RuBP). sugar, called ribulose biphosphate (RuBP). This is called This is called carbon fixation carbon fixation because carbon because carbon dioxide gas is “fixed” into an organic molcule.dioxide gas is “fixed” into an organic molcule.

This produces an unstable 6 carbon molecule, This produces an unstable 6 carbon molecule, which instantly breaks down into two 3 carbon which instantly breaks down into two 3 carbon molecules called phosphoglyceric acid (PGA).molecules called phosphoglyceric acid (PGA).

Catalyzed by Rubisco


2. Each molecule of PGA is reduced to the 3 2. Each molecule of PGA is reduced to the 3 carbon sugar-phosphate molecule, carbon sugar-phosphate molecule, phoshpoglyceraldehyde (PGAL).phoshpoglyceraldehyde (PGAL).

This requires 1 ATP and 1 NADPH (from the This requires 1 ATP and 1 NADPH (from the light reactions).light reactions).


3. A series of enzymes catalyzes the 3. A series of enzymes catalyzes the combination and rearrangement of the PGAL, combination and rearrangement of the PGAL, producing a 5 carbon sugar-phosphate, producing a 5 carbon sugar-phosphate, ribulose 5-phosphate.ribulose 5-phosphate.


4. An ATP molecule is used to add a 24. An ATP molecule is used to add a 2ndnd phosphate group to the Ribulose 5-phosphate, phosphate group to the Ribulose 5-phosphate, producing a molecule of the starting product, producing a molecule of the starting product, RuBP (completing the cycle).RuBP (completing the cycle).

4.4 The Calvin Cycle4.4 The Calvin Cycle Three turns of the cycle (which uses 3 COThree turns of the cycle (which uses 3 CO22

molecules) results in 6 PGAL molecules.molecules) results in 6 PGAL molecules. 5 of those PGAL are used to regenerate RuBP 5 of those PGAL are used to regenerate RuBP

& the last one is available to the organism to & the last one is available to the organism to use for maintenance & growth.use for maintenance & growth.

The PGAL is removed from the Calvin Cycle The PGAL is removed from the Calvin Cycle to synthesize other compounds like complex to synthesize other compounds like complex carbohydrates or amino acids.carbohydrates or amino acids.

Animation of The Calvin CycleAnimation of The Calvin Cycle

http://www.sinauer.com/cooper/4e/http://www.sinauer.com/cooper/4e/animations0305.htmlanimations0305.html

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Photosynthesis: The Light Reactions & The Calvin Cycle