Chapter 6 - Chapter 6 - PhotosynthesisPhotosynthesis

6-1: The Light Reactions6-1: The Light Reactions 6-2: The Calvin Cycle6-2: The Calvin Cycle

6-1: The Light Reactions6-1: The Light Reactions

All organisms use All organisms use ENERGYENERGY to carry out to carry out life processeslife processes

The energy comes directly or indirectly The energy comes directly or indirectly from the from the SUNSUN

Obtaining EnergyObtaining Energy Organisms are classified by how they get Organisms are classified by how they get

energyenergy AutotrophsAutotrophs

Organisms that use energy from sunlight or Organisms that use energy from sunlight or from chemical bonds in inorganic from chemical bonds in inorganic substances to make organic substancessubstances to make organic substances

Use the process of Use the process of photosynthesisphotosynthesis Convert light energy (sun) to chemical energy Convert light energy (sun) to chemical energy

(carbohydrates)(carbohydrates) Examples: Trees, algaeExamples: Trees, algae

Obtaining EnergyObtaining Energy HeterotrophsHeterotrophs

Animals or other organisms that must get Animals or other organisms that must get energy from energy from foodfood, not the sun or inorganic , not the sun or inorganic substancessubstances

Examples:Examples: CaterpillarsCaterpillars BirdsBirds HumansHumans

Biochemical PathwayBiochemical Pathway A series of linked A series of linked

reactions where the reactions where the productsproducts of the 1 of the 1stst reaction are the reaction are the reactantsreactants for the next for the next reactionreaction

PhotosynthesisPhotosynthesis is a is a biochemical pathway biochemical pathway with with Cellular Cellular RespirationRespiration

Overview of Overview of PhotosynthesisPhotosynthesis

PhotosynthesisPhotosynthesis produces organic produces organic compounds from compounds from COCO22 + + HH22OO

Oxygen and some organic compounds Oxygen and some organic compounds are used in a process known as are used in a process known as Cellular Cellular RespirationRespiration, which produces , which produces COCO22 + + HH22OO

Products of Products of PhotosynthesisPhotosynthesis are reactants are reactants in in Cellular RespirationCellular Respiration

Products of Products of Cellular Respiration Cellular Respiration are are reactants in reactants in PhotosynthesisPhotosynthesis

Photo./C.R. CyclePhoto./C.R. Cycle

Two Stages of PhotosynthesisTwo Stages of Photosynthesis

LightLight ReactionsReactions Light energy is absorbed and converted + Light energy is absorbed and converted +

stored to chemical energy (stored to chemical energy (ATPATP + + NADPHNADPH))

CalvinCalvin CycleCycle Organic compounds are made using Organic compounds are made using COCO22, ,

ATPATP, + , + NADPHNADPH

Biochemical EquationsBiochemical Equations

Photosynthetic EquationPhotosynthetic Equation 66 COCO22 + + 66 HH22OO + + nrgnrg 11 C C66HH1212OO66 + + 66 O O22

Cellular Respiration EquationCellular Respiration Equation 11 C C66HH1212OO6 6 + + 66 O O22 66 COCO22 + + 66 HH22OO + + nrgnrg

Capturing Light EnergyCapturing Light Energy LightLight ReactionsReactions – 1– 1stst rxns in rxns in

PhotosynthesisPhotosynthesis Begins with the absorption of Begins with the absorption of

lightlight Occurs in the Occurs in the chloroplastschloroplasts

Structure:Structure: Surrounded by double membraneSurrounded by double membrane Thylakoids – flattened sacs (inner Thylakoids – flattened sacs (inner

membrane)membrane) Grana – stacks of thylakoidsGrana – stacks of thylakoids Stroma – fluid surrounding granaStroma – fluid surrounding grana

Light travels from the Sun to Light travels from the Sun to Earth in waves of energyEarth in waves of energy ROYGBIVROYGBIV

When white light hits an When white light hits an object, its component colors object, its component colors can be:can be: ReflectedReflected TransmittedTransmitted AbsorbedAbsorbed

LIGHT & PIGMENTSLIGHT & PIGMENTS

LIGHT & PIGMENTSLIGHT & PIGMENTS If an object contains a If an object contains a pigmentpigment

((compound that absorbs lightcompound that absorbs light), the ), the colors will react differentlycolors will react differently

The pigments will absorb certain colors, The pigments will absorb certain colors, removing it from the visible spectrumremoving it from the visible spectrum

Chloroplast PigmentsChloroplast Pigments CHLOROPHYLLSCHLOROPHYLLS

Most important pigments in PhotosynthesisMost important pigments in Photosynthesis

Chlorophyll aChlorophyll a Absorbs more Absorbs more redred light; less light; less blueblue light light Involved directly in the Involved directly in the Light RxnsLight Rxns

Chlorophyll bChlorophyll b Absorbs more Absorbs more blueblue light; less light; less redred light light An An accessory pigment accessory pigment – helps capture light for – helps capture light for

chlorophyll a chlorophyll a moleculesmolecules

**ChlorophyllChlorophyll does not absorb any does not absorb any greengreen light therefore most plants appear light therefore most plants appear greengreen in in color*color*

ChlorophyllsChlorophylls CarotenoidsCarotenoids

Other pigments found in thylakoid Other pigments found in thylakoid membrane such as yellow, brown, membrane such as yellow, brown, + orange+ orange

Acts as anActs as an accessory pigment accessory pigment to help to help capture light for capture light for chlorophyll achlorophyll a Most abundant in Most abundant in

nonphotosynthetic plant parts:nonphotosynthetic plant parts:

FruitsFruits FlowersFlowers

Most of these colors can be seen in Most of these colors can be seen in leaves when leaves when chlorophyllschlorophylls break break down in the falldown in the fall

Absorption SpectraAbsorption Spectra

Converting Light Energy to Converting Light Energy to ChemicalChemical EnergyEnergy

Once the pigments capture the light energy, it must get Once the pigments capture the light energy, it must get convertedconverted into chemical energy into chemical energy

The energy is temporarily stored in The energy is temporarily stored in ATPATP + + NADPHNADPH; ; OO22 is is

given off given off

(it’s importance will be discussed in Ch. 7)(it’s importance will be discussed in Ch. 7)

Converting Light Energy to Converting Light Energy to ChemicalChemical EnergyEnergy

Photosystems (I+II)Photosystems (I+II) Groups of chlorophyll + carotenoid pigments Groups of chlorophyll + carotenoid pigments

embedded in the thylakoid membraneembedded in the thylakoid membrane Trying to capture as much light as possibleTrying to capture as much light as possible

Light reactions begin when Light reactions begin when accessory accessory pigments pigments absorb light energy from the sunabsorb light energy from the sun

The captured energy must find its way to a The captured energy must find its way to a specificspecific pairpair of of chlorophyll a chlorophyll a molecules for molecules for photosynthesis to beginphotosynthesis to begin

Two Stages of the Light Two Stages of the Light ReactionsReactions

Electron Transport ChainElectron Transport Chain ChemiosmosisChemiosmosis

Electron Transport Chain Electron Transport Chain (5 Steps)(5 Steps)1.1. Light energy forces Light energy forces electronselectrons to enter a higher to enter a higher

energy level in two energy level in two chlorophyll achlorophyll a molecules of molecules of PSIIPSII (“ (“excited electronsexcited electrons”)”)

Energy allows Energy allows electronselectrons to leave to leave chlorophyll achlorophyll a moleculesmolecules

Chlorophyll a Chlorophyll a molecules have been oxidizedmolecules have been oxidized REDOX must occur!REDOX must occur!

2.2. The acceptor of the The acceptor of the electronselectrons from from chlorophyll chlorophyll aa is a molecule in the thylakoid membrane is a molecule in the thylakoid membrane called the called the primary electron acceptorprimary electron acceptor

Electron Transport Chain Electron Transport Chain (5 Steps)(5 Steps)

3.3. The The primary electron acceptor primary electron acceptor then donates then donates the electrons to the first the electrons to the first ETCETC

A series of molecules that as they pass the A series of molecules that as they pass the electronselectrons from one to another, cause the movement of from one to another, cause the movement of protonsprotons ((HH++) into the thylakoid) into the thylakoid

4.4. Light is absorbed by Light is absorbed by PSIPSI causing causing electronselectrons to to become excited. become excited. Chlorophyll a Chlorophyll a loses those loses those electronselectrons to another to another primary electron primary electron acceptoracceptor, which then donates them to a , which then donates them to a second second ETCETC

Electron Transport Chain Electron Transport Chain (5 Steps)(5 Steps)

5.5. As the electrons make it to the end of the As the electrons make it to the end of the second second ETCETC, they are combined with a proton , they are combined with a proton and NADPand NADP++

ee-- + + HH++ + NADP+ NADP++ = = NADPHNADPH

Replacing Electrons in Light Replacing Electrons in Light ReactionsReactions

In order to keep the In order to keep the ETCETC going, going, waterwater is split is split to provide to provide PSII PSII with with electronselectrons

As As waterwater splits, it releases splits, it releases OO22 molecules to the molecules to the

environment environment It also helps to build up a concentration It also helps to build up a concentration

gradient of gradient of protonsprotons ((HH++) inside the thylakoid) inside the thylakoid The The purposepurpose of the of the ETCETC is to make: is to make:

NADPHNADPH

Restoring PSII (cont.)Restoring PSII (cont.)

For every 2 For every 2 waterwater molecules that are molecules that are split, 4 split, 4 ee-- are available for are available for PSIIPSII

ETC REVIEWETC REVIEW

After the ETCs, Chemiosmosis After the ETCs, Chemiosmosis takes placetakes place

ATP is main energy currency of cellsATP is main energy currency of cells

ChemiosmosisChemiosmosis – synthesis of – synthesis of ATPATP Relies on concentration gradient of Relies on concentration gradient of protonsprotons

((HH++) across thylakoid membrane ) across thylakoid membrane Where does Where does HH++ come from?come from?

Splitting of Splitting of waterwater PumpedPumped in from stroma to interior of in from stroma to interior of

thylakoidthylakoid Known as Known as Proton Pump Proton Pump (needs (needs nrgnrg to work) to work)

Proton pumpProton pump

Chemiosmosis cont.Chemiosmosis cont. ATP synthase ATP synthase – –

an enzyme that catalyzes the synthesis of an enzyme that catalyzes the synthesis of ATP from ADP + PATP from ADP + Pii

Converts potential energy (Converts potential energy (protonsprotons) into ) into chemical energy (chemical energy (ATPATP))

The The purposepurpose of Chemiosmosis is to of Chemiosmosis is to make:make:

ATPATP

Let’s look at Let’s look at Chemiosmosis…Chemiosmosis…

So what have we So what have we accomplish thus far?accomplish thus far?

Together, Together, ATPATP + + NADPHNADPH will provide will provide the energy needed for the 2the energy needed for the 2ndnd set of set of reactions for Photosynthesis to reactions for Photosynthesis to occur…..the occur…..the Calvin CycleCalvin Cycle

ATP

NADPH

Photosynthesis VideoPhotosynthesis Video

Video Recap