Overview of Photosynthesis

Transcription

1 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. Chemistry 104 Prof. Shapley page 1

2 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 Chemistry 104 Prof. Shapley page 2

3 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 sp 2 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. Chemistry 104 Prof. Shapley page 3

4 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. Chemistry 104 Prof. Shapley page 4

5 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. Chemistry 104 Prof. Shapley page 5

6 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 kj/mol. The change in Gibbs free energy is even higher with a ΔGrxn of kj/mol. ATP is stored in cells and can be used in the hydrolysis reaction whenever energy is needed for an endothermic reaction. Chemistry 104 Prof. Shapley page 6

7 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 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: Chemistry 104 Prof. Shapley page 7

8 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) total < number of valence electrons, the atom has a positive oxidation state (+1 for every electron below the valence number) 4. 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). Chemistry 104 Prof. Shapley page 8

9 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. Chemistry 104 Prof. Shapley page 9

10 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. Chemistry 104 Prof. Shapley page 10