Division of Labor in Chloroplasts

Green thylakoids• Capture light

• Liberate O2 from H2O

• Form ATP from ADP and phosphate

• Reduce NADP+ to NADPH

Colorless stroma• Contains water-soluble

enzymes

• Captures CO2

• Uses energy from ATP and NADPH for sugar synthesis

Light reactions

Dark reactions

Light(-dependent) reactions

Fig. 10-5, p. 152Wavelength (nm)400 500 600 700

0

20

40

60

80

100

chlorophyll b

chlorophyll a

Perc

ent

of

ligh

t abso

rbed

Absorption spectra of Chlorophyll a and b

Fig. 10-7, p. 154

pote

nti

al to

tra

nsfe

r ele

ctr

on

s (

measu

red

in

volt

s)

+0.8

+0.4

0

-0.6

ADP + Pi e

−

e−

e−

e−

e−

e−

NADPH

H+ + NADP+

P700

*

P70

0photosystem

I

photosystem IIreleased energy used to form ATPfrom ADP and phosphate

sunlightenergy

H2

O

sunlightenergy

P680

*

NONCYCLIC ELECTRON TRANSPORT

photolysis

P680: reaction center of photosystem II P700: reaction center of photosystem I

Pigments from the light harvesting complex

Fig. 10-7, p. 154

pote

nti

al to

tra

nsfe

r ele

ctr

on

s (

measu

red

in

volt

s)

+0.8

+0.4

0

-0.6

ADP + Pi e

−

e−

e−

e−

e−

e−

NADPH

H+ + NADP+

P700

*

P70

0

photosystem I

photosystem IIreleased energy used to form ATPfrom ADP and phosphate

sunlightenergy

H2

O

sunlightenergy

P680

*

CYCLIC ELECTRON TRANSPORT

photolysis

Fig. 10-3, p. 151

sunlight energy

oxygen released

H2O is split

H+

e−

H+

H+

NADP+

e−

H+

H+

carbohydrate end product (e.g. sucrose, starch, cellulose)

carbon dioxide used

Light-independe

nt reactions

sugar phosphate

Light-dependent reactionslumen

(H+ reservoir)

ADP + Pi

Stroma

electron transport system

photosystem II

photosystem I

electron transport system

Compare to respiration

Fig. 9-8c, p. 142

inner membranepyruvate from cytoplasm

Coenzymes give up electrons, hydrogen (H+) to transport system

NADH

NADH

FADH2

acetyl-CoA

TCA cycle

ATP

carbon dioxide

2

As electrons pass through system, H+ is pumped out from matrix

Oxygen accepts electrons, joins with 2H+, forms water

oxygen

INTERMEMBRANEspace

ATPsynthesized

ADPPi

H+

H+ H+ flows inH+

H+

H+

H+

electron transport system

H+

MATRIX

e−

e−

e−

Dark reactions

or

Light-independent reactions

Fig. 10-9, p. 157

(CO2 from the air)

stroma

Carbon dioxidefixation

(intermediates)

(PGA)

(RuBP)

rubiscoH2O

ADPPi

NADP+

(PGAL)

cyclic production of intermediate sugar phosphates

Calvin cycle

ADP

(PGAL)Pi sugar phosphate synthesis

typically used at once to form carbohydrates (mainlysucrose, starch, cellulose)sugar

phosphate

The Calvin cycle (C3 pathway of photosynthesis)

PGA: phosphoglyceric acidPGAL: phosphoglyceraldehydeRuBP: ribulose bisphosphateRubisco: ribulose bisphosphate carboxylase

The energy carriers ATP and NADPH (formed by photosystems I and II) are used to form high energy containing C-C and C-H bonds starting from H2O and CO2.

Through the Calvin cycle, plants capture CO2 and H2O and transform low energy containing C=O and H-O bonds into the high energy containing C-C and C-H bonds of sugar.

Rubisco is the worlds most abundant protein!

Fig. 10-9, p. 157

(CO2 from the air)

stroma

Carbon dioxidefixation

(intermediates)

(PGA)

(RuBP)

rubiscoH2O

ADPPi

NADP+

(PGAL)

cyclic production of intermediate sugar phosphates

Calvin cycle

ADP

(PGAL)Pi sugar phosphate synthesis

typically used at once to form carbohydrates (mainlysucrose, starch, cellulose)sugar

phosphate

Using ATP and NADPH to generate high energy containing covalent bondsPGA: phosphoglyceric acidPGAL: phosphoglyceraldehyde

C C OH

H C O

O H

HH

P

C C OH

H C O

H

HH

P

PGA

PGAL

ATP + NADPH

Low energy electrons

High energy electrons

Photorespiration

• When Rubisco uses O2, this will result in one molecule of PGA and one molecule of phosphoglycolate (a two-carbon molecule), instead of two PGA molecules (see the Calvin Cycle).

• Phosphoglycolate cannot be used in the calvin cycle and thus represents a loss of efficiency in photosynthesis.

• Photorespiration can cause up to a 25% reduction

in photosynthesis in C3 plants.

Photorespiration

C3 Plants

High rates of photorespiration (particularly on hot, bright days)

Produce less sugar during hot, bright days of summer

C4 Plants

Show little or no photorespiration

Produce 2 or 3 times more sugar than C3 plants during hot, bright days of summer

Fig. 10-10, p. 158

Corn, a C4 plant (right), is able to survive at a lower CO2 concentration than bean, a C3 plant (left), when they are grown together in a closed chamber in light for 10 days.

Fig. 10-12, p. 159

C4 cycle

AMP

mesophyll cells

C3 cycle

bundle sheath cells

Interaction between the C4 cycle and the C3 cycle

The C4 pathway concentrates CO2

Fig. 10-11, p. 159

airspace

mesophyll cells

vascular bundle

CO2 movementguardcell lower

epidermis

upperepidermis

bundlesheath cell

The C4 pathway concentrates CO2

In C4 plants, CO2 is first captured by PEP carboxylase in mesophyll cells to make oxaloacetate which is subsequently turned into malate. This malate then diffuses into the chloroplasts of bundle sheath cells where it releases CO2. Thus, bundle sheath chloroplasts contain higher CO2 concentrations compared to chloroplasts in mesophyll cells and therefore have higher photosynthesis and lower photorespiration rates.

However!

– The C4 pathway requires additional ATP for CO2 fixation.

– Thus, C4 plants only grow better than C3 plants under hot and dry environmental conditions.

Transforming CO2 and H2O into food

Light energy is captured to make ATP and NADPH via the action of photosystems I and II.

This ATP and NADPH is used via the Calvin cycle to transform the low energy containing C-O and H-O bonds of CO2 and H2O into the high energy containing C-C and C-H bonds of sugar.

In other words: Light energy from the sun is used by plants to increase the potential energy of electrons in the bonding orbitals of covalent bonds. This is done by replacing oxygen in C-O and H-O bonds by carbon or hydrogen, leading to the production of O2 and carbohydrates (sugars, starch, etc…).

SUMMARY: Transforming Light Energy into Chemical Energy

Consumption of photosynthesis products

1. AgricultureAnnual accumulation of light energy as C-H and C-C bonds (FOOD).

2. Fossil fuelsAccumulation of light energy as C-C and C-H bonds over millions of years

(accumulation of photosynthesis products over millions of years).

3. Energy intensive agriculture use of fossil fuels to increase agricultural yields (fertilizer and pesticide

production, irrigation, harvest, storage, transportation, etc…). Use of photosynthesis products of the past to increase FOOD yields (present photosynthesis productivity).

How do we maintain present levels of food production when fossil fuel sources become depleted?