Photosynthesis hl
Sources: Chapter 8 Metabolism pdf Pearson book 1rst edition.
Important terminology to know: Plastoquinone:The electron acceptor for the photosystems
Ferredoxin: Protein in the fluid outside the thylakoid. Thylakoids: are regular membranes with very small fluid-filled spaces inside which contains the structures:
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light-dependent and light-independent parts
8.3.1 Light-dependent reactions take place in the intermembrane space of the thylakoids.
Research shown that photosynthesis consist on two very different parts, the one that uses light directly (light dependent) and the one that does not uses light directly (light-independent). The chloroplast has an outer membrane and an inner membrane.
The light-independent reactions can only carry on in darkness for a few seconds because they depend on substances produced by the light-dependent reactions which rapidly run out.
The inner membrane has a third system of interconnected membranes called thylakoid membranes. Within the the thylakoid is a compartment called thylakoid space which is the place where the light-dependent reactions take place.
8.3.2 Light-independent reactions take place in the stroma
The inner membrane of the chloroplast encloses a compartment called stroma:
A thick protein-rich medium containing enzymes for use in the light-independent reactions, also known as the Calvin cycle.
*Calvin cycle: is an anabolic pathway that requires endergonic reactions to be coupled to the hydrolysis of ATP and the oxidation of reduced NADP.
8.3.3 Reduced NADP and ATP are produced in the light-dependent reactions.
The products of those reactions are reduced NADP and ATP. Light energy is converted into chemical energy in the form of ATP and reduced NADP in the light reactions. They serve as energy sources for the light-independent.
Research shown that photosynthesis consist on two very different parts, the one that uses light directly (light dependent) and the one that does not uses light directly (light-independent). The chloroplast has an outer membrane and an inner membrane.
The light-independent reactions can only carry on in darkness for a few seconds because they depend on substances produced by the light-dependent reactions which rapidly run out.
The inner membrane has a third system of interconnected membranes called thylakoid membranes. Within the the thylakoid is a compartment called thylakoid space which is the place where the light-dependent reactions take place.
8.3.2 Light-independent reactions take place in the stroma
The inner membrane of the chloroplast encloses a compartment called stroma:
A thick protein-rich medium containing enzymes for use in the light-independent reactions, also known as the Calvin cycle.
*Calvin cycle: is an anabolic pathway that requires endergonic reactions to be coupled to the hydrolysis of ATP and the oxidation of reduced NADP.
8.3.3 Reduced NADP and ATP are produced in the light-dependent reactions.
The products of those reactions are reduced NADP and ATP. Light energy is converted into chemical energy in the form of ATP and reduced NADP in the light reactions. They serve as energy sources for the light-independent.
photoactivation
8.3.4 Absorption of light by photosystems generates excited electrons.
Chlorophyll and accessory pigments are grouped in large light-harvesting arrays called photosystems. Those photosystems are located in the thylakoids (an arrangement of membranes inside the chloroplast), in addition to light-harvesting arrays the photosystems have also reaction centres. (figure bellow)
There are two types of light-harvesting arrays (Photosystems I and II) and both types contain many chlorophyll molecules, which absorb the light energy and pass it to 2 special chlorophyll molecules in the reaction centre of the photosystem. When these special chlorophyll molecules absorb the energy from a photon of light, an electron within the molecule becomes excited this is when the Chlorophyll is then Photoactivated. Now the chlorophyll at the reaction centre is able to donate excited electrons to an electron acceptor (special property of chlorophylls).
In the photosystem II is where the light-dependent reactions of photosynthesis begin. Absorption of 2 photons of light causes the production of 1 reduced plastoquinone with one of the chlorophylls at the reaction centre, having lost 2 electrons to a plastoquinone molecule. Photosystem II repeats this process to produce a second reduced plastoquinone so the chlorophyll at the reaction centre losses 4 electrons and 2 plastoquinone molecules have been reduced.
Chlorophyll and accessory pigments are grouped in large light-harvesting arrays called photosystems. Those photosystems are located in the thylakoids (an arrangement of membranes inside the chloroplast), in addition to light-harvesting arrays the photosystems have also reaction centres. (figure bellow)
There are two types of light-harvesting arrays (Photosystems I and II) and both types contain many chlorophyll molecules, which absorb the light energy and pass it to 2 special chlorophyll molecules in the reaction centre of the photosystem. When these special chlorophyll molecules absorb the energy from a photon of light, an electron within the molecule becomes excited this is when the Chlorophyll is then Photoactivated. Now the chlorophyll at the reaction centre is able to donate excited electrons to an electron acceptor (special property of chlorophylls).
In the photosystem II is where the light-dependent reactions of photosynthesis begin. Absorption of 2 photons of light causes the production of 1 reduced plastoquinone with one of the chlorophylls at the reaction centre, having lost 2 electrons to a plastoquinone molecule. Photosystem II repeats this process to produce a second reduced plastoquinone so the chlorophyll at the reaction centre losses 4 electrons and 2 plastoquinone molecules have been reduced.
photolysis
8.3.5 Photolysis of water generates electrons for use in the light-dependent reactions.
Once the platoquinone becomes reduced the chlorophyll in the reaction centre is then a powerful oxidizing agent and causes the water molecules nearest to it to split and give up electrons to replace those that it has lost:
Once the platoquinone becomes reduced the chlorophyll in the reaction centre is then a powerful oxidizing agent and causes the water molecules nearest to it to split and give up electrons to replace those that it has lost:
The splitting of water is called Photolysis which is how oxygen is generated in photosynthesis . Oxygen is a waste product and diffuses away. The useful product of the photosystem II is the reduced plastoquinone which not only carries a pair of electrons but also much of the energy absorbed from light, that energy drives all the subsequent reactions of the photosynthesis process.
the electron transport chain
8.3.6 Transfer of excited electrons occurs between carriers in thylakoid membranes.
The production of ATP using energy derived from light is called photophosphorylation. It is carried out by the thylakoids. These are regular membranes with very small fluid-filled spaces inside which contains the structures:
The production of ATP using energy derived from light is called photophosphorylation. It is carried out by the thylakoids. These are regular membranes with very small fluid-filled spaces inside which contains the structures:
- Photosystem II
- ATP synthase
- A chain of electron carries
- Photosystem I
proton gradient
8.3.7 Excited electrons from photosystem II are used to generate a proton gradient
Once the plastoquinone transfer its electron, the electrons are then passed from the carrier to carrier in the chain. As the electrons pass energy is released which is used to pump protons across the thylakoid membrane into the space inside the thylakoid.
A concentration gradient of protons develops across the thylakoid membrane which is a store of potential energy.
*Photolysis which takes places in the fluid inside the thylakoids also contributes to the proton gradient
Once the plastoquinone transfer its electron, the electrons are then passed from the carrier to carrier in the chain. As the electrons pass energy is released which is used to pump protons across the thylakoid membrane into the space inside the thylakoid.
A concentration gradient of protons develops across the thylakoid membrane which is a store of potential energy.
*Photolysis which takes places in the fluid inside the thylakoids also contributes to the proton gradient
Chemiosmosis
8.3.8 ATP synthase in thylakoids generates ATP using the proton gradient.
The protons are able to travel back across the membrane down the concentration gradient by passing through the enzyme ATP synthase.
The energy released by the passage of protons down their concentration gradient is used to make ATP, from ADP and inorganic phosphates.
This process is really similar to the one that occurs in the mitochondria and it is given the same name ( Chemiosmosis )
When the electrons reach the end of the chain of carriers they are passed to the plastocyanin.
*Reduced plastocyanin is needed in the next stage of photosynthesis
The protons are able to travel back across the membrane down the concentration gradient by passing through the enzyme ATP synthase.
The energy released by the passage of protons down their concentration gradient is used to make ATP, from ADP and inorganic phosphates.
This process is really similar to the one that occurs in the mitochondria and it is given the same name ( Chemiosmosis )
When the electrons reach the end of the chain of carriers they are passed to the plastocyanin.
*Reduced plastocyanin is needed in the next stage of photosynthesis
reduction of nadp
8.3.9 Excited electrons from Photosystem II are used to contribute to generate a proton gradient.
The remaining parts of the light-dependent reactions involve the photosystem I. The useful product of these reactions is reduced NADP, which is needed in the light-independent reactions of photosynthesis. Reduced NADP has a similar role to reduced NAD in cell respiration: it carry a pair of electrons that can be used to carry out reduction reactions.
The chlorophyll molecules in the photosystem I absorb light energy and pass it to the special 2 chlorophyll molecules in the reaction centre. That raises an electron in one of the chlorophylls to a high energy level which as in the photosystem II is called photoactivation. The excited electron passes along a chain of carries in photosystem I and at the end it passes to Ferredoxin. Two reduced molecules of ferredoxin are then used to reduce NADP to form reduced NADP.
The electron that photosystem I donated to the chain of electron is replaced by an electron carried by the plastocyanin.
Photosystem I and II are therefore linked: the electrons excited in photosystem II are passed along the chain of carriers to the plastocyanin which transfers them to the photosystem I. The electrons now get excited with light energy and are used to reduce more NADP.
*The supply of NADP can run out and when this happens the electrons returns to the electron transport chain that links the 2 photosystems rather than being passed to NADP. When the electron flows back to the electron transport chain to photosystem I it causes pumping of protons which creates ATP. This process is called CYCLIC PHOTOPHOSPHORYLATION
The remaining parts of the light-dependent reactions involve the photosystem I. The useful product of these reactions is reduced NADP, which is needed in the light-independent reactions of photosynthesis. Reduced NADP has a similar role to reduced NAD in cell respiration: it carry a pair of electrons that can be used to carry out reduction reactions.
The chlorophyll molecules in the photosystem I absorb light energy and pass it to the special 2 chlorophyll molecules in the reaction centre. That raises an electron in one of the chlorophylls to a high energy level which as in the photosystem II is called photoactivation. The excited electron passes along a chain of carries in photosystem I and at the end it passes to Ferredoxin. Two reduced molecules of ferredoxin are then used to reduce NADP to form reduced NADP.
The electron that photosystem I donated to the chain of electron is replaced by an electron carried by the plastocyanin.
Photosystem I and II are therefore linked: the electrons excited in photosystem II are passed along the chain of carriers to the plastocyanin which transfers them to the photosystem I. The electrons now get excited with light energy and are used to reduce more NADP.
*The supply of NADP can run out and when this happens the electrons returns to the electron transport chain that links the 2 photosystems rather than being passed to NADP. When the electron flows back to the electron transport chain to photosystem I it causes pumping of protons which creates ATP. This process is called CYCLIC PHOTOPHOSPHORYLATION
Summary of the light-dependent reactions of photosynthesis.
Carbon fixation
8.3.12 In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate.
Carbon dioxide is the carbon source for all organisms that carry out photosynthesis. The carbon fixation reaction in which it is converted into another carbon compound is arguably the most important in all living organisms.
In plants and algae it occurs in the stroma. The product of this carbon fixation reaction is a 3 carbon compound: glycerate 3 phosphate.
* Carbon dioxide does not react with a 2-carbon compound to produce glycerate 3 phosphate. Instead it reacts with a 5 carbon compound called ribulose bisphosphate RuBP to produce 2 molecules of glycerate 3 phosphate. The enzyme that catalyses this reaction is called ribulose bisphosphate carboxylase. The stroma contains large amounts of it to maximize carbon fixation.
Carbon dioxide is the carbon source for all organisms that carry out photosynthesis. The carbon fixation reaction in which it is converted into another carbon compound is arguably the most important in all living organisms.
In plants and algae it occurs in the stroma. The product of this carbon fixation reaction is a 3 carbon compound: glycerate 3 phosphate.
* Carbon dioxide does not react with a 2-carbon compound to produce glycerate 3 phosphate. Instead it reacts with a 5 carbon compound called ribulose bisphosphate RuBP to produce 2 molecules of glycerate 3 phosphate. The enzyme that catalyses this reaction is called ribulose bisphosphate carboxylase. The stroma contains large amounts of it to maximize carbon fixation.
the role of reduced nadp and atp in the calvin cycle
8.3.13 Glycerate-3-phosphate is reduced to triose phosphate using reduced NADP and ATP.
RuBP is a 5-carbon sugar derivate, and when it is converted to glycerate 3 phosphate by adding carbon and oxygen the amount of hydrogen in relation to oxygen is reduced. In carbohydrates the ratio of hydrogen to oxygen is 2:1. Hydrogen had to be added to glycerate 3 phosphate by a reduction reaction to produce carbohydrate. This involves both ATP and reduced NADP produced by the light-dependent reactions of photosynthesis. The ATP provides the energy needed to perform the reduction and reduced NADP provides the hydrogen atoms. The product is a 3 carbon sugar derivative, triose phosphate.
RuBP is a 5-carbon sugar derivate, and when it is converted to glycerate 3 phosphate by adding carbon and oxygen the amount of hydrogen in relation to oxygen is reduced. In carbohydrates the ratio of hydrogen to oxygen is 2:1. Hydrogen had to be added to glycerate 3 phosphate by a reduction reaction to produce carbohydrate. This involves both ATP and reduced NADP produced by the light-dependent reactions of photosynthesis. The ATP provides the energy needed to perform the reduction and reduced NADP provides the hydrogen atoms. The product is a 3 carbon sugar derivative, triose phosphate.
The fate of triose phosphate
8.3.14 Triose phosphate is used to generate RuBP and produce carbohydrates.
The first carbohydrate produced by the light-independent reactions of photosynthesis is triose phosphate. Two triose phosphate molecules can be combined to form hexose phosphate and hexose phosphate can be combined by condensation reactions to form starch. However if all of the triose phosphate produced by photosynthesis was converted to hexose and starch the supplies of RuBP in the chloroplast would ran out. Some triose phosphate in the chloroplast has to be used to regenerate RubBP, that process is a conversion of 3-carbon sugars into 5-carbon sugars and it cannot be done in a single step, a series of reactions have to take place:
As RuBP is both consumed and produced in the light-independent reactions of photosynthesis they form a cycle. It is called the calvin cycle and for it to continue indefinitely as much RuBP must be produced as consumed. If 3 RuBP molecules are used, six triose phosphates are produced. Five of these are needed to regenerate the 3 RuBP molecules. This leaves just one triose phosphate for conversion to hexose, starch and other products. To produce one molecule of glucose for example six turns of the Calvin cycle are needed each of which contributes one of the fixes carbon atoms in the glucose.
The first carbohydrate produced by the light-independent reactions of photosynthesis is triose phosphate. Two triose phosphate molecules can be combined to form hexose phosphate and hexose phosphate can be combined by condensation reactions to form starch. However if all of the triose phosphate produced by photosynthesis was converted to hexose and starch the supplies of RuBP in the chloroplast would ran out. Some triose phosphate in the chloroplast has to be used to regenerate RubBP, that process is a conversion of 3-carbon sugars into 5-carbon sugars and it cannot be done in a single step, a series of reactions have to take place:
As RuBP is both consumed and produced in the light-independent reactions of photosynthesis they form a cycle. It is called the calvin cycle and for it to continue indefinitely as much RuBP must be produced as consumed. If 3 RuBP molecules are used, six triose phosphates are produced. Five of these are needed to regenerate the 3 RuBP molecules. This leaves just one triose phosphate for conversion to hexose, starch and other products. To produce one molecule of glucose for example six turns of the Calvin cycle are needed each of which contributes one of the fixes carbon atoms in the glucose.
rUbp regeneration
8.3.15 Ribulose bisphosphate is reformed using ATP
In the last phase of the Calvin cycle a series of enzyme-catalyzed reactions convert triose phosphate molecules into RuBP. After the RuBP is regenerated it can serve to fix the CO2 and begin the cycle again.
In the last phase of the Calvin cycle a series of enzyme-catalyzed reactions convert triose phosphate molecules into RuBP. After the RuBP is regenerated it can serve to fix the CO2 and begin the cycle again.
Chloroplast structure and function
8.3.16 The structure of the chloroplast is adapted to its function in photosynthesis
Chloroplast are quite variable in structure but share certain features:
Chloroplast are quite variable in structure but share certain features:
- A double membrane forming the outer chloroplast envelope
- an extensive system of internal membranes called thylakoid which re intense green colour
- small fluid-filled spaces inside the thylakoid
- a colourless fluid around the thylakoids called stroma that contains many different enzymes
- In most chloroplasts there are stacks of thylakoids called GRANA. If a chloroplast had been phtosynthesizing rapidly then there may be starch grains or lipid droplets in the stroma
Applications and skills
8.3.17A Calvin’s experiment to elucidate the carboxylation of RuBP.
8.3.18S Annotation of a diagram to indicate the adaptations of a chloroplast
Theory of knowledge:
The lollipop experiment used to work out the biochemical details of the Calving cycle shows considerable creativity. To what extent is the creation of an elegant protocol similar to the creation of a work of art?
Works of art just as elegant protocols have something in similar, they are both made in the way that they are attractive to the eye. The creation of this things goes to a certain process which can last for days, weeks, months or even years and all the time that it takes to get the best idea to create the work of art or an elegant protocol. In my opinion the creation of a elegant protocol and the work of art are similar to the extent that the protocols has one more objective than the work of art, and it is to work, and be useful for experiments and activities while works of arts are mostly to stare at without touching it or using it to get results in terms of science.
8.3.17A Calvin’s experiment to elucidate the carboxylation of RuBP.
8.3.18S Annotation of a diagram to indicate the adaptations of a chloroplast
Theory of knowledge:
The lollipop experiment used to work out the biochemical details of the Calving cycle shows considerable creativity. To what extent is the creation of an elegant protocol similar to the creation of a work of art?
Works of art just as elegant protocols have something in similar, they are both made in the way that they are attractive to the eye. The creation of this things goes to a certain process which can last for days, weeks, months or even years and all the time that it takes to get the best idea to create the work of art or an elegant protocol. In my opinion the creation of a elegant protocol and the work of art are similar to the extent that the protocols has one more objective than the work of art, and it is to work, and be useful for experiments and activities while works of arts are mostly to stare at without touching it or using it to get results in terms of science.