C10 photosynthesis
Photosynthesis converts light energy to the chemical energy of food
Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
light reactions
Calvin cycle
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The light reactions (in the thylakoids)
Split H2O
Release O2
Reduce the electron acceptor NADP+ to NADPH
Generate ATP from ADP by photophosphorylation
The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
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Photosynthesis as a Redox Process
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Photosynthesis reverses the direction of electron flow compared to respiration
Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced
Photosynthesis is an endergonic process; the energy boost is provided by light
Chloroplasts: The Sites of Photosynthesis in Plants
Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
CO2 enters and O2 exits the leaf through microscopic pores called stomata
Thylakoids are connected sacs in the chloroplast that compose a third membrane system
Chlorophyll, the pigment that gives leaves their green color, resides in the thylakoid membranes
Photosynthesis is a complex series of reactions that can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product
The light reactions convert solar energy to the chemical energy of ATP and NADPH
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
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Electron Flow
In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH
Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy
Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP
Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities
In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix
In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma
ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place
cyclic
linear
the primary pathway, involves both photosystems and produces ATP and NADPH using light energy
8 steps
A photon hits a pigment in a light-harvesting complex of PS II, and its energy is passed among pigment molecules until it excites P680
An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)
H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680
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P680+ is the strongest known biological oxidizing agent
The H+ are released into the thylakoid space
O2 is released as a by-product of this reaction
Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
Potential energy stored in the proton gradient drives production of ATP by chemiosmosis
In PS I (like PS II), transferred light energy excites P700, which loses an electron to the primary electron acceptor
Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
NADP+ reductase catalyzes the transfer of electrons to NADP+, reducing it to NADPH
electrons cycle back from Fd to the PS I reaction center via a plastocyanin molecule (Pc)
Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH
No oxygen is released
Some organisms such as purple sulfur bacteria have PS I but not PS II
Cyclic electron flow is thought to have evolved before linear electron flow
Cyclic electron flow may protect cells from light-induced damage
A Photosystem
A Reaction-Center Complex Associated with Light-Harvesting Complexes
reaction-center complex
an association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor
consists of pigment molecules bound to proteins
transfer the energy of photons to the chlorophyll a molecules in the reaction-center complex
These chlorophyll a molecules are special because they can transfer an excited electron to a different molecule
primary electron acceptor
accepts excited electrons and is reduced as a result
Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions
Photosynthetic Pigments: The Light Receptors
Excitation of Chlorophyll by Light
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or transmitted
Leaves appear green because chlorophyll reflects and transmits green light
Chloroplasts are solar-powered chemical factories
Their thylakoids transform light energy into the chemical energy of ATP and NADPH
When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable
When excited electrons fall back to the ground state, excess energy is released as heat
The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar
The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
anabolic
The Calvin cycle is anabolic; it builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
three phases:
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Reduction
Regeneration of the CO2 acceptor (RuBP)
Carbon fixation (catalyzed by rubisco)
Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Photorespiration
In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound
consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
limits damaging products of light reactions that build up in the absence of the Calvin cycle
C4 Plants
minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds
Sugar production in C4 plants occurs in a three-step process:
1.The production of the four-carbon precursors is catalyzed by the enzyme PEP carboxylase in the mesophyll cells
2.These four-carbon compounds are exported to bundle-sheath cells
3.Within the bundle-sheath cells, they release CO2 that is then used in the Calvin cycle
climates
Suitable agricultural land is decreasing due to the effects of climate change, while the world demand for food continues to increase
Increasing levels of CO2 may affect C3 and C4 plants differently, perhaps changing the relative abundance of these species
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C4 photosynthesis uses less water and resources than C3 photosynthesis
Scientists have genetically modified rice, a C3 plant, to carry out C4 photosynthesis
They estimate 30–50% increase in yield compared to C3 rice
CAM Plants
crassulacean acid metabolism (CAM)
Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon
CAM plants open their stomata at night, incorporating CO2 into organic acids that are stored in the vacuoles
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
Life depends on photosynthesis
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The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
Plants store excess sugar as starch in chloroplasts and other structures such as roots, tubers, seeds, and fruits