ecture 16 Oct 7, 2005

1

Lecture 16 Oct 7, 2005

Photosynthesis I. Light Reactions

Lecture Outline Lecture Outline

1. Importance of Photosynthesis to all life on earth - primary producer, generates oxygen, ancient 2. What needs to be accomplished in photosynthesis 3. Structure of the chloroplast – 3 functional spaces 4. How light energy is harvested

– antenna complex, pigments, light spectrum - splitting of water, excitation of e^-

5. What work is done with capture light energy - “light” reactions

- noncyclic e^-transport - ATP and NADPH + H⁺ - cyclic electron transport – primarily ATP, limit O₂ 6. How ATP and NADPH + H⁺power anabolic pathways

- “dark” reactions – the Calvin Cycle

3 Figure 10.1

Photo

Photoauto autotrophs trophs Make their own “food” by light Heter

Heterotrophs otrophs Obtain “food” from “other” sources

∆G < 0

Light energy

Motion Heat

Photo

trophsAuto Hetero trophs

4

Photosynthesis

Photosynthesis is the ultimate energy sourceis the ultimate energy source for almost all life on earth

for almost all life on earth Solar

Energy Input

Herbivore Biomass Production

(Reflection/Heat)

(Heat Motion)

Carnivore Omnivore Biomass (Heat)

Plant Biomass Production

EnergyNet Absorbed

utilizedAnd

producersEat EnergyNet

utilized Net utilized

5

Photosynthesis Photosynthesis

H H

O O CO CO

O O

C C

H H

O O

Carbohydrate Carbohydrate Light

Light

Oxidized Carbon

Input

Reduced Carbon Output

Waste Product Basis for

Basis for Heterotroph Heterotroph Respiration Respiration

Photosynthesis is a remarkably similar process

at the molecular/cell biology level in a wide diversity of organisms

Evolutionarily Related Process, or an Evolutionarily Conserved Process

“ancient”

(a) Plants

(b) Multicellular algae

(c) Unicellular protist_{10 µm}

40 µm (d) Cyanobacteria

1.5 µm (e) Pruple sulfur

bacteria

Figure 10.2

Cyanobacteria

“blue-green algae”

Prokaryotes Euglena

Euglena Chlamydamonas Chlamydamonas Photosyntheic

Protists

Plants Plants

Non-Vascular Plants true algae bryophytes

-liverworts -mosses Vascular Plants

Ferns Gymnosperms

-conifers Angiosperms -monocots - dicots

Photosynthetic

Organisms

Photosynthesis ^–

is comprised of TWO Distinct Processes which occur simultaneously

(in most photosynthetic organisms)

Energy Capture Processes

Energy Utilization Processes

Make Carbohydrate Make Carbohydrate NEEDNEED ATP and NADPHATP and NADPH use

use light to light to

Make Make

“Light”

Reactions

“Dark” Reactions

Calvin Cycle

9

H2O CO2

Light

LIGHT REACTIONS

CALVIN CYCLE

Chloroplast

[CH2O]

(sugar) NADPH

NADP ⁺

ADP + P

O2

ATP

Figure 10.5

Light Light Reactions Reactions

(energy (energy capture) capture)

“Dark Reactions”

Calvin Cycle

(energy utilization)

Interdependent Interdependent

10

Structures all Photosynthetic Eukaryotes have in common

The organelle called the Chloroplast

This organelle is the SITE of photosynthesis where ALL photosynthetic reactions occur

Blue green algae (cyanobacteria)

do not have internal membranes (they are prokaryotes!) but they themselves

resemble chloroplasts

The extensively folded plasma membrane of cyanobacteria lays the same role

as thylakoid membrane in chloroplasts

11

Chloroplast

Mesophyll Cell

5 µm

Outer membrane

Intermembrane space Inner membrane

Thylakoid Thylakoid Granum

Granum Stroma

Stroma

1 µm

Vein Leaf cross section

Figure 10.3

Mesophyll

CO2 O2

Stomata

12

13

Chloroplasts Chloroplasts

-Contain their own DNA

-Contain bacterial-like ribosomes

-Believed derived from prokaryotic ancestor cyanobacterium = blue-green alga -Double membrane organelle

defines three three functional spaces functional spaces

14

Stroma

Thylakoid Space

Thylakoid Membrane

Intermembrane Space (transports things in and out of

the chloroplast, but not central to photosynthesis itself

Inner

Inner ChlorplastChlorplast Membrane Membrane

OuterChlorplast Membrane

3 Central Players

H

Thylakoid Space

Stroma

Stroma -

Thylakoid

Thylakoid SpaceSpace -- is the is the transient energy storage transient energy storage

shed for H shed for H⁺⁺ ionsions

generated generated in the in the light light reactions reactions

pH5.5 pH 8.5

Thylakoid

Thylakoid Membrane Membrane – Light Harvesting Complex Photosystem

Photosystem II II -

- Antenna Complex Antenna Complex

- - Water- Water -Splitting Complex Splitting Complex - - Reaction Center Reaction Center

“Excitation Complex”

“Excitation Complex”

17

Photon

Thylakoid Thylakoid

Photosystem PhotosystemIIII

STROMA STROMA

ThylakoidThylakoid membranemembrane

Transfer of energy

Pigment molecules

THYLAKOID SPACE THYLAKOID SPACE (INTERIOR OF THYLAKOID) (INTERIOR OF THYLAKOID)

Figure 10.12 Antenna

Antenna Light

Light--harvestingharvesting complexes complexes

Primary election acceptor

Reaction Reaction center center

Special chlorophyll a molecules

e^–

e^-

H₂O – O₂

Water Water Splitting Splitting Complex Complex

18

Photosystem Antenna Complex

- chlorophyll & accessory pigments

19

The Antenna Complex

proteins which hold PIGMENTS Pigments:

Chlorophylls - absorb all but greens Xanthophylls - absorb all but yellows Carotenoids - absorb all but orange/reds Phycocyanin - absorb all but blue-green

20

Reflected light

Reflected light - - the colors we see the colors we see

Light

Reflected Light Chloroplast

Absorbed

light Granum

Transmitted light Figure 10.7

21

The electromagnetic spectrum The electromagnetic spectrum

the higher the energy, the shorter the wavelength

Gamma

rays X-rays UV Infrared

Micro- waves

Radio waves 10^–5nm 10^–3nm 1 nm 10³nm 10⁶nm 1 m

10⁶nm 10³m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength Higher energy

Longer wavelength Lower energy

Figure 10.6 22

Absorption Spectra of Antenna Pigments Absorption Spectra of Antenna Pigments

Absorption of light byAbsorption of light by chloroplast pigmentschloroplast pigments

Chlorophyll Chlorophyll aa

Wavelength of light (nm)

Chlorophyll Chlorophyll bb

Carotenoids Carotenoids

Figure 10.9

Excitation of Chlorophyll by Light Excitation of Chlorophyll by Light

C CH CH2

C C CC

C C N N C H3C

C

C C

C C

C C C N C C C C N Mg H H3C

H

C CH2 CH₃

H CH3 C HH CH₂

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 molecule note magnesium atom at center

Hydrocarbon tail:

interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown

Figure 10.10 Excited

state

Energy of election

Ground e^–

HeatHeat

Photon Photon (fluorescence) (fluorescence)

Isolated chlorophyll chlorophyll when illuminated

– will fluoresce red fluoresce red, giving off light and heat

Blue light Blue light absorbed absorbed

e

Red light

Red light

Emitted

Emitted

With Heat

With Heat

25 Excited

state

Energy of election

Heat

Photon (fluorescence)

Chlorophyll molecule

Ground state Photon

e^–

Figure 10.11 A

Reaction Center Chlorophyll electron boosted to high energy level

Capture

x x x x

need replacement electron e^- transferred

to an

electron transport chain

26

Water splitting complex

(a protein in thylakoid membrane)

H O H

H O H O=O

H

H

H

H

e

e

e

e

Discard this, yuk

These e^- go to replace electron lost by chlorophyll

We’ll save H⁺in the thylakoid space

27

Stroma

Thylakoid Space Thylakoid

Membrane

H⁺

e^-

HO 2H

O=O (a gas)

H⁺

H⁺ H⁺

e^- e^-

e^-

HO H

HO^- H⁺

OH^- H O^- HO^-

e^- e^-

PS II

e^-

PS I

e^-

NADP⁺ NADPH

HH⁺⁺ ATPase ATPase

H⁺

H⁺ H⁺

H⁺ H⁺ H⁺

An “H

An “H⁺⁺pump”pump”

ADP+ P_i ATP

pH 5.5 pH 5.5 pH 8.5 pH 8.5

28

Key Players in the light reactions a. photosystem II: captures light

energy “boost” e

to a higher energy level, splits water into H

e

and O

b. Electron transport H

pump: lets e

“fall” to lower energy level, uses

energy to form H

gradient

29

c. another photosystem:

photosystem I: captures light

energy re-“boosts” e

to a higher energy level – forms NADPH + H

*makes reducing equivalents*

d. ATP synthase (H

ATPase): uses H

gradient to power ATP synthesis

30

• Produces NADPH, ATP, and oxygen

Figure 10.13 Photosystem II (PS II)

Photosystem-I (PS I) ATP

NADPH NADP⁺ ADP

CALVIN CYCLE CO₂ H2O

O2 [CH2O] (sugar) LIGHT

REACTIONS Light

Primary acceptor

Pq

Cytochrome complex

PC e

P680 e–

e^– O2 +

H2O 2 H+

Light

ATP

Primary acceptor

Fd ee–

NADP⁺ reductase Electron Tranchainsport

Electron transport chain

P700

Light

NADPH NADP⁺ + 2 H⁺

+ H+

1

5

7

2

3

4

6 8

Photosystem II -Light Energy used to

Form H

gradient (ATP Synthesis)

Photosystem I -Light Energy used

to make reducing equivalents (NADPH + H

)

Non- Non -Cyclic Cyclic Electron Electron

Flow Flow

Non Non -Cyclic Electron Flow - Cyclic Electron Flow

Mill makes

ATP ATP

e^–

e^– e^– e^–

e^–

e^–

e^– NADPH

ntohoP

Photosystem I -Light Energy can

also be used to make H

gradient)

cyclic electron flow cyclic electron flow

– photosystem I is used primarily – Primarily ATP is produced – Little O₂produced

Primary acceptor

Pq

Fd

Cytochrome complex

Pc

Primary acceptor

Fd

NADP⁺ reductase

NADPH NADP⁺

33

Cyclic e

flow

Photosystem Photosystem

II II Photosystem

Photosystem I I

Electron Electron Transport Transport

HH⁺⁺gradientgradient (ATP (ATP synthsynth))

NADP NADP

Reductase Reductase

34

LIGHT REACTOR

NADP⁺ ADP

ATP NADPH

CALVIN CYCLE

[CH2O] (sugar) STROMA

(Low H⁺concentration)

Photosystem II LIGHT

H2O CO2

Cytochrome complex

O2

H2O O2 1

1⁄₂ 2

Photosystem I Light

THYLAKOID SPACE (High H⁺concentration)

STROMA (Low H⁺concentration)

Thylakoid membrane

ATP synthase Pq

Pc

Fd NADP⁺ reductase

NADPH + H+ NADP++ 2H+

To Calvin cycle

ADP

P

ATP 3

H+ 2 H+

+2 H+ 2 H+

Figure 10.17

Light Dependent

Reactions Produce NADPH NADPH And ATP ATP To power

Calvin The Cycle

35

Next Time:

Next Time:

the DARK Side the DARK Side the Light

the Light independent reactions in dependent reactions The Calvin Cycle

The Calvin Cycle

36

Summary Summary

1.1. Photosynthesis ultimate source of energy for life Photosynthesis ultimate source of energy for life On earth

On earth

2.2. Ancient Process –Ancient Process – highly conservedhighly conserved

3.3. ThylakoidThylakoid membrane, Thylakoidmembrane, Thylakoid Space, StromaSpace, Stroma 4.4. Photosynthetic light reactionsPhotosynthetic light reactions

-capture energy from sunlight -capture energy from sunlight –– light harvesting pigmentslight harvesting pigments -use energy to -use energy to “split” water“split” water

-

-use energy to boost electron to high energy level use energy to boost electron to high energy level (PS II)(PS II) -

-electron transport lets electron fall to low energyelectron transport lets electron fall to low energy state, energy used to make

state, energy used to make HH⁺⁺ gradientgradient ((ATPATP)) -

-electron reelectron re--boosted by light absorption to highboosted by light absorption to high energy state

energy state (PS I)(PS I)

--high energy electron used to reduce NADPhigh energy electron used to reduce NADP⁺⁺ to to NADPH + H

NADPH + H⁺⁺

5.5. Can vary relative amount of ATP/NADPH madeCan vary relative amount of ATP/NADPH made by cyclic electron flow

by cyclic electron flow