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Questions and Answers
What is the primary function of photosynthesis?
What is the primary function of photosynthesis?
- To convert light energy into chemical energy (correct)
- To convert chemical energy into light energy
- To break down glucose into carbon dioxide and water
- To produce oxygen from carbon dioxide
Why do plants utilize a variety of pigments in photosynthesis?
Why do plants utilize a variety of pigments in photosynthesis?
- To reflect different colors of light
- To increase the rate of electron transport
- To protect chlorophyll from excessive light
- To absorb a wider range of wavelengths of light (correct)
In noncyclic photophosphorylation, what is the role of P680 and P700?
In noncyclic photophosphorylation, what is the role of P680 and P700?
- They are special chlorophyll a molecules that absorb maximum light at specific wavelengths and energize electrons. (correct)
- They directly fix carbon dioxide into glucose.
- They catalyze the production of ATP from ADP.
- They transport electrons from water to NADPH.
What is the role of the primary electron acceptor in noncyclic photophosphorylation?
What is the role of the primary electron acceptor in noncyclic photophosphorylation?
What is the direct source of electrons that replenish those lost by P680 in photosystem II during noncyclic photophosphorylation?
What is the direct source of electrons that replenish those lost by P680 in photosystem II during noncyclic photophosphorylation?
How do the electron transport chains in photosynthesis contribute to ATP production?
How do the electron transport chains in photosynthesis contribute to ATP production?
What is the primary difference between cyclic and noncyclic photophosphorylation?
What is the primary difference between cyclic and noncyclic photophosphorylation?
What is the main purpose of the Calvin cycle?
What is the main purpose of the Calvin cycle?
Which enzyme is responsible for the initial fixation of carbon dioxide in the Calvin cycle?
Which enzyme is responsible for the initial fixation of carbon dioxide in the Calvin cycle?
What is the role of ATP and NADPH in the Calvin cycle?
What is the role of ATP and NADPH in the Calvin cycle?
What happens to the majority of G3P molecules produced during the Calvin cycle?
What happens to the majority of G3P molecules produced during the Calvin cycle?
In the process of chemiosmosis in chloroplasts, where do protons accumulate to create a concentration gradient?
In the process of chemiosmosis in chloroplasts, where do protons accumulate to create a concentration gradient?
How does C4 photosynthesis minimize photorespiration compared to C3 photosynthesis?
How does C4 photosynthesis minimize photorespiration compared to C3 photosynthesis?
How do CAM plants temporally segregate carbon fixation?
How do CAM plants temporally segregate carbon fixation?
In C4 photosynthesis, what is the initial carbon fixation product in the mesophyll cells?
In C4 photosynthesis, what is the initial carbon fixation product in the mesophyll cells?
Flashcards
Photosynthesis
Photosynthesis
The process of converting sunlight into chemical bond energy, especially glucose.
Photosynthetic pigments
Photosynthetic pigments
Chlorophyll a, chlorophyll b, and carotenoids. Absorb different wavelengths of light to optimize energy absorption.
Photosystems I and II
Photosystems I and II
Chlorophyll a molecules (P680 and P700) associated with nearby pigments.
Noncyclic Photophosphorylation
Noncyclic Photophosphorylation
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Photosystem II
Photosystem II
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Primary Electron Acceptor
Primary Electron Acceptor
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Electron transport chain (photosynthesis)
Electron transport chain (photosynthesis)
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Phosphorylation
Phosphorylation
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Photosystem I
Photosystem I
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NADPH
NADPH
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Splitting of water
Splitting of water
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Cyclic Photophosphorylation
Cyclic Photophosphorylation
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Calvin Cycle
Calvin Cycle
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Carboxylation
Carboxylation
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Reduction (Calvin Cycle)
Reduction (Calvin Cycle)
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Study Notes
- Photosynthesis converts sunlight into chemical bond energy, mainly glucose.
- The general equation is 6CO2 + 6H2O + light → C6H12O6 + 6O2.
- This equation is the reverse of cellular respiration, with light providing the energy.
Light Absorption
- Photosynthesis begins with light-absorbing pigments that can only absorb specific wavelengths of light.
- Plant cells use a variety of pigments, including chlorophyll a, chlorophyll b, and carotenoids (red, orange, or yellow), to optimize the bandwidth of light absorbed.
- Light energy absorbed by pigments energizes electrons.
- Energized electrons re-emit energy which is reabsorbed by nearby pigment molecules.
- Energy bounces from one pigment molecule to another until absorbed by special chlorophyll a molecules (P680 and P700).
- P680 and P700 absorb maximum light at wavelengths of 680 nm and 700 nm.
- P700 forms photosystem I (PS I) while chlorophyll P680 forms photosystem II (PS II).
Noncyclic photophosphorylation
- Noncyclic photophosphorylation uses light energy to make ATP from ADP and Pi.
- Begins with PS II:
- Electrons trapped by P680 in photosystem II become energized by light.
- Two energized electrons are passed to the primary electron acceptor, the first in a chain of acceptors.
- Electrons pass through an electron transport chain involving proteins like ferredoxin and cytochrome.
- As electrons move down the electron transport chain they lose energy and are used to phosphorylate, producing 1.5 ATP molecules on average.
- The electron transport chain terminates with PS I (P700) where electrons are re-energized by sunlight and passed to a primary electron acceptor different from the acceptor associated with PS II.
- The two electrons then pass through a short electron transport chain where they combine with NADP+ and H+ to form NADPH, a coenzyme.
- The two electrons that originated in PS II are replaced when Hâ‚‚O is split into two electrons, 2 H+ and 1/2 O2, catalyzed by a manganese-containing protein complex.
- The electrons from H2O replace those lost from PS II, H+ provides the H in NADPH, and 1/2 O2 contributes to the oxygen gas that is released.
- Takes the energy in light and the electrons in Hâ‚‚O to make ATP and NADPH.
- Because the reactions require light, they are called the light-dependent reactions or light reactions.
- Summarized as: H2O + ADP + Pᵢ + NADP+ + light → ATP + NADPH + O2 + H+
Cyclic Photophosphorylation
- Occurs when electrons energized in PS I are recycled.
- Energized electrons from PS I join protein carriers and generate ATP as they pass along the electron transport chain.
- Electrons return to PS I and can be re-energized to participate in cyclic or noncyclic photophosphorylation.
- Cyclic photophosphorylation occurs simultaneously with noncyclic photophosphorylation to generate additional ATP (two electrons yield about 1 ATP).
Calvin Cycle
- Fixes inorganic CO2 into an organic molecule.
- Biosynthetic pathway yields a single glucose molecule (C6H12O6); the cycle must repeat six times using 6 CO2 molecules.
- The most important molecules processes described below:
- 6 CO2 combine with 6 RuBP to produce 12 PGA, catalyzed by rubisco (ribulose bisphosphate carboxylase/oxygenase); also known as the Calvin-Benson cycle or the carbon reduction cycle.
- 12 ATP and 12 NADPH are used to convert 12 PGA to 12 G3P. It releases ADP, Páµ¢, and NADP+ which are re-energized in noncyclic photophosphorylation.
- 6 ATP are used to convert 10 G3P to 6 RuBP, regenerating the 6 RuBP.
- Two remaining G3P molecules are used to build glucose, and other monosaccharides like fructose and maltose, or disaccharides like sucrose, and polysaccharides like starch and cellulose.
- Reactions are called light-independent reactions, or dark reactions, but require light since they depend on ATP and NADPH from photophosphorylation.
- Takes CO2 from the atmosphere and the energy in ATP and NADPH to create a glucose molecule.
- Summarized as: 6CO2 + 18ATP + 12NADPH + H+ → 18ADP + 18Pᵢ + 12NADP+ + 1 glucose.
Chloroplasts
- Sites of both light-dependent and light-independent reactions.
- Composed of:
- Outer membrane: Double layer of phospholipids
- Intermembrane space: Narrow area between the inner and outer membranes.
- Inner membrane: Double phospholipid bilayer
- Stroma: Fluid material inside the inner membrane where the Calvin cycle occurs, fixing carbon from CO2 to generate G3P.
- Thylakoids: Suspended within the stroma are stacks of pancake-like membranes where individual layers are thylakoids; an entire stack of thylakoids is a granum (plural, grana).
- Thylakoid lumen: Inside of the thylakoid where H+ ions accumulate.
- The photosynthetic processes arrangement is similar to mitochondrial respiratory processes.
Chemiosmosis in Chloroplasts
- Mechanism of ATP generation which stores energy in the form of a proton concentration gradient across a membrane.
- H+ ions accumulate inside thylakoids.
- H+ are released into the lumen when water is split by PS II. Also, H+ are carried from the stroma into the lumen by a cytochrome in the ETC between PS II and PS I.
- Creates a pH and electrical gradient across the thylakoid membrane.
- As H+ accumulates inside, pH decreases; as H+ come from outside, pH increases in the stroma.
- This consists of differences in the concentration of H+ across the thylakoid membrane from a stroma pH 8 to a thylakoid pH 5 (a factor of 1000).
- The accumulation of positively charged H+ inside the thylakoid creates an electric gradient (or voltage).
- ATP synthases generate ATP.
- Channel proteins, called ATP synthases, allow H+ to flow through the thylakoid membrane and out to the stroma.
- With the energy generated by H+ passage the turbine phosphorylate ADP to ATP.
- ~3 H+ are required to generate one ATP.
- Calvin cycle produces G3P using NADPH and CO2 and ATP.
- Electrons combine with NADP+ and H+ to produce NADPH, then with ATP and COâ‚‚, G3P are produced.
Photorespiration
- Rubisco can also fix oxygen, therefore it's not efficient, leading to two problems:
- Reduced CO2-fixing efficiency.
- Products formed when O2 is combined with RuBP do not lead to useful, energy-rich molecules like glucose where peroxisomes near chloroplasts break down photorespiration products.
C4 Photosynthesis
- An add-on feature to C3 photosynthesis where CO2 combines with PEP using PEP carboxylase to form OAA in mesophyll cells.
- OAA (four carbon atoms) is converted to malate and shuttled to bundle sheath cells where it is converted to pyruvate and CO2.
- Pyruvate is shuttled back to mesophyll cells, requiring one ATP to convert back to PEP,.
- CO2 is spatially segregated as the purpose for moving CO2 to bundle sheath cells is to increase the efficiency of photosynthesis by not having oxygen compete with the fixation of CO2.
- Occurs in hot, dry climates where C4 plants have an adaptive advantage over C3 plants, minimizing photorespiration and reducing water loss.
- Requires additional energy (1 ATP to AMP) for C4 photosynthesis.
- Examples include sugarcane, corn, and crab grass.
CAM Photosynthesis
- Another "add-on" feature to C3 photosynthesis, which takes place in hot, dry environments with cool nights like deserts.
- Similar to C4 photosynthesis, but CO2 is temporally segregated.
- PEP carboxylase fixes COâ‚‚ to OAA, as in C4 - OAA is converted to malic acid
- Malic acid is shuttled to the vacuole of the cell
- At night, stomata are open, PEP carboxylase is active, and malic acid accumulates in the cell's vacuole.
- During the day, stomata are closed where malic acid is converted back to OAA (requiring 1 ATP to ADP), releasing CO2, which is now fixed by rubisco and the Calvin cycle proceeds.
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Description
Explanation of photosynthesis, including light absorption by pigments such as chlorophyll a and b, and carotenoids. Details noncyclic photophosphorylation, photosystems I and II (PS I and PS II). General equation for photosynthesis provided.