Photosynthesis
Also Known As…
All the reasons you’ll ever need to chop
down all the plants because we probably
don’t really need them anyway right?
Photosynthesis: The Details
 Photosynthesis is the result of two distinct
processes – the light reactions and the Calvin
cycle (dark reactions).
6CO2 + 6H2O  C6H12O6 + 6O2
 Remember…The light reactions provide the
energy molecules to the Calvin cycle which will
attach the carbons, oxygens and hydrogens
together to make glucose.
The Light Reactions

The light reactions provide the necessary molecules
needed to make glucose. In order to do this we must
do the following:
1. Photoexcitation – Absorb energy of sunlight using
chlorophyll molecules.
2. Electron transport – Move energized electrons
through a series of reactions that will release energy
that we can harness. These electrons eventually end
up with NADP to make NADPH – an electron carrier
(for our back pocket).
3. Chemiosmosis – Protons (H+) are pumped across
the membrane which builds a gradient that is relieved
by ATP synthase which makes ATP.
1. Photoexcitation
 Chlorophyll has double bonds that are made of
electrons.
 When a photon hits these double bonds,
electrons are given the energy and go from a
ground state to an excited state with more
energy.
 The electrons are released from the chlorophyll
and are picked up by a primary electron
acceptor. Chlorophyll is oxidized and the
acceptor is reduced.
Photosystems
 Photoexcitation takes place in the
photosystems – there are two in the light
reactions – P680 & P700. The numbers refer to
the wavelength of light each responds to best.
 P680 is also known as PSII.
 P700 is known as PSI.
 They are numbered as I and II in order of their
discovery – not their occurrence.
 A photosystem is made of a chlorophyll
molecule, antenna chlorophylls, accessory
pigments and a primary electron acceptor.
2. Electron Flow
 There are two paths that electrons may take to
get through the photosystems – non-cyclic and
cyclic.
 Non-cyclic electron flow occurs when there is
ample ATP available and it gets the electrons
to NADP+ so it can form NADPH as soon as
possible.
 Cyclic electron flow occurs when ATP levels
are low. The electrons follow a cyclic path that
yields extra ATP molecules and then eventually
gives the electrons to NADP+.
Non-cyclic Electron Flow
1.
2.
3.
4.
Photons hit P680 – electrons are excited and go to
primary electron acceptor.
They are then passed on to PQ (plastoquinone) and
then to the b6f cytochrome complex – a proton pump.
Protons are pumped from the stroma into the thylakoid
lumen. This will form a proton gradient that will couple
with ATP Synthase to yield ATP. (a la cell respiration)
Electrons leave proton pump and go to PC
(plastocyanin) and then are dumped onto P700. Here
they receive another blast of energy from photons and
once again sent to a primary electron acceptor.
The electron acceptor passes the electrons to
ferredoxin (FD) and then the electrons are passed on to
NADP reductase which will become reduced and form
NADPH.
Cyclic Electron Flow
 Everything about the cyclic flow is the same until you
hit FD (ferredoxin).
 NOW…Instead of going on to NADP, the electrons are
re-inserted just before the proton pump (b6f complex)
so they can do the whole “proton gradient makes a
problem that is fixed with ATP synthase and we get
ATP from it” thing.
 This path can cycle several times before returning to
the non-cyclic mode.
 The next step, the Calvin cycle, requires more ATP
than NADPH in order to make a glucose molecule.
3. Chemiosmosis
 This step occurs during the electron movement
associated with the light reactions.
 You need both NADPH and ATP to run the
Calvin cycle.
 The ATP is made by chemiosmosis involving
the proton pump (b6f), the proton gradient it
produces and the enzyme ATP Synthase.
 You have already seen this as part of the
cellular respiration process inside the
mitochondrion.
The Light Reactions
The Dark Side



You just made a bunch of ATP and NADPH from
the light reactions.
You now use these electrons and energy to put a
bunch of carbons together to make glucose – this
is the job of the Calvin cycle.
The Calvin cycle can be broken down into three
phases:
1. Carbon fixation – Putting carbons together.
2. Reduction reactions – Adding electrons & energy.
3. Regeneration of RuBP – Getting back to the start – it
is a cycle after all.
Calvin Cycle: Step 1
Carbon Fixation
 We will be approaching this in class as if we have
six Calvin cycles all working together.
 The source of carbon is carbon dioxide (CO2).
 A carbon atom from CO2 is added to the 5-C
ribulose biphosphate (RuBP) to form an unstable
six-carbon molecule that quickly splits into two 3-C
molecules of PGA or 3-phosphoglycerate.
 The first molecule made in this process has three
carbons – this is why this pathway of
photosynthesis is known as the C3 pathway.
Calvin Cycle: Step 2
Reduction Reactions
 Each PGA is phosphorylated by ATP to form
1,3-biphosphoglycerate.
 Next, a pair of electrons from NADPH reduces
the 1,3-biphosphoglycerate to produce G3P glyceraldehyde 3-phosphate.
 What you have just done was add electrons to
make bonds and a source of hydrogen for the
glucose.
Calvin Cycle: Step 3
Regeneration of RuBP
 You now have 6 molecules of G3P – five of
them will be used to regenerate three
molecules of RuBP. This process consumes
three molecules of ATP. The RuBP can now be
reused to fix more carbon from CO2.
 The G3P that is left over is now used to make
carbohydrates.
 This explanation is usually doubled to explain
the formation of a glucose molecule that has
six carbons.
All Together Now!
FIN