UNIT 2: Metabolic Processes
Chapter 5: Photosynthesis: The Energy of Life pg. 210 - 240
5.2: Pathways of Photosynthesis
pg. 220 - 228
Light Dependent Reactions
Photosystem II and I are the two light capturing complexes used by
photoautotrophs to trap sunlight energy, transform it into ATP and NADPH.
Photosystem II
In photosystem II, water is split into electrons and protons and oxygen gas is
released. Sunlight energy (photons) strikes the antenna complex and excites
an electron in chlorophyll a molecule P680 becoming P680+. The electron
goes from ground state to excited state and then is passed onto the primary
electron acceptor.
The P680+ is now electronegative and requires an electron to become neutral
again. The P680+ molecule will remove an electron from a water molecule,
P680+ becomes P680, and water is split into a hydrogen proton and oxygen
molecule.
This process is enzyme facilitated in the water splitting complex inside the
thylakoid membrane. The electron released by P680 is accepted by
plastoquinone (PQ). Photosystem II can now start up again, accepting
photons. This process occurs twice to completely split a water molecule.
Figure 2: a) The actions of photosystem II begins when a photon energizes an electron in
P680, forming P680*. b) The energized chlorophyll (P680*) then transfers the high
energy electron to acceptor A in the reaction centre. P680 is now positively charged. c)
The positive P680* ion then oxidizes water, and the high energy electron is transferred
from the reaction centre to the carrier molecule plastoquinone (PQ). This process releases
both oxygen gas and protons into the lumen.
Linear Electron Transport and ATP Synthesis
The major steps of the Electron Transport System;
1. Oxidation P680: The absorption of light energy by photosystem II
results in the formation of an excited-state P680 (P680+) molecule.
This molecule is rapidly oxidized, transferring a high-energy electron
to the primary acceptor.
2. Oxidation-reduction of Plastoquinone: From the primary acceptor,
the electrons transfer to plastoquinone (PQ), which moves trough the
lipid bilayer and acts as an electron shuttle between photosystem II
and the Cytochrome complex. As plastoquinone accepts electrons
from photosystem II, it also gains protons (H+) from the stroma. When
PQ donates electron to the Cytochrome complex, it also releases
protons into the lumen, increasing the proton concentration there.
3. Electron transfer from Cytochrome complex and shuttling by
plastocyanin: From the Cytochrome complex, electrons pass to the
mobile carrier plastocyanin, which shuttles electrons from the
cytochrome complex to photosystem I.
4. Oxidation-reduction of P700: When a photon of light is absorbed by
photosystem I, an electron is excited and P700+ forms. The P700+
chlorophyll transfers its electron to the primary electron acceptor of
photosystem I, forming P700+. P700+ can now act as an electron
acceptor and is reduced back to P700 by the oxidation of plastocyanin.
5. Electron transfer to NADP+ by ferredoxin: The first electron from
P700+ is transported by a short sequence of carriers within
photosystem I. It is then transferred to ferredoxin, an iron-sulfur
protein. The oxidation of ferredoxin results in the transfer of the
electron to NAHP+, reducing it to NADPH.
6. Formation of NADPH: A second electron is transferred to NADP by
another molecule of ferredoxin. This second electron and a proton (H+)
from the stroma are added to NADP from NADP+ reductase to form
NADPH. NADPH is now carrying two high-energy electrons. The
concentration of protons in the stroma decreases as a result of this
NADPH formation. Along with the movement of protons from stroma
to lumen by plastoquinone and the splitting of water into protons,
these three processes create a much higher proton concentration inside
the lumen than outside in the stroma.
** This pathway is known as the non-cyclic pathway or linear.
H2O → 4 H+ + 4 e- + O2
For each electron passed from through the electron transport chain 2 photons
are required. One photon for each photosystem II and I. The oxidation of
water to release oxygen, therefore to get 4 electrons, it requires 8 photons.
Figure 3: Electron Transport in the Thylakoid, This model of the eukaryotic thylakoid
membrane illustrates the major protein and redox cofactors required for non-cyclic
electron transport and ATP synthesis by Chemiosmosis. The four major protein
complexes are photosystem II, the cytochrome complex, photosystem I, and ATP
synthase. The blue arrow illustrates the pathway of electron transport.
Chemiosmotic Synthesis of ATP
In the thylakoid membrane there are three mechanisms to synthesize ATP;
1. Protons are taken into the lumen by the reduction and oxidation of
plastoquinone as it moves from photosystem II to the Cytochrome
complex and back again.
2. The concentration of protons inside the lumen is increased by the
addition of two protons for each water molecule that split in the lumen.
3. The removal of one proton from the stroma for each NADPH
molecule formed decreases the concentration of protons in the stroma
outside the thylakoid.
There is a higher concentration of hydrogen protons in the thylakoid lumen
then in the stroma, creating a substantial proton-motive force. Hydrogen
protons are unable to pass through the thylakoid membrane but are forced to
leave the lumen and enter the stroma, passing through protein complexes of
ATP synthase, embedded in the membrane, by a process known as
Chemiosmosis. The free energy released by the protons is used to synthesize
ATP.
Figure 4: 1) a proton gradient is established by the carrying of protons across the
membrane by PQ, 2) by the releasing of protons into the lumen during the oxidation of
water, and 3- 4) by the removal of protons from the stroma during the reduction of
NADP+. 5) ATP is then synthesized as protons move through the ATP synthase complex.
In photosynthetic electron transport is the opposite of electron transport in
cellular respiration. Photosynthesis starts with a low energy molecule of
water, and ends in a high energy molecule of NADPH, created from the
movement of protons because of the proton gradient. Each step of the
represents an energy level change. Light energy is the input of energy that
drives the production of NADPH and ATP.
Figure 5: Energy Levels in the Electron Transport Chain, In the electron transport chain
of the light-dependent reactions, the energy level of the electrons is increased
dramatically by the light-absorbing actions of both photosystem II and I. The high energy
electrons at the end of the electron transport chain ultimately produce NADPH. pg. 224
Light Independent Reactions
Carbon dioxide is a low energy molecule, it is a waste molecule from
cellular respiration because is has no usable free energy. Sugar molecules,
such as; glucose (C6H12O6) store higher levels of free energy that can be
utilized by the cell for energy.
Photosynthesis is an eleven step process that uses NADPH and CO2 to
produce a high energy molecule in the form of a sugar molecule. This
endergonic reaction is facilitated by an enzyme during each step. The Calvin
cycle is responsible for the production of G3P and eventually glucose.
How the Calvin cycle Produces Carbohydrates
The Calvin cycle is divided into three phases: carbon fixation, reduction and
regeneration.
1. Carbon Fixation: During carbon fixation, carbon dioxide (inorganic)
is transformed into glucose (organic). Carbon dioxide enters the
Calvin cycle as an inorganic compound and reacts with ribulose 1-5
bisphosphate (RuBP), a 5 carbon sugar, to produce three carbon
molecules of 3-phosphoglycerate. (C3 metabolism)
2. Reduction: The molecules of 3-phosphoglycerate, are phosphorylated
by the hydrolysis of ATP, and oxidation of NADPH, creating
glyceraldehydes-3-phosphate (G3P) molecules. One molecule of G3P
is removed from the Calvin Cycle.
3. Regeneration: In this multi-step process, G3P molecules are
combined and rearranged to regenerate molecules of RuBP. RuBP is
reused over and over again in the Calvin Cycle.
Figure 7: The Calvin cycle reactions consist of three phases: carbon fixation (each CO2 is
incorporated into a 6-C compound), reduction (G3P is produced), and regeneration
(RuBP is re-formed). The sum of three turns of the cycle (three CO2 molecules)
ultimately produces one molecule of a 3-carbon sugar, G3P, which can then return to
participate in the light-dependent reactions.
Rubisco: The Most Abundant Protein on Earth
Rubisco – is ribulose 1.5-bisphosphate carboxylase oxygenase; a critical
enzyme that acts as a catalyst for the reduction of carbon dioxide in the
Calvin cycle of photosynthesis.
Ribulose-1,5-bisphosphate carboxylase oxygenase is the enzyme that
catalyzes the first reaction of the Calvin cycle. It catalyzes the CO2 fixation
in all photoautotrophs, which provides the organic carbon molecule (glucose)
for most of the world’s organisms.
A Diversity of Organic Products