10 photosynthesis text

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Photosynthesis

• Plants are photoautotrophs

– They use the energy of sunlight to make organic molecules from water and carbon dioxide

Figure 10.1


• Photosynthesis

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

These organisms use light energy to drive the synthesis of organic molecules from carbon dioxideand (in most cases) water. They feed not onlythemselves, but the entire living world. (a) Onland, plants are the predominant producers offood. In aquatic environments, photosyntheticorganisms include (b) multicellular algae, suchas this kelp; (c) some unicellular protists, suchas Euglena; (d) the prokaryotes calledcyanobacteria; and (e) other photosyntheticprokaryotes, such as these purple sulfurbacteria, which produce sulfur (sphericalglobules) (c, d, e: LMs).

(a) Plants

(b) Multicellular algae

(c) Unicellular protist 10 m

40 m(d) Cyanobacteria

1.5 m(e) Pruple sulfurbacteria

Figure 10.2


Chloroplasts: The Sites of Photosynthesis in Plants

• The leaves of plants

– Are the major sites of photosynthesis

Vein

Leaf cross section

Figure 10.3

Mesophyll

CO2 O2Stomata


• Chloroplasts

– Are the organelles in which photosynthesis occurs

– Contain thylakoids and grana

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

ThylakoidGranumStroma

1 µm


Tracking Atoms Through Photosynthesis: Scientific Inquiry

• Photosynthesis is summarized as6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

Photosynthesis converts light energy (inorganic) to the chemical energy (organic, potential) of food

Chloroplasts split Chloroplasts split water into water into Hydrogen and Hydrogen and oxygen, oxygen, incorporating the incorporating the electrons of electrons of hydrogen into hydrogen into sugar moleculessugar molecules

6 CO2

12 H2O

Reactants:

Products: C6H12O66 H2O 6 O2

Figure 10.4


Photosynthesis as a Redox Process

• Photosynthesis is a redox process

– Water is oxidized, carbon dioxide is reduced

Photosynthesis consists of two processes

The light reactions-Occur in the grana-Split water, release oxygen, produce ATP,

and form NADPH

The Calvin cycle-Occurs in the stroma-Forms sugar from carbon dioxide, using ATP

for energy and NADPH for reducing power


• An overview of photosynthesis

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

[CH2O](sugar)

NADPH

NADP

ADP

+ P

O2Figure 10.5

ATP


light reactions

• The light reactions convert solar energy to the chemical energy of ATP and NADPH

LightIs a form of electromagnetic energy, which travels in waves

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

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 wavelengthHigher energy

Longer wavelengthLower energyFigure 10.6


• The visible light spectrum

– Includes the colors of light we can see

– Includes the wavelengths that drive photosynthesis

PigmentsAre substances that absorb visible light

LightReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight Figure 10.7

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 wavelengthHigher energy

Longer wavelengthLower energyFigure 10.6


• Chlorophyll a

– Is the main photosynthetic pigment

• Chlorophyll b

– Is an accessory pigment 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

Other accessory pigmentsAbsorb different wavelengths of light and pass the energy to chlorophyll a


Excitation of Chlorophyll by Light

• When a pigment absorbs light

– It goes from a ground state to an excited state, which is unstable

Excitedstate

Ene

rgy

of e

lect

ion

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A


• The spectrophotometer

– Is a machine that sends light through pigments and measures the fraction of light transmitted at each 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

Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.

EXPERIMENT

RESULTS

Ab

sorp

tion

of

ligh

t b

ych

loro

pla

st p

igm

en

ts

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


photosystem

• A photosystem

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

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–

The light-harvesting complexes consist of pigment molecules bound to particular proteins and help funnel the energy of photons of light to the reaction center

http://vcell.ndsu.nodak.edu/animations/photosynthesis/

movie.htm


• The thylakoid membrane

– Is populated by two types of photosystems, I and II

Figure 10.13Photosystem II

(PS II)

Photosystem-I(PS I)

ATP

NADPH

NADP+

ADPCALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

Pq

Cytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O2 H+

Light

ATP

Primaryacceptor

Fd

ee–

NADP+

reductase

ElectronTransportchain

Electron transport chain

P700

Light

NADPH

NADP+

+ 2 H+

+ H+


Light Phase1a. The pigments get excited by the photons

2a. All pigments will pass on their energy to a chlorophyll a in photosystem II-P680

3a. 2 chlorophyll a electrons become so excited they escape

4a. The electrons are captured by a primary electron acceptor

Figure 10.13 Photosystem II(PS II)

Photosystem-I(PS I)

ATPNADPH

NADP+

ADPCALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

PqCytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O2 H+

Light

ATP

Primaryacceptor

Fdee–

NADP+

reductase



P700Light

NADPH

NADP+

+ 2 H+

+ H+



Photosystem-I(PS I)

ATPNADPH

NADP+

ADPCALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

PqCytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O2 H+

Light

ATP

Primaryacceptor

Fdee–

NADP+

reductase



P700Light

NADPH

NADP+

+ 2 H+

+ H+

Photolysis (same time as 1-4 on last slide)1b. Water is split in a process called photolysis

2b. The water is split into 2 electrons, 2 protons (H+) and an O

3b. The O will bind to another O from a 2nd water molecule and escape from the thylakoid and plant as O2

4b. The water’s electrons are passed to PII to replace P680’s electrons that go to PI


Electron Transport Chain

1. Each photoexcited electron passes from PII to PI via an electron transport chain

2. The “fall” creates energy to help move H+ through protein channel from the stroma to the thylakoid space, which generates a concentration gradient of H+ (refer to information on chemiosmosis)


Photosystem-I(PS I)

ATPNADPH

NADP+

ADPCALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

PqCytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O2 H+

Light

ATP

Primaryacceptor

Fdee–

NADP+

reductase



P700Light

NADPH

NADP+

+ 2 H+

+ H+

http://vcell.ndsu.noda

k.edu/animations/

photosystemII/movie.htm


Chemiosmosis

LIGHTREACTOR

NADP+

ADP

ATP

NADPH

CALVINCYCLE

[CH2O] (sugar)STROMA(Low H+ concentration)

Photosystem II

LIGHT

H2O CO2

Cytochromecomplex

O2

H2O O21

1⁄2

2

Photosystem ILight

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane

ATPsynthase

PqPc

Fd

NADP+

reductase

NADPH + H+

NADP+ + 2H+

ToCalvincycle

ADP

PATP

3

H+

2 H++2 H+

2 H+

Figure 10.17


Chemiosmosis

3. H+ is pumped from stroma into thylakoid space (lumen)

4. The H+ then diffuse back into stroma through ATP-synthase channels-ATP is produced

LIGHTREACTOR

NADP+

ADP

ATP

NADPH

CALVINCYCLE

[CH2O] (sugar)STROMA(Low H+ concentration) Photosystem II

LIGHTH2O CO2

Cytochromecomplex

O2

H2O O21

1⁄22

Photosystem ILight



Thylakoidmembrane

ATPsynthase

PqPc

Fd

NADP+

reductase

NADPH + H+

NADP+ + 2H+

ToCalvincycle

ADP

PATP

3

H+

2 H++2 H+

2 H+

Figure 10.17


Chemiosmosis

5. ATP goes to Calvin cycle

LIGHTREACTOR

NADP+

ADP

ATP

NADPH

CALVINCYCLE

[CH2O] (sugar)STROMA(Low H+ concentration)

Photosystem II

LIGHT

H2O CO2

Cytochromecomplex

O2

H2O O21

1⁄2

2

Photosystem ILight



Thylakoidmembrane

ATPsynthase

PqPc

Fd

NADP+

reductase

NADPH + H+

NADP+ + 2H+

ToCalvincycle

ADP

PATP

3

H+

2 H++2 H+

2 H+

Figure 10.17



Photosystem-I(PS I)

ATPNADPH

NADP+

ADPCALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

PqCytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O2 H+

Light

ATP

Primaryacceptor

Fdee–

NADP+

reductase



P700Light

NADPH

NADP+

+ 2 H+

+ H+

photosystem I• 1. Pigments in photosystem I are energized by photons and

pass on energy to a P700 chlorophyll a

• 2. P 700 oxidizes

• 3. The electrons from P700 are replaced by the electrons coming from PII that were passed along the electron transport chain

4. The electrons from P700 are passed through a short electron transport chain reducing NADP+ to NADPH


• 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


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

+

H2O2 H+

Light

ATP

Primaryacceptor

Fd

ee–

NADP+

reductase



P700

Light

NADPH

NADP+

+ 2 H+

+ H+

Light Phase PII and PI Products• Produces NADPH, ATP, (both go to Calvin Cycle) and oxygen (by-

product)

1

5

7

2

3

4

6

8


Calvin cycle

• The Calvin cycle uses ATP and NADPH to convert CO2 to sugar

• The Calvin cycle

– Is similar to the citric acid cycle

– Occurs in the stroma

Also known aslight-independent phaseThe dark phaseThe Calvin-Benson cycle


• The Calvin cycle has three phases

– Carbon fixation

– Reduction

– Regeneration of the CO2 acceptor


• The Calvin cycle

(G3P)

Input(Entering one

at a time)CO2

3

Rubisco

Short-livedintermediate

3 P P

3 P P

Ribulose bisphosphate(RuBP)

P

3-Phosphoglycerate

P6 P

6

1,3-Bisphoglycerate

6 NADPH

6 NADPH+

6 P

P6

Glyceraldehyde-3-phosphate(G3P)

6 ATP

3 ATP

3 ADP CALVINCYCLE

P5

P1

G3P(a sugar)Output

LightH2O CO2

LIGHTREACTION

ATP

NADPH

NADP+

ADP

[CH2O] (sugar)

CALVINCYCLE

Figure 10.18

O2

6 ADP

Glucose andother organiccompounds

Phase 1: Carbon fixation

Phase 2:Reduction

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)


Steps….• Carbon dioxide is reduced and bound to RuBP (ribulose biphosphate)

forming a 6C sugar

• This is called carbon fixation

• The 6C is unstable and breaks into 2 3C molecules of 3-PGA (3-phosphoglycerate)

• This all happens with the help of the enzyme rubisco (ribulose biphosphate carboxylase)

(G3P)

Input(Entering one

at a time)CO2

3

Rubisco


3 P P

3 P PRibulose bisphosphate(RuBP)

P3-Phosphoglycerate

P6 P

6

1,3-Bisphoglycerate6 NADPH

6 NADPH+

6 P

P6Glyceraldehyde-3-phosphate(G3P)

6 ATP

3 ATP3 ADP CALVIN

CYCLE

P5

P1 G3P(a sugar)Output

LightH2O CO2

LIGHTREACTIONATP

NADPH

NADP+

ADP

[CH2O] (sugar)

CALVINCYCLE

Figure 10.18

O2

6 ADP



Assuming we started with 3 CO2…• 6 molecules of 3-PGA consume energy from 6 molecules of ATP and

are reduced by electrons from 6 NADPH

• The reduction of the 3-PGA forms G3P

• 1 G3P (2 needed) will be used by the cell to form glucose and be used for cellular respiration

• 5 G3Ps will use 3 ATP molecules to be rearranged into 3 RuBPs (15 carbons total)

• The Calvin cycle will start again

(G3P)

Input(Entering one

at a time)CO2

3

Rubisco


3 P P


P3-Phosphoglycerate

P6 P

6


6 NADPH+

6 P


6 ATP

3 ATP3 ADP CALVIN

CYCLE

P5


LightH2O CO2

LIGHTREACTIONATP

NADPH

NADP+

ADP

[CH2O] (sugar)

CALVINCYCLE

Figure 10.18

O2

6 ADP



Photorespiration

• When there’s no CO2 present (when stomata are closed, such as when it is hot or dry to conserve water), RuBP will bind to O2 and produce a 2 carbon molecule that is broken down into CO2 and H2O

• No ATP or glucose are produced

• This is called photorespiration instead of photosynthesis

(G3P)

Input(Entering one

at a time)CO2

3

Rubisco


3 P P


P3-Phosphoglycerate

P6 P

6


6 NADPH+

6 P


6 ATP

3 ATP3 ADP CALVIN

CYCLE

P5


LightH2O CO2

LIGHTREACTIONATP

NADPH

NADP+

ADP

[CH2O] (sugar)

CALVINCYCLE

Figure 10.18

O2

6 ADP


http://vcell.ndsu.nodak.edu/animations/photosynthesis/movie.htm


C4 plants

• 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


C4 plants

• Some plants will not allow photorespiration to occur

• They fix CO2 to a C4 compound using a special enzyme, store it in bundle sheaf cells near leaf veins, and then break down the C4 to CO2 when needed

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

• CAM plants are similar to C4 plants, in that they can carry on photosynthesis even when their stomata are closed

• They only open their stomata at night to preserve water (they are found in very dry climates, named for the cacti species it was discovered in)

• They fix CO2 into a 4C at night, then break down and use the CO2 in the sunlight


CAM

• The CAM pathway is similar to the C4 pathway

Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in differenttypes of cells.

(a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times.

(b)

PineappleSugarcane

Bundle-sheath cell

Mesophyll Cell

Organic acid

CALVINCYCLE

Sugar

CO2 CO2

Organic acid

CALVINCYCLE

Sugar

C4 CAM

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Night

Day

1

2 Organic acidsrelease CO2 toCalvin cycle

Figure 10.20


The Importance of Photosynthesis: A Review

• A 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 3-Phosphoglycerate

Amino acidsFatty acids

Starch(storage)

Sucrose (export)

G3P

Photosystem IIElectron transport chain

Photosystem I

Chloroplast

Figure 10.21


• Organic compounds produced by photosynthesis

– Provide the energy and building material for ecosystems


Photosynthesis/Cellular Respiration Evolutionary Delimmas

• H2O in the PSII is broken to produce O2 which in evolutionary terms provided the atmosphere with

oxygen for the first time. Since oxygen is a necessary component of water, how did H2O exist

(70% of earth’s surface) before O2 was produced as a byproduct of PSII?

• Photosynthesis and Cellular Respiration are complete as one cycle (utilizing each other’s

products as reactants, etc.) and are in perfect harmony as co-systems. How could one

(photosynthesis for example) exist before the other, as evolutionists suspect?