Overview of Photosynthesis
In photosynthesis, green plants absorb energy from the sun and use the
energy to drive an endothermic reaction, the reaction between carbon
dioxide and water that produces glucose and oxygen.
The process converts the light energy to chemical energy and stores it
in the bonds of the sugar.
Outline
•
Absorbing Light
•
Making Oxygen
•
Oxidation States
•
Homework
Absorbing Light with Organic Molecules
Representations of Organic Molecules
In this unit, we'll be concerned with many complex organic molecules. Organic molecules consist mainly
of carbon and hydrogen, but can also contain other elements. Because the molecules contain many atoms,
we need a way to represent the structures without having to draw all the atoms.
• A line in a structure will represent a carbon-carbon bond unless it is labeled with another atom.
• All carbon atoms will make 4 bonds (some combination of single, double, and triple bonds). Any
bonds not specified will be to hydrogen atoms.
For information on names of organic molecules, click here.
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Chlorophyll and Other Light-Absorbing Molecules
Chlorophyll is the pigment in green plants. You should remember from the discussion on color and
absorption spectroscopy that this pigment is green because it reflects green light and absorbs light in the
red and blue regions of the spectrum.
What is it about this molecule that causes it to absorb visible light? Let's look at the structure of
chlorophyll and several other light-absorbing molecules.
chlorophyll, green
hemoglobin, red
melanins, yellow and brown
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beta-carotene, orange
These are very different molecules but they have something in common. What is the common structural
motif? Each has conjugated multiple bonds, that is alternating single bonds and double bonds. There is a
long string of connected atoms (mostly carbon atoms) that, according to the hybrid orbital bonding
scheme, make sigma bonds with sp2 hybrid orbitals.
Each one of these atoms has one remaining p orbital and 1 electron. These p orbitals combine over the
whole molecule. When they combine, there is 1 pi orbital formed for every atomic p orbital. As the
number of p orbitals increases, the spacing between the orbitals decreases.
As you can see in the figure above, as the number of p orbitals that combine to form pi bonding and pi
antibonding orbitals increases, the spacing between them decreases. Chlorophyll has 24 p orbitals in
conjugation from 23 carbon atoms and 1 oxygen atom. Hemoglobin, the red pigment in blood, has 26 p
orbitals in conjugation. Beta-carotene, the colored molecule in carrots, has 22 p orbitals in conjugation.
One of the melanin molecules, the pigment in human skin, has 32 p orbitals in conjugation.
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The lowest energy light (in the red region) that
chlorophyll absorbs corresponds to the energy gap
between the highest filled pi orbital and the lowest
empty pi orbital.
The next band absorbed by the chlorophyll
molecule (in the blue region) corresponds to light
with the same energy as the gap between the
highest filled pi orbital and the second lowest
empty pi orbital.
Chlorophyll can absorb higher energy light, too,
but that light is in the UV.
There are chlorophyll molecules with slightly
different structures that absorb light that has slightly lower and slightly higher bands. Taken together, the
chlorophyll molecules do a good job of absorbing all the red and blue light, leaving only the middle
region of the spectrum to be reflected to our eyes.
When chlorophyll absorbs light an electron is promoted to a high energy state. The resulting excited state
molecule can transfer its energy and its electron to other molecules.
Making Oxygen
The light reaction of photosynthesis involves:
1.
Photoexcitation: the transfer of energy from light to form a high energy state of a molecule
2.
Reduction: a reaction in which a molecule adds an electron. This may involve adding a proton,
too, so the charge on the molecule doesn't necessarily change.
3.
Oxidation: a reaction in which a molecule loses an electron. This may involve losing a proton as
well so the charge on the molecule doesn't necessarily change.
4.
Energy transfer: using energy stored in a molecule to provide the needed energy for an
endothermic reaction.
Photoexcitation
We've seen that chlorophyll absorbs photons of light. Light causes an electron from an occupied orbital to
be promoted to a higher energy empty orbital. The excited state chlorophyll molecule has extra energy
that it can transfer to other molecules and provide the energy necessary for an endothermic reaction.
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The high energy electron makes chlorophyll a reducing agent. A reducing agent transfers electrons to
other molecules and reduces them. Chlorophyll transfers its electron to other molecules.
Because one electron was removed from a lower energy filled orbital, chlorophyll can also act as an
oxidizing agent. It can accept an electron from another molecule (oxidizing it) to fill its half empty pi
bonding orbital.
Let's represent the photoexcited chlorophyll molecule with its extra energy, oxidizing ability, and
reducing ability as:
Reduction
To talk about reduction, it is necessary to introduce you to a rather large biological molecule called
nicotinamide adenine dinucleotide phosphate or NADP+. The structure is shown below left. We'll
abbreviate most of this and focus on a smaller part.
The high energy electron from photoexcitation of chlorophyll passes from one molecule to the next and
eventually finds its way to NADP+. The intermediate is neutral and has an unpaired electron on a carbon
atom. Another electron, from a second photoexcited chlorophyll, adds to form an intermediate with a
negative charge on carbon. A proton adds in the final step to give NADPH.
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Two equivalents of photoexcited chlorophyll are required to make each equivalent of NADP+. Another
way to write the equation above is:
Oxidation
Once the photoexcited chlorophyll transfers its high energy electron to another molecule, it is left with an
electron hole. It can accept an electron from another molecule.
Below you see the net reaction. The source of the electrons is water. Transferring 2 electrons to 2
equivalents of the chlorophyll molecule gives 2 protons and 1/2 equivalent of molecular oxygen. This is
the way in which green plants make oxygen in photosynthesis.
Energy Transfer
Many biological reactions are energetically uphill and require an energy source. The energy given off by
the reaction of adenosine triphosphate, or ATP, with water is commonly used in cells to provide energy
for endothermic reactions.
Below is the reaction of ATP and water to make a phosphate and ADP (adenosine diphosphate). This
releases heat energy with a ΔHrxn of -20.5 kJ/mol. The change in Gibbs free energy is even higher with a
ΔGrxn of -30.5 kJ/mol. ATP is stored in cells and can be used in the hydrolysis reaction whenever energy
is needed for an endothermic reaction.
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In photosynthesis, the extra energy from the photoexcitation of chlorophyll is ultimately used to drive the
synthesis of ATP from ADP and phosphate.
The ΔGrxn of this reaction is +30.5 kJ/mol so it can only go forward when it is coupled with the reaction
of excited state chlorophyll to ground state chlorophyll. The TOTAL Gibbs free energy for the sum of 2
reactions is a negative number.
It is actually a complicated process with several energy changes but we can think of it as this:
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Overall Reaction
The light reaction of photosynthesis takes water, ADP, and NADP+and converts them to protons, ATP, and
NADPH. All of these are used in the next step, reduction of carbon dioxide and formation of a sugar.
There is a YouTube video that goes into greater detail on the light reaction of photosynthesis.
Oxidation States
Oxidation States from Lewis Structures
The oxidation state of an atom gives us an indication of the
electron density around the atom and it helps to keep track
of the electron change in oxidation-reduction reactions. In
the oxidation state formalism, we consider that each atom in
a molecule or ion is bonded to the others through ionic
bonds.
For each atom in the structure:
1.
Break all 2-electron bonds and give both electrons
to the more electronegative of the bonding pair.
2.
Sum all electrons around the atom.
3.
Compare that number to the number of valence
electrons of that atom.
◦
total = number of valence electrons,
oxidation state is zero
◦
total > number of valence electrons, the
atom has a negative oxidation state (-1 for
every additional electron above the valence
number)
◦
4.
total < number of valence electrons, the
atom has a positive oxidation state (+1 for
every electron below the valence number)
Use Roman Numerals to indicate the oxidation
state.
Oxidation-Reduction Half Reactions
Let's look at the transformation of NADP to NADPH and focus on the 3 carbon atoms and the nitrogen
atom that seem to change their bonds (in red).
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The oxidation state of the nitrogen atom doesn't change but 2 of the carbon atoms decrease in oxidation
state. The carbon that bonds to the additional proton goes from -I to -II in oxidation state. One of the
carbons bonded to nitrogen goes from I to 0 oxidation state. Overall, the addition of the 2 electrons causes
the total of the oxidation states to decrease by 2 units.
In the conversion of water to 1/2 equivalent of molecular oxygen, we can see that the oxidation state of
the oxygen atom changes from -II to 0. Two electrons are lost and the sum of the oxidation states of the
atoms increases by 2 units.
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Balanced Oxidation-Reduction Reaction
The net, balanced reaction has the increases in oxidation states of some atoms equal to the decrease in
oxidation states of others.
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