PhotosynthesisPhotosynthesis

• Light ReactionsLight Reactions• Calvin CycleCalvin Cycle

PhotosynthesisPhotosynthesis• “ “ Construction by light”Construction by light”

–Basic EquationBasic Equation

6CO2 + 6H2O C6H12O6 + 6O2

Take home message: The atoms that make up the reactants are the EXACT SAME atoms that make up the products.

PhotosynthesisPhotosynthesis• Occurs in Occurs in ProducersProducers or or

AutotrophsAutotrophs: (make their food): (make their food)

– Algae, Plants, some bacteriaAlgae, Plants, some bacteria

– uses SMALL portion of the uses SMALL portion of the sun’s energy sun’s energy Visible Light Visible Light

PhotosynthesisPhotosynthesis

•ConsumersConsumers or or HeterotrophsHeterotrophs can’t use can’t use the sun’s light to make foodthe sun’s light to make food–Get glucose by eating other Get glucose by eating other plants or animals which eat plants or animals which eat plantsplants

–Herbivores,Herbivores, Carnivores,Carnivores, OmnivoresOmnivores

There are actually poems There are actually poems written about written about

photosynthesis!photosynthesis!• http://www.pbs.org/wgbh/nova/http://www.pbs.org/wgbh/nova/

nature/photosynthesis.htmlnature/photosynthesis.html

Structures for Structures for PhotosynthesisPhotosynthesis

Can all cells photosynthesize? - NO

In plants, only:MESOPHYLL cells (found in the leaf) can do PS. ANIMATION!!

http://dendro.cnre.vt.edu/forestbiology/photosynthesis.swf

Structure of the leafStructure of the leaf

Photosynthesis – The Photosynthesis – The LeafLeaf

•Body of LeafBody of Leaf–light absorption & CO2 light absorption & CO2 diffusiondiffusion

•Veins – transport tissueVeins – transport tissue–Xylem:Xylem: H20 & minerals from H20 & minerals from roots to leaf roots to leaf

–Phloem:Phloem: newly made newly made glucose to rest of plantglucose to rest of plant

Structures of the LeafStructures of the Leaf•Spongy MesophyllSpongy Mesophyll

–Middle layer made of cells Middle layer made of cells loaded with chloroplastsloaded with chloroplasts

–Spaces for COSpaces for CO22 /O2 /O2 exchange and water vaporexchange and water vapor

•Epidermis – top and bottom of Epidermis – top and bottom of the leafthe leaf

•Secretes a Secretes a waxywaxy cuticle cuticle

Structures of the LeafStructures of the Leaf•StomaStoma

–Openings on underside of Openings on underside of leaf through which gases leaf through which gases diffusediffuse•Guard cellsGuard cells are located on are located on either side and control either side and control whether stoma is open or whether stoma is open or closedclosed

StomatesStomates

Role of the StomatesRole of the Stomates•When open When open

–Oxygen outOxygen out–Transpiration: letting water Transpiration: letting water vapor outvapor out

–COCO22 in in

•Stomates close when water is Stomates close when water is lowlow

•When there is enough H20: open When there is enough H20: open during day, closed at nightduring day, closed at night

Role of the StomatesRole of the Stomates

–Rate of photosynthesis Rate of photosynthesis depends on COdepends on CO22 levels in levels in leafleaf

–Stomates closedStomates closed no CO no CO22 no PS no PS

Structure of a chloroplast:

In the ChloroplastsIn the Chloroplasts•Pigments: molecules that absorb Pigments: molecules that absorb

lightlight –Main pigment is chlorophyll Main pigment is chlorophyll (green)(green)

–Found in thylakoid membraneFound in thylakoid membrane

•Light energy converted to Light energy converted to Chemical energyChemical energy

•Chem. Energy stored as glucose Chem. Energy stored as glucose and starchand starch

In the ChloroplastsIn the Chloroplasts•Different pigments Different pigments

absorb/reflect different absorb/reflect different wavelengths with in visible wavelengths with in visible light spectrumlight spectrum

•Why do we see green?Why do we see green?

In th

e C

hlor

opla

sts

PigmentPigment AbsorbeAbsorbed Colorsd Colors

Reflected Reflected ColorsColors

ChlorophyChlorophyll All A

Yellow/Yellow/

GreenGreenChlorophyChlorophy

ll Bll BRed/Red/GreenGreen

CarotenoiCarotenoidd

OrangeOrange

XanthophXanthophyllsylls

YellowYellow

The key Players in PSThe key Players in PS• We already know: chlorophyll, We already know: chlorophyll,

CO2, water and lightCO2, water and light

• New players: New players: –ElectronsElectrons

–NADP+/NADPH: an electron carrierNADP+/NADPH: an electron carrier

http://dendro.cnre.vt.edu/forestbiology/photosynthesis.swf

ANIMATION!!!!

–Photosystems are groups of Photosystems are groups of pigments that absorb light pigments that absorb light energyenergy

–These groups of pigments are These groups of pigments are embedded in the thylakoid embedded in the thylakoid membranemembrane

Step one: Light Reactions – Step one: Light Reactions – convert visible light into convert visible light into

chemical energychemical energy

–Light energy splits H20 freeing Light energy splits H20 freeing HH++ (protons) and electrons (protons) and electrons

–A Proton gradient is established A Proton gradient is established to make ATP using ATP to make ATP using ATP synthasesynthase

–Electrons are given to NADP+ to Electrons are given to NADP+ to make NADPHmake NADPH

Step one: Light Reactions – Step one: Light Reactions – convert visible light into convert visible light into

chemical energychemical energy

–ATP and NADPH provide energy ATP and NADPH provide energy for step twofor step two

http://www.sumanasinc.com/webcontent/animations/http://www.sumanasinc.com/webcontent/animations/content/harvestinglight.htmlcontent/harvestinglight.html

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

– http://highered.mcgraw-hill.com/sites/0072437316/student_view0/http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter10/animations.html#chapter10/animations.html#

Step one: Light Reactions – Step one: Light Reactions – convert visible light into convert visible light into

chemical energychemical energy

Step two: light independentStep two: light independent

• Calvin CycleCalvin Cycle

–Energy in NADPH and ATP is Energy in NADPH and ATP is used to build 2 3-carbon sugars used to build 2 3-carbon sugars and then 1 glucose from COand then 1 glucose from CO22

•Energy is now stored in the Energy is now stored in the sugarssugars

• If not needed, turned to starchIf not needed, turned to starch

Three stages in Calvin Three stages in Calvin CycleCycle

• Carbon FixationCarbon Fixation– COCO22 attaches to RuBP attaches to RuBP 2 molecules of 3-PGA 2 molecules of 3-PGA

• Chemical ReshufflingChemical Reshuffling– 3-PGA 3-PGA G3P G3P

• Reforming RuBPReforming RuBP– For every 6 molecules of G3P, created, For every 6 molecules of G3P, created,

• One is used toward the creation of sugarsOne is used toward the creation of sugars• Five are converted back to RuBPFive are converted back to RuBP

Key things to knowKey things to know

• What is the purpose of the light reactionsWhat is the purpose of the light reactions• What are the reactants and the final What are the reactants and the final

productsproducts• What is the purpose of the Calvin Cycle?What is the purpose of the Calvin Cycle?• What is/are the reactants and the final What is/are the reactants and the final

products?products?• Why is carbon fixation and why do we Why is carbon fixation and why do we

need different methods to fix carbon?need different methods to fix carbon?

After PhotosynthesisAfter Photosynthesis• glucose can be used immediately glucose can be used immediately

–Broken down; mitochondria converts Broken down; mitochondria converts energy to ATPenergy to ATP

• Glucose can be stored for laterGlucose can be stored for later–Linked together to make starch Linked together to make starch

• Stored glucose/starch can become Stored glucose/starch can become food for other organismsfood for other organisms

Carbon Fixation Carbon Fixation AlternativesAlternatives

•Used by plants in Hot, Dry Used by plants in Hot, Dry ClimatesClimates–PhotorespirationPhotorespiration

•Occurs at high light intensities Occurs at high light intensities and temperatures > 30 C and temperatures > 30 C

•Low CO2, and High O2 Low CO2, and High O2 decrease in carbon fixation decrease in carbon fixation (use O2, lose CO2)-(use O2, lose CO2)- inefficent inefficent (low) production of sugars.(low) production of sugars.

Carbon Fixation Carbon Fixation AlternativesAlternatives

• C4 plants C4 plants – Carbon fixed into 4-carbon compoundsCarbon fixed into 4-carbon compounds– Occurs during the day, but uses two types of Occurs during the day, but uses two types of

cells (mesophyll and BSC)cells (mesophyll and BSC)•““same time, different place”same time, different place”

• CAM Pathway (another modification of C4 CAM Pathway (another modification of C4 metabolism)metabolism)– Stomates open at night, closed in the dayStomates open at night, closed in the day– Within the same cellsWithin the same cells

•Different time , same placeDifferent time , same place

C4 plants (25% of all PS C4 plants (25% of all PS on land)on land)

• high daytime temperatures high daytime temperatures • intense sunlight. intense sunlight.

– Crabgrass, corn (maize), sorghum, sugar caneCrabgrass, corn (maize), sorghum, sugar cane

• CAM - CAM - – high daytime temperatures high daytime temperatures – intense sunlight intense sunlight – low soil moisture.low soil moisture.

• Cacti, pineapple, sedumsCacti, pineapple, sedums

Why aren’t all plants C4?Why aren’t all plants C4?

• More energy intensive with less productivityMore energy intensive with less productivity– CAM PLANTS GROW SLOWLY!!!!CAM PLANTS GROW SLOWLY!!!!– WHY…need to make starch or malate, then break WHY…need to make starch or malate, then break

it down (PEP CARBOXYLASE)it down (PEP CARBOXYLASE)

• C3 is EFFICIENT…C3 is EFFICIENT…

• Actually, if C4 plants are given enough Actually, if C4 plants are given enough water…will revert to C3 (can do both)water…will revert to C3 (can do both)

C4 PathwayC4 Pathway

• Examples: Sugar cane, Corn, CrabgrassExamples: Sugar cane, Corn, Crabgrass

– Stomata partially closed during the dayStomata partially closed during the day– Lose less water than C3 plants, but Lose less water than C3 plants, but

produce same amount of CHO (water produce same amount of CHO (water efficient, growth efficient)efficient, growth efficient)

– Part of carbon fixation done in a “different” Part of carbon fixation done in a “different” cell (CO2 is “concentrated in these cells to cell (CO2 is “concentrated in these cells to increase the likelihood of creating sugars, increase the likelihood of creating sugars, instead of photorespiration. instead of photorespiration.

CAM PATHWAYCAM PATHWAY

• Stomates open at night, closed in the Stomates open at night, closed in the dayday

• Take in CO2 at night and fix it into Take in CO2 at night and fix it into organic compoundsorganic compounds

• Slow growth, but very water Slow growth, but very water efficientefficient

Hmmm what about water Hmmm what about water plants? plants?

Hydrophytic leaves are adapted to an aquatic existence.   Note that the upper palisade mesophyll is well developed for photosynthesis while the lower portion of the mesophyll is very loosely arranged with considerable open space which helps the leaf float to the surface.  The loosely arranged tissue of the lower mesophyll is aerenchyma.  stoma are located in the upper epidermis.

The pine leaf is well adapted to dry conditions with a thick epidermis with stoma recessed into the into the surface.  Beneath the epidermis is the thick walled cells of the hypodermis which helps reduce water evaporation from the leaf.  The pine leaf has an endodermis inside the mesophyll which is not seen in either of the other leaf types observed.