PhotosynthesisPhotosynthesis

How Plants Make Food from How Plants Make Food from Sunlight and Low Energy Sunlight and Low Energy

MoleculesMolecules

PhotoautotrophsPhotoautotrophs

PhotoautotrophsPhotoautotrophs

Carbon source is carbon dioxideCarbon source is carbon dioxide

Energy source is sunlightEnergy source is sunlight

HeterotrophsHeterotrophs

Get carbon and energy by eating Get carbon and energy by eating

autotrophs or one anotherautotrophs or one another

Carbon and Energy Carbon and Energy SourcesSources

Photoautotrophs Photoautotrophs

Capture sunlight energy and use it to Capture sunlight energy and use it to carry out photosynthesiscarry out photosynthesis

PlantsPlants

Some bacteriaSome bacteria

Many protistansMany protistans

T.E. Englemann’s T.E. Englemann’s Experiment Experiment

Background Background Certain bacterial cells will move Certain bacterial cells will move

toward places where oxygen toward places where oxygen concentration is highconcentration is high

Photosynthesis produces oxygenPhotosynthesis produces oxygen

T.E. Englemann’s T.E. Englemann’s ExperimentExperiment

HypothesisHypothesis Movement of bacteria can be used to Movement of bacteria can be used to

determine optimal light wavelengths determine optimal light wavelengths for photosynthesis for photosynthesis

T.E. Englemann’s T.E. Englemann’s ExperimentExperiment

MethodMethod Algal strand placed on microscope Algal strand placed on microscope

slide and illuminated by light of slide and illuminated by light of varying wavelengthsvarying wavelengths

Oxygen-requiring bacteria placed on Oxygen-requiring bacteria placed on same slidesame slide

T.E. Englemann’s T.E. Englemann’s ExperimentExperiment

T.E. Englemann’s T.E. Englemann’s ExperimentExperiment

Results Results Bacteria congregated where red and Bacteria congregated where red and violet wavelengths illuminated algaviolet wavelengths illuminated alga

ConclusionConclusionBacteria moved to where algal cells Bacteria moved to where algal cells released more oxygen--areas released more oxygen--areas illuminated by the most effective illuminated by the most effective light for photosynthesislight for photosynthesis

Linked ProcessesLinked Processes

PhotosynthesisPhotosynthesis

Energy-storing Energy-storing pathway pathway

Releases oxygenReleases oxygen

Requires carbon Requires carbon dioxidedioxide

Aerobic Aerobic RespirationRespiration

Energy-releasing Energy-releasing pathwaypathway

Requires oxygenRequires oxygen

Releases carbon Releases carbon dioxidedioxide

Focusing in on the location of photosynthesis in a plantFocusing in on the location of photosynthesis in a plant

Location and structure of chlorophyll molecules in plantsLocation and structure of chlorophyll molecules in plants

Photosynthesis EquationPhotosynthesis Equation

12H2O + 6CO2 6O2 + C6H12O6 + 6H2Owater carbon

dioxideoxygen glucose water

LIGHT ENERGY

Two Stages of Two Stages of PhotosynthesisPhotosynthesis

sunlight water uptake carbon dioxide uptake

ATP

ADP + Pi

NADPH

NADP+

glucoseP

oxygen release

LIGHT INDEPENDENT-

REACTIONS

LIGHT DEPENDENT-REACTIONS

new water

Continual input of solar energy into Continual input of solar energy into

Earth’s atmosphere Earth’s atmosphere

Almost 1/3 is reflected back into Almost 1/3 is reflected back into

spacespace

Of the energy that reaches Earth’s Of the energy that reaches Earth’s

surface, about 1% is intercepted by surface, about 1% is intercepted by

photoautotrophsphotoautotrophs

Sunlight EnergySunlight Energy

Electromagnetic Electromagnetic Spectrum Spectrum

Shortest Shortest Gamma raysGamma rays

wavelength wavelength X-raysX-rays

UV radiationUV radiation

Visible lightVisible light

Infrared radiationInfrared radiation

MicrowavesMicrowaves

LongestLongest Radio wavesRadio waves

wavelengthwavelength

Visible Light Visible Light

Wavelengths humans perceive as Wavelengths humans perceive as different colorsdifferent colors

Violet (380 nm) to red (750 nm) Violet (380 nm) to red (750 nm) Longer wavelengths, lower energyLonger wavelengths, lower energy

PhotonsPhotons

Packets of light energyPackets of light energy

Each type of photon has fixed Each type of photon has fixed amount of energyamount of energy

Photons having most energy travel Photons having most energy travel as shortest wavelength (blue-green as shortest wavelength (blue-green light)light)

PigmentsPigments

Light-absorbing Light-absorbing moleculesmolecules

Absorb some Absorb some wavelengths and wavelengths and transmit otherstransmit others

Color you see are Color you see are the wavelengths the wavelengths NOT absorbed NOT absorbed

Wavelength (nanometers)

chlorophyll b

chlorophyll a

Pigments in Pigments in PhotosynthesisPhotosynthesis

BacteriaBacteria Pigments in plasma membranesPigments in plasma membranes

PlantsPlants Pigments embedded in thylakoid Pigments embedded in thylakoid

membrane systemmembrane system Pigments and proteins organized into Pigments and proteins organized into

photosystemsphotosystems Photosystems located next to electron Photosystems located next to electron

transport systemstransport systems

Pigments in a Pigments in a PhotosystemPhotosystem

reaction center (a specialized chlorophyll a molecule)

Pigments absorb light energy, give Pigments absorb light energy, give up eup e-- which enter electron transport which enter electron transport systemssystems

Water molecules are split, ATP and Water molecules are split, ATP and NADH are formed, and oxygen is NADH are formed, and oxygen is releasedreleased

Pigments that gave up electrons get Pigments that gave up electrons get replacementsreplacements

Light-Dependent Light-Dependent ReactionsReactions

Photosystem Function: Photosystem Function: Harvester Pigments Harvester Pigments

Most pigments in photosystem are Most pigments in photosystem are harvester pigmentsharvester pigments

When excited by light energy, these When excited by light energy, these pigments transfer energy to adjacent pigments transfer energy to adjacent pigment moleculespigment molecules

Each transfer involves energy loss Each transfer involves energy loss

Photosystem Function: Photosystem Function: Reaction Center Reaction Center

Energy is reduced to level that can be Energy is reduced to level that can be captured by molecule of chlorophyll captured by molecule of chlorophyll aa

This molecule (P700 or P680) is the This molecule (P700 or P680) is the reaction center of a photosystemreaction center of a photosystem

Reaction center accepts energy and Reaction center accepts energy and donates electron to acceptor molecule donates electron to acceptor molecule

Cyclic Electron FlowCyclic Electron Flow

electron acceptor electron transport system

e–

e–

e–

e–

ATP

Electron Transport Electron Transport SystemSystem

Adjacent to photosystem Adjacent to photosystem Acceptor molecule donates electrons Acceptor molecule donates electrons

from reaction centerfrom reaction center

As electrons flow through system, As electrons flow through system, energy they release is used to energy they release is used to produce ATP and, in some cases, produce ATP and, in some cases, NADPHNADPH

Cyclic Electron FlowCyclic Electron Flow

Electrons Electrons are donated by P700 in photosystem I to are donated by P700 in photosystem I to

acceptor moleculeacceptor molecule flow through electron transport system flow through electron transport system

and back to P700and back to P700

Electron flow drives ATP formationElectron flow drives ATP formation No NADPH is formedNo NADPH is formed

Energy ChangesEnergy ChangesP

ote

nti

al

to t

ran

sfer

en

erg

y (v

oid

s)

H2O 1/2 O2 + 2H+

(PHOTOSYSTEM II)

(PHOTOSYSTEM I)

e–

e–

e–e–

secondtransport

system

NADPHfirst

transport

system

Noncyclic Electron FlowNoncyclic Electron Flow

Two-step pathway for light Two-step pathway for light

absorption and electron excitationabsorption and electron excitation

Uses two photosystems: type I and Uses two photosystems: type I and

type II type II

Produces ATP and NADPHProduces ATP and NADPH

Involves photolysis - splitting of Involves photolysis - splitting of

waterwater

Figure 10.4 An overview of photosynthesis: cooperation of the light reactions Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 1)and the Calvin cycle (Layer 1)

Figure 10.4 An overview of photosynthesis: cooperation of the light reactions Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 2)and the Calvin cycle (Layer 2)

Synthesis part of Synthesis part of

photosynthesisphotosynthesis

Can proceed in the darkCan proceed in the dark

Take place in the stromaTake place in the stroma

Calvin-Benson cycleCalvin-Benson cycle

Light-Independent Light-Independent ReactionsReactions

Calvin-Benson Cycle Calvin-Benson Cycle

Overall reactantsOverall reactants Carbon dioxideCarbon dioxide

ATPATP

NADPHNADPH

Overall productsOverall products GlucoseGlucose

ADPADP

NADPNADP++

Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated

Melvin CalvinMelvin Calvin

The Calvin cycle (Layer 1)The Calvin cycle (Layer 1)

The Calvin cycle (Layer 2)The Calvin cycle (Layer 2)

The Calvin cycle (Layer 3)The Calvin cycle (Layer 3)

Using the Products of Using the Products of PhotosynthesisPhotosynthesis

Phosphorylated glucose is the Phosphorylated glucose is the building block for:building block for:

sucrosesucrose The most easily transported plant The most easily transported plant

carbohydratecarbohydrate starchstarch

The most common storage formThe most common storage form

In Calvin-Benson cycle, the first In Calvin-Benson cycle, the first stable intermediate is a three-carbon stable intermediate is a three-carbon PGAPGA

Because the first intermediate has Because the first intermediate has three carbons, the pathway is called three carbons, the pathway is called the C3 pathwaythe C3 pathway

The C3 PathwayThe C3 Pathway

Photorespiration in C3 Photorespiration in C3 PlantsPlants

On hot, dry days stomata closeOn hot, dry days stomata close Inside leaf Inside leaf

Oxygen levels riseOxygen levels rise Carbon dioxide levels dropCarbon dioxide levels drop

Rubisco attaches RuBP to oxygen Rubisco attaches RuBP to oxygen instead of carbon dioxideinstead of carbon dioxide

Only one PGAL forms instead of twoOnly one PGAL forms instead of two

C4 Plants C4 Plants

Carbon dioxide is fixed twiceCarbon dioxide is fixed twice In mesophyll cells, carbon dioxide is In mesophyll cells, carbon dioxide is

fixed to form four-carbon oxaloacetate fixed to form four-carbon oxaloacetate

Oxaloacetate is transferred to bundle-Oxaloacetate is transferred to bundle-

sheath cellssheath cells

Carbon dioxide is released and fixed Carbon dioxide is released and fixed

again in Calvin-Benson cycleagain in Calvin-Benson cycle

Figure 10.18 CFigure 10.18 C4 4 leaf anatomy and the C leaf anatomy and the C4 4 pathway pathway

CAM PlantsCAM Plants

Carbon is fixed twice (in same cells)Carbon is fixed twice (in same cells) Night Night

Carbon dioxide is fixed to form organic Carbon dioxide is fixed to form organic acidsacids

DayDay Carbon dioxide is released and fixed in Carbon dioxide is released and fixed in

Calvin-Benson cycleCalvin-Benson cycle

Figure 10.19 CFigure 10.19 C4 4 and CAM photosynthesis comparedand CAM photosynthesis compared

Figure 10.20 A review of photosynthesisFigure 10.20 A review of photosynthesis