Almost all plants are photosynthetic autotrophs, as are some bacteria and protistsAutotrophs generate their own organic matter through photosynthesisSunlight energy is transformed to energy stored in the form of chemical bonds(a) Mosses, ferns, andflowering plants(b) Kelp(c) Euglena(d) CyanobacteriaTHE BASICS OF PHOTOSYNTHESIS

Light Energy Harvested by Plants & Other Photosynthetic Autotrophs6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2

SUN LIGHT

SUNLIGHT SPECTRUM

SUNLIGHT OBSORBED BY PLANTS

SUN LIGHTABSORBED BY PIGMENTSApigmentis a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption.

VARIOUS PLANT PIGMENT

GREEN PIGMENT IN PLANT CELLS (CHLOROPLAST)

WHY ARE PLANTS GREEN? Plant Cells have Green Chloroplasts

The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).

Chloroplasts absorb light energy and convert it to chemical energyLightReflectedlightAbsorbedlightTransmittedlightChloroplastTHE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED

Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water AN OVERVIEW OF PHOTOSYNTHESISCarbon dioxideWaterGlucoseOxygen gasPHOTOSYNTHESIS

PHOTOSYNTHESISSunlight provides ENERGYCO2 + H2O produces Glucose + Oxygen6CO2 + 6H2OC6H12O6 + 6O2

In most plants, photosynthesis occurs primarily in the leaves, in the chloroplastsA chloroplast contains: stroma, a fluid grana, stacks of thylakoids The thylakoids contain chlorophyllChlorophyll is the green pigment that captures light for photosynthesisPhotosynthesis occurs in chloroplasts

The location and structure of chloroplastsLEAF CROSS SECTIONMESOPHYLL CELLLEAFChloroplastMesophyllCHLOROPLASTIntermembrane spaceOutermembraneInner membraneThylakoid compartmentThylakoidStromaGranumStromaGrana

Chloroplasts contain several pigmentsChloroplast PigmentsChlorophyll a Chlorophyll b CarotenoidsXanthophyllFigure 7.7

Chlorophyll a & bChl a has a methyl group Chl b has a carbonyl group

Different pigments absorb light differently

TWO STEPS OF PHOTOSYNTHESISLIGHT REACTIONDARK REACTION

Steps of PhotosynthesisLight hits reaction centers of chlorophyll, found in chloroplastsChlorophyll vibrates and causes water to break apart.Oxygen is released into air

Hydrogen remains in chloroplast attached to NADPHTHE LIGHT REACTION

Two connected photosystems collect photons of light and transfer the energy to chlorophyll electronsThe excited electrons are passed from the primary electron acceptor to electron transport chainsTheir energy ends up in ATP and NADPHIn the light reactions, electron transport chains generate ATP, NADPH, & O2

The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-)Plants produce O2 gas by splitting H2O

PhotonPhotonWater-splittingphotosystemNADPH-producingphotosystemATPmillTwo types of photosystems cooperate in the light reactions

Steps of PhotosynthesisThe DARK Reactions= Calvin CycleCO2 from atmosphere is joined to H from water molecules (NADPH) to form glucoseGlucose can be converted into other molecules with yummy flavors!

The Calvin cycle makes sugar from carbon dioxideATP generated by the light reactions provides the energy for sugar synthesisThe NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucoseLightChloroplastLightreactionsCalvincycleNADPADP+ PThe light reactions convert solar energy to chemical energyProduce ATP & NADPH

AN OVERVIEW OF PHOTOSYNTHESIS

The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membraneThe flow of H+ back through the membrane is harnessed by ATP synthase to make ATPIn the stroma, the H+ ions combine with NADP+ to form NADPHChemiosmosis powers ATP synthesis in the light reactions

2 H + 1/2Water-splittingphotosystemReaction-centerchlorophyllLightPrimaryelectronacceptorEnergyto makeElectron transport chainPrimaryelectronacceptorPrimaryelectronacceptorNADPH-producingphotosystemLightNADP123How the Light Reactions Generate ATP and NADPH

SummaryLight Dependent Reactions

a. Overall inputlight energy, H2O. b. Overall output ATP, NADPH, O2.

Animation is of the Calvin Cycle Note what happens to the carbon dioxide and what the end product is. Second animation of the Calvin Cycle is very clear and even does the molecular bookkeeping for you.

Light Independent Reactions aka Calvin CycleCarbon from CO2 is converted to glucose

(ATP and NADPH drive the reduction of CO2 to C6H12O6.)

Light Independent Reactions aka Calvin CycleCO2 is added to the 5-C sugar RuBP by the enzyme rubisco.This unstable 6-C compound splits to two molecules of PGA or 3-phosphoglyceric acid. PGA is converted to Glyceraldehyde 3-phosphate (G3P), two of which bond to form glucose. G3P is the 3-C sugar formed by three turns of the cycle.

SummaryLight Independent Reactions a. Overall input CO2, ATP, NADPH. b. Overall output glucose.

Review: Photosynthesis uses light energy to make food moleculesLightChloroplastPhotosystem II Electron transport chains Photosystem ICALVIN CYCLEStromaElectronsLIGHT REACTIONSCALVIN CYCLECellular respirationCelluloseStarchOther organic compoundsA summary of the chemical processes of photosynthesis

Types of Photosynthesis C3

C4

CAM Rubisco: the worlds busiest enzyme!

Competing ReactionsRubisco grabs CO2, fixing it into a carbohydrate in the light independent reactions. O2 can also react with rubisco, inhibiting its active sitenot good for glucose outputwastes time and energy (occupies Rubisco)

PhotorespirationWhen Rubisco reacts with O2 instead of CO2Occurs under the following conditions:Intense Light (high O2 concentrations)High heatPhotorespiration is estimated to reduce photosynthetic efficiency by 25%

Why high heat?When it is hot, plants close their stomata to conserve waterThey continue to do photosynthesis use up CO2 and produce O2 creates high O2 concentrations inside the plant photorespiration occurs

C4 PhotosynthesisCertain plants have developed ways to limit the amount of photorespiration C4 Pathway*CAM Pathway** Both convert CO2 into a 4 carbon intermediate C4 Photosynthesis

Leaf AnatomyIn C3 plants (those that do C3 photosynthesis), all processes occur in the mesophyll cells.

Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C4 PathwayIn C4 plants photosynthesis occurs in both the mesophyll and the bundle sheath cells.

Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C4 PathwayCO2 is fixed into a 4-carbon intermediate Has an extra enzyme PEP Carboxylase that initially traps CO2 instead of Rubisco makes a 4 carbon intermediate

C4 PathwayThe 4 carbon intermediate is smuggled into the bundle sheath cellThe bundle sheath cell is not very permeable to CO2CO2 is released from the 4C malate goes through the Calvin Cycle

How does the C4 Pathway limit photorespiration?Bundle sheath cells are far from the surface less O2 accessPEP Carboxylase doesnt have an affinity for O2 allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is)

CAM PathwayFix CO2 at night and store as a 4 carbon molecule Keep stomates closed during day to prevent water loss Same general process as C4 Pathway

How does the CAM Pathway limit photorespiration?Collects CO2 at night so that it can be more concentrated during the dayPlant can still do the calvin cycle during the day without losing water

Summary of C4 PhotosynthesisC4 PathwaySeparates by space (different locations)CAM PathwaySeparates reactions by time (night versus day)

***************Affinity of PEP Carboxylase for CO2 is much higher than its affinity for O2.