The Alternative Oxidase

http://www.plantphys.net/article.php?ch=11&id=148

Relationship of oxygen consumption rate and inflorescence (spadix) temperature
in Eastern skunk cabbage Symplocarpus foetidus measured in the field in
Ontario, Canada. The points were gathered at 24-minute intervals from a single
plant over a 5-day period. Peak respiration occurs at a spadix temperature of
about 18°C, which occurs at an ambient temperature of about 3°C. Above this
level, the cluster of points at the upper right demonstrates the inverse
relationship between respiration and spadix temperature. When ambient
temperature dropped below about 3°C, however, heat production could not be
sustained, and spadix temperature dropped quickly to just above ambient,
resulting in the cluster of points at the lower left.

http://www.plantphys.net/printer.php?
ch=11&id=126

Lecture 20: Photosynthesis

 

By the end of this lecture you

should be able to…

Describe photosynthesis as an

ecological as well as a
physiological process.

Provide an overview of the

mechanics of photosynthesis
and discuss its importance as a
physiological and ecological
process.
Concepts and terms: light and
dark reactions, redox reaction

 Discuss anatomical features of

leaves important for
photosynthesis.
Concepts and terms: leaf
anatomy, chloroplast structure,
grana, stroma

 

Describe the energy

transformations in the light

reactions.
Concepts and terms: light

absorption, chlorophyll,

photosystems I and II, action

spectrum, NADPH, ATP, cyclic

and non-cyclic e flow.

Describe chemiosmosis.
Describe the Calvin cycle.

Concepts and terms: RUBISO,

3PG

Assigned reading: Chapter 10 in

the text.

From Larcher.

fs

From

Photons

to a

Plant

ps

ns



s

ms

s

hr

wk yr

EXCITED STATE

CHROMOPHORE

REACTION CENTER

MEMBRANE

CHLOROPLAST

LEAF

PLANT

GOV 92

4e

C

2

H

5

V

H

C

H

H

CH

3

CH

3

N

N

Mg

N

N

H

C

H

3

C

H

Light absorption

Primary photochemistry

Electron transport

O2 evolution

CO2 fixation

Metabolism

Productivity

?

?

C

6

H

12

O

6

+ 6O

2

+ 6H

2

O <->

12H

2

O + 6 CO

2

Respiration vs. photosynthesis…

C

6

H

12

O

6

+ 6H

2

O > 6CO

2

+ 6H

2

0

G = -686 kcal/mol; an exergonic reaction.

Exergonic versus endergonic reactions.

For a reaction to occur
spontaneously, H must decrease, S
must increase, or both must occur.
Spontaneous reaction have
negative free energy.

G = G

final

– G

initial

…spontaneous reactions proceed if
the change in free energy is
negative. At equilibrium G = 0.

Photosynthesis can be broadly separated into:

• light reactions - light dependent
• dark reactions - light independent

Light reactions are involved in:

• the oxidation of water to oxygen
• reduction of NADP

+

to NADPH and ATP

• occur in thylakoid membranes

Dark reactions :

• use NADPH and ATP to fix CO

2

into CH

2

O

• occur in stroma matrix of chloroplasts

Photosynthesis and respiration are redox reactions.

Photosynthetically active radiation.. 400 to 700 nm.

RUBISCO

It is a notoriously inefficient enzyme

About 10

7

tons of this enzyme is present on Earth;

this amounts to 29 kilograms of this enzyme per
person on Earth!

The active site, i.e., where CO

2

and O

2

are added,

is in the large subunit; it is formed at the
interface between two large subunits.

The enzyme RUBISCO in higher plants is

made up of 8 large (53 kilodaltons) and 8 small
(13 kilodaltons) subunits.

CARBOXYLA

TION

Sucrose

Starch

REGENERATION

Rubisco

GOV 94

REDUCTION

&

Phosphoglyceric

acid

Ribulose 1,5-

bisphosphate

Triose

phosphate

REGENERA

TION

Ribulose 5-

monophosphate

PHOSPHOR

YLATION

PHOSPHOR

YLATION

On thylakoid membrane:

1. Light absorption
2. Excitation energy transfer
3. Primary photochemistry
4. Electron transport leading to…

oxygen evolution
production of reducing power, NADPH
proton translocation

5. Formation of ATP

In stroma matrix:

1. CO

2

fixation by RUBISCO (C

3

and C

4

plants) and PEPcase in C

4

pants.

2. Reduction and phosphorylation of

phosphoglyceric acids to triose
phosphate using NADPH and ATP.

3. Regeneration of receptors of CO

2

(rubp)

4. Formation 6-C sugars and starch

RUBISCO=Ribulose bis phosphate carboxylase-

oxygenase

PEPC’ase = Phosphoenolpyruvate carboxylase

P

h

ATP

STROMA

2H

+

3H+

Q

A

LUMEN

2H

+

PQH2

2

PQH

PQH2

PQ

PC

PC

P700

A

0

FeS

Fd

2H

+

PQ

NADPH

3H+

ADP + P

i

PQ

H O

2

OEC

+

1

2

–

2

2H +

O

P

680

P

h

Q

A

Q

B

FeS

Cyt f

Cyt b

6

6

Cyt b

ADP

ATP

Cyt b

6

f

ATP

synthase

NADP+

+H+

3-phosphoglycerate

REDUCTION

Ribulose 1,5-

Bisphosphate

Sucrose

Synthesis

PSII

REGENERATION

FNR

CYTOSOL

CO

2

CARBOXYLA TION

Triose

phosphate

9

10

11

12

1

2

4

5

6

8

7

3

PSI

Starch

Synthesis