Photosynthesis Photosynthesis is the process of converting energy from sunlight to energy in chemical bonds. The general equation describing photosynthesis

Photosynthesis

Photosynthesis is the process of converting energy from sunlight to energy in chemical bonds.

The general equation describing photosynthesis is:

light + 6 H2O + 6 CO2 C6H12O6 + 6 O2

C6H12O6 is glucose. Sometimes you will see CH2O or (CH2O)n. These are general formulas for glucose or any carbohydrate.

Dr. Abboud ElKichaoui

Islamic University- Biology and Biotechnology Department.

General Biology

The process of photosynthesis begins with light-absorbing pigments in plant cells. A pigment molecule is able to absorb the energy from light only within a narrow range of wavelengths. In order to absorb as much of the entire bandwidth from sunlight as possible, different pigments, capable of absorbing different wavelengths, act together to optimize energy absorption



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These pigments include the green chlorophyll a and chlorophyll b and the carotenoids, which are red, orange, or yellow. When the light is absorbed into one of these pigments, the energy from the light is incorporated into electrons within the atoms that make up the molecule.

These energized electrons (or “excited” electrons) are unstable and almost immediately re-emit the absorbed energy. The energy is then reabsorbed by electrons of a nearby pigment molecule.

The process of energy absorption, followed by re-emission of energy, continues, with the energy bouncing from one pigment molecule to another. The process ends when the energy is absorbed by one of two special chlorophyll a molecules, P680 and P700



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Chlorophyll a molecules (2)

P680

P700

or



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These two chlorophyll molecules, named with numbers that represent the wavelengths at which they absorb their maximum amounts of light (680 and 700 nanometers), are different from other chlorophyll molecules because of their association with various nearby pigments.

Together with these other pigments, chlorophyll P700 forms a pigment cluster called photosystem I (PS I). Chlorophyll P680 forms photosystem II (PS II).

• During the reactions, there are two possible routes for electron flow:

• Cyclic Photophosphorylation and

• Noncyclic Photophosphorylation



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1. Photosystem II.

Electrons trapped by P680 in photosystem II are energized by light. Two electrons are shown “moving” up, signifying an increase in their energy.

Light Reactions A) Noncyclic Photophosphorylation

Photophosphorylation is the process of making ATP from ADP and Pi (phosphorylation) using energy derived from light (photo). Noncyclic photophosphorylation begins with PS II and follows the steps:



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2. Primary electron acceptor.Two energized electrons are passed to a molecule called the primary electron acceptor. This electron acceptor is called “primary” because it is the first in a chain of electron acceptors.



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3. Electron transport chain. Electrons pass through an electron transport chain. This chain consists of proteins that pass electrons from one carrier protein to the next. Some carrier proteins, like ferredoxin and cytochrome, include nonprotein parts containing iron.



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• 4. Phosphorylation. As the two electrons move “down” the electron transport chain, they lose energy. The energy lost by the electrons as they pass along the electron transport chain is used to phosphorylate, on average, about 1.5 ATP molecules.



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5. Photosystem I. The electron transport chain terminates with PS I (with P700). Here the electrons are again energized by sunlight and passed to a primary electron acceptor (different from the one associated with PS II).



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6. NADPH. The two electrons pass through a short electron transport chain. At the end of the chain, the two electrons combine with NADP+ and H+ to form NADPH. NADPH is a coenzyme. Since the electrons have a considerable amount of energy left, NADPH is an energy-rich molecule.



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7. Photolysis.The two electrons that originated in PS II are now incorporated into NADPH. The loss of these two electrons from PS II is replaced when H2O is split into two electrons, 2 H+ and 1⁄2 O2. The process is called photolysis and literally means decomposition (lysis) by light (photo). A manganese-containing protein complex catalyzes the reaction. The two electrons from H2O replace the lost electrons from PS II. One of the H+ provides the H in NADPH.



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In summary, photophosphorylation takes the energy in light and the electrons in H2O to make the energy-rich molecules ATP and NADPH.

Because the reactions require light, they are often called the light- dependent reactions or, simply, light reactions.

The following equation informally summarizes the process:

H2O + ADP + Pi + NADP+ + light ATP + NADPH + O2 + H+



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A second photophosphorylation sequence occurs when the electrons energized in PS I are “recycled.”

In this sequence, energized electrons from PS I join with protein carriers and generate ATP as they pass along the electron transport chain. In contrast to noncyclic photophosphorylation where electrons become incorporated into NADPH, electrons in cyclic photophosphorylation return to PS I. Here they can be energized again to participate in cyclic or noncyclic photophosphorylation.

Cyclic photophosphorylation is considered a primitive form of photosynthesis but occurs simultaneously with noncyclic photophosphorylation.

B) Cyclic Photophosphorylation



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Calvin-Benson Cycle (Dark Reaction)

The Calvin-Benson cycle “fixes” CO2. That is, it takes chemically unreactive, inorganic CO2 and incorporates it into an organic molecule that can be used in biological systems. The biosynthetic pathway involves over a dozen products. The function of the pathway is to produce a single molecule of glucose (C6H12O6). In order to accomplish this, the Calvin-Benson cycle must repeat six times, and use 6 CO2 molecules. Thus, all the molecules involved have been multiplied by 6. Only the most important molecules are discussed.

C3



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1. Carboxylation: 6 CO2 combine with 6 RuBP to produce 12 PGA.

The enzyme RuBP carboxylase, or rubisco, catalyzes the merging of CO2 and RuBP (ribulose bisphosphate). The Calvin-Benson cycle is referred to as C3 photosynthesis because the first product formed, PGA (phosphoglycerate), contains three carbon atoms. Other names are the Calvin cycle and the carbon reduction cycle.

C3



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2. Reduction: 12 ATP and 12 NADPH are used to convert 12 PGA to 12 PGAL.

The energy in the ATP and NADPH molecules is incorporated into PGAL (glyceride 3-phosphate), thus making PGAL a very energy-rich molecule. ADP, Pi, and NADP+ are released and then re-energized in noncyclic photophosphorylation.

C3



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3. Regeneration: 6 ATP are used to convert 10 PGAL to 6 RuBP.

Regenerating the 6 RuBP originally used to combine with 6 CO2 allows the cycle to repeat.

C3



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4. Carbohydrate synthesis.

Note that 12 PGAL were created in step 2, but only 10 were used in step 3. What happened to the remaining two?

These two remaining PGAL are used to build glucose, a common energy-storing molecule. Other monosaccharides like fructose and maltose can also be formed. In addition, glucose molecules can be combined to form disaccharides like sucrose and polysaccharides like starch and cellulose.

C3



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You should recognize that no light is directly used in the Calvin-Benson cycle. Thus, these reactions are often called the light-independent reactions or even the dark reactions. But be careful—the all process (Photosynthesis) cannot occur in the dark. This is because it is dependent upon the energy from ATP and NADPH, and these two energy-rich molecules can be created only during photophosphorylation, which can occur only in light.



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• In summary, the Calvin-Benson cycle takes CO2 from the atmosphere and the energy in ATP and NADPH to create a glucose molecule. Of course, the energy in ATP and NADPH represents energy from the sun captured during photophosphorylation. The Calvin-Benson cycle can be informally summarized as follows:

6CO2 + 18ATP + 12NADPH + H

18ADP + 18Pi + 12NADP + 1 glucose

• Keep in mind that the reactions that occur during photosynthesis (and in any biosynthetic pathway) do not occur spontaneously. Every product formed in every reaction is catalyzed by an enzyme. In some reactions, coenzymes or metal-ion cofactors may also be involved.



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ChloroplastsChloroplasts are the sites where both the light-dependent and light-independent reactions of photosynthesis occur. Chloroplasts consist of an outside phospholipid bilayer membrane enclosing a fluid called the stroma (Figure 3-2). Suspended within the stroma are stacks of pancakelike

membranes. Individual membrane layers (the “pancakes”) are thylakoids; an entire stack of thylakoids is a granum (plural, grana). Within the thylakoids are the light-absorbing pigments and enzymes for the light-dependent reactions. In the surrounding stroma are the enzymes for the Calvin-Benson cycle. Thus, the light reactions occur on the thylakoid membranes, and the dark reactions occur in the stroma.



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Chemiosmotic TheoryChemiosmotic theory describes the mechanism by which ADP is phospho-rylated to ATP. The process illustrated in the figure listed below.



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1. H+ ions (protons) accumulate inside thylakoids.

The light reactions of photosynthesis occur in the membranes of the thylakoids. As photolysis occurs, H+ are created and released to the inside of the thylakoids (the oxygen is released to the outside). Also, H+ accompany the electrons as they pass along the electron transport chain between PS II and PS I. These H+ come from the stroma (outside the thylakoids) and are released to the inside of the thylakoids.



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2. A pH and electrical gradient across the thylakoid membrane is created.

As H+ accumulate inside the thylakoid, the pH decreases. Since some of these H+ come from outside the thylakoids (from the stroma), the H+ concentration decreases in the stroma and its pH increases. This creates a pH gradient consisting of differences in the concentration of H+ across the thylakoid membrane from a stroma pH 8 to a thylakoid pH 5 (a factor of 1000). Since H+ are positively charged, their accumulation on the inside of the thylakoid creates an electric gradient (or voltage) as well.



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3. ATP synthases generate ATP.

The pH and electrical gradient represent potential energy like water behind a dam. Similar to a dam, channel proteins, called ATP synthases, allow the H+ to flow through the thylakoid membrane and out to the stroma. The energy generated by the passage of the H+ (like the water through turbines in a dam) provides the energy for the ATP synthases to phosphorylate ADP to ATP. The passage of about three H+ is required to generate one ATP.



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Photorespiration

Because of its critical function in catalyzing the fixation of CO2 in all photosynthesizing plants, rubisco is the most common protein on earth.

However, it is not a particularly efficient molecule. In addition to its CO2-fixing capabilities, it is also able to fix oxygen. The biosynthetic pathway that leads to the fixation of oxygen is called photorespiration.



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Photorespiration leads to two problems.:

• The first problem is that the CO2-fixing efficiency is reduced because, instead of fixing only CO2, rubisco fixes some O2 as well.

• The second problem is that the products formed when O2 is combined with RuBP do not lead to the production of useful, energy-rich molecules like glucose. Instead, specialized cellular organelles, the peroxisomes, are found near chloroplasts, where they function to break down photorespiration products. Thus, considerable effort is made by plants to rid the cell of the products of photorespiration.

• Since the early atmosphere in which primitive plants originated contained very little oxygen, it is hypothesized that the early evolution of rubisco was not influenced by its O2-fixing handicap. Dr. Abboud ElKichaoui


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C4 PhotosynthesisImproving upon photosynthetic efficiency, some plants have evolved a special “add-on” feature to C3 photosynthesis.

When CO2 enters the leaf, it is absorbed by the usual photosynthesizing cells, the mesophyll cells. Instead of being fixed by rubisco into PGA, the CO2 combines with PEP (phosphoenolpyruvate) to form OAA (oxaloacetate or oxaloacetic acid).



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C4 Photosynthesis pathway



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The fixing enzyme is PEP carboxylase. OAA, the first product of this pathway, has 4 carbon atoms, thus the name C4 photosynthesis. OAA is then converted to malate, and the malate is shuttled to specialized cells within the leaf, the bundle sheath cells.

mesophyll

bundle sheath



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Here malate is converted to pyruvate and CO2. The pyruvate is then shuttled back to the mesophyll cells where one ATP (broken down to AMP, instead of ADP) is required to convert the pyruvate back to PEP. Then the process repeats. The overall effect is to move CO2 from mesophyll cells to the bundle sheath cells.

When malate delivers CO2 to a bundle sheath cell, rubisco begins the Calvin-Benson cycle (C3 photosynthesis). Because little oxygen is present, rubisco can fix CO2 without competition from O2. Thus, little hotorespiration takes place, and photosynthesis is more efficient.



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• The purpose for moving CO2 to bundle sheath cells is to increase the efficiency of photosynthesis.

• The bundle sheath cells surround the leaf veins and are themselves surrounded by densely packed mesophyll cells. Since bundle sheath cells rarely make contact with an intercellular space, very little oxygen reaches them.



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In order for photosynthesis to occur, the stomata must be open to allow CO2 to enter. However, when the stomata are open, H2O can escape. The higher rate of photosynthesis among C4 plants allows them to reduce the time that the stomata are open, thereby, reducing H2O loss. Thus, C4 plants are found in hot, dry climates, where they possess an adaptive advantage over C3 plants.

This advantage apparently compensates for the additional energy requirement (1 ATP to AMP) for C4 photosynthesis.C4 photosynthesis occurs in about a dozen plant families. Sugarcane and crab grass are two examples.

Sugarcanecrab grass



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CAM Photosynthesis

Another “add-on” feature to C3 photosynthesis is crassulacean acid metabolism (CAM). The physiology of this pathway is almost identical to C4 photosynthesis, with the changes that follow:

1. PEP carboxylase still fixes CO2 to OAA, as in C4. Instead of malate, however, OAA is converted to malic acid. (This is a minor difference, since malate is merely the ionized form of malic acid).



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• 2. Malic acid is shuttled to the vacuole of the cell (not moved out of the cell to bundle sheath cells as in regular C4).

• 3. Stomata are open at night. During the night, PEP carboxylase is active and malic acid accumulates in the cell’s vacuole.



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• 4. Stomata are closed during the day (the reverse of other plants). At this time, malic acid is shuttled out of the vacuole and converted back to OAA (requiring 1 ATP to ADP), releasing CO2.

• The CO2 is now fixed by rubisco, and the Calvin-Benson cycle proceeds.

• The advantage of CAM is that photosynthesis can proceed during the day while the stomata are closed, greatly reducing H2O loss.

• As a result, CAM provides an adaptation for plants that grow in hot, dry environments with cool nights (such as deserts).

• The name crassulacean acid metabolism comes from the early discovery of CAM in the succulent plants of the family Crassulaceae and the discovery of the accumulation of malic acid in vacuoles during the night.

•In addition to the Crassulaceae, CAM is found among plants in over a dozen different families, including cacti.



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Photosynthesis Photosynthesis is the process of converting energy from sunlight to energy in chemical bonds. The general equation describing photosynthesis