Photosynthesis
Content :
Chloroplasts
In the previous sections we saw how glucose was degraded to CO2 and H2O in an oxidation process, yielding NADH and consequently ATP. Now we will look at the reverse reaction, the synthesis of glucose from CO2 and H2O. This process is called photosynthesis, without it life on earth as it exists now would not be possible. The location of photosynthesis in eukaryotic cells are the chloroplasts.
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Chloroplasts are organelles with an inner and outer membrane, which have similar properties as those of the mitochondria . The inside structures of the chloroplasts are called thylakoids. They are membranous sacs which can stack into multiple layers called grana. Grana can make bridges to neighboring grana called stroma lamellae. The entire structure, which has a green color is suspended in a liquid medium called stroma. When illuminated, the thylakoid membranes produce oxygen.
Photosynthesis includes light absorbing reactions and chemical reactions leading to the synthesis of glucose. The overall reaction can be formulated:
6 CO2 + 6 H2 O ® C6H12O6 + 6O2
This reaction requires chemical energy in form of ATP and NADPH . NADPH is a form of NADH which exists in the chloroplasts an like NADH it is a compound which is a strong reducing agent. A large amount of energy is required to make NADPH and chloroplast harvest this energy in form of light energy. We will now look at the process by which light energy is converted into chemical energy. All the reactions take place on enzyme complexes located in the thylokoid membrane.
Light reaction
Photosynthesis can be divided into two distinct events, reactions which require light and reactions which proceed without light. The dark reaction is involves the fixation of carbon dioxide, whereas the light reaction is associated with oxygen evolution.
The most important component of the light harvesting system (converts light energy into chemical energy) is chlorophyll . Eventhough they have a green color, there are different types of chlorophylls. For example plants have chlorophyll a and chlorophyll b, whereas chlorophyll c is found mainly in marine phytoplankton. The different chlorophylls have different absorption spectra, chlorophyll a absorbs light strongly at 680 nm (red light) and chlorophyll b at 660 nm. Absorption spectra
These wavelengths may change depending on the association of chlorophylls with protein. All chlorophylls have similar chemical structure consisting of a porphyrin group which captures the light energy and a very non polar phytol side chain which helps to solubilize the chlorophyll in the lipid bilayer.
In addition to chlorophyll other pigments are part of the light harvesting system. Carotenoids absorb light at blue wavelength and pass the enrergy to chlorophyll. Carotenoids are yellow to red, which are the fall collors of leaves, since carotenoids are more stable than the green chlorophyll.
Photosystems
Between 50 and 100 chlorophyll molecules and a somewhat smaller number of carotenoids and peptides are organized into a large complex called photosystem . In photosynthetic eukaryotes there are two different photosystems, photosystem I and II. The components of photosystem II are shown below.
The light energy harvested by the chlorophyll and carotenoid pigments is funneled to a special chlorophyll a molecule called P680. Together with certain enzymes it forms a complex called reaction center. P680 in this reaction center is the site, can transfer electrons excited by light energy to substances which can accept electrons . In addition to the photosystem there are chlorophyll and other pigment containing assemblies, which are called light harvesting complexes. They can funnel additional excitation energy to the photosystem. Photosystem II with is P680 reaction center can absorb light most efficiently at 680 nm Exitation of the photosystem
Electron flow in the thylakoid membrane
The scheme above shows how the reaction center in P680 can extract electrons from water. First light is absorbed by the chlorophyll molecules of photosystem II and the energy is funneled to chlorophyll a of P680. One electron of P680 becomes excited and in its excited state it has a high energy. It can reduce an electron carrier called plastoquinone. In this reduction P680 has given up one electron and becomes positively charged. P680 returns from its excited, positive state to the ground state and is now very positive. It is so positive that it can remove an electron from water, forming oxygen. Since it takes 2 H2O to form 1 O2 + 4 H+ + 4e-, the above cycle has to be repeated four times in order to remove the four electrons from two water molecules.
The scheme above (called Z-scheme) shows the continued passage of the electrons through several electron carrierers and another photosystem. The electrons are passed from plastoquinone to a so called b6f - complex, which is coupled to a proton pump, moving a proton from the stroma into the inner space of the thylakoid. Then the electrons are handed to plastocyanin and given to a second photosystem, the photosystem I. This system has a reaction center P700, which absorbs light most efficiently at 700 nm. In a similar mechanism as in P680, the energy is used to boost up the energy of the electrons, so they can be passed to a protein called ferredoxin. Now the electrons have such a high energy that they can be added to NADP+ to form the high energy compound NADPH. In an alternative pathway ferredoxin can return electrons to the b6f - complex, where they can move another proton. This pathway is called a cyclic electron flow, because the same electrons are boosted by P700 to continuously move electrons across the thylakoid membrane. Only when they are given to NADP+ they will be replaced by new electrons extracted from water. Because of this proton pumping and the production of protons from water when oxygen is formed, the inside of the thylakoids becomes very acidic. The energy of this gradient is used to drive an ATPase (ATP synthase) located in the thylakoid membrane. The protons move through the ATPase into the stroma (pH=7) producing ATP in the stroma. As a result of light absorption, reducing power in from of NADPH and chemical energy in form of ATP is formed as.
Dark reaction
During the light reaction NADPH (reducing power) and ATP (chemical energy) have been produced. In the dark reaction, which does not require any light, CO2 is converted into glucose. The reactions take place in the stroma and are known as the Calvin cycle. A highly simplified version of the Calvin cycle is shown below
The first step in carbon fixation is the addition of CO2 to a 5 carbon compound called ribulose 1,5-bisphosphate. This reaction does not require energy, only carbon dioxide and water. The reaction products are 2 molecules of 3-phosphoglycerate. The actual numbers used in the above sequence are 6 ribulose 1,5-bisphosphate + 6 CO2 because we formulated initially the overall reaction as:
6 CO2 + 6O2 ® C6H12O6 + 6O2 . The enzyme which catalyzes this first step is called ribulose 1,5-bisphosphocarboxylase (RuBP carboxylase). It is present in all photosynthetic organisms and is probably the most abundant protein on earth. The enzyme is very complex and its activity is controlled by a variety of factors . It is inhibited by high ADP and low NADPH concentrations and is active only at a pH close to 8. The second reaction in the cycle requires ATP to form 1,3-diphosphoglycerate followed by a reduction to yield glyceraldehyde 3-phosphate. Tow molecules of glyceraldehyde 3-phosphate will react to form glucose as outlined above. The remaining 10 molecules of glyceraldhyde-3-phosphate are recycled to ribulose 1,5-bisphosphate under the consumption of additional ATP. The reactions from 3-phosphoglycerate to glucose is in essence the glycolysis pathway in a reverse sequence.
Let’s examine how the activity of RuBP carboxylase is tied to the light cycle. When the light is on, NADPH is produced causing the NADPH concentration to increase. This activates RuBP carboxylase. Protons are moved into the thylakoid space which causes the pH in the stroma to raise. (the enzyme is only active at pH>7) . Finally the ADP concentration decreases (ADP was an inhibitor) since ATP is continuously made. This means that the Calvin cycle can only function if RuBP carboxylase is active and it is only active when ADP concentration is low, NADPH concentration is high and pH of stroma is high. All this is the case only if the light is on. The conditions which exist when the light is off (low NADPH, high ADP and pH=7) inhibit RuBP carboxylase.
Photorespiration
The affinity of RuBP carboxylase to oxygen is 10 times higher than to carbon dioxide. In order to prevent this reaction the carbon dioxide concentration in the chloroplasts must be kept high. In a complex reaction which involves three organelles and under consumption of energy plant cells have to convert the 2 carbon compound phoshoglycolate into the 3 carbon compound 3-phosphoglycerate. Under certain circumstances the plant can lose 50 % of the fixed carbon through photorespiration. photorespiration slide
Hatch Slack Cycle
Carbon dioxide is added to phosphoenolpyruvate to form oxaloacetate. The enzyme phosphoenolpyruvate carboxylase has a very high affinity for CO2. Oxaloacetate is reduced to malate consuming NADPH. Malate leaves the mesophyll cells and enters the bundle sheath cells, where malate gives up CO2 to from pyruvate. The CO2 is then fed into the Calvin cycle, which leads to the formation of glucose. Pyruvate leaves the bundle sheath cells and enters the mesophyll cell here it is converted into phosphoenolpyruvate and can now accept another CO2. This conversion consumes much energy in form of ATP. Through this mechanism C4 plants keep a very high CO2 pressure in their bundle sheath cells and can completely prevent photorespiration. C4 plants.
When the oxygen concentration is high, RuBP carboxylase can catalyzed the oxidation of ribulose 1,5-bisphosphate to phosphoglycolate and 3-phosphoglycerate instead of the normal reaction with CO
There a specialized plants which have a mechanism by which the can keep the carbon dioxide concentration in the chloroplasts very high and thus prevent photorespiration. These plants are known as C4 plants, since they use a pathway (called the Hatch Slack Cycle) in which the first metabolite containing the added CO2 is a 4 carbon atom compound. All other plants which don’t have this pathway are called C3 plants , since their first step in carbon fixations is the formation of a 3 carbon compound (3-phosphoglycerate). The leaves : of C4 plants are composed of an epidermis (outer cell layer), mesophyll cell and the bundle sheath cells. The Hatch Slack Cycle takes place in the mesophyll cells. 
At elevated temperatures the oxygen solubility in cellular fluids increases, which causes an increase in photorespiration in C3 plants. Thus in tropical rain forests with high plant densities and increased competition for CO2 C4 plants do much better, since C3 plants lose carbon through photorespiration. However, in cooler climates where the oxygen solubility is lower in cellular fluids and photorespiration is low, C3 plants do much better than C4 plants, because they don’t need any energy in the first carbon fixation step. Sugar cane, corn and many tropical plants are C4 plants. A variation of the CO2 metabolism in C4 plants is found in so called CAM plants
