Chapter 10:

PHOTOSYNTHESIS

2. Light Absorption

1. Overview of Photosynthesis

3. The Light Reactions

4. The Calvin Cycle

1. Overview of Photosynthesis

Chapter Reading – pp. 185-190, 206-207

What is Photosynthesis?

The process of converting light energy (kinetic)

into energy stored in the covalent bonds of

glucose molecules (potential).

• carried out by photoautotrophs• plants, phytoplankton, cyanobacteria (any

photosynthetic organism)

• the basis of almost all ecosystems• all “food energy” ultimately comes from the sun

• source of all atmospheric oxygen (O2)

Light

energy

PHOTOSYNTHESIS

6 CO2 6+ H2O

Carbon dioxide Water

C6H12O6 6+ O2

Glucose Oxygen gas

(a) Plants

(c) Unicellular protist10 µm

1.5 µm

40 µm(d) Cyanobacteria

(e) Purple sulfur

bacteria

(b) Multicellular alga

Photosynthetic Organisms

5 µm

Mesophyll cell

StomataCO2 O2

Chloroplast

Mesophyll

Vein

Leaf cross section

Photosynthesis occurs in Chloroplasts

1 µm

Thylakoid

space

Chloroplast

Granum

Intermembrane

space

Inner

membrane

Outer

membrane

Stroma

Thylakoid

Chloroplast

Structure

Reactants: 6 CO2

Products:

12 H2O

6 O26 H2OC6H12O6

The Fate of Atoms Involved

in Photosynthesis

Revealed by experiments involving radioactive

isotopes in key molecules:

• 14C in CO2 and 18O in H2O and CO2

Light

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

[CH2O]

(sugar)

Two Stages of Photosynthesis

2. Light Absorption

Chapter Reading – pp. 190-193

UV

Visible light

InfraredMicro-waves

RadiowavesX-rays

Gamma

rays

103 m1 m

(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energyHigher energy

Shorter wavelength

The Electromagnetic Spectrum

Reflectedlight

Absorbedlight

Light

Chloroplast

Transmittedlight

Granum

Chlorophyll absorbs “non-green”

light

Green light passes

on through or is

reflected, causing

the leaves to

appear green

• only wavelengths

with exact amount

of energy to excite

an e- to a higher

orbital are absorbed

Galvanometer

Slit moves topass lightof selectedwavelength

Whitelight

Greenlight

Bluelight

The low transmittance(high absorption)reading indicates thatchlorophyll absorbsmost blue light.

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

Refractingprism

Photoelectrictube

Chlorophyllsolution

TECHNIQUE

1

2 3

4

Spectrophotometry

Spectrophotometers

measure the amount

of light passing

through a sample:

• measures %

absorbance

• measures %

transmittance

OR

Wavelength of light (nm)

(b) Action spectrum

(a) Absorption spectra

(c) Engelmann’sexperiment

Aerobic bacteria

RESULTS

Filamentof alga

Chloro-

phyll a Chlorophyll b

Carotenoids

500400 600 700

700600500400

Light-Absorbing Pigments

Chlorophyll a & b,

and carotenoids

Porphyrin ring:light-absorbing“head” of molecule;note magnesiumatom at center

in chlorophyll aCH3

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

CHOin chlorophyll b

(a) Excitation of isolated chlorophyll molecule

Heat

Excitedstate

(b) Fluorescence

PhotonGroundstate

Photon(fluorescence)

En

erg

y o

f ele

ctr

on

e–

Chlorophyllmolecule

Electrons absorb Photons• electrons excited to higher energy orbitals

3. The Light Reactions

Chapter Reading – pp. 194-199

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

e–

Pigmentmolecules

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Th

yla

ko

id m

em

bra

ne

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvesting

complexes

Photosystems

Each photosystem

in the thylakoid

membrane

consists of:

• an array of light-

absorbing pigments

• a reaction center

containing 2

molecules of

chlorophyll a and a

primary e- acceptor

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fd

NADP+

reductase

NADP+

+ H+

NADPH

8

7

e–

e–

6

Photosystem II(PS II)

The Light Reactions

Produces ATP (chemical energy) & NADPH (reducing power).

1

H2O ½ O2 + 2 H+ + 2 *e-

2e- transport chain

(ETC) pumps H+

into thylakoid

PS I

PS II

3 ATP Synthase

uses H+ flow to

make ATP

42 e- to

NADPHPS II

PS I

4 Stages of the Light Reactions

1) H2O split to O, 2 H+ & 2 high energy e- (*e-) in PS II

H2O O2 + H+ + *e-sunlight

2) Energy released by a series of *e- transfers is

used to generate H+ gradient

• H+ accumulates inside the thylakoid membrane

• *e- ends up in NADPH (an electron carrier)

3) H+ gradient used to make ATP (chemiosmosis)

4) *e- “re-energized” in PS I, passed on to NADP+

Mill

makes

ATP

e–

NADPH

e–

e–

e–

e–

e–

ATP

Photosystem II Photosystem I

e–

Electron Energy Levels

Key

Mitochondrion Chloroplast

CHLOROPLAST

STRUCTURE

MITOCHONDRION

STRUCTURE

Intermembrane

space

Inner

membrane

Electrontransport

chain

H+ Diffusion

Matrix

Higher [H+]

Lower [H+]

Stroma

ATP

synthase

ADP + Pi

H+ATP

Thylakoid

space

Thylakoid

membrane

ATP in Respiration vs Photosynthesis

Light

Fd

Cytochrome

complex

ADP

+

iH+

ATP

P

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

Summary of the Light Reactions

4. The Calvin Cycle

Chapter Reading – pp. 199-204

Overview of the Calvin Cycle

A series of reactions called the Calvin cycle

that synthesize glucose from CO2 and H2O:

CO2 + H2O C6H12O6 (glucose)ATP, NADPH

• can occur in dark (doesn’t require light directly)

• also occurs during daylight!

• takes place in the stroma of chloroplasts

• outside the thylakoids

• uses energy stored in ATP and NADPH

• produced by the light reactions

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

Input

CO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P6

1,3-Bisphosphoglycerate

6

P

P6

6

6 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 P

Output G3P(a sugar)

Glucose andother organiccompounds

CalvinCycle

3

3 ADP

ATP

5 P

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

G3P

The Calvin

Cycle

C4 leaf anatomy

Mesophyll cellPhotosyntheticcells of C4

plant leafBundle-sheathcell

Vein(vascular tissue)

Stoma

The C4 pathway

Mesophyllcell CO2

PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

PEP (3C)

ADP

ATP

Pyruvate (3C)

CO2

Bundle-sheathcell

CalvinCycle

Sugar

Vasculartissue

C4 Pathway helps retain H2O

Mechanism to store carbon

from CO2 in soluble form,

allowing stomata closure

during day to conserve water

CO2

Sugarcane

Mesophyllcell

CO2

C4

Bundle-sheathcell Organic acids

release CO2 to Calvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Pineapple

Night

Day

CAM

SugarSugar

Calvin

Cycle

CalvinCycle

Organic acid Organic acid

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

CO2 CO2

1

2

C4 compared to CAM

Crassulacean

Acid

Metabolism

• a slightly

different way

to fix CO2 at

night for use

during the day

without

opening

stomata

LightReactions:

Photosystem IIElectron transport chain

Photosystem IElectron transport chain

CO2

NADP+

ADP

Pi+

RuBP3-Phosphoglycerate

CalvinCycle

G3PATP

NADPHStarch(storage)

Sucrose (export)

Chloroplast

Light

H2O

O2

Summary of Photosynthesis

Key Terms for Chapter 10

• chlorophyll, carotenoids

• ATP synthase

• ATP, NADPH, photosystem, reaction center

• electron transport chain (ETC)

• photoautotroph

• chloroplast, thylakoid, stroma

• Light reactions, Calvin cycle

• C4 and CAM carbon fixation

Relevant

Chapter

Questions 1-8, 10, 12