
Light‐‐Dependent Reactions
Light
The Light Reactions
Non‐cyclic photophosphorylation

Non‐cyclic:
Non
cyclic: flow of electrons is unidirectional from water to NADP+
 Photophosphorylation: use of photons of light to drive phosphorylation of ADP to produce ATP via chemiosmosis
 produces ATP, NADPH and oxygen gas  referred to as “Z diagram” – shape of energy flow





Photosystems are embedded in the thylakoid
membrane.
membrane
They contain chlorophyll and accessory pigments that are associated with proteins.
A photosystem consists of an antenna complex
and a reaction centre.
section focuses on how eukaryotes carry out photosynthesis in chloroplasts
photosynthetic prokaryotes undergo similar processes, but occur in cytosol and along folds in cell membrane
Photosystems I & II
•
Of the many chlorophyll a molecules, only one can trigger the light reactions by donating its excited electron to a primary electron acceptor
•
The other chlorophyll a, chlorophyll b and carotenoid molecules function collaboratively as a light‐gathering antenna that absorbs photons and passes the energy from pigment to pigment until it reaches the one chlorophyll a molecule in an area called the reaction centre
1
Photosystem I and II



ETC of Photosynthesis
Photosystem I contains a specialized chlorophyll a molecule known as P700 since it best absorbs light with an average wavelength of 700 nm
Photosystem II contains a specialized chlorophyll a molecule known as P680 since it best absorbs light with an average wavelength of 680 nm
P700 and P680 chlorophyll a molecules are identical P700 and P680 chlorophyll a molecules are identical –
they simply absorb at slightly different wavelengths because of the effects of the proteins they are associated with in the reaction centre
3
1
H+
to the
Calvin Cycle
4
H+
H+
H+
H+
H+
H+
+
H+ H
H+
ADP + Pi
ATP
ETC of Photosynthesis
H+
ETC of Photosynthesis
electron carrier
6
3
2
1
H+
4
H+
H+
H+
H+
H+
H+
+
H+ H
H+
ADP + Pi
ATP
H+
to the
Calvin Cycle
5
to the
Calvin Cycle
$$ in the bank…
reducing power
2
Steps of Light
Steps of Light‐‐Dependent Reactions
Step 1: Photosystem II (PSII)



• PSII passes electrons to PSI via an electron transport system, which contains the b6‐f complex
• complex acts as a proton pump to produce a proton gradient across the thylakoid membrane
• electrons lost from the reaction centre of PSII are replenished by the oxidation of water
• PSI uses the electrons to reduce NADP+ to NADPH


Steps of Light‐‐Dependent Reactions
Steps of Light
Steps of Light
Steps of Light‐‐Dependent Reactions
Step 1: cont…d
 water
water‐splitting complex holds 2 water molecules in place splitting complex holds 2 water molecules in place as enzyme strips 4 electrons, one at a time
+
 P680 accepts electrons, one at a time and passes them on to primary electron carrier
 process occurs 4 times to form 1 oxygen molecule
 4
4 H+ (p
(protons) from the 2 water molecules remain in the )
thylakoid space or lumen
 oxygen atoms from water immediately form oxygen molecule
 oxygen is released by plants into environment
 this process can occur more than 200 times a second
P680 molecule in reaction centre of PSII absorbs a photon of light, exciting an electron
high‐energy electron is transferred to primary electron acceptor, P680 molecule has been oxidized
resulting positive ion P680+ is extremely electronegative and can remove electron from molecule of water
reduction of P680+ to P680 by electrons in water is facilitated by an enzyme subunit of PSII, called water‐
splitting complex
water‐splitting complex oxidizes a molecule of water, passing an electron to the P680+ to make it neutral again


from the primary electron acceptor, energized electrons are transferred one by one, along electron transport chain
transferred one‐by‐one, along electron transport chain
with each transfer of electrons, small amounts of energy are released
Step 2: Oxidation‐Reduction Plastoquinone (PQ)
 electrons are transferred from primary electron acceptor to PQ  PQ acts as an electron shuttle between PSII and b6‐f cytochrome complex
3
Steps of Light‐‐Dependent Reactions
Steps of Light
Step 3: b6‐f complex – proton gradient formed
 released energy is used by b6‐f cytochrome complex to pump f cytochrome complex to pump H+ from stroma, across thylakoid membrane and into thylakoid space
 generates a hydrogen ion concentration or proton gradient across thylakoid membrane
Step 4: Electron transfer from b6‐f complex to plastocyanin
 from b6‐f complex, electrons pass to mobile carrier
plastocyanin
 plastocyanin shuttles electrons from b6‐f complex to PSI
Steps of Light‐‐Dependent Reactions
Steps of Light
Step 5: Photosystem I (PSI)
 photon of light is absorbed by PSI  energy is transferred to reaction centre P700 molecule and electrons are excited
 P700 transfers electron to the primary electron acceptor of PSI, forming P700+ (oxidation)
+  P700 can now act as an electron acceptor and is reduced back to P700 by the oxidation of plastocyanin (reduction)
7
y
p
y
(
)
Step 6: Electron transfer to NADP+ by ferredoxin
st
 1 electron from primary electron acceptor of PSI is transferred to ferredoxin (Fd), an iron‐sulfur protein
+
 oxidation of Fd results in transfer of electron to NADP , reducing it to NADP
Steps of Light‐‐Dependent Reactions
Steps of Light
Step 7: Formation of NADPH
nd electron is transferred to NADP by another  2
molecule of Fd
nd electron and a proton (H+) from the stroma  this 2
are added to NADP by the NADP+ reductase to form NADPH
 NADPH is now carrying 2 high‐energy electrons
y g
g
gy
 Reducing power of NADPH will be used in the light‐
independent reactions
 concentration of protons in the stroma decreases due to NADPH formation
4
Making ATP by Chemiosmosis




Recall…during cellular respiration, a hydrogen ion or proton gradient across the inner mitochondrial membrane is used as a source of energy to produce ATP by chemiosmosis
in the electron transport chain of photosynthesis, a similar proton gradient is established across thylakoid membrane
like mitochondria chloroplasts also have an ATP synthase like mitochondria, chloroplasts also have an ATP synthase enzyme
ATP synthase is embedded in the thylakoid membrane and provides the only pathway for hydrogen ions to move down their concentration gradient
Making ATP by Chemiosmosis



Balance Sheet





to pass a single electron through the ETC from PSII to NADP+ requires 2 photons of light (one absorbed by PSII and a second by PSI)
process begins with oxidation of water and production of oxygen gas
How many photons need to be absorbed to produce a single molecule of oxygen?
2 H
H2O 4H
H+ + 4 e
‐ + O
O2
2 molecules of water must be oxidized to energize and remove 4 e‐
Therefore, to get 4 e‐ (one O2 molecule), need to absorb 8 photons of light (4 by each PS)
As hydrogen ions move down their concentration gradient through the ATP synthase molecule energy gradient through the ATP synthase molecule, energy of the gradient is used to generate ATP molecules
electrochemical gradient drives the photophosphorylation of ADP to ATP
1 ATP forms for every 4 protons that pass through the ATP synthase from the thylakoid lumen into the h
f
h h l k dl
h
stroma
Summary


noncyclic electron flow pushes electrons from water, where they are at a low state of potential energy to where they are at a low state of potential energy, to NADPH, where they are stored at a high state of potential energy
equipment of thylakoid membrane converts light energy to chemical energy stored in NADPH and ATP (oxygen is a by product)
(oxygen is a by‐product)
5
Cyclic Photophosphorylation
Cyclic Photophosphorylation





PSI can function independently of PSII through cyclic electron transport
In cyclic electron flow, the electron transport from PSI to ferredoxin (Fd) is not followed by an electron donation to NADP+ reductase complex
instead, reduced Fd donates electron back to b6‐f complex where it then is transferred to P700
generates proton gradient for ATP synthesis
does NOT generate NADPH or oxygen
Cyclic Photophosphorylation
Cyclic Photophosphorylation




energy absorbed from light is converted into chemical energy of ATP without oxidation of water or reduction of NADP+ to NADPH
ONLY occurs in PSI and ONLY produces ATP
electrons cycle and therefore, no source of electrons (water) is required
activities of cyclic and non‐cyclic pathways vary y
y
p
y
y
depending on amounts of ATP and NADPH required by reactions in chloroplasts
Photophosphorylation cyclic
photophosphorylation
In cyclic photophosphorylation, an electron in P700 is excited by a photon and begins taking the same path that it took in noncyclic photophosphorylation. However, the electron is not used to reduce NADP+ but instead is passed back to the b6‐f complex, where the energy is used to generate the proton gradient.
noncyclic
photophosphorylation
6
The Calvin Cycle
Occurs in the stroma of chloroplasts.
 Cyclical reactions similar to the Krebs Cycle.
C li l i
i il h K b C l

Divided into three phases:
1. Carbon Fixation
2. Reduction Reactions
3. Regeneration of RuBP

The Calvin Cycle
Phase 2: Reduction Reactions
 6 PGAs are phosphorylated by 6 ATPs to form 6 molecules of 1, 3‐BPG.
 6 NADPH molecules reduce the six 1,3‐BPG to 6 G3P or PGAL.
 One molecule of G3P exits the cycle as a final product.
The Calvin Cycle
Phase 1: Carbon Fixation
 3 CO2 are added to RuBP to form 3 unstable 6‐carbon intermediates.
 The intermediates split into six 3‐carbon molecules called PGA.
 These reactions are catalyzed by rubisco.
These reactions are catalyzed by rubisco
The Calvin Cycle
Phase 3: Regeneration of RuBP
 3 ATP are used to rearrange the remaining 5 G3P into 3 molecules of RuBP.
 The cycle continues with the RuBP fixing more CO2.
7
Calvin cycle
To Produce One G3P…
C
C
1C
C C C C C
3. Regeneration
of RuBP
3 RuBP + 3 CO2 + 9 ATP + 6 NADPH + 5 H2O
 9 ADP + 8 Pi + 6 NADP+ + G3P + 3 RuBP
C C C C C
C C C C C
RuBP
ribulose bisphosphate
starch,
sucrose,
cellulose
& more
C= C= C
H H H
|
| |
C– C– C
H
|
H
|
Stroma of chloroplast
H
|
Calvin Cycle
C
C
C
C
C
C
C
C
C
C
C
C
1. Carbon fixation
Rubisco
ribulose bisphosphate carboxylase
3 ADP
C
C
C
C
C
C
CO2
C C C C C C
5C
C
3 ATP
used
to make
glucose
C
C C C C C C
6C
C C C C C C
glyceraldehyde‐3‐P
G3P
3C
PGA
C C C
phosphoglycerate
3C
3C
6 NADP
C
C
C
C
C
C
C
C
C
C
C
C
6 ATP
2. Reduction
6 NADPH
C
C
C
C
C
C
6 ADP
To G‐3‐P and Beyond!
Carbon dioxide ( CO2)

Rubisco
Ribulose 1,5‐bisphosphate (RuBP) (5C)
 end product of Calvin cycle
d d t f C l i l
3‐phosphoglycerate (3C) (PGA)
Carbon fixation
3 ADP
Glyceraldehyde‐3‐P
6 ATP
 energy rich 3 carbon
sugar
6 ADP
Reforming
RuBP
3 ATP
2Pi
1,3‐bisphosphoglycerate (3C)
6 NADPH
Reverse of
glycolysis
6Pi
Glyceraldehyde 3‐phosphate (3C) (G3P)
Glyceraldehyde 3‐phosphate (3C)
y
y 3p
p
(3 )
Glyceraldehyde 3‐phosphate (3C) (G3P)

6 NADP+

G‐3‐P = important intermediate
G‐3‐P   glucose   carbohydrates
  lipids
  amino acids
  nucleic acids
Glucose and
other sugars
8