UNDERSTANDINGS:
● Light-dependent reactions take place in the intermembrane space of the thylakoids.
● Light-independent reactions take place in the stroma.
● Reduced NADP and ATP are produced in the light-dependent reactions.
● Absorption of light by photosystems generates excited electrons.
● Photolysis of water generates electrons for use in the light-dependent reactions.
● Transfer of excited electrons occurs between carriers in thylakoid membranes.
● Excited electrons from Photosystem II are used to contribute to generate a proton gradient.
● ATP synthase in thylakoids generates ATP using the proton gradient.
● Excited electrons from Photosystem I are used to reduce NADP.
● In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate.
● Glycerate 3-phosphate is reduced to triose phosphate using reduced NADP and ATP.
● Triose phosphate is used to regenerate RuBP and produce carbohydrates.
● Ribulose bisphosphate is reformed using ATP.
● The structure of the chloroplast is adapted to its function in photosynthesis.
APPLICATIONS AND SKILLS:
● Application: Calvin’s experiment to elucidate the carboxylation of RuBP.
● Skill: Annotation of a diagram to indicate the adaptations of a chloroplast to its function.
The chloroplast;
Some people refer to the chloroplast as a photosynthetic machine. They are not wrong. Unlike respiration, where some of the steps occur outside the mitochondrion, all of the photosynthetic process occurs within the chloroplast. Chloroplasts, along with mitochondria, represent possible evidence for the theory of endosymbiosis. Both organelles have an extra outer membrane (indicating a need for protection in a potentially hostile environment), their own DNA, and they are very near in size to a typical prokaryotic cell.
● Light-dependent reactions take place in the intermembrane space of the thylakoids.
● Light-independent reactions take place in the stroma.
● Reduced NADP and ATP are produced in the light-dependent reactions.
● Absorption of light by photosystems generates excited electrons.
● Photolysis of water generates electrons for use in the light-dependent reactions.
● Transfer of excited electrons occurs between carriers in thylakoid membranes.
● Excited electrons from Photosystem II are used to contribute to generate a proton gradient.
● ATP synthase in thylakoids generates ATP using the proton gradient.
● Excited electrons from Photosystem I are used to reduce NADP.
● In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate.
● Glycerate 3-phosphate is reduced to triose phosphate using reduced NADP and ATP.
● Triose phosphate is used to regenerate RuBP and produce carbohydrates.
● Ribulose bisphosphate is reformed using ATP.
● The structure of the chloroplast is adapted to its function in photosynthesis.
APPLICATIONS AND SKILLS:
● Application: Calvin’s experiment to elucidate the carboxylation of RuBP.
● Skill: Annotation of a diagram to indicate the adaptations of a chloroplast to its function.
The chloroplast;
Some people refer to the chloroplast as a photosynthetic machine. They are not wrong. Unlike respiration, where some of the steps occur outside the mitochondrion, all of the photosynthetic process occurs within the chloroplast. Chloroplasts, along with mitochondria, represent possible evidence for the theory of endosymbiosis. Both organelles have an extra outer membrane (indicating a need for protection in a potentially hostile environment), their own DNA, and they are very near in size to a typical prokaryotic cell.
Chloroplasts occur mostly within the cells of the photosynthetic factory of the plant, the leaves. However, some plants have chloroplasts in cells of other organs.
THE OVERALL PROCESS OF PHOTOSYNTHESIS
During the discussion on respiration, we considered the means by which the cell breaks down chemical bonds in glucose to produce ATP. In this section, the discussion centers on the establishment of chemical bonds to produce organic compounds. Using light energy, the raw materials of photosynthesis are carbon dioxide and water. Many enzymes are involved to enable the formation of products that include glucose, more water, and oxygen. The overall equation is:
Light
6 CO2 + 12 H2O → C6H12O6 + 6 H2O + 6 O2
Water occurs on both sides because 12 molecules are consumed and 6 molecules are produced. Clearly, photosynthesis is essentially the reverse of respiration. Whereas respiration is, in general, a catabolic process, photosynthesis is, in general, an anabolic process. Photosynthesis occurs in organisms referred to as autotrophs. These organisms make their own food. Non-photosynthetic and non-chemosynthetic organisms are referred to as heterotrophs. They must obtain their food from other organisms.
Photosynthesis involves two major stages:
• The light-dependent reaction
• The light-independent reaction.
THE LIGHT-DEPENDENT REACTION
The light-dependent reaction occurs in the thylakoids or grana of the chloroplast. A stack of thylakoids makes up a granum. Light supplies the energy for this reaction to occur. The ultimate source of light is the Sun. Even though plants may survive quite well when they receive light from sources other than the Sun, most plants on our planet rely on the Sun for the energy necessary to drive photosynthesis.
To absorb light, plants have special molecules called pigments. There are several different pigments in plants, and each effectively absorbs photons of light at different wavelengths. The two major groups are the chlorophylls and the carotenoids.
These pigments are organized on the membranes of the thylakoids. The regions of organization are called photosystems and include:
• Chlorophyll a molecules
• Accessory pigments
• A protein matrix.
The reaction center is the portion of the photosystem that contains:
• A pair of chlorophyll a molecules
• A matrix of protein
• A primary electron acceptor.
Bacteria that carry out photosynthesis have only one type of photosystem. However, modern-day plants have two types of photosystem. Each absorbs light most efficiently at a different wavelength. Photosystem I is most efficient at 700 nanometers (nm) and is labeled as P700. Photosystem II is most efficient at 680 nm and is labeled as P680. These two photosystems work together to bring about a non-cyclical electron transfer.
(The numbered descriptions that follow refer to the numbered steps in the following picture)
1. A photon of light is absorbed by a pigment in Photosystem II and is transferred to other pigment molecules until it reaches one of the chlorophyll a (P680) molecules in the reaction center. The photon energy excites one of the chlorophyll a electrons to a higher energy state.
2. This electron is captured by the primary acceptor of the reaction center.
3. Water is split by an enzyme to produce electrons, hydrogen ions, and oxygen atom. This process is driven by the energy from light and is called photolysis. The electrons are supplied one by one to the chlorophyll a molecules of the reaction center.
4. The excited electrons pass from the primary acceptor down an electron transport chain, losing energy at each exchange. The first of the three carriers is plastoquinone (PQ). The middle carrier is a cytochrome complex.
5. The energy lost from the electrons moving down the electron transport chain drives chemiosmosis (similar to that in respiration) to bring about phosphorylation of ADP to produce ATP.
6. A photon of light is absorbed by a pigment in Photosystem I. This energy is transferred through several accessory pigments until received by a chlorophyll a (P700) molecule. This results in an electron with a higher energy state being transferred to the primary electron acceptor. The de-energized electron from Photosystem II fills the void left by the newly energized electron.
7. The electron with the higher energy state is then passed down a second electron transport chain that involves the carrier ferredoxin.
8. The enzyme NADP reductase catalyzes the transfer of the electron from ferredoxin to the energy carrier NADP+. Two electrons are required to reduce NADP+ fully to NADPH.
THE OVERALL PROCESS OF PHOTOSYNTHESIS
During the discussion on respiration, we considered the means by which the cell breaks down chemical bonds in glucose to produce ATP. In this section, the discussion centers on the establishment of chemical bonds to produce organic compounds. Using light energy, the raw materials of photosynthesis are carbon dioxide and water. Many enzymes are involved to enable the formation of products that include glucose, more water, and oxygen. The overall equation is:
Light
6 CO2 + 12 H2O → C6H12O6 + 6 H2O + 6 O2
Water occurs on both sides because 12 molecules are consumed and 6 molecules are produced. Clearly, photosynthesis is essentially the reverse of respiration. Whereas respiration is, in general, a catabolic process, photosynthesis is, in general, an anabolic process. Photosynthesis occurs in organisms referred to as autotrophs. These organisms make their own food. Non-photosynthetic and non-chemosynthetic organisms are referred to as heterotrophs. They must obtain their food from other organisms.
Photosynthesis involves two major stages:
• The light-dependent reaction
• The light-independent reaction.
THE LIGHT-DEPENDENT REACTION
The light-dependent reaction occurs in the thylakoids or grana of the chloroplast. A stack of thylakoids makes up a granum. Light supplies the energy for this reaction to occur. The ultimate source of light is the Sun. Even though plants may survive quite well when they receive light from sources other than the Sun, most plants on our planet rely on the Sun for the energy necessary to drive photosynthesis.
To absorb light, plants have special molecules called pigments. There are several different pigments in plants, and each effectively absorbs photons of light at different wavelengths. The two major groups are the chlorophylls and the carotenoids.
These pigments are organized on the membranes of the thylakoids. The regions of organization are called photosystems and include:
• Chlorophyll a molecules
• Accessory pigments
• A protein matrix.
The reaction center is the portion of the photosystem that contains:
• A pair of chlorophyll a molecules
• A matrix of protein
• A primary electron acceptor.
Bacteria that carry out photosynthesis have only one type of photosystem. However, modern-day plants have two types of photosystem. Each absorbs light most efficiently at a different wavelength. Photosystem I is most efficient at 700 nanometers (nm) and is labeled as P700. Photosystem II is most efficient at 680 nm and is labeled as P680. These two photosystems work together to bring about a non-cyclical electron transfer.
(The numbered descriptions that follow refer to the numbered steps in the following picture)
1. A photon of light is absorbed by a pigment in Photosystem II and is transferred to other pigment molecules until it reaches one of the chlorophyll a (P680) molecules in the reaction center. The photon energy excites one of the chlorophyll a electrons to a higher energy state.
2. This electron is captured by the primary acceptor of the reaction center.
3. Water is split by an enzyme to produce electrons, hydrogen ions, and oxygen atom. This process is driven by the energy from light and is called photolysis. The electrons are supplied one by one to the chlorophyll a molecules of the reaction center.
4. The excited electrons pass from the primary acceptor down an electron transport chain, losing energy at each exchange. The first of the three carriers is plastoquinone (PQ). The middle carrier is a cytochrome complex.
5. The energy lost from the electrons moving down the electron transport chain drives chemiosmosis (similar to that in respiration) to bring about phosphorylation of ADP to produce ATP.
6. A photon of light is absorbed by a pigment in Photosystem I. This energy is transferred through several accessory pigments until received by a chlorophyll a (P700) molecule. This results in an electron with a higher energy state being transferred to the primary electron acceptor. The de-energized electron from Photosystem II fills the void left by the newly energized electron.
7. The electron with the higher energy state is then passed down a second electron transport chain that involves the carrier ferredoxin.
8. The enzyme NADP reductase catalyzes the transfer of the electron from ferredoxin to the energy carrier NADP+. Two electrons are required to reduce NADP+ fully to NADPH.
NADPH and ATP are the final products of the light-dependent reaction. They supply chemical energy for the light-independent reaction to occur.
ATP production in photosynthesis is very similar to ATP production in respiration. Chemiosmosis allows the process of phosphorylation of ADP. In this case, the energy to drive chemiosmosis comes from light. As a result, we refer to the production of ATP in photosynthesis as photophosphorylation.
THE LIGHT-INDEPENDENT REACTION
The light-independent reaction occurs within the stroma or cytosol-like region of the chloroplast.
The ATP and NADPH produced by the light-dependent reaction provide the energy and reducing power for the light-independent reaction to occur. Up to this point there has been no mention of carbohydrate production. Therefore, as we know glucose is a product of photosynthesis, the result of the light-independent reaction must be the production of glucose.
The light-independent reaction involves the Calvin cycle (see Figure below), which occurs in the stroma of the chloroplast. Because it is a cycle, it begins and ends with the same substance. You should recall that a similar cyclic metabolic pathway occurred in respiration: the Krebs cycle.
ATP production in photosynthesis is very similar to ATP production in respiration. Chemiosmosis allows the process of phosphorylation of ADP. In this case, the energy to drive chemiosmosis comes from light. As a result, we refer to the production of ATP in photosynthesis as photophosphorylation.
THE LIGHT-INDEPENDENT REACTION
The light-independent reaction occurs within the stroma or cytosol-like region of the chloroplast.
The ATP and NADPH produced by the light-dependent reaction provide the energy and reducing power for the light-independent reaction to occur. Up to this point there has been no mention of carbohydrate production. Therefore, as we know glucose is a product of photosynthesis, the result of the light-independent reaction must be the production of glucose.
The light-independent reaction involves the Calvin cycle (see Figure below), which occurs in the stroma of the chloroplast. Because it is a cycle, it begins and ends with the same substance. You should recall that a similar cyclic metabolic pathway occurred in respiration: the Krebs cycle.
1. Ribulose bisphosphate (RuBP), a 5-carbon compound, binds to an incoming carbon dioxide molecule in a process called carbon fixation. This fixation is catalyzed by an enzyme called RuBP carboxylase (rubisco). The result is an unstable 6-carbon compound.
2. The unstable 6-carbon compound breaks down into two 3-carbon compounds called glycerate 3-phosphate (GP).
3. The 3-carbon molecules of GP are acted upon by ATP and NADPH from the light- dependent reaction to form two other 3-carbon molecules called triose phosphate (TP). This is a reduction reaction.
4. The molecules of TP may then go in either of two directions. Some leave the cycle to become sugar phosphates that may become more complex carbohydrates. Most, however, continue in the cycle to reproduce the originating compound of the cycle, RuBP.
5. In order to regain RuBP molecules from TP, the cycle uses ATP.
Spheres are used to represent the carbon atoms so that they can be tracked through the cycle. The coefficients (numbers) in front of each compound involved show what it takes to produce one molecule of a 6-carbon sugar. It is clear that for every 12 TP molecules, the cycle produces one 6-carbon sugar and six molecules of the 5-carbon compound RuBP. All the carbons are accounted for, and the law of conservation of mass is demonstrated. Also, it is important to note that 18 ATPs and 12 NADPH are necessary to produce six RuBP molecules and one molecule of a 6-carbon sugar.
TP is the pivotal compound in the Calvin cycle. It may be used to produce simple sugars such as glucose, disaccharides such as sucrose, or polysaccharides such as cellulose or starch. However, most of it is used to regain the starting compound of the Calvin cycle, RuBP.
SUMMARY OF PHOTOSYNTHESIS
In summary, the process of photosynthesis includes the light-dependent and the light- independent reactions. The products of the light-dependent reaction are ATP and NADPH, which are needed to allow the light-independent reaction to proceed. Thus it is clear that light is needed for the light-independent reaction to occur, but not directly.
2. The unstable 6-carbon compound breaks down into two 3-carbon compounds called glycerate 3-phosphate (GP).
3. The 3-carbon molecules of GP are acted upon by ATP and NADPH from the light- dependent reaction to form two other 3-carbon molecules called triose phosphate (TP). This is a reduction reaction.
4. The molecules of TP may then go in either of two directions. Some leave the cycle to become sugar phosphates that may become more complex carbohydrates. Most, however, continue in the cycle to reproduce the originating compound of the cycle, RuBP.
5. In order to regain RuBP molecules from TP, the cycle uses ATP.
Spheres are used to represent the carbon atoms so that they can be tracked through the cycle. The coefficients (numbers) in front of each compound involved show what it takes to produce one molecule of a 6-carbon sugar. It is clear that for every 12 TP molecules, the cycle produces one 6-carbon sugar and six molecules of the 5-carbon compound RuBP. All the carbons are accounted for, and the law of conservation of mass is demonstrated. Also, it is important to note that 18 ATPs and 12 NADPH are necessary to produce six RuBP molecules and one molecule of a 6-carbon sugar.
TP is the pivotal compound in the Calvin cycle. It may be used to produce simple sugars such as glucose, disaccharides such as sucrose, or polysaccharides such as cellulose or starch. However, most of it is used to regain the starting compound of the Calvin cycle, RuBP.
SUMMARY OF PHOTOSYNTHESIS
In summary, the process of photosynthesis includes the light-dependent and the light- independent reactions. The products of the light-dependent reaction are ATP and NADPH, which are needed to allow the light-independent reaction to proceed. Thus it is clear that light is needed for the light-independent reaction to occur, but not directly.