CO2

Triose-PSugars

Electron Transp ortPhotop hosphory la tion

Sinks

C a lv in C yc le P ho to resp ira tion

CO2The photosynthetic apparatus must:

• Efficiently store solar energy in chemical bonds• Provide the correct ratio of products (NADPH, ATP)• Avoid over-excitation of the reaction centers which

can lead to photodamage.

The energetic intermediate is an electrochemical gradient of H+.

ADP + Pi

ATP

In the formulation of Mitchell, p = -(F

= mV(pH)

p is also called pmf or proton-motive force.

An electrochemical gradient of protons is the essential energetic intermediate in ATP synthesis

The Ratio of NADPH:ATP produced by photosynthesis is determined by:

1) The ratio of H+ pumped per e- transferred (H+/e-)2) The ratio of H+ transferred through the ATP synthase: ATP produced (n)

ADP + Pi

ATP

H+/e-

n

electron transfer(exergonic)

SH2

O2e-

ATP

ATP synthesis(endergonic)

In this case, the pH of the matrix and IMS are nearly the same, andessentially all pmf is stored as

ADP + Pi

ATP

It is commonly believed that, in chloroplasts, the component collapses as Cl- ions move in response to the electric field.Consequently, it is widely thought that pmf in thylakoids is stored solely as a pH

p = -(F

= mV(pH)

= mV(pH)

ADP + Pi

ATP

The proton gradient also plays a central role in the regulation of photosynthesis.

ADP + Pi

ATP

-Xanthophyll cycleqE quenching

heat

photochemistry

The relationships between the proton gradient and regulatory phenomena are fundamental to understanding how plants convert energy and respond to the environment.

Several new discoveries have altered our view of these relationships:•Structure/function of

•cytochrome bc complexes•ATP synthase

•The ability to probe key energy conserving reactions in vivo in the steady-state.

Historical Perspective of the H+/e-

1965 1970 1975 1980 1985 1990 1995

0

1

2

3

4

5

6

3: Low light2: High light

1976: Mitchell introduces the Q cycle

H+ /

e-

Year

1/3reductionof ATP

Linear H+/e-: Electron Pool

0.00 0.01 0.02 0.03

0.0

0.1

0.2

0.3

0.4

0.5

d (Electron Pool) / dt (a.u.)

d

dt

(a.u

.)

High-resolution structures provide an explanation for the proton pumping activity of the cytochrome bc1 (and related b6f complexes

• Assuming that pmf is in the form of pH and using a H+/ATP of n=4 1-5, the pH required to sustain measured values of GATP range between 1.7 and 2.2, suggesting the minimal lumen pH required ranges from 5.4 to 5.9.

• If a steady state component exists (pH=0.2 to 0.7), see 6a&b, this would increase the minimal pH range to 5.6 and 6.6.

How large of a pH is required to sustain photosynthesis?4

What is the value of n?

f

0 500 1000 1500 2000 2500 30000

20

40

60

80 Pea Tobacco Cucumber

Cyt

ochr

ome

R

educ

tion

half-

time

(ms)

Light Intensity ( mole m-2 sec-1)

5.5 6.0 6.5 7.0 7.5 8.0

20

40

60

80

pH

Cyt

ochr

ome

f R

educ

tion

half-

time

(ms)

Comparison of cytochrome f rereduction rates in intact leaves from pea (squares), tobacco (circles) and cucumber (triangles) with those from isolated thylakoids suspended at different pH values, adapted from12 (Inset). Observed half-times ranged from 20ms to 28ms for the entire range of light intensitities up to 2800 umoles m-2 s-1. Comparison to thylakoid data (dotted lines in inset12) suggests that the lumen pH is regulated so that it remains above ~ pH 6.

3b

Upper and Lower Bounds of Lumen pH Estimated by in situ Probes

Stromal pH

7.8

VDE1

B6f3

qE quenching7

4 5 6 7 8PC2

DegradesPSII2 OEC loses Ca2+

n=4 + 6

n=44

ATP5 activation

-20 0 20 40 60-10

0

10

20C

Figure 6

time (s)

048

12B

- I /

I 0 X

100

0

0

10

20A

-1 0 1 2 56 57 58 59 60-10

0

10

20 D

time (s)

-20 0 20 40 60 80-10

0

10

20

(a

.u.)

time (s)

500 550

-6-5-4-3-2-1012345

-I/I

X 1

000

(nm)

-60 -40 -20 0 20 40 60 80 100 120 140

0

2

4

(a

.u.)

time (s)

Summary