PPT- Light Class 7 Notes | EduRev

Science Class 7

Class 7 : PPT- Light Class 7 Notes | EduRev

 Page 1


Chapter 8: Photosynthesis: Energy From the Sun
I Identifying Photosynthetic Reactants and Products.
• In the 1800's, it was known that there were three principle ingredients for
photosynthesis: water, carbon dioxide, and light.
• There were two products: carbohydrates and oxygen.
• The water, which came from the soil, was transported through the roots.
• The CO
2
 and oxygen were taken up from the air.
• Light was an absolute necessity in order to produce oxygen and carbohydrates.
• By 1804, scientists summarized photosynthesis: CO
2
 + H
2
O + light energy ?
sugar + O
2
. (See Figure 8.1)
• More recently, using radioactive isotopes, it has been determined that the actual
reaction is: 6CO
2
 + 12H
2
O ? C
6
H
12
O
6
 + 6O
2
 + 6 H
2
O.
• Water appears on both sides of the equation because water is both used as a
reactant and released as a product. (See Figure 8.2)
II The Two Pathways of Photosynthesis: An Overview
• Photosynthesis occurs as a result of many and not just a single step.
• When looked at as a whole, it can be separated into two different pathways.
• The first pathway is called the light reaction, and is driven by light. It
produces ATP and the reduced electron carrier, NADPH+H
+
.
• The second pathway, called the Calvin-Benson cycle, does not use light
directly. It uses ATP, NADPH+H
+
 and CO
2
 to produce sugar.
• The light reactions are mediated by molecular assembles called photosystems.
• These systems pass electrons from one molecule to another and some of this
flow is coupled to synthesis of ATP.
• The pathway is referred to as photophosphorylation. Both NADPH + H
+
 and
ATP are produced by the light reactions.
• The second path is the Calvin-Benson Cycle, which uses the energy stored in
NADPH + H
+
 and ATP to fix CO
2
 into carbohydrates.
III Properties of Light Pigments
Light is the source of the energy required to drive photosynthesis
A. Light comes in packets called photons
• Visible light is part of the electromagnetic radiation spectrum. It comes in
discreet packets called photons.
• Light also behaves as if it were a wave. The wavelength of light is the distance
between one of the peaks and the next peak of the waves. (See Figure 8.4)
Page 2


Chapter 8: Photosynthesis: Energy From the Sun
I Identifying Photosynthetic Reactants and Products.
• In the 1800's, it was known that there were three principle ingredients for
photosynthesis: water, carbon dioxide, and light.
• There were two products: carbohydrates and oxygen.
• The water, which came from the soil, was transported through the roots.
• The CO
2
 and oxygen were taken up from the air.
• Light was an absolute necessity in order to produce oxygen and carbohydrates.
• By 1804, scientists summarized photosynthesis: CO
2
 + H
2
O + light energy ?
sugar + O
2
. (See Figure 8.1)
• More recently, using radioactive isotopes, it has been determined that the actual
reaction is: 6CO
2
 + 12H
2
O ? C
6
H
12
O
6
 + 6O
2
 + 6 H
2
O.
• Water appears on both sides of the equation because water is both used as a
reactant and released as a product. (See Figure 8.2)
II The Two Pathways of Photosynthesis: An Overview
• Photosynthesis occurs as a result of many and not just a single step.
• When looked at as a whole, it can be separated into two different pathways.
• The first pathway is called the light reaction, and is driven by light. It
produces ATP and the reduced electron carrier, NADPH+H
+
.
• The second pathway, called the Calvin-Benson cycle, does not use light
directly. It uses ATP, NADPH+H
+
 and CO
2
 to produce sugar.
• The light reactions are mediated by molecular assembles called photosystems.
• These systems pass electrons from one molecule to another and some of this
flow is coupled to synthesis of ATP.
• The pathway is referred to as photophosphorylation. Both NADPH + H
+
 and
ATP are produced by the light reactions.
• The second path is the Calvin-Benson Cycle, which uses the energy stored in
NADPH + H
+
 and ATP to fix CO
2
 into carbohydrates.
III Properties of Light Pigments
Light is the source of the energy required to drive photosynthesis
A. Light comes in packets called photons
• Visible light is part of the electromagnetic radiation spectrum. It comes in
discreet packets called photons.
• Light also behaves as if it were a wave. The wavelength of light is the distance
between one of the peaks and the next peak of the waves. (See Figure 8.4)
• Visible light fits into the overall electromagnetic spectrum between ultraviolet
and infrared radiation. (See figure 8.5).
• Humans perceive light as having distinct colors. The colors relate to the
wavelength of the light as shown in Figure 8.5.
• The frequency of light is inversely related to its wavelength.
• The shorter the wavelength the higher the frequency, the longer the
wavelength the lower the frequency.
• The amount of energy contained in a single photon is directly proportional
to its frequency.
• The shorter the wavelength, the greater the energy of the photon.
• A photon of red light has a wavelength of 660?m, and has less energy than
a photon of blue light, which has a wavelength of 430 ?m.
• Brightness is a measure of the photons striking an area per unit time, such as
one cm
2
 per second. Light intensity is often expressed in energy units, such as
calories per square centimeter per second.
B. Absorption of a proton puts a pigment in an excited state
• When a photon and a pigment molecule meet, one of three things happens:
• The photon may bounce off the molecule. This is reflection.
• The photon may pass through the molecule. This is transmission.
• If neither of these outcomes occur, the photon is absorbed by the
molecule. This is excitation.
• If absorbed, the photon disappears, but the energy it possessed can be neither
created nor destroyed and is therefore absorbed by the electron.
• The electron is raise from its ground state to an excited state of higher
energy.
• The difference between the excited and the ground state is precisely equal
to the energy of the absorbed proton. (See Figure 8.6)
• All molecules absorb electromagnetic radiation, but differ in the specific
wavelengths absorbed.
• Molecules that absorb wavelengths in the visible range are called pigments.
• When a beam of white light shines on an object, and the object appears to be
red in color, it is because it has absorbed other colors from the white light
except for the color red.
• In the case of chlorophyll, plants look green because they absorb green less
effectively than the other colors found in sunlight.
C. Light absorption and biological activity vary with wavelength
• A given type of molecule can absorb radiant energy of only certain
wavelengths.
• If we plot the absorption of the compound as a function of wavelength, the
result is an absorption spectrum. (See Figure 8.7)
Page 3


Chapter 8: Photosynthesis: Energy From the Sun
I Identifying Photosynthetic Reactants and Products.
• In the 1800's, it was known that there were three principle ingredients for
photosynthesis: water, carbon dioxide, and light.
• There were two products: carbohydrates and oxygen.
• The water, which came from the soil, was transported through the roots.
• The CO
2
 and oxygen were taken up from the air.
• Light was an absolute necessity in order to produce oxygen and carbohydrates.
• By 1804, scientists summarized photosynthesis: CO
2
 + H
2
O + light energy ?
sugar + O
2
. (See Figure 8.1)
• More recently, using radioactive isotopes, it has been determined that the actual
reaction is: 6CO
2
 + 12H
2
O ? C
6
H
12
O
6
 + 6O
2
 + 6 H
2
O.
• Water appears on both sides of the equation because water is both used as a
reactant and released as a product. (See Figure 8.2)
II The Two Pathways of Photosynthesis: An Overview
• Photosynthesis occurs as a result of many and not just a single step.
• When looked at as a whole, it can be separated into two different pathways.
• The first pathway is called the light reaction, and is driven by light. It
produces ATP and the reduced electron carrier, NADPH+H
+
.
• The second pathway, called the Calvin-Benson cycle, does not use light
directly. It uses ATP, NADPH+H
+
 and CO
2
 to produce sugar.
• The light reactions are mediated by molecular assembles called photosystems.
• These systems pass electrons from one molecule to another and some of this
flow is coupled to synthesis of ATP.
• The pathway is referred to as photophosphorylation. Both NADPH + H
+
 and
ATP are produced by the light reactions.
• The second path is the Calvin-Benson Cycle, which uses the energy stored in
NADPH + H
+
 and ATP to fix CO
2
 into carbohydrates.
III Properties of Light Pigments
Light is the source of the energy required to drive photosynthesis
A. Light comes in packets called photons
• Visible light is part of the electromagnetic radiation spectrum. It comes in
discreet packets called photons.
• Light also behaves as if it were a wave. The wavelength of light is the distance
between one of the peaks and the next peak of the waves. (See Figure 8.4)
• Visible light fits into the overall electromagnetic spectrum between ultraviolet
and infrared radiation. (See figure 8.5).
• Humans perceive light as having distinct colors. The colors relate to the
wavelength of the light as shown in Figure 8.5.
• The frequency of light is inversely related to its wavelength.
• The shorter the wavelength the higher the frequency, the longer the
wavelength the lower the frequency.
• The amount of energy contained in a single photon is directly proportional
to its frequency.
• The shorter the wavelength, the greater the energy of the photon.
• A photon of red light has a wavelength of 660?m, and has less energy than
a photon of blue light, which has a wavelength of 430 ?m.
• Brightness is a measure of the photons striking an area per unit time, such as
one cm
2
 per second. Light intensity is often expressed in energy units, such as
calories per square centimeter per second.
B. Absorption of a proton puts a pigment in an excited state
• When a photon and a pigment molecule meet, one of three things happens:
• The photon may bounce off the molecule. This is reflection.
• The photon may pass through the molecule. This is transmission.
• If neither of these outcomes occur, the photon is absorbed by the
molecule. This is excitation.
• If absorbed, the photon disappears, but the energy it possessed can be neither
created nor destroyed and is therefore absorbed by the electron.
• The electron is raise from its ground state to an excited state of higher
energy.
• The difference between the excited and the ground state is precisely equal
to the energy of the absorbed proton. (See Figure 8.6)
• All molecules absorb electromagnetic radiation, but differ in the specific
wavelengths absorbed.
• Molecules that absorb wavelengths in the visible range are called pigments.
• When a beam of white light shines on an object, and the object appears to be
red in color, it is because it has absorbed other colors from the white light
except for the color red.
• In the case of chlorophyll, plants look green because they absorb green less
effectively than the other colors found in sunlight.
C. Light absorption and biological activity vary with wavelength
• A given type of molecule can absorb radiant energy of only certain
wavelengths.
• If we plot the absorption of the compound as a function of wavelength, the
result is an absorption spectrum. (See Figure 8.7)
• Absorption spectrums are good fingerprints of compounds. Sometimes an
absorption spectrum contains enough information to enable us to identify an
unknown compound.
• If absorption results in an activity of some sort, then a plot of the effectiveness
of the light as a function of wavelength is called an action spectrum. Figure
8.8 shows the action spectrum of photosynthesis by Anacharis, a freshwater
plant.
D. Photosynthesis uses chlorophylls and accessory pigments
• Chlorophylls are important pigments in photosynthesis.
• Plants have two predominant chlorophylls: chlorophyll A and chlorophyll B.
These molecules differ slightly in their structure.
• Both have a similar ring structure.
• In the center of the chlorophyll ring is a magnesium atom. At the
peripheral location of the ring is a long hydrocarbon tail that can associate
with the hydrophobic region of the thylakoid membrane. (See Figure 8.9)
• These chlorophylls absorb blue and red wavelengths, which are near the ends
of the spectrum.
• Other accessory pigments absorb photons that are intermediate in energy,
between the red and blue wavelengths, and then transfer a portion of that
energy to chlorophylls.
• Examples of accessory pigments are the carotenoids such as beta carotene,
which absorb photons in the blue and blue-green wavelength, and appear deep
yellow in color.
IV Light reactions:
A. Light absorption
• A pigment molecule enters an excited state when it absorbs a photon.
• The excited state is unstable. One of two things will happen.
• The molecule might return to ground state, emitting a photon of light. This is
called fluorescence. (See Figure 8.6)
• When the molecule fluoresces, it emits a photon of a longer wavelength.
(See Figure 8.10)
• This is because of the second law of thermodynamics, which says that
during an energy transfer, not all the energy is available to do work.
• The molecule might pass some of the absorbed energy to other pigment
molecules.
• Pigments in the photosynthetic organisms arrange into energy absorbing
antennae systems.
• In these systems, complex proteins hold the molecules in the correct
orientation to enable the transfer of energy.
Page 4


Chapter 8: Photosynthesis: Energy From the Sun
I Identifying Photosynthetic Reactants and Products.
• In the 1800's, it was known that there were three principle ingredients for
photosynthesis: water, carbon dioxide, and light.
• There were two products: carbohydrates and oxygen.
• The water, which came from the soil, was transported through the roots.
• The CO
2
 and oxygen were taken up from the air.
• Light was an absolute necessity in order to produce oxygen and carbohydrates.
• By 1804, scientists summarized photosynthesis: CO
2
 + H
2
O + light energy ?
sugar + O
2
. (See Figure 8.1)
• More recently, using radioactive isotopes, it has been determined that the actual
reaction is: 6CO
2
 + 12H
2
O ? C
6
H
12
O
6
 + 6O
2
 + 6 H
2
O.
• Water appears on both sides of the equation because water is both used as a
reactant and released as a product. (See Figure 8.2)
II The Two Pathways of Photosynthesis: An Overview
• Photosynthesis occurs as a result of many and not just a single step.
• When looked at as a whole, it can be separated into two different pathways.
• The first pathway is called the light reaction, and is driven by light. It
produces ATP and the reduced electron carrier, NADPH+H
+
.
• The second pathway, called the Calvin-Benson cycle, does not use light
directly. It uses ATP, NADPH+H
+
 and CO
2
 to produce sugar.
• The light reactions are mediated by molecular assembles called photosystems.
• These systems pass electrons from one molecule to another and some of this
flow is coupled to synthesis of ATP.
• The pathway is referred to as photophosphorylation. Both NADPH + H
+
 and
ATP are produced by the light reactions.
• The second path is the Calvin-Benson Cycle, which uses the energy stored in
NADPH + H
+
 and ATP to fix CO
2
 into carbohydrates.
III Properties of Light Pigments
Light is the source of the energy required to drive photosynthesis
A. Light comes in packets called photons
• Visible light is part of the electromagnetic radiation spectrum. It comes in
discreet packets called photons.
• Light also behaves as if it were a wave. The wavelength of light is the distance
between one of the peaks and the next peak of the waves. (See Figure 8.4)
• Visible light fits into the overall electromagnetic spectrum between ultraviolet
and infrared radiation. (See figure 8.5).
• Humans perceive light as having distinct colors. The colors relate to the
wavelength of the light as shown in Figure 8.5.
• The frequency of light is inversely related to its wavelength.
• The shorter the wavelength the higher the frequency, the longer the
wavelength the lower the frequency.
• The amount of energy contained in a single photon is directly proportional
to its frequency.
• The shorter the wavelength, the greater the energy of the photon.
• A photon of red light has a wavelength of 660?m, and has less energy than
a photon of blue light, which has a wavelength of 430 ?m.
• Brightness is a measure of the photons striking an area per unit time, such as
one cm
2
 per second. Light intensity is often expressed in energy units, such as
calories per square centimeter per second.
B. Absorption of a proton puts a pigment in an excited state
• When a photon and a pigment molecule meet, one of three things happens:
• The photon may bounce off the molecule. This is reflection.
• The photon may pass through the molecule. This is transmission.
• If neither of these outcomes occur, the photon is absorbed by the
molecule. This is excitation.
• If absorbed, the photon disappears, but the energy it possessed can be neither
created nor destroyed and is therefore absorbed by the electron.
• The electron is raise from its ground state to an excited state of higher
energy.
• The difference between the excited and the ground state is precisely equal
to the energy of the absorbed proton. (See Figure 8.6)
• All molecules absorb electromagnetic radiation, but differ in the specific
wavelengths absorbed.
• Molecules that absorb wavelengths in the visible range are called pigments.
• When a beam of white light shines on an object, and the object appears to be
red in color, it is because it has absorbed other colors from the white light
except for the color red.
• In the case of chlorophyll, plants look green because they absorb green less
effectively than the other colors found in sunlight.
C. Light absorption and biological activity vary with wavelength
• A given type of molecule can absorb radiant energy of only certain
wavelengths.
• If we plot the absorption of the compound as a function of wavelength, the
result is an absorption spectrum. (See Figure 8.7)
• Absorption spectrums are good fingerprints of compounds. Sometimes an
absorption spectrum contains enough information to enable us to identify an
unknown compound.
• If absorption results in an activity of some sort, then a plot of the effectiveness
of the light as a function of wavelength is called an action spectrum. Figure
8.8 shows the action spectrum of photosynthesis by Anacharis, a freshwater
plant.
D. Photosynthesis uses chlorophylls and accessory pigments
• Chlorophylls are important pigments in photosynthesis.
• Plants have two predominant chlorophylls: chlorophyll A and chlorophyll B.
These molecules differ slightly in their structure.
• Both have a similar ring structure.
• In the center of the chlorophyll ring is a magnesium atom. At the
peripheral location of the ring is a long hydrocarbon tail that can associate
with the hydrophobic region of the thylakoid membrane. (See Figure 8.9)
• These chlorophylls absorb blue and red wavelengths, which are near the ends
of the spectrum.
• Other accessory pigments absorb photons that are intermediate in energy,
between the red and blue wavelengths, and then transfer a portion of that
energy to chlorophylls.
• Examples of accessory pigments are the carotenoids such as beta carotene,
which absorb photons in the blue and blue-green wavelength, and appear deep
yellow in color.
IV Light reactions:
A. Light absorption
• A pigment molecule enters an excited state when it absorbs a photon.
• The excited state is unstable. One of two things will happen.
• The molecule might return to ground state, emitting a photon of light. This is
called fluorescence. (See Figure 8.6)
• When the molecule fluoresces, it emits a photon of a longer wavelength.
(See Figure 8.10)
• This is because of the second law of thermodynamics, which says that
during an energy transfer, not all the energy is available to do work.
• The molecule might pass some of the absorbed energy to other pigment
molecules.
• Pigments in the photosynthetic organisms arrange into energy absorbing
antennae systems.
• In these systems, complex proteins hold the molecules in the correct
orientation to enable the transfer of energy.
• The excited electrons are passed to the reaction center of the antenna
complex.
• The pigment molecule in the center is always a molecule of chlorophyll A.
• There are other chlorophyll A molecules in the antenna, but they absorb
light at shorter wavelength.
B. Excited chlorophyll acts as a reducing agent
• The ground state chlorophyll, symbolized by Chl, is not much of a reducing
agent, but excited Chl* is a good one.
• The reducing capability of Chl* is because excited molecules have electrons
zipping around further away from the nucleus. (See Figure 8.11)
• Less tightly held, the electron is more likely to be passed on in a redox
reaction to an oxidizing agent.
• Chl* can react with an oxidizing agent in a reaction like Chl*A ? Chl
+
 + A
-
.
• This is the first biochemical consequence of light; chlorophyll becomes a
reducing agent after having absorbed a photon.
• The electron is passed on to an oxidizing agent. Chlorophyll then becomes a
positively charged ion, which is missing an electron.
V Electron Flow Phosphorylation and Reductions
• The passing of an electron from chlorophyll to an electron acceptor begins an
electron flow.
• The electrons flow through a series of carriers, where redox reactions occur,
one after another, and the energy of these redox reactions is used to pump
protons.
•  The process is referred to as photophosphorylation.
• Another high energy product that is generated is NADPH+H
+
. Similar to the NAD
found in cellular respiration, NADP
+
 is nicotinamide adenine dinucleotide
phosphate.
• There are two different systems for flow of electrons in photosynthesis.
• The noncyclic electron flow produces NADPH+H
+
 and ATP. (See Figure 8.12)
• Noncyclic electron flow also produces the oxygen that we find in the atmosphere.
• Cyclic electron flow produces just ATP.
The oxygen we breathe:
• All the oxygen in the atmosphere is generated by non-cyclic
photophosphorylation.
• When the excited electron is passed from chlorophyll P680 (photosystem II),
it leaves the chlorophyll molecule oxidized and charged, Chl
+
.
• The electrons to restore the chlorophyll come from water.
• H
2
O ? 2e
-
 + 2H
+
 + 1/2O
2
• The oxidized chlorophyl A is reduced by an electron from a manganese
containing protein called protein Z.
Page 5


Chapter 8: Photosynthesis: Energy From the Sun
I Identifying Photosynthetic Reactants and Products.
• In the 1800's, it was known that there were three principle ingredients for
photosynthesis: water, carbon dioxide, and light.
• There were two products: carbohydrates and oxygen.
• The water, which came from the soil, was transported through the roots.
• The CO
2
 and oxygen were taken up from the air.
• Light was an absolute necessity in order to produce oxygen and carbohydrates.
• By 1804, scientists summarized photosynthesis: CO
2
 + H
2
O + light energy ?
sugar + O
2
. (See Figure 8.1)
• More recently, using radioactive isotopes, it has been determined that the actual
reaction is: 6CO
2
 + 12H
2
O ? C
6
H
12
O
6
 + 6O
2
 + 6 H
2
O.
• Water appears on both sides of the equation because water is both used as a
reactant and released as a product. (See Figure 8.2)
II The Two Pathways of Photosynthesis: An Overview
• Photosynthesis occurs as a result of many and not just a single step.
• When looked at as a whole, it can be separated into two different pathways.
• The first pathway is called the light reaction, and is driven by light. It
produces ATP and the reduced electron carrier, NADPH+H
+
.
• The second pathway, called the Calvin-Benson cycle, does not use light
directly. It uses ATP, NADPH+H
+
 and CO
2
 to produce sugar.
• The light reactions are mediated by molecular assembles called photosystems.
• These systems pass electrons from one molecule to another and some of this
flow is coupled to synthesis of ATP.
• The pathway is referred to as photophosphorylation. Both NADPH + H
+
 and
ATP are produced by the light reactions.
• The second path is the Calvin-Benson Cycle, which uses the energy stored in
NADPH + H
+
 and ATP to fix CO
2
 into carbohydrates.
III Properties of Light Pigments
Light is the source of the energy required to drive photosynthesis
A. Light comes in packets called photons
• Visible light is part of the electromagnetic radiation spectrum. It comes in
discreet packets called photons.
• Light also behaves as if it were a wave. The wavelength of light is the distance
between one of the peaks and the next peak of the waves. (See Figure 8.4)
• Visible light fits into the overall electromagnetic spectrum between ultraviolet
and infrared radiation. (See figure 8.5).
• Humans perceive light as having distinct colors. The colors relate to the
wavelength of the light as shown in Figure 8.5.
• The frequency of light is inversely related to its wavelength.
• The shorter the wavelength the higher the frequency, the longer the
wavelength the lower the frequency.
• The amount of energy contained in a single photon is directly proportional
to its frequency.
• The shorter the wavelength, the greater the energy of the photon.
• A photon of red light has a wavelength of 660?m, and has less energy than
a photon of blue light, which has a wavelength of 430 ?m.
• Brightness is a measure of the photons striking an area per unit time, such as
one cm
2
 per second. Light intensity is often expressed in energy units, such as
calories per square centimeter per second.
B. Absorption of a proton puts a pigment in an excited state
• When a photon and a pigment molecule meet, one of three things happens:
• The photon may bounce off the molecule. This is reflection.
• The photon may pass through the molecule. This is transmission.
• If neither of these outcomes occur, the photon is absorbed by the
molecule. This is excitation.
• If absorbed, the photon disappears, but the energy it possessed can be neither
created nor destroyed and is therefore absorbed by the electron.
• The electron is raise from its ground state to an excited state of higher
energy.
• The difference between the excited and the ground state is precisely equal
to the energy of the absorbed proton. (See Figure 8.6)
• All molecules absorb electromagnetic radiation, but differ in the specific
wavelengths absorbed.
• Molecules that absorb wavelengths in the visible range are called pigments.
• When a beam of white light shines on an object, and the object appears to be
red in color, it is because it has absorbed other colors from the white light
except for the color red.
• In the case of chlorophyll, plants look green because they absorb green less
effectively than the other colors found in sunlight.
C. Light absorption and biological activity vary with wavelength
• A given type of molecule can absorb radiant energy of only certain
wavelengths.
• If we plot the absorption of the compound as a function of wavelength, the
result is an absorption spectrum. (See Figure 8.7)
• Absorption spectrums are good fingerprints of compounds. Sometimes an
absorption spectrum contains enough information to enable us to identify an
unknown compound.
• If absorption results in an activity of some sort, then a plot of the effectiveness
of the light as a function of wavelength is called an action spectrum. Figure
8.8 shows the action spectrum of photosynthesis by Anacharis, a freshwater
plant.
D. Photosynthesis uses chlorophylls and accessory pigments
• Chlorophylls are important pigments in photosynthesis.
• Plants have two predominant chlorophylls: chlorophyll A and chlorophyll B.
These molecules differ slightly in their structure.
• Both have a similar ring structure.
• In the center of the chlorophyll ring is a magnesium atom. At the
peripheral location of the ring is a long hydrocarbon tail that can associate
with the hydrophobic region of the thylakoid membrane. (See Figure 8.9)
• These chlorophylls absorb blue and red wavelengths, which are near the ends
of the spectrum.
• Other accessory pigments absorb photons that are intermediate in energy,
between the red and blue wavelengths, and then transfer a portion of that
energy to chlorophylls.
• Examples of accessory pigments are the carotenoids such as beta carotene,
which absorb photons in the blue and blue-green wavelength, and appear deep
yellow in color.
IV Light reactions:
A. Light absorption
• A pigment molecule enters an excited state when it absorbs a photon.
• The excited state is unstable. One of two things will happen.
• The molecule might return to ground state, emitting a photon of light. This is
called fluorescence. (See Figure 8.6)
• When the molecule fluoresces, it emits a photon of a longer wavelength.
(See Figure 8.10)
• This is because of the second law of thermodynamics, which says that
during an energy transfer, not all the energy is available to do work.
• The molecule might pass some of the absorbed energy to other pigment
molecules.
• Pigments in the photosynthetic organisms arrange into energy absorbing
antennae systems.
• In these systems, complex proteins hold the molecules in the correct
orientation to enable the transfer of energy.
• The excited electrons are passed to the reaction center of the antenna
complex.
• The pigment molecule in the center is always a molecule of chlorophyll A.
• There are other chlorophyll A molecules in the antenna, but they absorb
light at shorter wavelength.
B. Excited chlorophyll acts as a reducing agent
• The ground state chlorophyll, symbolized by Chl, is not much of a reducing
agent, but excited Chl* is a good one.
• The reducing capability of Chl* is because excited molecules have electrons
zipping around further away from the nucleus. (See Figure 8.11)
• Less tightly held, the electron is more likely to be passed on in a redox
reaction to an oxidizing agent.
• Chl* can react with an oxidizing agent in a reaction like Chl*A ? Chl
+
 + A
-
.
• This is the first biochemical consequence of light; chlorophyll becomes a
reducing agent after having absorbed a photon.
• The electron is passed on to an oxidizing agent. Chlorophyll then becomes a
positively charged ion, which is missing an electron.
V Electron Flow Phosphorylation and Reductions
• The passing of an electron from chlorophyll to an electron acceptor begins an
electron flow.
• The electrons flow through a series of carriers, where redox reactions occur,
one after another, and the energy of these redox reactions is used to pump
protons.
•  The process is referred to as photophosphorylation.
• Another high energy product that is generated is NADPH+H
+
. Similar to the NAD
found in cellular respiration, NADP
+
 is nicotinamide adenine dinucleotide
phosphate.
• There are two different systems for flow of electrons in photosynthesis.
• The noncyclic electron flow produces NADPH+H
+
 and ATP. (See Figure 8.12)
• Noncyclic electron flow also produces the oxygen that we find in the atmosphere.
• Cyclic electron flow produces just ATP.
The oxygen we breathe:
• All the oxygen in the atmosphere is generated by non-cyclic
photophosphorylation.
• When the excited electron is passed from chlorophyll P680 (photosystem II),
it leaves the chlorophyll molecule oxidized and charged, Chl
+
.
• The electrons to restore the chlorophyll come from water.
• H
2
O ? 2e
-
 + 2H
+
 + 1/2O
2
• The oxidized chlorophyl A is reduced by an electron from a manganese
containing protein called protein Z.
• Protein Z regains its lost electron by oxidizing a water molecule.
• The protons (H
+
) generated are released into the thylakoid lumen, where it
helps to build the proton gradient.
A. Noncyclic Events summary:
• Photons are absorbed by chlorophyll P680 molecules.
• The P680 excited electrons transfer to the electron transport chain.
• The P680 is reduced by Protein Z and electrons from the hydrogen atoms of
water.
• Free energy of P680 electrons power proton (H
+
) transport from stroma to
lumen of chloroplast.
• Absorption of photons by P700 of photosystem I occur.
• The P700 electrons are replaced by those flowing from P680.
• Electrons from P700 are transferred to the electron transport chain.
• The P700 electrons and free protons from the stroma are used to reduce
NADP
+
 to NADPH + H
+
.
• See Figure 8.12 for details.
B. Cyclic electron flow produces ATP but no NADPH
• Photosystem I acts on its own. (See Figure 8.13)
• Cyclic refers to the circular pathway of P700 electrons.
• Energy produced must be stored in ATP and NADPH+H
+
 to drive the Calvin-
Benson cycle. This cycle uses more ATP than NADPH+H
+
.
• To keep balance, cyclic electron flow makes ATP without making
NADPH+H
+
.
• No photolysis of water occurs during cyclic events.
• No oxygen is generated by the cyclic events.
• The P700 molecule, the reaction center chlorophyll, starts at ground state.
• It absorbs a photon and becomes P700*.
• The P700* molecule reduces ferredoxin (Fd
red
)
• In contrast to non-cyclic photophosphorylation where Fd
red
 reduces
NADP
+
, Fd
red
 passes its electron to plastoquinone (PQ).
• The PQ molecule passes the electron to the cytochrome complex.
• The electron continues down a redox chain, pumping protons as it goes.
• The P700
+
, the chlorophyll that lost an electron, gets it back again at the
end of the chain.
C. Chemiosmosis is the source or ATP
• The ATP molecules are produced by a mechanism similar to what is described
in Chapter 7 for mitochondria.
• The same type of chemiosmotic mechanism operates in photosynthesis, but is
called photophosphorylation. (See Figure 8.14)
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