Photosynthesis is simply the process by which organisms convert solar
energy to chemical energy
12H20 + 6CO2
C6H12O6 + 6O2 + 6H2O
or, in a more balanced form:
6H20 + 6CO2
C6H12O6 + 6O2
This is an energy requiring reaction - the energy source is sunlight
Plants produce sugars as a source of food. However, they produce way
more than they need to survive. This is good because all other life on earth
must survive on the food energy obtained by this excess
All photosynthesis occurs in the chloroplast, so let's review the anatomy of a
Electron Micrograph of a Chloroplast
- The innermost membrane of the chloroplast is called the thylakoid
- The thylakoid membrane is folded upon itself forming many disks called grana
(singular = granum).
- The "cytoplasm" of the chloroplast is called the stroma
The photosynthesis reactions can be broken down into two components:
- The light-dependent reactions (the "light" reactions) - occur on
the thylakoid membranes
- The light-independent reactions (the "dark" reactions) - occur
in the stroma
The light-dependent reactions
- Goal: To trap sunlight energy and store it as chemical energy to use in
all life functions
- Why: ATP is a good source of energy, but it does not store well
- This is like money, if you had 1 million dollars, would you rather
carry around singles or gold?
- Where do these reactions occur?: on the membranes of the thylakoids
- How is it done?
- Wavelengths of light absorbed?
How do the light-dependent reactions proceed?
- A photon is absorbed by photosystem II (P680)
- An electron is raised from a low energy state to a high energy state
- the electron then falls down to the low energy state, releasing its
energy. However, this energy is not lost, it is picked up by an adjacent
pigment molecule where it is used to raise an electron to a higher energy
state, etc. etc., until this energy reaches the photosystem (like a bucket
brigade or "the wave" at a football game)
- At the photosystem, the electron is raised, but instead
of falling back down, it is stolen by another, electron defficient molecule
in the electron transport chain
- Meanwhile the photosystem's stolen electron is replenished by photolysis,
or the splitting of H2O to form H+and O2
(note: the H+ is kept in side the thylakoid membrane). The O2
resulting is the source of all oxygen in our atmosphere
- The electron travels down the electron transport system (ETS). Along the
way, more H+ is pumped into the thylakoid compartment.
- The electron eventually reaches photosystem I (P700), where it waits until
the electron is excited by another photon
- The electron is stolen by another electron acceptor from a second ETS
- The final fate of the electron is in converting NADP+ to NADPH
- The H+ is released to generate ATP
Non-Cyclic Photophosphorylation - A More Detailed Look
The form of photosynthesis with which we are most familiar is non-cyclic
photophosphorylation. It consists of two sets of pigments to excite. They are
called PS1, or photosystem 1, and PS2, or photosystem 2. PS1 is better excited
by light at about 700 nm, and is thus sometimes called P- 700. PS2 cannot use
photons of wavelength longer than 680 nm, and is thus sometimes called P- 680.
Energy enters the system when PS2 becomes excited by light. Electrons are
shed by the excited PS2 (oxidation), which grabs electrons from water, producing
a molecule of oxygen gas for every two waters split. PS2 thus returns it to its
unexcited state (reduction) . The electrons are passed through a chain of
oxidation-reduction reactions. Each arrow in the diagram above actually
represents a reaction like this one:
Each element in the pathway is reduced by the electrons, and turns right
around to reduce its neighbor in the pathway by giving it the electrons, thus
becoming reoxidized and ready for the next electrons to pass through the
Electrons leaving photosystem II are transferred transferred through a series
of molecules and ultimately end up transferred to to a molecule called
plastoquinone (PQ). The PQ reacts with two hydrogen ions from the stroma
and the two electrons from photosystem II to form PQH2.
2H+stroma + 2e- + PQ
The PQH2 diffuses across the thylakoid membrane, passes the two
electrons to the next electron carrier and releases the two hydrogen ions into
PQ + 2H+lumen + 2e-
PQ can then diffuse back across the membrane to repeat the process. The net
result of the Q cycle is to move two hydrogen ions from the stroma to the lumen.
Part of Photosystem II has the ability to split water and release oxygen.
PSII is the only known biological molecule capable of oxidizing water. The
electrons produced by the oxidation of water supply a steady source of electrons
for Photosystem II.
4H+ + 2e- + O2
Sometimes an organism has all the reductive power (NADPH) that it needs to
synthesize new carbon skeletons, but still needs ATP to power other activities
in the chloroplast. Many bacteria can shut off PS2, allowing the production of
ATP in the absence of glucose . A proton gradient is generated across the
membrane using the mechanisms of photosynthesis. This type of energy generation
is called cyclic photophosphorylation.
This may seem counter-intuitive. It appeared from noncyclic
phtotphosphorylation that PS1 was responsible for NADPH production, while in
cyclic photophosphorylation it is important for ATP production. This apparent
dichotomy can be resolved when we understand what makes PS1 both a good
candidate for noncyclic photophosphorylation and for NADPH production. PS1 is
very good at transferring an electron, whether it be to NADP or to ferredoxin (fd).
It is a powerful reductant. PS2, on the other hand, is better at grabbing
electrons from water to transfer them to quinone (Q). It is a good oxidant.
As you can see, the electron transferred is not derived from water, but from
PS1 itself. It therefore must be recycled to PS1.
The light-independent reactions
- Goal: to take the recently created NADPH and ATP and store their energy by
constructing sugars from CO2
- Where: in the stroma of the chloroplast
- Where does the CO2 come from?
- The atmosphere - the leaf opens up its stomates and
lets CO2 in
- When this happens, H2O is inadvertently released
- The plant must always balance its carbon intake with water loss
- How is CO2 converted into sugar?
- The energy is stored by converting CO2 into sugars in the Calvin-Benson
The Calvin-Benson Cycle
- A molecule of CO2 is taken in by the cell and is combined with RuBP
(a five carbon sugar, abbreviated as 5C) to form a 6C intermediate sugar via
an enzyme called RuBisCo. The 6C
then breaks down to form 2 PGA's (3-phosphoglycerate) - each a 3C
- The PGA's undergoes a cyclic pathway, the Calvin-Benson cycle, which will
eventually spit out 2 PGAL's (phosphoglyceraldehyde G3P in your book). Two
PGAL's can form a sugar phosphate, which can then form a sugar
One of the biggest faux pas (that's French for big
"mistakes") of evolution RuBisCo is not only attracted to CO2,
but it can also use
O2 in the Calvin-Benson Cycle
Why does photorespiration occur?
- When O2 is used in the Calvin-Benson Cycle, no energy is stored
- in fact, energy is lost!
- The reaction is as follows: O2 + RUBP
1 PGA + 1 Phosphogylcolate
- There is very little use for phosphoglycolate in the plant, so the
plant must spend energy to convert the phosphoglycolate back to a useful
molecule and reclaim the two carbons
- The conversion of phosphyglycolate occurs in the peroxysome
Rubisco can utilize O2 as a substrate instead of CO2
- When this evolved, the concentration of O2 was low - this was
not a problem
- Plants have since evolved ways to reduce the damage caused by O2
in the Calvin-Benson Cycle
- Plants must spend up to 40% of their energy stored in sugars to deal with
the damage created by RuBisCo fixing O2 in the Calvin-Benson
- O2 and CO2 bind at the same active site
- if O2 and CO2 are present in equal concentration, CO2
is fixed 80x faster
BUT the ratio of CO2/O2 in water in equilibrium with
air at 25oC=1/24
Therefore, for every 3 CO2 incorporated, there is 1 O2
- [CO2] in air=0.035%
- [O2] in air=21%
Major efforts have been made to modify the properties of Rubisco to eliminate
the oxygenation reaction, especially using molecular genetics
- The plant must then undergo a complex series of reactions to remove the O2
from the phosphoglycolate
Nature, however, has worked out a system to avoid photorespiration, it is called
the C4 photosynthetic pathway. We will discuss this next lecture -
- all the results so far indicate that the two reactions cannot be
- modifications in Rubisco that reduce the oxygenase activity also reduce
the carboxylase activity