Chapter 10Photosynthesis

Overview:

• The Process That Feeds the Biosphere• Photosynthesis– Is the process that converts solar

energy into chemical energy

THEMES!

•1. Science as a process•2. Evolution•3. Energy Transfer•4. Continuity & Change•5. Structure Function•6. Regulation•7. Interdependence in Nature•8. Science, Tech, & Society

CONCEPTS

• 10.1 = Photosynthesis = Light E Chem E

• 10.2 = Light Rx = Solar E ATP + NADPH

• 10.3 = Calvin Cycle = ATP + NADPH + CO2

Sugar!• 10.4 = Alternatives = CAM, C4

• "...the cells of lifeBound themselves into clans,a multitude of cellsTo make one being-asthe molecules beforeHad made of many one cell. Meanwhile they had inventedChlorophyll and ate sunlight,cradles in peaceOn the warm waves."

• - Robinson Jeffers, The Beginning and the End

Autotrophs (like plants)

– Are the producers of the biosphere– Photoautotrophs use light as energy

source to synthesize organic molecules– Are the producers of the biosphere– Chemoautotrophs harvest energy from

oxidizing substances such as sulfur or ammonia

Plants are photoautotrophs

• They use the energy of sunlight to make organic molecules from water and carbon dioxide

Figure 10.1

Photosynthesis

• Occurs in plants, algae, certain other protists, and some prokaryotes

These organisms use light energy to drive the synthesis of organic molecules from carbon dioxideand (in most cases) water. They feed not onlythemselves, but the entire living world. (a) Onland, plants are the predominant producers offood. In aquatic environments, photosyntheticorganisms include (b) multicellular algae, suchas this kelp; (c) some unicellular protists, suchas Euglena; (d) the prokaryotes calledcyanobacteria; and (e) other photosyntheticprokaryotes, such as these purple sulfurbacteria, which produce sulfur (sphericalglobules) (c, d, e: LMs).

(a) Plants

(b) Multicellular algae

(c) Unicellular protist 10 m

40 m(d) Cyanobacteria

1.5 m(e) Pruple sulfurbacteria

Figure 10.2

*Heterotrophs

– Obtain their organic material from other organisms

– Are the consumers of the biosphere– Are the consumers of the biosphere-

fungi, animals, and many prokaryotes

Concept 10.1:

• Photosynthesis converts light energy to the chemical energy of food

Chloroplasts: The Sites of Photosynthesis in Plants

Vein

Leaf cross section

Mesophyll

CO2 O2Stomata

• The leaves of plants are the major sites of photosynthesis– What is function of the stomata?– What is function of leaf veins?– What is function of the mesophyll cells?

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

Chloroplasts

• Are the organelles in which photosynthesis occurs

• Contain thylakoids and grana• About a half million chloroplasts

per square mm of leaf surface• Found mainly in mesophyll

laver of leaf • Each cell contains about 30-40

chloroplasts)• Identify structures within a

chloroplast

Go fg. 10.19

Photosynthetic membranes!

Stomata

Evapotranspiration

Maple Tree

•500 ft2 of leaves•weighing 500 lbs•total chloroplast surface area = 140 miles2

•can produce 2 tons of sugar in one day!

Tracking Atoms Through Photosynthesis: Scientific

Inquiry• Photosynthesis is summarized as

CO2 + H2O [CH2O] + O2

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

The Splitting of Water

• Chloroplasts split water into– Hydrogen and oxygen, incorporating

the electrons of hydrogen into sugar molecules

6 CO2 12 H2OReactants:

Products: C6H12O66 H2O 6 O2

Figure 10.4

Photosynthesis as a RedOx Process

•Photosynthesis is a redox process– 1. Water is oxidized, carbon dioxide is

reduced– 2. Reverses flow of electrons in

respiraton– 3. Water is split and electrons

transferred with H+ from water to carbon dioxide, reducing it to sugar.

– 4. Electrons increase in potential energy as they move from water to sugar, the process requires energy. That energy boast is sunlight.

Figure 8.6 Free energy changes (G) in exergonic and endergonic reactions

(a) Exergonic reaction: energy released

Reactants

ProductsEnergy

Progress of the reaction

Amount ofenergy

released (∆G<0)F

ree

en

erg

y

Energy

Products

Amount ofenergy

released (∆G>0)

Progress of the reaction

Reactants

Fre

e e

ne

rgy

(b) Endergonic reaction: energy required

The Two Stages of Photosynthesis: A Preview

• Photosynthesis consists of two processes– The light reactions (photo) convert

solar energy to chemical energy– The Calvin cycle (synthesis) uses

sunlight energy to corporate CO2 from the atmosphere into sugar

Overview of photosynthesis

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

[CH2O](sugar)

NADPH

NADP

ADP

+ P

O2Figure 10.5

ATP

The light reactions

• The light reactions– 1. Occur in the thylakoids (analogies?)– 2. Split water, release oxygen, produce ATP, and

form NADPH– 3. Light energy absorbed by chlorophyll in the

thylakoids drives the transfer of electrons and hydrogen from water to NADP+ forming NADPH

– 4. NADPH, an electron acceptor, provides reducing power via energized electrons to the Calvin cycle

– 5. Water is split in the process and oxygen is released as a by-product.

– 6. Light reaction also generates ATP using a process called photophosphorylation (chemiosmosis)

– 7. Light energy is initially converted to chemical energy in two forms- NADPH and ATP

Light Reactions

H2O + NADP+ + ADP O2 + ATP + NADPH2

– Light is absorbed by chlorophyll a which "excites" the electrons

– Electrons are passed through a series of carriers (electron transport chain), ATP is produced.

– Water is split (photolysis), giving off oxygen.

– 2H2O 4H+ + 4e- + O2

The Calvin cycle

•The Calvin cycle– 1. Occurs in the stroma (analogies?)– 2. Forms sugar from carbon dioxide, using

ATP for energy and NADPH for reducing power

– 3. Named for Melvin Calvin who worked out many of the steps in the 1940s

– 4. Cycle begins with the incorporation of CO2 into organic molecules, a process called carbon fixation

– 5. The fixed carbon molecule is reduced with electrons provided by NADPH

– 6. ATP from the light reactions also powers part of the Calvin cycle.

– 7. Light independent reactions

Calvin Cycle

ATP + NADPH2 + CO2 C6H12O6 + NADP+ + ADP

Overview of photosynthesis

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

[CH2O](sugar)

NADPH

NADP

ADP

+ P

O2Figure 10.5

ATP

Concept 10.2:

• The light reactions convert solar energy to the chemical energy of ATP and NADPH

The Nature of Sunlight• Light– Is a form of electromagnetic energy,

which travels in waves– Transmitted– Reflected– Absorbed– While light travels as a wave, many of its

properties are those of a discrete particle, the photon.

• Wavelength– Is the distance between the crests of

waves– Determines the type of electromagnetic

energy– The amount of energy in a photon is

inversely related to its wavelength. Photons with shorter wavelength pack _____ energy.

Frequency & Wavelength

The electromagnetic spectrum

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energyFigure 10.6

Visible light spectrum

The most important segment for life is a narrow band between 380 to 760 nm, the band of visible light– Includes the colors of light we can see– Includes the wavelengths that drive

photosynthesis– When light meets matter, it may be

reflected, transmitted, or absorbed

Photosynthetic Pigments: The Light Receptors

• Pigments– Are substances that absorb visible

light– Different pigments absorb photons of

different wavelengths, and the wavelengths that are absorbed disappear

– A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.

Chlorophyll

C55H70MgN4O6

Light

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight

Figure 10.7

Spectrophotometer

– a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength

• An absorption spectrum– Is a graph plotting light absorption versus wavelength

Figure 10.8

Whitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

Galvanometer

Slit moves topass lightof selectedwavelength

Greenlight

The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.

The low transmittance(high absorption) readingchlorophyll absorbs most blue light.

Bluelight

1

2 3

40 100

0 100

• The absorption spectra of chloroplast pigments– Provide clues to the relative

effectiveness of different wavelengths for driving photosynthesis

Absorption Spectra• The absorption spectra of three types of pigments in chloroplasts

Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.

EXPERIMENT

RESULTSA

bso

rptio

n o

f lig

ht

by

chlo

rop

last

pig

me

nts

Chlorophyll a

(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.

Wavelength of light (nm)

Chlorophyll b

Carotenoids

Figure 10.9

Action Spectrum• The action spectrum of a pigment

– Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis

Rat

e o

f ph

otos

ynth

esis

(mea

sure

d by

O2 r

elea

se)

Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.

(b)

• The action spectrum for photosynthesis– Was first demonstrated by Theodor W. Engelmann

400 500 600 700

Aerobic bacteria

Filamentof alga

Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.

(c)

Light in the violet-blue and red portions of the spectrum are most effective in driving

photosynthesis.

CONCLUSION

Chlorophyll a• Is the main photosynthetic

pigment that participates in the light reactions

• Other accessory pigments absorb different wavelengths of light

• Chlorophyll b is an accessory pigment, has a slightly different structure, has a different absorption spectrum and can funnel energy to chlorophyll a

• Carotenoids can funnel energy from other wavelengths to chlorophlyll a and also participate in photoprotection against excessive light

C

CH

CH2

CC

CC

C

CNNC

H3C

C

CC

C C

C

C

C

N

CC

C

C N

MgH

H3C

H

C CH2 CH3

H

CH3C

HHCH2

CH2

CH2

H CH3

C O

O

O

O

O

CH3

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown

Figure 10.10

Excitation of Chlorophyll by Light

• When a pigment absorbs light– It goes from a ground state to an excited state, which is unstable– The only photons that a molecule can absorb are those whose energy matches exactly

the energy difference between the ground state and the excited stare of this electron– A particular compound absorbs only photons corresponding to specific wavelengths.

Excitedstate

Ene

rgy

of e

lect

ion

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A

Fluorescence

• If an isolated solution of chlorophyll is illuminated– It will fluoresce, giving off light and heat

Figure 10.11 B

Photosystems

-In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems.-A photosystem is composed of a reaction center surrounded by a number of light-harvesting complexes

Primary electionacceptor

Photon

Thylakoid

Light-harvestingcomplexes

Reactioncenter

Photosystem

STROMA

Thy

lako

id m

embr

ane

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)Figure 10.12

e–

light-harvesting complexes

– Consist of pigment molecules bound to particular proteins

– Funnel the energy of photons of light to the reaction center “antennae complexes”

• When a reaction-center chlorophyll molecule absorbs energy:– One of its electrons gets bumped up to

a primary electron acceptor (accepts electron from chlorophyll a

– This is first step of light reactions

Thylakoid membrane

– Is populated by two types of photosystems, I and II

– Photosystem I has a reaction center, chlorophyll a that has an absorption peak at 680nm

– Photosystem II has a reaction center, chlorphyll a that has an absortion peak at 680nm.

– These two centers work together to use light energy to generate ATP and NADPH

Noncyclic Electron Flow• Noncyclic electron flow, the predominant route, produces both

ATP and NADPH.• Photosystem II absorbs a photon of light. One of the electrons of

P680 is excited to a higher energy state.• This electron is captured by the primary electron acceptor, leaving

the reaction center oxidized.• An enzyme extracts electrons from water and supplies them to the

oxidized reaction center. This reaction splits water into two hydrogen ions and an oxygen atom that combines with another oxygen atom to form O2.

• Photoexcited electrons pass along an electron transport chain before ending up at an oxidized photosystem I reaction center.

• As these electrons “fall” to a lower energy level, their energy is harnessed to produce ATP.

• Meanwhile, light energy has excited an electron of PS I’s P700 reaction center. The photoexcited electron was captured by PS I’s primary electron acceptor, creating an electron “hole” in P700. This hole is filled by an electron that reaches the bottom of the electron transport chain from PS II.

• Photoexcited electrons are passed from PS I’s primary electron acceptor down a second electron transport chain through the protein ferredoxin (Fd).

• The enzyme NADP+ reductase transfers electrons from Fd to NADP+. Two electrons are required for NADP+’s reduction to NADPH. NADPH will carry the reducing power of these high-energy electrons to the Calvin cycle.

Figure 10.13Photosystem II

(PS II)

Photosystem-I(PS I)

ATP

NADPH

NADP+

ADP

CALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

Pq

Cytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O2 H+

Light

ATP

Primaryacceptor

Fd

ee–

NADP+

reductase

ElectronTransportchain

Electron transport chain

P700

Light

NADPH

NADP+

+ 2 H+

+ H+

Noncyclic Electron Flow

• Produces NADPH, ATP, and oxygen

1

5

7

2

3

4

6

8

A mechanical analogy for the light Rx

MillmakesATP

ATP

e–

e–e–

e–

e–

Pho

ton

Photosystem II Photosystem I

e–

e–

NADPH

Pho

ton

Figure 10.14 

Cyclic Electron Flow

• Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow.– Excited electrons cycle from their reaction

center to a primary acceptor, along an electron transport chain, and return to the oxidized P700 chlorophyll.

– As electrons flow along the electron transport chain, they generate ATP by cyclic photophosphorylation.

– There is no production of NADPH and no release of oxygen.

cyclic electron flow

• Only photosystem I is used– Only ATP is produced

Primaryacceptor

Pq

Fd

Cytochromecomplex

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

ATPFigure 10.15

Photosystem II Photosystem I

NADP+

A Comparison of Chemiosmosis in Chloroplasts and

Mitochondria– Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis.

– In both organelles, an electron transport chain pumps protons across a membrane as electrons are passed along a series of increasingly electronegative carriers.

– This transforms redox energy to a proton-motive force in the form of an H+ gradient across the membrane.

– ATP synthase molecules harness the proton-motive force to generate ATP as H+ diffuses back across the membrane.

Today’s Bellringer

• Compare Oxidative Phosphorylation in Mitochondria and Photophosphorylation in Chlorplasts

• Consult fgs.

• Include Drawings, similarities and differences. Chemiosmosis

• [be prepared for an essay question like this]

Chemiosmosis• The spatial organization of chemiosmosis differs in chloroplasts and

mitochondria

• Mitochondria transfers chemical energy from food to ATP. Chloroplasts transfers light energy into chemical energy of ATP

Key

Higher [H+]Lower [H+]

Mitochondrion Chloroplast

MITOCHONDRIONSTRUCTURE

Intermembrancespace

Membrance

Matrix

Electrontransport

chain

H+ DiffusionThylakoidspace

Stroma

ATPH+

PADP+

ATPSynthase

CHLOROPLASTSTRUCTURE

Figure 10.16

• In both organelles– Redox reactions of electron transport

chains generate a H+ gradient across a membrane

• ATP synthase– Uses this proton-motive force to make

ATP

Chemiosmosis• The inner membrane of the mitochondrion pumps protons from

the mitochondrial matrix out to the intermembrane space. The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space inside the thylakoid.

• The thylakoid membrane makes ATP as the hydrogen ions diffuse down their concentration gradient from the thylakoid space back to the stroma through ATP synthase complexes, whose catalytic knobs are on the stroma side of the membrane.

• The proton gradient, or pH gradient, across the thylakoid membrane is substantial.

• When chloroplasts are illuminated, the pH in the thylakoid space drops to about 5 and the pH in the stroma increases to about 8, a thousandfold different in H+ concentration.

• The light-reaction “machinery” produces ATP and NADPH on the stroma side of the thylakoid.

• Noncyclic electron flow pushes electrons from water, where they have low potential energy, to NADPH, where they have high potential energy.

• This process also produces ATP and oxygen as a by-product

Chemiosmosis in Thylakoids

pH Differences?5 – 8 = 1000x [H+]

Concept 10.3:

• The Calvin cycle uses ATP and NADPH to convert CO2 to sugar

• The Calvin cycle– Is similar to the citric acid cycle– Occurs in the stroma– Regenerates its starting materials

after molecules enter and leave the cycle

Melvin Calvin

•MelCal for short!

Calvin Cycle (Details)

– The Calvin cycle regenerates its starting material after molecules enter and leave the cycle.

– The Calvin cycle is anabolic, using energy to build sugar from smaller molecules.

– Carbon enters the cycle as CO2 and leaves as sugar.

– The cycle spends the energy of ATP and the reducing power of electrons carried by NADPH to make sugar.

– The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).

– Each turn of the Calvin cycle fixes one carbon.– For the net synthesis of one G3P molecule, the cycle

must take place three times, fixing three molecules of CO2.

– To make one glucose molecule requires six cycles and the fixation of six CO2 molecules.

Calvin Cycle = 3 phases

• 1. Carbon fixation• 2. Reduction• 3. Regeneration of the CO2 acceptor

The Calvin Cycle

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

Phase 1: Carbon Fixation

In the carbon fixation phase, each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).

• This is catalyzed by RuBP carboxylase or rubisco.

• Rubisco is the most abundant protein in chloroplasts and probably the most abundant protein on Earth.

• The six-carbon intermediate is unstable and splits in half to form two molecules of 3-phosphoglycerate for each CO2

Rubisco

Phase 2: Reduction• During reduction, each 3-phosphoglycerate

receives another phosphate group from ATP to form 1,3-bisphosphoglycerate.

• A pair of electrons from NADPH reduces each 1,3-bisphosphoglycerate to G3P.

• The electrons reduce a carboxyl group to the aldehyde group of G3P, which stores more potential energy.

• If our goal was the net production of one G3P, we would start with 3CO2 (3C) and three RuBP (15C).

• After fixation and reduction, we would have six molecules of G3P (18C).

• One of these six G3P (3C) is a net gain of carbohydrate.

• This molecule can exit the cycle and be used by the plant cell.

Phase 3: Regeneration

• The other five G3P (15C) remain in the cycle to regenerate three RuBP. In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle to regenerate three molecules of RuBP.

• For the net synthesis of one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH.

• The light reactions regenerate ATP and NADPH.• The G3P from the Calvin cycle is the starting

material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates.

Concept 10.4:

• Alternative mechanisms of carbon fixation have evolved in hot, arid climates

•“It is not the strongest species that survive, nor the most intelligent, but the most responsive to change.”

– Charles Darwin On the Origin of Species

• On hot, dry days, plants close their stomata– Conserving water but limiting access

to CO2

– Causing oxygen to build up– Stomata are major routes for gas and

water exchange– On hot dry days, stoma close– C3 fix rubisco as initial step to Calvin

cycle (rice, wheat, soybeans)

Photorespiration:An Evolutionary Relic?

• In photorespiration– O2 substitutes for CO2 in the active site of

the enzyme rubisco– The photosynthetic rate is reduced– RuBP splits into a 2 carbon fragment –

photorespiration– This fragment is then exported from

chloroplast and degraded to carbon dioxide– Produces no ATP– Does not produce any organic molecules– Evolutionary baggage???– Can drain away about 50% of carbon fixed

on a hot dry day.

C4 Plants

• C4 plants minimize the cost of photorespiration– By incorporating CO2 into four carbon

compounds in mesophyll cells(sugar cane, corn)

• These four carbon compounds– Are exported to bundle sheath cells,

where they release CO2 used in the Calvin cycle

C4 Leaf Anatomy & Pathway

CO2

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue)

Photosyntheticcells of C4 plantleaf

Stoma

Mesophyllcell

C4 leaf anatomy

PEP carboxylase

Oxaloacetate (4 C) PEP (3 C)

Malate (4 C)

ADP

ATP

Bundle-Sheathcell CO2

Pyruate (3 C)

CALVINCYCLE

Sugar

Vasculartissue

Figure 10.19

CO2

Compare with C3 Leaf Structure

CAM Plants

• CAM plants– Open their stomata at night,

incorporating CO2 into organic acids• During the day, the stomata close– And the CO2 is released from the

organic acids for use in the Calvin cycle

CAM & C4 similarities

Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in differenttypes of cells.

(a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times.

(b)

PineappleSugarcane

Bundle-sheath cell

Mesophyll Cell

Organic acid

CALVINCYCLE

Sugar

CO2 CO2

Organic acid

CALVINCYCLE

Sugar

C4 CAM

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Night

Day

1

2 Organic acidsrelease CO2 toCalvin cycle

Figure 10.20

The Importance of Photosynthesis: A Review

Light reactions:• Are carried out by molecules in the thylakoid membranes• Convert light energy to the chemical energy of ATP and NADPH• Split H2O and release O2 to the atmosphere

Calvin cycle reactions:• Take place in the stroma• Use ATP and NADPH to convert CO2 to the sugar G3P• Return ADP, inorganic phosphate, and NADP+ to the light reactions

O2

CO2H2O

Light

Light reaction Calvin cycle

NADP+

ADP

ATP

NADPH

+ P 1

RuBP 3-Phosphoglycerate

Amino acidsFatty acids

Starch(storage)

Sucrose (export)

G3P

Photosystem IIElectron transport chain

Photosystem I

Chloroplast

Figure 10.21

• Organic compounds produced by photosynthesis– Provide the energy and building

material for ecosystems