10: Photosynthesis
Photosynthesis: The process by which plants convert solar energy into chemical energy.
Autotrophs/Producers ("self-nourishing"): self-sustaining organisms, all materials for making their own food derived from environment
-ultimate source of all organic materials for all heterotrophs
Heterotrophs/consumers or decomposers( "nourishment from others"): unable to make their own food, obtain organic material through diet
- all dependent either directly or indirectly on autotrophs for obtaining nutrients
-early photosynthetic bacteria contained infolded plasma membranes containing clusters of photosynthetic enzymes and molecules.
-the endosymbiont theory suggests that choloroplasts existed as prokaryotes prior to being engulfed by and eukaryotic cell and gradually becoming dependent on one another, forming a cell containing the first cholorplast as an organelle
Chloroplasts: The Site of Photosynthesis in Plants
-Mesophyll cells: cells composing the tissue in the interior of the leaf and containing the majority of chloroplasts
-Stomata: pores in the leaf that are the entrance point for carbon dioxide (reactant of photosynthesis) and exit point of oxygen (product of photosynthesis)
-Water is delivered to the leaves from the roots by veins. Veins are also used to transport sugar to the roots and other non-photosynthetic portions of a plant
Anatomy of a chloroplast:
-Chloroplasts have two membranes, an inner membrane and an outer membrane. Between the two is the intermembrane space
-Stroma: dense fluid filled space inside the inner membrane
-Thylakoid: stacked, flattened, interconnected, chlorophyll-containing membranous sacs suspended within the stroma.
-Thylakoid space (lumen): space inside the membranous sacs of the thylakoid
-Granum: stack of thylakoids
-Chlorophyll: green pigment responsible for absorbing the sunlight which drives photosynthesis.
Photosynthesis: The process by which plants convert solar energy into chemical energy.
Autotrophs/Producers ("self-nourishing"): self-sustaining organisms, all materials for making their own food derived from environment
-ultimate source of all organic materials for all heterotrophs
Heterotrophs/consumers or decomposers( "nourishment from others"): unable to make their own food, obtain organic material through diet
- all dependent either directly or indirectly on autotrophs for obtaining nutrients
- 10.1: Photosynthesis converts light energy to the chemical energy of food
-early photosynthetic bacteria contained infolded plasma membranes containing clusters of photosynthetic enzymes and molecules.
-the endosymbiont theory suggests that choloroplasts existed as prokaryotes prior to being engulfed by and eukaryotic cell and gradually becoming dependent on one another, forming a cell containing the first cholorplast as an organelle
Chloroplasts: The Site of Photosynthesis in Plants
-Mesophyll cells: cells composing the tissue in the interior of the leaf and containing the majority of chloroplasts
-Stomata: pores in the leaf that are the entrance point for carbon dioxide (reactant of photosynthesis) and exit point of oxygen (product of photosynthesis)
-Water is delivered to the leaves from the roots by veins. Veins are also used to transport sugar to the roots and other non-photosynthetic portions of a plant
Anatomy of a chloroplast:
-Chloroplasts have two membranes, an inner membrane and an outer membrane. Between the two is the intermembrane space
-Stroma: dense fluid filled space inside the inner membrane
-Thylakoid: stacked, flattened, interconnected, chlorophyll-containing membranous sacs suspended within the stroma.
-Thylakoid space (lumen): space inside the membranous sacs of the thylakoid
-Granum: stack of thylakoids
-Chlorophyll: green pigment responsible for absorbing the sunlight which drives photosynthesis.
Tr(acking Atoms Through Photosynthesis
Equation for photosynthesis:
6Co2 + 6 H2O + light energy --> C6H12O6 + 6O2 + 6H20
The Splitting of Water
Hydrogen electrons: go to replace electrons transferred from P680 and P700
Hydrogen protons: contribute to the proton gradient by being pumped into the thylakoid space (lumen)
Oxygen atoms: release of O2 gas into the atmosphere as a waste product
Photosynthesis as a Redox Process
-water is split and electrons are transferred along with hydrogen ions from water to the carbon dioxide, reducing it to a sugar.
-this involves an increase in the potential energy of the electrons, and is therefore an endergonic process.
-the energy required for this anabolic process is provided by light energy
The Two Stages of Photosynthesis: A Preview
1) light reactions: convert solar energy to chemical energy
-water is split: provides source of electrons, H+ ions, and Oxygen byproduct
-light absorbed by chlorophyll drives transfer of electrons and H+ ions to acceptor NADP+.
-Reduced, NADP+---> NADPH (in the stroma)
-ATP produced via chemiosmosis :photophosphorylation (in the stroma)
-light energy initially stored as chemical energy in ATP and NADPH
-No sugar produced during light reactions
2) Calvin cycle (light independent reactions/dark reactions): CO2 from atmosphere incorporated into organic compounds (carbon fixation/assimilation)
-The fixed carbon is reduced to a carbohydrate by the addition of electrons, provided by NADPH
-This process also requires the ATP formed during the light reactions
-Light is electromagnetic energy, and exists as waves (disturbances in the electric and magnetic fields)
-Wavelength: distance between two crests (nanometer for gamma rays)
-Electromagnetic spectrum: the entire range of radiation.
-The most important part for living organisms is that between 380 nm and 750 nm,
this portion of the spectrum is known as visible light (can be detected by human eye)
Equation for photosynthesis:
6Co2 + 6 H2O + light energy --> C6H12O6 + 6O2 + 6H20
The Splitting of Water
Hydrogen electrons: go to replace electrons transferred from P680 and P700
Hydrogen protons: contribute to the proton gradient by being pumped into the thylakoid space (lumen)
Oxygen atoms: release of O2 gas into the atmosphere as a waste product
Photosynthesis as a Redox Process
-water is split and electrons are transferred along with hydrogen ions from water to the carbon dioxide, reducing it to a sugar.
-this involves an increase in the potential energy of the electrons, and is therefore an endergonic process.
-the energy required for this anabolic process is provided by light energy
The Two Stages of Photosynthesis: A Preview
1) light reactions: convert solar energy to chemical energy
-water is split: provides source of electrons, H+ ions, and Oxygen byproduct
-light absorbed by chlorophyll drives transfer of electrons and H+ ions to acceptor NADP+.
-Reduced, NADP+---> NADPH (in the stroma)
-ATP produced via chemiosmosis :photophosphorylation (in the stroma)
-light energy initially stored as chemical energy in ATP and NADPH
-No sugar produced during light reactions
2) Calvin cycle (light independent reactions/dark reactions): CO2 from atmosphere incorporated into organic compounds (carbon fixation/assimilation)
-The fixed carbon is reduced to a carbohydrate by the addition of electrons, provided by NADPH
-This process also requires the ATP formed during the light reactions
- 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
-Light is electromagnetic energy, and exists as waves (disturbances in the electric and magnetic fields)
-Wavelength: distance between two crests (nanometer for gamma rays)
-Electromagnetic spectrum: the entire range of radiation.
-The most important part for living organisms is that between 380 nm and 750 nm,
this portion of the spectrum is known as visible light (can be detected by human eye)
-Photons: particles of light with fixed amounts of energy.
-The amount of energy is inversely related to the wavelength:
-larger wavelengths= lower energy (750 nm)
-smaller wavelengths= higher energy (380 nm)
-The sun radiates the full spectrum, but the atmosphere selectively screens some forms of radiation.
-visible light is allowed to pass, and is the type of radiation that drives photosynthesis.
Photsynthetic Pigments: The Light Receptors
-Light can be reflected, transmitted or absorbed by matter.
-Pigment: a substance that absorbs visible light.
-Different pigments absorb different wavelengths of light, colors that are absorbed are not seen
-Those wavelengths that are not absorbed are reflected or transmitted, and it is that color, or mixture of colors that you see.
-Chlorophyll (pigment in thylakoids), absorbs violet-blue and red light, and reflect/transmit green light.
-This is why chloroplasts are green, and so are the leaves and other portions of a plant.
-Spectrophotometer: instrument used to measure the ability of a solution to absorb various wavelengths of light.
-absorption spectrum: a graph of these results plotting light absorption versus wavelength
-action spectrum: relative effectiveness of of different wavelengths in driving a process (such as photosynthesis), plot wavelength versus measure of reaction rate
-There are different kinds of chlorophyll that absorb light differently. The combined absorption spectra of different pigment molecules broadens the spectrum of colors that can be used to drive photosynthesis.
Chlorophyll a : key light capturing pigment that directly participates in light reactions (appears blue-green)
Chlorophyll b: accessory pigment of light reactions (appears olive green)
-one slight molecular difference between the two chlorophyll molecules changes how they absorb light.
-both absorb violet-blue light and red light best
Carotenoids: separate group of accessory pigments (appears in shades of yellow or orange)
-absorb violet and blue-green light
-may contribute to photosynthesis, but also have photoprotective function
- photoprotective molecules absorb excessive light that may otherwise be damaging.
-Observe the absorption spectra of the 3 different pigments below to see how they absorb light differently, expanding the overall ability of chloroplasts to absorbs light.
-The amount of energy is inversely related to the wavelength:
-larger wavelengths= lower energy (750 nm)
-smaller wavelengths= higher energy (380 nm)
-The sun radiates the full spectrum, but the atmosphere selectively screens some forms of radiation.
-visible light is allowed to pass, and is the type of radiation that drives photosynthesis.
Photsynthetic Pigments: The Light Receptors
-Light can be reflected, transmitted or absorbed by matter.
-Pigment: a substance that absorbs visible light.
-Different pigments absorb different wavelengths of light, colors that are absorbed are not seen
-Those wavelengths that are not absorbed are reflected or transmitted, and it is that color, or mixture of colors that you see.
-Chlorophyll (pigment in thylakoids), absorbs violet-blue and red light, and reflect/transmit green light.
-This is why chloroplasts are green, and so are the leaves and other portions of a plant.
-Spectrophotometer: instrument used to measure the ability of a solution to absorb various wavelengths of light.
-absorption spectrum: a graph of these results plotting light absorption versus wavelength
-action spectrum: relative effectiveness of of different wavelengths in driving a process (such as photosynthesis), plot wavelength versus measure of reaction rate
-There are different kinds of chlorophyll that absorb light differently. The combined absorption spectra of different pigment molecules broadens the spectrum of colors that can be used to drive photosynthesis.
Chlorophyll a : key light capturing pigment that directly participates in light reactions (appears blue-green)
Chlorophyll b: accessory pigment of light reactions (appears olive green)
-one slight molecular difference between the two chlorophyll molecules changes how they absorb light.
-both absorb violet-blue light and red light best
Carotenoids: separate group of accessory pigments (appears in shades of yellow or orange)
-absorb violet and blue-green light
-may contribute to photosynthesis, but also have photoprotective function
- photoprotective molecules absorb excessive light that may otherwise be damaging.
-Observe the absorption spectra of the 3 different pigments below to see how they absorb light differently, expanding the overall ability of chloroplasts to absorbs light.
Excitation of Chlorophyll by Light
-When a pigment molecule absorbs a photon of light one of the molecule's electrons is elevated to an orbital with more potential energy.
-The elevated electron is said to be in an excited state.
-Only photons that have exactly the amount of energy that is the difference between that molecule's excited and ground state can be absorbed by that molecule. (this accounts for the difference in light absorption between molecules)
-We know from earlier chapters that higher potential energy means more unstable and more likely to change (to a more stable state).
-Excited electrons are very unstable and must return to ground state.
A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes
-Photosystem: complexes of chlorophyll molecules organized along with other small organic molecules and proteins
-These complexes are broken down into
1) reaction-center complex: complex containing proteins holding two special molecules of chlorophyll a and a molecule capable of accepting electrons (called the primary electron acceptor)
2) light-harvesting complex: consist of pigment molecules that may include chlorophyll a, chlorophyll b and carotenoids bound to proteins
-this collection of pigments expands the types of light the chloroplast can absorb and the surface area
for light absorption.
-When a pigment molecule in a light-harvesting complex absorbs a photon, it becomes excited, as it returns to ground state it passes the energy to another pigment molecule in the light-harvesting complex, exciting it. This continues until the energy reaches chlorophyll a molecules of the reaction-center.
-When excited, these chlorophyll a molecules not only promote their electrons, but they then pass them to the electron acceptor, this is a redox reaction and the first step in the light reactions.
-There are two photosystems within thylakoid membranes: Photosystem II (PS II) and Photosystem I (PS I)
-Photosystem II functions first in the light reactions.
-Each system has a characteristic reaction-center complex containing a special pair of chlorophyll a molecules.
-They both contain identical molecules of chlorophyll a, but the molecules absorb light differently due to the proteins they are associated with in the different reaction-centers of each system.
-The reaction-complex of PS II contains chlorophyll a molecules called P680 (because they absorb 680 nm light best)
-The reaction-complex of PS I contains chlorophyll a molecules called P700 (because they absorb 700 nm light best)
Linear Electron Flow
Linear electron flow:flow of electrons through the photosystems and other molecular components built into the thylakoid membrane during the light reactions of photosynthesis.
Light Reactions:
1) a pigment molecule in the light-harvesting complex absorbs a photon of light, exciting its electron. As this electron returns to its ground state, an electron nearby in another pigment molecule is excited... this process continues as pigment molecules relay the energy, until the energy reaches the P680 pair of chloropyll a molecules in the PS II reaction-center complex, exciting electrons.
2) These electrons are then transferred to the primary electron acceptor, resulting in oxidized P680 (P680+)
3) An enzyme catalyzes the split of water into two electrons, two hydrogen ions and an oxygen atom.
-The electrons go to replace those lost by P680+, returning it to P680
-The H+ ions are released into the thylakoid space (lumen), contributing to the proton gradient
-The oxygen atom combines with another oxygen atom and forms O2.
4) The electron acceptor delivers the photoexcited electrons from PS II to PS I using an electron transport chain.
-This chain consists of plastiquinone (Pq), a cytochrome complex, and the protein plastocyanin (Pc)
5) The exergonic fall of electrons to a lower energy state provides energy for the synthesis of ATP. The flow of electrons through the cytochrome complex results in H+ being pumped into the lumen, contributing to the proton gradient used for chemiosmosis.
6) While electrons are being transferred, so is the light energy, which proceeds to PS700 in the reaction center of PSI via the light-harvesting complexes of this system.
-The photoexcited electron of P700 is then transferred to PSI's primary electron acceptor, oxidizing P700 to P700 +
-P700+ then acts as the final electron acceptor at the bottom of the 1st electron transport chain.
7) Electrons are passed from the primary acceptor of PSI to a second electron transport chain where they proceed to participate in another series of redox reactions
-This electron transport chain does not contribute to the proton gradient, and therefore do not make ATP.
8) The enzyme NADP+ reductase catalyzes the transfer of electrons from ferredoxin (Fd) in the electron transport chain to NADP+, reducing it to NADPH, making NADP+ the final electron acceptor of the second electron transport chain.
BOTTOM LINE: the light reactions use solar power to generate ATP and NADPH, which provide the chemical energy needed for the Calvin cycle.
-When a pigment molecule absorbs a photon of light one of the molecule's electrons is elevated to an orbital with more potential energy.
-The elevated electron is said to be in an excited state.
-Only photons that have exactly the amount of energy that is the difference between that molecule's excited and ground state can be absorbed by that molecule. (this accounts for the difference in light absorption between molecules)
-We know from earlier chapters that higher potential energy means more unstable and more likely to change (to a more stable state).
-Excited electrons are very unstable and must return to ground state.
A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes
-Photosystem: complexes of chlorophyll molecules organized along with other small organic molecules and proteins
-These complexes are broken down into
1) reaction-center complex: complex containing proteins holding two special molecules of chlorophyll a and a molecule capable of accepting electrons (called the primary electron acceptor)
2) light-harvesting complex: consist of pigment molecules that may include chlorophyll a, chlorophyll b and carotenoids bound to proteins
-this collection of pigments expands the types of light the chloroplast can absorb and the surface area
for light absorption.
-When a pigment molecule in a light-harvesting complex absorbs a photon, it becomes excited, as it returns to ground state it passes the energy to another pigment molecule in the light-harvesting complex, exciting it. This continues until the energy reaches chlorophyll a molecules of the reaction-center.
-When excited, these chlorophyll a molecules not only promote their electrons, but they then pass them to the electron acceptor, this is a redox reaction and the first step in the light reactions.
-There are two photosystems within thylakoid membranes: Photosystem II (PS II) and Photosystem I (PS I)
-Photosystem II functions first in the light reactions.
-Each system has a characteristic reaction-center complex containing a special pair of chlorophyll a molecules.
-They both contain identical molecules of chlorophyll a, but the molecules absorb light differently due to the proteins they are associated with in the different reaction-centers of each system.
-The reaction-complex of PS II contains chlorophyll a molecules called P680 (because they absorb 680 nm light best)
-The reaction-complex of PS I contains chlorophyll a molecules called P700 (because they absorb 700 nm light best)
Linear Electron Flow
Linear electron flow:flow of electrons through the photosystems and other molecular components built into the thylakoid membrane during the light reactions of photosynthesis.
Light Reactions:
1) a pigment molecule in the light-harvesting complex absorbs a photon of light, exciting its electron. As this electron returns to its ground state, an electron nearby in another pigment molecule is excited... this process continues as pigment molecules relay the energy, until the energy reaches the P680 pair of chloropyll a molecules in the PS II reaction-center complex, exciting electrons.
2) These electrons are then transferred to the primary electron acceptor, resulting in oxidized P680 (P680+)
3) An enzyme catalyzes the split of water into two electrons, two hydrogen ions and an oxygen atom.
-The electrons go to replace those lost by P680+, returning it to P680
-The H+ ions are released into the thylakoid space (lumen), contributing to the proton gradient
-The oxygen atom combines with another oxygen atom and forms O2.
4) The electron acceptor delivers the photoexcited electrons from PS II to PS I using an electron transport chain.
-This chain consists of plastiquinone (Pq), a cytochrome complex, and the protein plastocyanin (Pc)
5) The exergonic fall of electrons to a lower energy state provides energy for the synthesis of ATP. The flow of electrons through the cytochrome complex results in H+ being pumped into the lumen, contributing to the proton gradient used for chemiosmosis.
6) While electrons are being transferred, so is the light energy, which proceeds to PS700 in the reaction center of PSI via the light-harvesting complexes of this system.
-The photoexcited electron of P700 is then transferred to PSI's primary electron acceptor, oxidizing P700 to P700 +
-P700+ then acts as the final electron acceptor at the bottom of the 1st electron transport chain.
7) Electrons are passed from the primary acceptor of PSI to a second electron transport chain where they proceed to participate in another series of redox reactions
-This electron transport chain does not contribute to the proton gradient, and therefore do not make ATP.
8) The enzyme NADP+ reductase catalyzes the transfer of electrons from ferredoxin (Fd) in the electron transport chain to NADP+, reducing it to NADPH, making NADP+ the final electron acceptor of the second electron transport chain.
BOTTOM LINE: the light reactions use solar power to generate ATP and NADPH, which provide the chemical energy needed for the Calvin cycle.
Cyclic Electron Flow
Cyclic electron flow: alternative path of photoexcited electron flow that only uses PS I.
-no NADPH produced, no oxygen release, but ATP is generated by cycling electrons back through the cytochrome complex.
-several photosynthetic bacteria utilize only a single photosystem in this way.
-it is theorized that these bacteria are descendants of ancestral bacteria, before a second photosystem evolved.
-Cyclic electron flow is therefore considered an "evolutionary leftover," though some organisms with 2 photosystems still utilize it.
-There is evidence that the process may have a photoprotective role
-Mutant plants that are unable to carry out cyclic electron flow, cannot survive in high intensity light
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
-Both chloroplasts and mitochondria generate ATP using chemiosmosis.
-An electron transport chain couples redox reactions to the transport of H+ across a membrane to create a proton motive force.
-The proton gradient is utilized by ATP synthase, allowing H+ to diffuse down its gradient, supplying energy for the formation of ATP.
-chloroplasts= photophosphorylation (plants do not need food to make ATP, just light energy)
-ETC embedded in thylakoid membrane
- H+ pumped into thylakoid space (lumen), and diffuse back into the stroma
-ATP synthesized on stroma side of membrane
- the electrons that supply the ETC are derived from water
-Final electron acceptor = NADP+
-mitochondria= oxidative phosphorylation (mitochondria need food to make ATP)
-ETC embedded in inner mitochondrial membrane
-H+ pumped from matrix into intermembrane space, and diffuse back into the matrix
-ATP synthesized in matrix
-the electrons that supply the ETC are derived from the oxidation of organic compounds
-Final electron acceptor = O2
Cyclic electron flow: alternative path of photoexcited electron flow that only uses PS I.
-no NADPH produced, no oxygen release, but ATP is generated by cycling electrons back through the cytochrome complex.
-several photosynthetic bacteria utilize only a single photosystem in this way.
-it is theorized that these bacteria are descendants of ancestral bacteria, before a second photosystem evolved.
-Cyclic electron flow is therefore considered an "evolutionary leftover," though some organisms with 2 photosystems still utilize it.
-There is evidence that the process may have a photoprotective role
-Mutant plants that are unable to carry out cyclic electron flow, cannot survive in high intensity light
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
-Both chloroplasts and mitochondria generate ATP using chemiosmosis.
-An electron transport chain couples redox reactions to the transport of H+ across a membrane to create a proton motive force.
-The proton gradient is utilized by ATP synthase, allowing H+ to diffuse down its gradient, supplying energy for the formation of ATP.
-chloroplasts= photophosphorylation (plants do not need food to make ATP, just light energy)
-ETC embedded in thylakoid membrane
- H+ pumped into thylakoid space (lumen), and diffuse back into the stroma
-ATP synthesized on stroma side of membrane
- the electrons that supply the ETC are derived from water
-Final electron acceptor = NADP+
-mitochondria= oxidative phosphorylation (mitochondria need food to make ATP)
-ETC embedded in inner mitochondrial membrane
-H+ pumped from matrix into intermembrane space, and diffuse back into the matrix
-ATP synthesized in matrix
-the electrons that supply the ETC are derived from the oxidation of organic compounds
-Final electron acceptor = O2
- 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar