Download 3.3 Photosynthesis… The Details

3.3 Photosynthesis…
The Details
•
Photosynthesis is divided into 2
sequential processes
1. Light reaction (stages 1 and 2)
2. Carbon Fixation (Calvin Cycle) (stage 3)
Overview of Photosynthesis
6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O
• Photosynthesis, like cellular respiration, is a series of redox reactions!
Tracking Atoms in Photosynthesis…
Reactants
6 CO2
Products
C6H12O6
12 H2O
6H2O
6O2
What does this suggest about photosynthesis?
The sugars that result from photosynthesis are
produced by adding the hydrogen ions and electrons
from water to carbon dioxide, NOT by splitting CO2 and
adding water!
oxidized
H+ and e-
6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O
reduced
The stages of photosynthesis…
Light Dependent Reactions
• Occurs along the thylakoid
membrane
• this is where H2O is
oxidized.
• chemiosmotic production of
ATP!
• electron carrier (NADPH) is synthesized
Light Reaction
• Divided into 3 parts;
1. Photoexcitation-absorption of a photon
by an electron of chlorophyll
2. Photosystems and the ETC
3. Chemiosmosis (sound familiar?)
Photoexcitation
• Before photons strike chlorophyll, their electrons
are at ground state (lowest potential energy)
• Once the photon strikes the electron it becomes
“excited” and moves to a higher potential
energy level
• The electron eventually goes back to ground
state (loss of potential energy appears as heat
and light-fluorescence if it is not used in
photosynthesis)
Photoexcitation
• Most chlorophyll do not fluoresce while
in a thylakoid memebrane b/c the
excited electron is captured by the
primary electron acceptor
• This is a redox rxn-the chlorphyll is
oxidized and the primary acceptor is
reduced
Photosystems and the ETC
ETC of Photosynthesis
Chloroplasts transform light energy
into chemical energy of ATP

generates O2
use electron carrier NADPH
Photosystem
Photon
PETC
Photosynthetic Electron Transfer Chain (PETC)
• series of electron carriers
which take electrons from
photosystem, and..
• ultimately carry electrons to
NADP+
Photosystems
• Experiments in the 1940’s suggested that light photons are
absorbed at 2 different points along the same PETC.…
• In fact, there are two Photosystems in operation
chlorophyll a
ETC of Photosynthesis
Photosystem II
chlorophyll b
Photosystem I
ETC of Photosynthesis
sun
1
e
e
Photosystem II
P680
chlorophyll a
Inhale, baby!
ETC of Photosynthesis
thylakoid
chloroplast
+H+ H+ H+
+ + +
H+ H+H
+H+ H H H
H
H+
ATP
+H+ H+ H+
+
H
H + + H+H+ H+
HH
SPLIT water!
H H
1
O
H
e-
e e
Photosystem II
P680
chlorophyll a
fill the e– vacancy
H+
e-
+H
OO
e
e
H
2
ETC of Photosynthesis
thylakoid
chloroplast
H+
+H+ H+ H+
+
H
H + + H+H+ H+
HH
+H+ H+ H+
H+ H+H
+ + + +
H+H H H H
ATP
3
2
1
e
e
H+
4
ATP
H+
to Calvin Cycle
H+
H+
H+
Photosystem II
P680
chlorophyll a
H+
H+
+
H+ H
ADP + Pi
ATP
H+
H+
energy to build
carbohydrates
ETC of Photosynthesis
e
e
5
e e
Photosystem II
P680
chlorophyll a
Photosystem I
P700
chlorophyll b
sun
ETC of Photosynthesis
electron carrier
6
e
e
5
sun
Photosystem II
P680
chlorophyll a
Photosystem I
P700
chlorophyll b
$$ in the bank…
reducing power!
ETC of Photosynthesis
sun
sun
+
+
+ H
H
+
+
H+ H +
H H
H+H+ H+ H
+
H
to Calvin Cycle
O
split H2O
ATP
Terms
• Primary electron acceptor-pheophytin (PSII)
• PQ-Plastoquinone (PSII)
• Z protein-splits water (PSII) the 2 electrons are used to replace
the ones lost in P680, oxygen leaves as “waste” and the
protons remain adding to the gradient
• B6-f cytochrome complex (PSII)
• Pc-Plastocyanin (PSII)
• Fd-ferredoxin (PSI)
• NADP reductase (PSI) uses 2 electrons and H+ to reduce
NADP+ to NADPH (b/c 2 electrons are needed this process
must occur twice)
• ATP is created by chemiosmosis due to the proton gradient in
the thylakoid lumen-this drives ATP synthase. 4 protons are
move into the lumen for each pair of electrons that move
through the ETC
ETC of Photosynthesis
• ETC uses light energy to produce
– ATP & NADPH (called noncyclic electron flow)
• go to Calvin cycle
• PS II absorbs light
– excited electron passes from chlorophyll to
“primary electron acceptor-pheophytin”
– need to replace electron in chlorophyll
– Enzyme (z protein) extracts electrons from H2O &
supplies them to chlorophyll
• splits H2O (h protons)
• O combines with another O to form O2
• O2 released to atmosphere
• and we breathe easier!
Noncyclic
Photophosphorylation
• Light reactions elevate
electrons in
2 steps (PS II & PS I)
– PS II generates
energy as ATP
– PS I generates
reducing power as NADPH
ATP
Cyclic photophosphorylation
• If PS I can’t pass electron
to NADP…it cycles back
to PS II & makes more
ATP, but no NADPH
– coordinates light reactions
to Calvin cycle
– Calvin cycle uses more
ATP than NADPH
18 ATP +
12 NADPH
 1 C6H12O6

ATP
Photophosphorylation
cyclic
photophosphorylation
NADP
NONcyclic
photophosphorylation
ATP
Light reactions
• Electron Transport Chain
• like in cellular respiration
– proteins in organelle membrane
– electron acceptors
• NADPH
– proton (H+)
gradient across
inner membrane
• find the double membrane!
– ATP synthase
enzyme
thylakoid
chloroplast
+H+ H+ H+
+ + +
H+ H+H
+H+ H H H
H
ATP
+H+ H+ H+
H+ H+H
+ + + +
H+H H H H
Photosynthesis summary
Where did the energy come from?
Where did the electrons come from?
Where did the H2O come from?
Where did the O2 come from?
Where did the O2 go?
Where did the H+ come from?
Where did the ATP come from?
What will the ATP be used for?
Where did the NADPH come from?
What will the NADPH be used for?
…stay tuned for the Calvin cycle
Practice Questions
• Q 1-8 p.166
The stages of photosynthesis…
Light Independent Reactions-Calvin Cycle
• Occurs in the stroma of the
chloroplast.
• Where CO2 is “fixed” into
sugars!
Whoops!
Wrong
Calvin…
The Calvin Cycle
Remember what it means to be a
plant…
• Need to produce all organic molecules
necessary for growth
– carbohydrates, lipids, proteins, nucleic acids
• Need to store chemical energy (ATP)
produced from light reactions
– in a more stable form
– that can be moved around plant
– saved for a rainy day
carbon + water + energy  glucose + oxygen
dioxide
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
Light reactions
• Convert solar energy to chemical
energy
ATP
– ATP
 energy
– NADPH
 reducing power
• What can we do now?
  build stuff !!
photosynthesis
How is that helpful?
• Want to make C6H12O6
– synthesis
– How? From what?
CO2 are available?
What raw materials
NADPH
carbon fixation
reduces CO2
NADP
C6H12O6
NADP
From CO2  C6H12O6
• CO2 has very little chemical energy
– fully oxidized
• C6H12O6 contains a lot of chemical energy
– highly reduced
• Synthesis = endergonic process
– put in a lot of energy
• Reduction of CO2  C6H12O6 proceeds in
many small uphill steps
– each catalyzed by a specific enzyme
– using energy stored in ATP & NADPH
From Light reactions to Calvin
cycle
• Calvin cycle
– chloroplast stroma
• Need products of light reactions to drive
stroma
synthesis reactions
– ATP
ATP
– NADPH
thylakoid
C
C
Calvin cycle
C C C C C
1C
C C C C C
3. Regeneration
C C C C C
of RuBP
RuBP
starch,
sucrose,
cellulose
& more
ribulose bisphosphate
3 ATP
H H H
|
| |
C–C–C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
CO2
1. Carbon fixation
C C C C C C
RuBisCo
ribulose
bisphosphate
carboxylase
3 ADP
used
to make
glucose
C=C=C
5C
C
C C C C C C
6C
C C C C C C
5C
glyceraldehyde-3-P
G3P
C C C
PGA
phosphoglycerate
3C
6 NADP
C
C
C
C
C
C
6 ATP
2. Reduction
6 NADPH
3C
C
C
C
C
C
C
3C
6 ADP
C
C
C
C
C
C
H
|
H
|
H
|
Remember
G3P?
glycolysis
glucose
C-C-C-C-C-C
2 ATP
2 ADP
fructose-1,6bP
P-C-C-C-C-C-C-P
DHAP
P-C-C-C
G3P
glyceraldehyde
3-phosphate
C-C-C-P
2 NAD+
2
4 ADP
Photosynthesis
pyruvate
C-C-C
4 ATP
To G3P and Beyond!
• Glyceraldehyde-3-P
– end product of Calvin cycle
– energy rich 3 carbon sugar
– “C3 photosynthesis”
• G3P is an important intermediate
• G3P
  glucose  
carbohydrates
  lipids   phospholipids, fats, waxes
  amino acids   proteins
  nucleic acids   DNA, RNA
RuBisCo
• Enzyme which fixes carbon from air
– ribulose bisphosphate carboxylase
– the most important enzyme in the world!
• it makes life out of air!
– definitely the most abundant enzyme
Accounting
• The accounting is complicated
– 3 turns of Calvin cycle = 1 G3P
– 3 CO2  1 G3P (3C)
– 6 turns of Calvin cycle = 1 C6H12O6 (6C)
– 6 CO2  1 C6H12O6 (6C)
– 18 ATP + 12 NADPH  1 C6H12O6
– any ATP left over from light reactions will
be used elsewhere by the cell
Photosynthesis summary
• Light reactions
– produced ATP
– produced NADPH
– consumed H2O
– produced O2 as byproduct
• Calvin cycle
– consumed CO2
– produced G3P (sugar)
– regenerated ADP
– regenerated NADP
ADP
NADP
Light Reactions
light  ATP + NADPH + O
2
energy
H 2O +
H2O
sunlight
Energy Building
Reactions
NADPH
ATP
O2
 produces ATP
 produces NADPH
 releases O2 as a
waste product
Calvin Cycle
CO2 + ATP + NADPH  C6H12O6 + ADP + NADP
CO2
ADP
NADP
Sugar
Building
Reactions
NADPH
ATP
sugars
 builds sugars
 uses ATP &
NADPH
 recycles ADP
& NADP
 back to make
more ATP &
NADPH
Putting it all together
light
CO2 + H2O + energy  C6H12O6 + O2
H2O
CO2
sunlight
ADP
Energy NADP
Building
Reactions
Sugar
Building
Reactions
NADPH
ATP
O2
sugars
Plants make both:
 energy
 ATP & NADPH
 sugars
even though
this equation
is a bit of a lie…
it makes a
better story
sun
Energy
cycle
Photosynthesis
light
CO2 + H2O + energy  C6H12O6 + O2
plants
CO2
glucose
H2O
animals, plants
ATP
C6H12O6 + O2  energy + CO2 + H2O
Cellular Respiration
The Great Circle
of Life,Mufasa!
ATP
O2
Summary of photosynthesis
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
•
•
•
•
•
•
•
•
•
•
Where did the CO2 come from?
Where did the CO2 go?
Where did the H2O come from?
Where did the H2O go?
Where did the energy come from?
What’s the energy used for?
What will the C6H12O6 be used for?
Where did the O2 come from?
Where will the O2 go?
What else is involved…not listed in this equation?
Supporting a biosphere
• On global scale,
photosynthesis is the
most important process
for the continuation of life on Earth
– each year photosynthesis…
• captures 121 billion tons of CO2
• synthesizes 160 billion tons of carbohydrate
– heterotrophs are dependent on plants as food
source for fuel & raw materials
The poetic perspective…
• All the solid material of every plant
was built by sunlight out of thin air
• All the solid material of every animal
was built from plant material
air
Then all the plants, cats,
dogs, elephants & people …
are really particles of air woven
together by strands of sunlight!
sun
Practice Questions
• Q 9-13 p.166