Overview: The Process That Feeds the Biosphere Photosynthesis

converts solar energy into chemical energy

Directly or indirectly, photosynthesis nourishes almost the entire living world

Autotrophs are the producers of the biosphere

Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules

Figure 10.1

These organisms feed not only themselves but also most of the living world

© 2011 Pearson Education, Inc.

BioFlix: Photosynthesis

(a) Plants(b) Multicellular

alga

(c) Unicellularprotists

(d) Cyanobacteria

(e) Purple sulfurbacteria

10 m

1 m

40 m

Figure 10.2

Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2

Figure 10.3

Photosynthesis converts light energy to the chemical energy of foodThe structure and organization of plant cells allows for photosynthesis to occur thanks to chloroplasts

Chloroplasts: The Sites of Photosynthesis in PlantsLeaves are the major

locations of photosynthesis

Their green color is from chlorophyll, the green pigment within chloroplasts

Chloroplasts

Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

Each mesophyll cell contains 30–40 chloroplasts

Outermembrane

IntermembranespaceInnermembrane

1 m

Thylakoidspace

ThylakoidGranumStroma

ChloroplastFigure 10.4b

Figure 10.4c

Mesophyllcell

20 m

Tracking Atoms Through Photosynthesis: Scientific Inquiry

Photosynthesis is a complex series of reactions that can be summarized as the following equation:

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

© 2011 Pearson Education, Inc.

The Splitting of Water

Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product

Figure 10.5

Reactants:

Products:

6 CO2

6 H2O 6 O2

12 H2O

C6H12O6

Photosynthesis as a Redox ProcessPhotosynthesis reverses the direction of electron

flow compared to respirationPhotosynthesis is an endergonic process; the

energy boost is provided by light

Energy 6 CO2 6 H2O C6 H12 O6 6 O2

becomes reduced

becomes oxidized

The Two Stages of Photosynthesis: A Preview

Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

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

Light

LightReactions

Chloroplast

NADP

ADP

+ P i

H2O

Light

LightReactions

Chloroplast

ATP

NADPH

NADP

ADP

+ P i

H2O

O2

Light

LightReactions

CalvinCycle

Chloroplast

ATP

NADPH

NADP

ADP

+ P i

H2O CO2

O2

Light

LightReactions

CalvinCycle

Chloroplast

[CH2O](sugar)

ATP

NADPH

NADP

ADP

+ P i

H2O CO2

O2

The light reactions convert solar energy to the chemical energy of ATP and NADPHThylakoids turn light into chemical energy

The Nature of Sunlight

Light is a form of electromagnetic energy

Wavelength determines the type of electromagnetic energy

Each color is seen due to them being different wavelengths

Figure 10.7

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

Visible light

Shorter wavelength Longer wavelength

Lower energyHigher energy

380 450 500 550 600 650 700 750 nm

105 nm 103 nm 1 nm 103 nm 106 nm (109 nm) 103 m1 m

Photosynthetic Pigments: The Light Receptors Pigments are

substances that absorb visible light

Wavelengths that are not absorbed are reflected or transmitted

Leaves appear green because chlorophyll reflects and transmits green light

Chloroplast

LightReflectedlight

Absorbedlight

Transmittedlight

Granum

© 2011 Pearson Education, Inc.

Animation: Light and Pigments

Right-click slide / select “Play”

A spectrophotometer measures a pigment’s ability to absorb various wavelengths

This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

Figure 10.9

Whitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

Galvanometer

Slit moves topass lightof selectedwavelength.

Greenlight

High transmittance(low absorption):Chlorophyll absorbsvery little green light.

Bluelight

Low transmittance(high absorption):Chlorophyll absorbsmost blue light.

TECHNIQUE

(b) Action spectrum

(a) Absorptionspectra

Engelmann’sexperiment

(c)

Chloro-phyll a Chlorophyll b

Carotenoids

Wavelength of light (nm)

Ab

so

rpti

on

of

lig

ht

by

ch

loro

pla

st

pig

me

nts

Ra

te o

f p

ho

tos

yn

the

sis

(m

ea

su

red

by

O2

rele

as

e)

Aerobic bacteria

Filamentof alga

400 500 600 700

400 500 600 700

400 500 600 700

RESULTS

action spectrum profiles the relative effectiveness of different wavelengths

absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

• The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann

• Areas receiving wavelengths favorable to photosynthesis produced excess O2

• He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production

Structure of Chlorophyll

Hydrocarbon tail(H atoms not shown)

Porphyrin ring

CH3

CH3 in chlorophyll aCHO in chlorophyll b

• Chlorophyll a is the main photosynthetic pigment

• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis, or to absorb excess energy

Excitation of Chlorophyll by LightWhen a pigment absorbs

light, it goes from a ground state to an excited state, which is unstable

When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence

Figure 10.12

Excitedstate

Heat

e

Photon(fluorescence)

Groundstate

PhotonChlorophyll

molecule

En

erg

y o

f el

ectr

on

Fluorescence

If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes

A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes

The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center

(a) How a photosystem harvests light

Th

ylak

oid

mem

bra

ne

PhotonPhotosystem STROMA

Light-harvestingcomplexes

Reaction-centercomplex

Primaryelectronacceptor

Transferof energy

Special pair ofchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

e

Filled with various pigments

REDOX

Figure 10.13b

(b) Structure of photosystem II

Th

ylak

oid

mem

bra

ne Chlorophyll STROMA

Proteinsubunits THYLAKOID

SPACE

Linear Electron Flow During the light reactions, there are two possible routes for electron flow:

cyclic and linear

Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy

Light Reactions

Primaryacceptor

P680

Light

Pigmentmolecules

Photosystem II(PS II)

1

2e

• Photosystem II (PS II) functions first and is best at absorbing a wavelength of 680 nm

• The reaction-center chlorophyll a of PS II is called P680

• It is P680+ naturally

Types of Photosystems in the Thylakoid Membrane

Primaryacceptor

H2O

O2

2 H

+1/2

P680

Light

Pigmentmolecules

Photosystem II(PS II)

1

2

3

e

e

e

P680+ is a very strong oxidizing agent

H2O is split by enzymes, and electrons are transferred from the H atoms to P680+ P680

O2 is released

P680+ Activation: Photolysis

Cytochromecomplex

Primaryacceptor

H2O

O2

2 H

+

1/2

P680

Light

Pigmentmolecules

Photosystem II(PS II)

Pq

Pc

ATP

1

2

3

5

Electron transport chain

e

e

e

4

Energy released by “the fall” creates a proton gradient across thylakoid membrane

Diffusion of H+ (protons) across the membrane drives ATP synthesis

Electrons Are Passed Between Photosystems by ETC & Generate ATP

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

Electron transport chain

P700

Light

e

e

4

e

e

PSI Accepts Electrons From PSII

Transferred light energy excites P700, which loses an electron to an electron acceptor, creating P700+ (ready to take on more e-)

Reaction Center for PSI is P700

The primary electron acceptor of PS I passes electrons to the protein ferredoxin (Fd)

The electrons are then transferred to NADP+ and reduce it to NADPH, for the reactions of the Calvin cycle

This process also removes an H+ from the stroma

Photosystem I Redox

Linear Electron Flow

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 Reaction: Review

Photosystem II Photosystem I

Millmakes

ATP

ATP

NADPH

e

e

e

ee

e

e

Ph

oto

n

Ph

oto

n

Photosystem I

Primaryacceptor

Cytochromecomplex

Fd

Pc

ATP

Primaryacceptor

Pq

Fd

NADPH

NADP

reductase

NADP

+ H

Photosystem II

Cyclic Electron Flow

• Only photosystem I used: produces ATP, but not NADPH, no O2 released

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

A Comparison of Chemiosmosis in Chloroplasts and MitochondriaChloroplasts and mitochondria generate ATP

by chemiosmosis, but use different sources of energy

Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP

Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities

Mitochondrion Chloroplast

MITOCHONDRIONSTRUCTURE

CHLOROPLASTSTRUCTURE

Intermembranespace

Innermembrane

Matrix

Thylakoidspace

Thylakoidmembrane

Stroma

Electrontransport

chain

H Diffusion

ATPsynthase

H

ADP P iKey Higher [H ]

Lower [H ]

ATP

Figure 10.17

Figure 10.18

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

ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH

The Light Reactions: in Summary

The Calvin CycleGenerates sugars and starting materials through the use of ATP and electrons from NADPH

Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)

For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2

The Calvin cycle has three phases Carbon fixation (catalyzed by rubisco) Reduction Regeneration of the CO2 acceptor (RuBP)

Ribulose – 1,5 bisphosphate Carboxylase/Oxygenase RuBisCO

The Rubisco enzyme is the most common enzyme on the planet!

Vital for Glucose formation!

Input

3 (Entering oneat a time)

CO2

Phase 1: Carbon fixation

Rubisco

3 P P

P6

Short-livedintermediate

3-Phosphoglycerate3 P P

Ribulose bisphosphate(RuBP)

Input

3 (Entering oneat a time)

CO2

Phase 1: Carbon fixation

Rubisco

3 P P

P6

Short-livedintermediate

3-Phosphoglycerate6

6 ADP

ATP

6 P P1,3-Bisphosphoglycerate

CalvinCycle

6 NADPH

6 NADP

6 P i

6 P

Phase 2: Reduction

Glyceraldehyde 3-phosphate(G3P)

3 P PRibulose bisphosphate

(RuBP)

1 PG3P

(a sugar)Output

Glucose andother organiccompounds

Input

3 (Entering oneat a time)

CO2

Phase 1: Carbon fixation

Rubisco

3 P P

P6

Short-livedintermediate

3-Phosphoglycerate6

6 ADP

ATP

6 P P1,3-Bisphosphoglycerate

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: Recap

The Importance of Photosynthesis: A ReviewThe energy entering chloroplasts as sunlight gets

stored as chemical energy in organic compoundsSugar made in the chloroplasts supplies chemical

energy and carbon skeletons to synthesize the organic molecules of cells

Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits

In addition to food production, photosynthesis produces the O2 in our atmosphere

Light

LightReactions:

Photosystem IIElectron transport chain

Photosystem IElectron transport chain

NADP

ADP

+ P i

RuBP

ATP

NADPH

3-Phosphoglycerate

CalvinCycle

G3PStarch(storage)

Sucrose (export)

Chloroplast

H2O CO2

O2

Figure 10.22

Figure 10.UN02

Primaryacceptor

Primaryacceptor

Cytochromecomplex

NADP

reductase

Photosystem II

Photosystem IATP

Pq

Pc

Fd

NADP

+ H

NADPH

H2O

O2

Electron transport

chain

Electron transport

chain

Regeneration ofCO2 acceptor

Carbon fixation

Reduction

CalvinCycle

1 G3P (3C)

5 3C

3 5C 6 3C

3 CO2

Figure 10.UN03

Figure 10.UN04

pH 7

pH 4

pH 4

pH 8

ATP

Figure 10.UN05

Figure 10.UN06

Figure 10.UN07

Figure 10.UN08