Chapter 7Chapter 7Photosynthesis:Photosynthesis:

Using light energy to produceUsing light energy to produce

organic moleculesorganic molecules

I. Study of PhotosynthesisI. Study of Photosynthesis

Before 1600Before 1600: Ancient Greeks proposed that : Ancient Greeks proposed that plants obtained all nourishment from soil.plants obtained all nourishment from soil.

Mid 1600sMid 1600s: Jan Baptista van Helmont : Jan Baptista van Helmont performed experiments to determine where performed experiments to determine where plants obtain nourishment from.plants obtain nourishment from. Planted small willow tree in a pot with soil.Planted small willow tree in a pot with soil. Added water only, for five years.Added water only, for five years. Plant gained 75 kilograms. Plant gained 75 kilograms. Soil had lost 60 grams.Soil had lost 60 grams.

Add water onlyfor five years

Plant: 75 kg gainedSoil: 0.06 kg lost

Helmont’s Experiment

I. Study of PhotosynthesisI. Study of Photosynthesis

Helmont’s conclusionsHelmont’s conclusions: : Plants Plants do notdo not obtain all nutrients from obtain all nutrients from soilsoil (correct) (correct) They obtain most nutrients from They obtain most nutrients from waterwater (wrong). (wrong).

I. Study of PhotosynthesisI. Study of Photosynthesis

17701770:: Joseph Priestley consistently observed Joseph Priestley consistently observed that a candle in a closed jar would not burn that a candle in a closed jar would not burn out if a mint plant was added to the jar. out if a mint plant was added to the jar.

Priestley’s conclusionPriestley’s conclusion:: Plants “restore” the air Plants “restore” the air depleted by a candle.depleted by a candle.

Late 1770sLate 1770s:: Jan Ingenhousz showed that Jan Ingenhousz showed that plants require light to “restore” the air in a plants require light to “restore” the air in a closed jar.closed jar.

1800s1800s:: Photosynthesis reaction elucidated. Photosynthesis reaction elucidated. 1950s1950s:: Discovered that oxygen produced by Discovered that oxygen produced by

plants comes from water.plants comes from water.

Photosynthesis Requires Water, Carbon Dioxide, and Light

II. Net Reaction of PhotosynthesisII. Net Reaction of Photosynthesis

6CO6CO22 + + 6H6H22OO + + LIGHT ENERGYLIGHT ENERGY ---> C ---> C66HH1212OO66 + 6O + 6O22

REDOX REACTIONREDOX REACTION Carried out by photosynthetic Carried out by photosynthetic autotrophs (autotrophs (plants, plants,

algae, and some bacteria). Also called producers.algae, and some bacteria). Also called producers. Produce 200 billion tons of organic material from COProduce 200 billion tons of organic material from CO22

every year.every year. Almost all living organisms obtain energy directly or Almost all living organisms obtain energy directly or

indirectly from photosynthesis.indirectly from photosynthesis. ExceptionsExceptions: A few bacteria metabolize sulfur or : A few bacteria metabolize sulfur or

iron and some animals that eat them, don’t depend iron and some animals that eat them, don’t depend on photosynthesis. on photosynthesis.

Photosynthesis is a Redox Reaction

III. Chloroplasts are site of photosynthesis in III. Chloroplasts are site of photosynthesis in eucaryoteseucaryotes

All green parts of a plant carry out photosynthesis.All green parts of a plant carry out photosynthesis.

Most Most chloroplastschloroplasts are found in leaves, specifically in are found in leaves, specifically in

mesophyllmesophyll, green tissue in interior of leaves., green tissue in interior of leaves.

Green color is due to Green color is due to chlorophyllchlorophyll,, a light absorbing a light absorbing

pigment.pigment.

In bacteria, photosynthesis occurs on extensions of the In bacteria, photosynthesis occurs on extensions of the

cell membrane.cell membrane.

StomataStomata: Pores in leaf for exchange of CO: Pores in leaf for exchange of CO22 and O and O22

Chloroplasts are the Site of Photosynthesis

Areas of Chloroplast Important for PhotosynthesisAreas of Chloroplast Important for Photosynthesis

ThylakoidsThylakoids: : Membrane “discs” arranged in Membrane “discs” arranged in

stacks (grana) which contain chlorophyll and stacks (grana) which contain chlorophyll and

other important molecules. other important molecules.

Site where solar energy is trapped and converted Site where solar energy is trapped and converted

into chemical energy (into chemical energy (lightlight reactions). reactions).

Thylakoid MembraneThylakoid Membrane: Site of ATP synthesis.: Site of ATP synthesis.

StromaStroma: : Thick fluid outside thylakoid Thick fluid outside thylakoid

membranes, surrounded by interior membrane. membranes, surrounded by interior membrane.

Site of sugar synthesis (Site of sugar synthesis (darkdark reactions). reactions).

Chloroplasts Have Three Sets of Membranes

IV. Plants produce oxygen by “splitting” waterIV. Plants produce oxygen by “splitting” water Water is used as a source of H and electrons to Water is used as a source of H and electrons to

reduce reduce COCO22

6CO6CO22 + 6H + 6H22O + O + ENERGYENERGY ---> C ---> C66HH1212OO66 + 6 + 6OO22

Where does the free oxygen come fromWhere does the free oxygen come from?? C COO22 or H or H22OO

Label the CLabel the COO22 or H or H22O O with radioactive with radioactive OO1818

CCOO22 + 2H + 2H22O -------> CHO -------> CH22OO + H + H22OO + O + O22

COCO22 + 2H + 2H22O O -------> CH -------> CH22O + HO + H22O + O + OO22

**** Free oxygen comes from the splitting of water, not **** Free oxygen comes from the splitting of water, not

COCO22..

Oxygen Generated by Photosynthesis Comes from Water

IV. Light reactions trap IV. Light reactions trap energyenergy and and electronselectrons required to make sugar from COrequired to make sugar from CO22

Light reactions:Light reactions: Require light. Convert light energy to Require light. Convert light energy to chemical energy of ATPchemical energy of ATP and and reducing power of NADPHreducing power of NADPH Occur in the Occur in the thylakoidthylakoid membranes of chloroplast membranes of chloroplast Water is split with energy from sun into Water is split with energy from sun into free Ofree O22, H and , H and

electrons.electrons. Reduce NADPReduce NADP++ to NADPH to NADPH: High energy electrons and : High energy electrons and

H obtained from splitting of HH obtained from splitting of H22OO

PhotophosphorylationPhotophosphorylation:: Light energy is used to produce Light energy is used to produce ATP from ADP + PATP from ADP + Pii

ATP synthesis is driven by ATP synthesis is driven by chemiosmosischemiosmosis

InputInput:: ADP, NADP+, water, and light. ADP, NADP+, water, and light.

OutputOutput:: ATP, NADPH, and O ATP, NADPH, and O22..

Light Dependent Reactions: Light Energy Trapped by Chlorophyll is Used to Split Water, Make NADPH & ATP

V. Light Independent (Dark) reactions (Calvin V. Light Independent (Dark) reactions (Calvin Cycle) make sugar from COCycle) make sugar from CO22

Calvin CycleCalvin Cycle: : Uses ATP and NADPH produced by light Uses ATP and NADPH produced by light reactions to reducereactions to reduce COCO22 to to glyceraldehyde-3-phosphateglyceraldehyde-3-phosphate

Occurs in the Occurs in the stromastroma of chloroplast of chloroplast Don’t need light directly.Don’t need light directly. Carbon fixationCarbon fixation: : Process of gradually reducing COProcess of gradually reducing CO22

gathered from atmosphere to organic moleculesgathered from atmosphere to organic molecules NADPH provides H and electrons to reduce CONADPH provides H and electrons to reduce CO22 and and

ATP provides energy.ATP provides energy.

InputInput:: CO CO22 , ATP, and NADPH. , ATP, and NADPH.

OutputOutput:: Sugars, ADP, and NADP+. Sugars, ADP, and NADP+.

Light Independent Reactions: Sugar Synthesis

NOTE:NOTE:

Light reactions:Light reactions: Transform light energy into Transform light energy into

usable form of chemical energy (ATP and usable form of chemical energy (ATP and

NADPH). Water is split to obtain H.NADPH). Water is split to obtain H.

Light independent reactions (Calvin cycle):Light independent reactions (Calvin cycle):

Use chemical energy (ATP and NADPH) to Use chemical energy (ATP and NADPH) to

drive the drive the endergonicendergonic reactions of sugar reactions of sugar

synthesis.synthesis.

Light and Dark Reactions of Photosynthesis

V. Properties of light energy: A portion of V. Properties of light energy: A portion of the electromagnetic spectrumthe electromagnetic spectrum

Visible lightVisible light:: A small portion of the A small portion of the electromagnetic energy spectrum which cells in electromagnetic energy spectrum which cells in our retina can detect ( wavelength: 380 - 750 nm).our retina can detect ( wavelength: 380 - 750 nm).

WavelengthWavelength:: Distance between two “crests” of Distance between two “crests” of light wavelight wave

PhotonPhoton: : Discrete “particles” of light energyDiscrete “particles” of light energy EnergyEnergy:: The amount of energy is The amount of energy is INVERSELYINVERSELY

proportional to the wavelength of lightproportional to the wavelength of light

Smaller wavelengthSmaller wavelength Longer wavelengthLonger wavelength

GammaGamma X-rays UV X-rays UV visiblevisible infrared microwaves radioinfrared microwaves radioraysrays light light lightlight light light waves waves

Visible light spectrumVisible light spectrum

Wavelength in nanometers:Wavelength in nanometers:

380 470 520 570 610 650380 470 520 570 610 650

VIOLETVIOLET BLUEBLUE GREENGREEN YELLOWYELLOW ORANGE ORANGE RED RED

Higher EnergyHigher Energy Lower EnergyLower Energy

White Light is a Spectrum of Different Lights

Isaac Newton (1642-1727) separated white light by passing itthrough a prism.

Chlorophyll Absorbs Portions of Visible Light Spectrum

Green light is reflected bychlorophyll

VI. Pigments allow plants to absorb various VI. Pigments allow plants to absorb various wavelengths of lightwavelengths of light

PigmentsPigments: Molecules that absorb light energy: Molecules that absorb light energy

Black objectBlack object:: All wavelengths are All wavelengths are absorbedabsorbed White objectWhite object:: All wavelengths are All wavelengths are reflectedreflected Green objectGreen object:: All wavelengths BUT green are All wavelengths BUT green are

absorbedabsorbed

Absorption spectrumAbsorption spectrum: Shows wavelengths : Shows wavelengths

absorbed by a certain pigmentabsorbed by a certain pigment

Plants use different Plants use different pigmentspigments to capture light to capture light energy, each has its own unique absorption energy, each has its own unique absorption spectrumspectrum

Chlorophyll aChlorophyll a:: PrimaryPrimary light absorption light absorption pigmentpigment

Chlorophyll bChlorophyll b:: AccessoryAccessory light absorption light absorption pigmentpigment

CarotenoidsCarotenoids:: AccessoryAccessory light absorption light absorption pigmentspigments

Structure of a Chlorophyll Molecule

IX. How is ATP produced in photosynthesis?IX. How is ATP produced in photosynthesis?

AnswerAnswer: : By By chemiosmosischemiosmosis through the through the development of a development of a proton gradientproton gradient in the in the chloroplast.chloroplast.

NOTENOTE:: The synthesis of ATP in both The synthesis of ATP in both oxidative oxidative

phosphorylationphosphorylation and and photosynthesisphotosynthesis is linked to the is linked to the

production of a production of a proton gradient.proton gradient.

Electron transport chain pumps HElectron transport chain pumps H++ to create gradient to create gradient

ATP synthaseATP synthase:: Uses energy as H Uses energy as H++ flows “downhill” to flows “downhill” to

drive the synthesis of ATP from ADP and Pdrive the synthesis of ATP from ADP and Pii

ATP Production Requires a Proton Gradient

Photosynthesis Helps Counteract the Greenhouse Effect

The earth’s atmosphere contains about 0.03% of carbon dioxide.

Carbon dioxide traps solar energy in the atmosphere, making

the earth about 10oC warmer than it would otherwise be.

Since the mid 1800s, the atmospheric levels of carbon dioxide

have risen steadily due to the burning of fuels and forests.

The “Greenhouse Effect” refers to the global warming that is

caused by increased atmospheric carbon dioxide levels.

Global warming may cause polar ice caps to melt, which in turn

could cause massive coastal flooding and other problems.

Plants use up about half of carbon dioxide generated by humans

and other organisms.

Greenhouse Effect: Heat is Trapped by Carbon Dioxide