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