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Capturing Solar Energy:Capturing Solar Energy:PhotosynthesisPhotosynthesis

Chapter 7Chapter 7

Photosynthesis• The process of converting light energy into

chemical energy.

• Nourish most living organisms directly orindirectly.

• Plants are autotrophs, self feeders, producers of organic molecules from CO2.

Earth’sTimelineHumans haveonly existed for1 second ofEarth’s 24 hourclock.

Phototrophicbacteria (andphotosynthesis)evolved closeto the beginningof life.

Pre-Biotic Earth The very earlyatmosphere of theearth containedmostly CO2,about 80%.

The concentrationof CO2 graduallydropped to about20% by 3,500 Ma(million years ago).

The evolution ofthe first bacteriaoccurred about3,500 Ma.

age of earth oldest rocks

oldest stromatolites

earliest bacteria first green algae(Eukaryotic)

earliest photosynthetic bacteria

earliest biogenic carbon

Carbon Dioxide

HydrogenOxygen

Nitrogen

stromatolites abundant

oldest "red beds"

red beds abundant

oldest chemical evidence ofcomplex cells

banded iron formations(with reduced iron)







Carbon Dioxide

HydrogenOxygen

Nitrogen


oldest "red beds"

red beds abundantbanded iron formations(with reduced iron)

Oxygen generated after life began.

No Oxygen present in the early atmosphere.

Earliest Evidence ofMicrobial Life

• Graphite bands found 3.8 billion years ago.– Graphite is a material that can be produced when

organic carbon, found in all living organisms, isaltered by heat and pressure over time.

(i.e. evidence ofBiogenic Carbon)







Carbon Dioxide

HydrogenOxygen

Nitrogen


oldest "red beds"


Bands ofGraphite madefrom compressedorganic carbon.


• Stromatolites found 2.7 billion years ago.– Stromatolites formed when sediments accumulated

over sticky colonies of underwater cyanobacteria.- After buried by sediment, the

bacteria moved upward towardneeded sunlight.

- The cycle begins again with theaccumulation of more sediment,forming layers over time.

Earliest Evidence of Life







Carbon Dioxide

HydrogenOxygen

Nitrogen


oldest "red beds"


Stromatolite formationindicates cyanobacteria,photosynthetic organismsstriving for sunlight.


• Banded iron formations (BIFs) found between3.5 billion and two billion years ago.– BIFs are formed when oxygen released by

photosynthetic bacteria create rust, oxidized iron, inshallow ocean shores.


Earliest Evidence of Photosynthesis







Carbon Dioxide

HydrogenOxygen

Nitrogen


oldest "red beds"


Banded Iron Formationsindicate the presence of anoxidizing environment:Oxygen is present locally!


• The Cambrian Explosion, 543 million years ago– Nearly all the major animal groups we are familiar

with first appeared in this period.


Earliest Evidence of Photosynthesis

Evidence for HighConcentrations ofOxygen in theAtmosphere

Photosynthesis6CO2 + 6H2O + light C6H12O6 + 6O2

PhotosyntheticOrganisms

Photosynthetic Adaptations

• Leaves– Designed to Capture Sunlight

• Thin (only a few cells thick)

• Large surface area (to expose all cells to thesun)

– Designed to Prevent Water Loss and allowGas Exchange

• Mesophyll Cells

• Chloroplasts

Leaf Adaptations

1. Capturing Sunlight2. Prevention of Water Loss

•Cuticle–Waxy waterproofcovering that reducesevaporation

•Upper & LowerEpidermis

Leaf Adaptations

1. Capturing Sunlight2. Prevention of Water Loss3. Gas Exchange

•Stoma-Pores, open & close

•Guard Cells- Move to open & closethe ‘stoma’

Leaf Adaptations

Stoma open when humidity is high.Guard cells absorb water and swell.Open stoma let CO2 in and O2 out.

Stoma deflate and closewhen humidity is low toprevent loss of water.

Vascular Bundle and Sheath

• Veins of plants.• Supply water and minerals to mesophyll cells.• Carry sugar from mesophyll cells to other parts

of the plant.

Mesophyll Cells

Palisade Mesophyll(carry-out photosynthesis)

Spongy Mesophyll(allow for gas exchange)

• “Middle of the Leaf”• Major site of photosynthesis in the leaf.• A single cell contains 40-200 chloroplasts.

Chloroplasts• Organelles where photosynthesis takes place.• Contain dish-shaped, interconnected membranous

sacs called thylakoid disks where the light-dependent photosynthetic reactions take place.

• Light-independent photosynthetic reactions takeplace in the stroma.

Chloroplasts

Light Dependent Reaction

Light Independent Reaction

The cuticle is a waxylayer that preventswater loss.

The upper andlower epidermis arethicker cells thatprotect the interiorleaf structure andprevent water andgas exchange withthe atmosphere.The stoma are pores in the lower

epidermis that allow gas exchange.They are regulated by guard cellswhich open and close according tolocal humidity and gas needs.

Chloroplasts are theeukaryotic organelleswhere photosynthesistakes place within theplant cell.

The Vascular Bundle transports water tothe leaves and sugars away from theleaves to other parts of the plant.Bundle Sheath cells protect the vascularbundle and regulate water and sugarmovement from the vascular bundle.

Mesophyll cells makeup most of the interiorof the leaf.Palisades mesophyllcells (top) carryoutmost of thephotosynthesis.Spongy mesophyllcells (bottom) storegasses and water forpalisades cells.

The Thylakoid disksare packed full ofchlorophyll (or otherphotosyntheticpigment).The location of thelight dependentreaction ofphotosynthesis.

The Stroma makes up theremaining interior space.The location of the lightindependent reaction ofphotosynthesis.

Reactions of Photosynthesis

• Light-Dependent Reaction– Takes place within the thylakoid membrane.

– Generates energy-carrier molecules(ATP, NADPH)

• Light-Independent Reaction– Takes place in the stroma.

6CO2 + 6H2O + light C6H12O6 + 6O2

COCO22 + H + H++ + ATP C + ATP C66HH1212OO66 + ADP + ADP

HH22O + O + lightlight + ADP O + ADP O22 + H + H++ + ATP + ATP

LightCaptured by

Pigments

•The visible spectra of energy isquite narrow.

•Light is composed of individualpackets of energy, photons.

•The shorter the wavelength, themore energetic the light.

LightCaptured by

Pigments• Photosynthetic

pigments absorbs lightenergy in the veryhigh and lowwavelengths.

• Light that is notabsorbed is reflectedand gives the objectits color.

Not all light is the samecolor or wavelength.

Chlorophyll a absorbs inthe violet and redwavelengths.

Chlorophyll b absorbs inthe blue and orangewavelengths.

Carotenoids absorb in theviolet to green wavelengths.

Most photosynthesis utilizesthe violet, blue, and redwavelengths of light.

PhotosyntheticPigments

• Chlorophyll absorbs violet, blue,and red light.– Two types of chlorophyll:

• Chlorophyll A, absorbs violet and red.• Chlorophyll B, absorbs blue and orange.

• Additional accessory pigments,carotenoids, absorb blue andgreen light while reflecting redand yellow.

• Chlorophyll breaks down in thecold before carotenoids, givingleaves red and yellow hues inautumn.

Photosystems of Photosynthesis

• Photosynthetic pigmentproteins, chlorophyll andcarotenoids, cluster togetheralong the thylakoid membranecreating photosystems.

• Two photosystems, PS I andPS II, contain clusters ofchlorophyll and carotenoids atreaction centers.


Electrons generated from light energy arepassed from the reaction center along an

electron transport chain (ETC), aseries of electron-carrying molecules

in the membrane, toproteins to generateATP and NADPH.

Photosystems ofPhotosynthesis

Light energy (1) excites electronsin the reaction center ofchloroplasts (2) sending themdown an electron transportchain (3,4) to generate ATP (5).

Excited electrons are furtherexcited (6) and sent down asecond ETC (7,8) to formNADPH (9).


Excitation of PS II generates H+ and electrons. Excitation of PS II generates H+ and electrons.

High H+ concentration is used to synthesize ATP. High H+ concentration is used to synthesize ATP.

Excitation of PS I generates electronsfor NADPH synthesis.

Energy-carrier molecules (ATP and NADPH) areused to drive light independent reactions.

PhotosyntheticLight-Dependent

Reaction

Generates a lot of ATP.

ATP Synthesis in Chloroplasts

• A gradient of H+

ions is built upthrough activetransport acrossthe ETC of PS II.

ATP Synthesis in Chloroplasts

• A gradient of H+

ions is built upthrough activetransport acrossthe ETC of PS II.

• Passive transportof H+ across themembrane iscoupled with ATPsynthesis.

Light-Independent Reactionsof Photosynthesis

• Generation of glucose and other largecarbon-storage molecules from CO2 andenergy generated in PS II and PS I iscalled the Calvin-Bensen cycle.

• Carbon fixation is the process ofcapturing CO2 from the atmosphere andincorporating (fixing) it into largermolecules.

Light-Independent Reactionsof Photosynthesis

Key:– RuBP: ribulose bisphosphate– PGA: phosphoglyceric acid– G3P: glyceraldehyde-3-phosphate

Bioflix on Photosynthesis

Adaptations of the Calvin Cycle• Cells require CO2 to gain energy from the

Calvin Cycle.• To minimize evaporative loss of H2O, cells

close their stomata in hot, dry weather.– CO2 cannot get in,– O2 cannot get out.

Adaptations of the Calvin Cycle

• Rubisco, the enzyme thatcatalyzes the reaction ofRuBP and CO2 is not

selective.• Photorespiration occurs

when O2 is combined withRuBP instead of CO2, generating CO2 and noadditional energy.

• Plants in hot, dry climates have adapted atwo-stage carbon-fixation pathway, the C4pathway.

C3 Pathway used by most plants

PhotosyntheticLight Independent

Reaction

Takes place in the stromaof the chloroplast.

Stores the energygenerated in the lightreaction in the formof glucose.

Rubisco requires CO2.Because of the non-specificity ofrubisco, cells undergo wastefulphotorespiration when CO2 isnot available.

C4 Pathway used by Grasses

Key:•PEP: phosphoenolpyruvate

Homework

Chapter 7Thinking Through the Concepts,

Review Questions 1 and 5.

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