Chapter 10

Photosynthesis

Updated Oct. 2008

Overview: The Process That Feeds the Biosphere

Photosynthesis Is the process that converts solar energy into

chemical energy nourishes almost all the living world directly or

indirectlyAll organisms use organic compounds for

energy & for carbon skeletonsOrganisms obtain organic compounds by

one of two major modes: autotrophic nutrition or heterotrophic nutrition

Autotrophs

produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment

Autotrophs are the ultimate sources of organic compounds for all heterotrophic organisms

Autotrophs are the producers of the biosphere

Autotrophs

Photoautotrophs use light as a source of energy to synthesize organic compounds

Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes

Chemoautotrophs harvest energy from oxidizing inorganic substances, such as sulfur and ammonia

Chemoautotrophy is unique to prokaryotes

Photosynthesis

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

(a) Plants

(b) Multicellular algae

(c) Unicellular protist 10 m

40 m(d) Cyanobacteria

1.5 m(e) Pruple sulfurbacteria

Figure 10.2

BioFlix: PhotosynthesisBioFlix: Photosynthesis

Heterotrophs live on organic compounds produced by other organisms

These organisms are the consumers of the biosphere

The most obvious type of heterotrophs feeds on other organisms.

Animals feed this way

Other heterotrophs decompose and feed on dead organisms or on organic litter, like feces and fallen leaves.

Most fungi and many prokaryotes get their nourishment this way

Concept 10.1: Photosynthesis

Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria

All green parts of a plant have chloroplasts However, the leaves are the major site of

photosynthesis for most plants The color of a leaf comes from chlorophyll, the

green pigment in the chloroplasts

Vein

Leaf cross section

Figure 10.3

Mesophyll

CO2 O2Stomata

Chloroplasts: The Sites of Photosynthesis in PlantsChloroplasts are found mainly in

mesophyll cells forming the tissues in the interior of the leaf

O2 exits & CO2 enters the leaf through pores called stomata in the leaf

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

Chloroplastshave two membranes around a central

aqueous space, the stroma

In the stroma is an elaborate system of interconnected membranous sacs, the thylakoids

Prokaryotes

Photosynthetic prokaryotes lack chloroplasts

Their photosynthetic membranes arise from infolded regions of the plasma membranes, folded in a manner similar to the thylakoid

membranes of chloroplasts

Tracking Atoms Through Photosynthesis: Scientific Inquiry

Photosynthesis is summarized as

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

Notes: Water is input and output It’s really 2 G3P that are output, not glucose

G3PGlycolysis

The Splitting of Water

One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not

CO2

This was not shown until the 1950s.

C. B. van Niel suggested water split

In the bacteria that he was studying, hydrogen sulfide (H2S), not water, is used in photosynthesis

These bacteria produce yellow globules of sulfur as a waste, rather than oxygen

Van Niel proposed this chemical equation for photosynthesis in sulfur bacteria:

CO2 + 2H2S [CH2O] + H2O + 2S

He generalized this idea and applied it to plants, proposing this reaction for their photosynthesis: CO2 + 2H2O [CH2O] + H2O + O2

In the 1950s…

Other scientists confirmed van Niel’s hypothesis twenty years later

They used 18O, a heavy isotope, as a tracer

They could label either C18O2 or H218O

They found that the 18O label only appeared in the oxygen produced in photosynthesis when

water was the source of the tracer

Photosynthesis as a Redox Process

Photosynthesis is a redox process Water is oxidized

Because electrons are more equally shared in O2 (potential energy has been lost)

Carbon dioxide is reduced Because it takes energy to move the electrons away

from oxygen and share equally in glucose (energy has been stored)

The Two Stages of Photosynthesis: A Preview

Photosynthesis consists of:

1. The light reactions (the photo part)

2. The Calvin cycle (the synthesis part)

The Two Stages of Photosynthesis: A Preview The light reactions (in the thylakoids):

Split H2O

Release O2

Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation

The Calvin cycle (in the stroma) Forms sugar from CO2, using ATP and NADPH The Calvin cycle begins with carbon fixation,

incorporating CO2 into organic molecules

An overview of photosynthesis

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

[CH2O](sugar)

NADPH

NADP

ADP

+ P

O2Figure 10.5

ATP

Concept 10.2: The thylakoids convert solar energy to the chemical energy of ATP & NADPH

Light Is a form of electromagnetic energy, which

travels in rhythmic waves

Wavelength Is the distance between the crests of waves Determines the type of electromagnetic energy The shorter the wavelength, the higher the

energy

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

The visible light spectrum

380 – 750 nm Includes the colors of light we can see Includes the wavelengths that drive photosynthesis

While light travels as a wave, many of its properties are those of a discrete particle, the photon

Photosynthetic Pigments: The Light Receptors When light meets matter, it may be:

Reflected Transmitted Absorbed

Pigments are substances that absorb visible light

Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or

transmitted Leaves appear green because chlorophyll

reflects and transmits green light

Animation: Light and PigmentsAnimation: Light and Pigments

Reflect light, which include the colors we see

Light

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight

Figure 10.7

The spectrophotometer

A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light

It beams narrow wavelengths of light through a solution containing the pigment & 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

Chlorophyll a, the dominant pigment, absorbs best in the red and violet-blue wavelengths and least in the green Other pigments with different structures have

different 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

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 of a pigment Profiles the relative effectiveness of different

wavelengths of radiation in driving photosynthesis

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

The action spectrum for photosynthesisWas first demonstrated by Theodor W.

Engelmann in 1883

C

CH

CH2

CC

CC

C

CNNC

H3C

C

CC

C C

C

C

C

N

CC

C

C N

MgH

H3C

H

C CH2CH3

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

Structure of chlorophyll molecules

Chlorophyll a Is the main photosynthetic pigment

Chlorophyll b Is an accessory pigment

Only chlorophyll a participates directly in the light reaction, but accessory photosynthetic pigments absorb light & transfer energy to chlorophyll a

Carotenoids

Participate in photoprotection against excessive light

These compounds absorb & dissipate excessive light energy that would otherwise: damage chlorophyll or interact with oxygen to form reactive

oxidative molecules that could damage the cell

Similar pigment in human eyes (so eat your 5 fruits and veggies a day!)

Excitation of Chlorophyll by Light

When a pigment absorbs light It goes from a

ground state to an excited state, which is unstable

A particular compound absorbs Only photons of a

specific wavelength

Excitedstate

Ene

rgy

of e

lect

ion

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A

If an isolated solution of chlorophyll is illuminated

It will fluoresce, giving off light and heat

Figure 10.11 B

A photosystem

Is composed of a reaction center surrounded by a number of light-harvesting complexes

What’s being reduced?

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–

In the photosystems...

The light-harvesting complexes Consist of pigment molecules bound to

particular proteins Funnel the energy of photons of light to the

reaction center

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

primary electron acceptor

Two types of photosystems

Photosystem II Discovered second, but comes first Most effectively absorbs light at 680 nm (P680)

Photosystem I Discovered first, but comes second Most effectively absorbs light at 700 nm (P700)

Use identical chlorophyll a but are associated with different proteins in the thylakoid membrane

Linear electron flow

Is the primary pathway of energy transformation in the light reactions

Also the most evolved, requiring both photosystem II and I to work together

Inputs: H20 and light/photons

Outputs: NADPH, ATP, and oxygen

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

+

H2O

2 H+

Light

ATP

Primaryacceptor

Fd

ee–

NADP+

reductase

ElectronTransportchain

Electron transport chain

P700

Light

NADPH

NADP+

+ 2 H+

+ H+

1

5

7

2

3

4

6

8

A mechanical analogy for the light reactions

MillmakesATP

ATP

e–

e–e–

e–

e–

Pho

ton

Photosystem II Photosystem I

e–

e–

NADPH

Pho

ton

Figure 10.14 

Cyclic Electron Flow

• Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH

Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

Fig. 10-15

ATPPhotosystem II

Photosystem I

Primary acceptor

Pq

Cytochromecomplex

Fd

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

NADP+

+ H+

Some organisms such as purple sulfur bacteria have PS I but not PS II

Cyclic electron flow is thought to have evolved before linear electron flow

Cyclic electron flow may protect cells from light-induced damage

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

Chloroplasts and mitochondria Generate ATP by the same basic mechanism:

chemiosmosis But use different sources of energy to

accomplish this (organic molecules vs. light energy)

Fig. 10-16

Key

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

MITOCHONDRIONSTRUCTURE

Intermembranespace

Innermembrane

Electrontransport

chain

H+ Diffusion

Matrix

Higher [H+]Lower [H+]

Stroma

ATPsynthase

ADP + P i

H+ATP

Thylakoidspace

Thylakoidmembrane

Chemiosmosis

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

Some of the electron carriers, including the cytochromes, are very similar in chloroplasts & mitochondria

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 the pH in the stroma increases to about 8 a thousandfold difference in H+ concentration

Light

Fd

Cytochromecomplex

ADP +

i H+

ATPP

ATPsynthase

ToCalvinCycle

STROMA(low H+ concentration)

Thylakoidmembrane

THYLAKOID SPACE(high H+ concentration)

STROMA(low H+ concentration)

Photosystem II Photosystem I

4 H+

4 H+

Pq

Pc

LightNADP+

reductase

NADP+ + H+

NADPH

+2 H+

H2OO2

e–

e–

1/21

2

3

Concept 10.3: The Calvin cycle uses ATP & NADPH to convert CO2 to sugar

The Calvin cycle Is similar to the citric acid cycle Occurs in the stroma

The Calvin cycle has three phases Carbon fixation (making G3P) Reduction Regeneration of the CO2 acceptor

The Calvin cycle

Each turn of the cycle fixes one carbon

The Calvin Cycle – What to Know

Know all the inputs & outputsKnow which molecules are the same in the

citric acid cycle & the Calvin cycleWhat is so special about G3P & Rubisco?Ribulose biphosphate (RuBP) is the CO2

acceptorWhat the complimentary compound to

RuBP in the citric acid cycle?

Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis

On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis

The closing of stomata reduces access to CO2

and causes O2 to build up These conditions favor a seemingly wasteful

process called photorespiration

cuticle

Internal leaf structure of a C3 plant

upperepidermis

mesophyllcells

lowerepidermis

chloroplasts

bundle sheathvascular bundle(vein)

stoma

C3 Plants & Photorespiration

C3 plants – Calvin cycles initial steps form a 3-carbon compound

C3 plants include rice, wheat & soybeans

On dry hot days photorespiration increases O2 substitutes for CO2 in the active site of the

enzyme rubisco

Photorespiration consumes ATP

Photorespiration: An Evolutionary Relic?The photosynthetic rate is reduced

sometimes as much as 50%This metabolic pathway seems

counterproductive for plantsIs it, or do we just not understand enough?If it is, how could it have evolved?Maybe on a planet with less O2, there was

no penalty for having an enzyme (rubisco) that sometimes processes O2 instead of CO2

C4 Plants

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

compound (oxaloacetate) in mesophyll cells Using PEP carboxylase

These 4-carbon compounds Are exported to bundle sheath cells, where they

release CO2 used in the Calvin cycle

C4 Plants - Adaptation

PEP carboxylase has no affinity for O2 unlike rubisco, so no photorespiration in mesophyll cells

CO2 is kept high in bundle-sheath cells so rubisco doesn’t process O2

This adaptation is especially useful in hot, dry climates

Corn, sugarcane and some invasive & weedy plants (like crabgrass) have this adaptation

C4 leaf anatomy and the C4 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

CAM (Crassulacean acid metabolism) Plants

From the plant family Crassulaceae (succulents like sedums)

During the night, the stomata open and they store CO2 in organic acids until day...

During the day, the stomata close And the CO2 is released from the organic acids

for use in the Calvin cycle

CAM Plants - Adaptations

Similar to C4, but the separation is temporal instead of spatial

Another adaptation to hot, dry climatesJade (and similar) plants, many cacti and

pineapples are some examples

Both C4 and CAM also conserve water

CO2

Sugarcane

Mesophyllcell

CO2

C4

Bundle-sheathcell

Organic acidsrelease CO2 to Calvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Pineapple

Night

Day

CAM

SugarSugar

CalvinCycle

CalvinCycle

Organic acid Organic acid

(a) Spatial separation of steps (b) Temporal separation of steps

CO2 CO2

1

2

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

On a global scale...

Photosynthesis is the most important process on Earth.

It is responsible for the presence of oxygen in our atmosphere.

Each year, photosynthesis synthesizes 160 billion metric tons of carbohydrate.

No process is more important than photosynthesis to the welfare of life on Earth.

You should now be able to:

1. Describe the structure of a chloroplast

2. Describe the relationship between an action spectrum and an absorption spectrum

3. Trace the movement of electrons in linear electron flow

4. Trace the movement of electrons in cyclic electron flow

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5. Describe the similarities and differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts

6. Describe the role of ATP and NADPH in the Calvin cycle

7. Describe the major consequences of photorespiration

8. Describe two important photosynthetic adaptations that minimize photorespiration

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings