PHOTOSYNTHESIS

Chapter 10

PHOTOSYNTHESIS

Overview: The Process That Feeds the BiospherePhotosynthesis

Is the process that converts light (sun) energy into chemical energy

Energy ConversionLight energy

ECOSYSTEM

CO2 + H2O

Photosynthesisin chloroplasts

Cellular respirationin mitochondria

Organicmolecules

+ O2

ATP

powers most cellular work

Heat

Summary Equation for Photosynthesis

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

Photosynthesis is a redox process

Water is oxidized, carbon dioxide is reduced

It is an anabolic pathway with endergonic reactions

It requires energy

Summary Equation for Photosynthesis

Energy 6 CO2 6 H2O C6 H12 O6 6 O2

becomes reduced

becomes oxidized

Autotrophs vs. Heterotrophs

AutotrophsCan covert inorganic carbon sources to organic materialAre the producers of the biosphere

HeterotrophsObtain their organic material from other organismsAre the consumers of the biosphere

WHERE PHOTOSYNTHESIS OCCURS

The leaves of plantsAre the major sites of photosynthesis

Vein

Leaf cross section

Mesophyll

CO2 O2Stomata

STRUCTURE OF CHLOROPLASTS

Chloroplast

Mesophyll

5 µm

Outermembrane

IntermembranespaceInner

membrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

The Nature of Sunlight

LightIs a form of electromagnetic energy, which travels in waves

WavelengthIs the distance between the crests of wavesDetermines the type of electromagnetic energy

The visible light spectrumIncludes the colors of light we can seeIncludes the wavelengths that drive photosynthesisIs in the 380 nm to 750 nm wavelength range

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 energy

ELECTROMAGNETIC SPECTRUM

PHOTOSYNTHETIC PIGMENTS

Absorb light waves that are used in photosynthesis orReflect light waves that are not used in photosynthesis

LightReflectedLight

Chloroplast

Absorbedlight

GranumTransmitt

edlight

Green light is NOT used!

ABSORPTION SPECTRA

The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments

.

Ab

sorp

tion

of

lig

ht

by

chlo

rop

last

pig

men

tsChlorophyl

l a

Wavelength of light (nm)

Chlorophyll b

Carotenoids

Photosynthesis consists of two processesLight reactions

Occur in the thylakoid membranesSplits waterReleases oxygenProduces ATP and NADPH

Calvin cycleOccurs in the stromaForms sugar from carbon dioxideUsing ATP (for energy) and NADPH (for reducing power) from the light reactions

STAGES OF PHOTOSYNTHESIS

Light

LightReactions

CalvinCycle

Chloroplast

[CH2O](sugar)

ATP

NADPH

NADP

ADP

+ P i

H2O CO2

O2

EXCITATION OF CHLOROPHYLL BY LIGHT

Excitedstate

En

erg

y of

ele

ctio

n

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

When a pigment absorbs lightIt goes from a ground state to an excited state, which is unstable

CHLOROPHYLL IN A PHOTOSYSTEM

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

Thylakoid membranes are populated by two types of photosystems, I (P700) and II (P680)

Primary electionacceptor

Photon

Thylakoid

Light-harvestingcomplexes

Reactioncenter

Photosystem

STROMA

Th

yla

koid

m

em

bra

ne

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

e–

LIGHT REACTIONS ANAOLGY

Millmakes

ATP

ATP

e–

e–e–

e–

e–

Photo

n

Photosystem II Photosystem I

e–

e–

NADPH

Photo

n

 

Cytochromecomplex

Primaryacceptor

Primaryacceptor

H2O

O2

2 H

+1/2

P680

Light

Pigmentmolecules

Photosystem II(PS II)

Photosystem I(PS I)

Pq

Pc

ATP

1

2

3

5

6

7

8

Electron transport chain

Electron

transport

chain

P700

Light

+ HNADP

NADPH

NADP

reductase

Fd

e

e

e

e

4

e

e

LIGHT REACTIONSLINEAR (NON-CYCLIC)

ELECTRON FLOW

LIGHT REACTIONSCYCLIC ELECTRON FLOW

Primaryacceptor

Pq

Fd

Cytochromecomplex

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

ATPPhotosystem II Photosystem I

NADP+

Occasionally, electrons take a different pathIn cyclic electron flow

Only photosystem I is usedOnly ATP is produced

Chemiosmosis and Photophosphorylation

STROMA(low H concentration)

STROMA(low H concentration)

THYLAKOID SPACE(high H concentration)

Light

Photosystem II

Cytochromecomplex Photosystem I

Light

NADP

reductase

NADP + H

ToCalvinCycle

ATPsynthase

Thylakoidmembrane

2

1

3

NADPH

Fd

Pc

Pq

4 H+

4 H++2 H+

H+

ADP+P i

ATP

1/2

H2OO2

CALVIN CYCLE

The Calvin cycle uses ATP and NADPH from light reactions to convert CO2 to sugar

Sometimes called “dark” reactions or light independent reactionsOccurs in the stroma

The Calvin cycle has 3 phases

1.Carbon fixation

2.Reduction

3.Regeneration of the CO2 acceptor

Input

3 (Entering oneat a time)

CO2

Phase 1: Carbon fixation

Rubisco

3 P P

P6

Short-livedintermediate

6

6 ADP

ATP

6 P P

CalvinCycle

6 NADPH

6 NADP

6 P i

6 P

Phase 2: Reduction

Glyceraldehyde 3-phosphate(G3P)

P5G3P

ATP

3 ADP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

3 P PRibulose bisphosphate

(RuBP)

1 PG3P

(a sugar)Output

Glucose andother organiccompounds

3

Calvin Cycle

REVIEW OF PHOTOSYNTHESIS

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

Amino acidsFatty acids

Starch(storage)

Glucose (export)

G3P

Photosystem IIElectron transport chain

Photosystem I

Chloroplast