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8.3 U 1 Light-dependent reactions take place in the thylakoid membranes and the space inside them.
8.3 U 1 Light-dependent reactions take place in the thylakoid membranes and the space inside them.
Be able to:
Annotate a diagram to explain light dependent reactions
explain what happens to an electron when it is excited by photons of light
define photolysis
identificy the products of the light dependent reactions that are to the light independent reactions.
Photosynthesis is the process by which cells synthesize organic molecules (e.g. glucose) from inorganic molecules (CO2 and H2O) in the presence of sunlight. This process requires a photosynthetic pigment (chlorophyll) and can only occur in certain organisms (plants, some bacteria). In plants, photosynthesis occurs within a specialized organelle called the chloroplast
8.3 U 2 Light-independent reactions take place in the stroma.
8.3 U 2 Light-independent reactions take place in the stroma.
Be able to:
State the name of the 5-Carbon compound present in the Calvin Cycle
Define carboxylation
State the name of the enzyme responsible for fixing CO2
Annotate a diagram that explains the cycle of reaction that occur in the light independent stages of photosynthesis
The light-independent reactions use the products produced by the light dependent reactions and CO2 to produce glucose.
The stroma contains enzymes necessary for the light-independent reactions (Calvin Cycle)
6 CO2 are required to create one 6 carbon sugar molecule.
ATP and hydrogen / electrons (carried by NADPH) are transferred to the site of the light independent reactions
The hydrogen / electrons are combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates)
The ATP provides the required energy to power these anabolic reactions and fix the carbon molecules together
8.3 U 3 Reduced NADP and ATP are produced in the light-dependent reactions.
8.3 U 3 Reduced NADP and ATP are produced in the light-dependent reactions.
The production of ATP by the light dependent reactions is called photophosphorylation, as it uses light as an energy source. Photophosphorylation may be either a cyclic process or a non-cyclic process
This process is the first stage of photosynthesis in which light energy is converted into chemical energy (NADPH and ATP). These products will be used in the light independent stage.
non-cyclic photophosphorylation:
Non-cyclic photophosphorylation involves two photosystems (PS I and PS II) and does involve the reduction of NADP+
one-way flow of 2 e-s from water to PsII to ETS to PsI to NADP+
2 main products: 1(NADPH + H+ 2) ATP
cyclic photophosphorylation:
involves the use of only one photosystem (PS I) and does not involve the reduction of NADP+cyclic flow of e-s from PsI to ETS back to PsI
1 main product: ATP
8.3 U 4 Absorption of light by photosystems generates excited electrons.
8.3 U 4 Absorption of light by photosystems generates excited electrons.
The light dependent reactions use photosynthetic pigments (organised into photosystems) to convert light energy into chemical energy (specifically ATP and NADPH)
Chlorophyll molecules are grouped together into photosystems contained within the thylakoid membranes.
Chlorophyll a within the photosystem II (PS II) absorbs a photon of light (most efficient at 680 nm).
This photon of light excites an electron from the chlorophyll a molecule to a higher energy state.
The chlorophyll is now in a photoactivated state.
The excited electron is released by the oxidized Chlorophyll a molecule to the primary electron acceptor in photosystem II.
This electron acceptor is called plastoquinone (PQ)
PQ accepts two exited electrons and transfers these electrons along a series of electron carriers in the thylakoid membrane
Photosystem II can repeat this process to produce a second reduced PQ molecule (total of 4e- are used to produce 2 reduced PQ molecules)
8.3.U 5 Photolysis of water generates electrons for use in the light-dependent reactions.
8.3.U 5 Photolysis of water generates electrons for use in the light-dependent reactions.
The lost electrons are replaced by the splitting of water through a process called photolysis. During this reaction oxygen is produced and released as a by-product.
oxidized chlorophyll a (chl+) has a lack of electrons
replaced as H2O is split: H2O ----> 2 H+ + 2 e-s + 1/2 O2 c. H+:
remain in thylakoid interior, lowering pH
contributing to chemiosmotic gradient
used in phosphorylation of ATP
2 electrons: replace electrons lost by chlorophyll a to ETS: chl a+ + 2 e-s ----> chl a
1/2 O2: lost to environment as a waste product
8.3 U 6 Transfer of excited electrons occurs between carriers in thylakoid membranes.
8.3 U 6 Transfer of excited electrons occurs between carriers in thylakoid membranes.
The electron moves along a series of electron carriers through a series of redox reactions from photosystem II (PSII) to photosystem I (PSI)
Electron Carriers: (plastoquinone (PQ) --> cytochrome complex b6f --> lastocyanin --> PS I).
8.3 U 7 Excited electrons from Photosystem II are used to contribute to generate a proton gradient.
8.3 U 7 Excited electrons from Photosystem II are used to contribute to generate a proton gradient.
As electrons move down the electron transport chain they lose energy. This energy is used to pump H+ (protons) across the thylakoid membrane into the thylakoid space. This creates an H+ concentration gradient and the potential energy needed, that will drive chemiosmosis and produce ATP from ADP during photophosphorylation (similar to oxidative phosphorylation in mitochondria). The photolysis of water also helps create the proton gradient as H+ is produced when water is split
8.3 U 8 ATP synthase in thylakoids generates ATP using the proton gradient.
8.3 U 8 ATP synthase in thylakoids generates ATP using the proton gradient.
The production of ATP from ADP and phosphate using energy derived from light is called photophosphorylation.. Photophosphorylation occurs on the thylakoid membrane of the chloroplasts.
As electrons pass along the electron transport chain between PS II and PS I, the electrons release energy that is used to pump protons from the stroma into the thylakoid space.
This builds up the concentration of H+ within the thylakoid space creating a proton concentration gradient.
Protons will flow across the membrane from the thylakoid space to the stroma, through ATP synthase following the concentration gradient.
As the protons flow through ATP synthase, the energy released from the flow of protons is used to combine ADP and inorganic phosphate to form ATP(photophosphorylation).
This whole process of producing ATP is called chemiosmosis.
8.3 U 9 Excited electrons from Photosystem I are used to reduce NADP.
8.3 U 9 Excited electrons from Photosystem I are used to reduce NADP.
A photon of light strikes photosystem I, re-exciting the electron to a higher energy state (photoactivation)
This electron is passed along a second electron transport chain (includes ferredoxin) until it is accepted by the final electron carrier NADP+.
A second electron that follows the same path is also accepted by NADP+.
These electrons reduce NADP+ to form NADPH (NADP+ + 2e- + H+ --> NADPH).
This reaction is catalyzed by an enzyme called NADP reductase
Since the electrons are not returned to photosystem II, this path for making ATP is called non-cyclic photophosphorylation.
New electrons are passed to PSI from PSII through a carrier called plastocyanin
If NADP+ runs out and it cannot accept the excited electron from photosystem I, electrons return to the electron transport chain (PQ) where they can reflow back to PS I thus pumping more protons into the thylakoid space; producing more ATP.
This is called cyclic photophosphorylation.
8.3 U 10 In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate.
8.3 U 10 In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate.
The light independent reactions use the chemical energy derived from light dependent reactions to form organic molecules
One CO2 molecule enters the Calvin cycle and combines with a 5 carbon molecule called Ribulose bisphosphate (RuBP) to temporarily form a 6C molecule.
This reaction is catalyzed by the enzyme RuBP carboxylase (rubisco).
This immediately breaks down into two 3C molecules called glycerate-3-phosphate (GP).
8.3 U 11 Triose phosphate is used to regenerate RuBP and produce carbohydrates.
8.3 U 11 Triose phosphate is used to regenerate RuBP and produce carbohydrates.
synthesis of carbohydrates and other products:
enzymes convert TP to various products
other products: lipids, amino acids, nucleic acids
polysaccharides: starch
disaccharides: sucrose
monosaccharides: glucose, fructose
8.3.U 12 Ribulose bisphosphate is reformed using ATP.
8.3.U 12 Ribulose bisphosphate is reformed using ATP.
Two TP molecules are used to produce one six carbon glucose phosphate molecule, which can eventually be combined with other glucose phosphate to form starch.
The other ten TP (3C) molecules are used to regenerate six RuBP (5C) using 6 ATP molecules for energy.
So for every 6 triose phosphate molecules produced, 5 of these triose (3C) sugars are used to reform 3 RuBP (5C) molecules using 3 ATP molecules The one remaining triose phosphate forms half a glucose phosphate
8.3 U 14 The structure of the chloroplast is adapted to its function in photosynthesis.
8.3 U 14 The structure of the chloroplast is adapted to its function in photosynthesis.
chloroplast has double membrane regulating internal conditions
chloroplast interior separated into thylakoids and stroma
thylakoids = site of light dependent reactions
large surface area to maximize light absorption
small space inside thylakoids allows for proton accumulation
thylakoid interior acidic/ pH = 4/ high proton concentration allowing for chemiosmotic gradient
stroma = site of light independent reactions
stroma pH = 8/ basic where Calvin cycle enzymes function optimally
thylakoid membranes hold photosystem pigments
pigments anchor in thylakoid membranes by hydrophobic/ hydrocarbon tails
photolysis enzymes on inner surface of thylakoid membrane for splitting water into oxygen and hydrogen
intrinsic proteins bound in thylakoid membranes allow electron transport/ ETC between PsII and PsI
ETC proteins between photosystems II and I pump protons into thylakoid interior adding to chemiosmotic gradient
NADP reductase bound to outer thylakoid membrane allowing reduction of NADP to NADPH for Calvin cycle
ATP synthase transmembrane complex bound across thylakoid membrane
allowing proton flow down chemiosmotic gradient/ photophosphorylation/ ATP production in stroma
chloroplast DNA allows for protein synthesis
chloroplast ribosomes allow for protein synthesis
chloroplast starch granules allow for storage of photosynthetic products
8.3 A 1 Calvin’s experiment to elucidate the carboxylation of RuBP.
8.3 A 1 Calvin’s experiment to elucidate the carboxylation of RuBP.
The light independent reactions are also collectively known as the Calvin cycle – named after American chemist Melvin Calvin. Calvin mapped the complete conversion of carbon within a plant during the process of photosynthesis. The experiments performed by Melvin Calvin explained the process that plants use to make food. Radioactively labeled carbon was introduced into algae and analyzed to see where it would be found in the cells by the use of two-dimensional paper chromatography. The results of these experiments showed that carbon dioxide was converted to carbohydrates during the light-independent reactions of photosynthesis.
Chlorella is a good model organism because the algal cell has only one large chloroplast.
From a series of autoradiographs, Calvin was able to determine when each of the metabolites is produced during the light-independent reactions.
Principle of two-dimensional chromatography
The chromatograms are dried and placed against photographic film in the dark. The radiation from the 14C-labelled compounds creates dark spots on the film. The images created are called autoradiographs.
8.3 S 1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.
8.3 S 1 Annotation of a diagram to indicate the adaptations of a chloroplast to its function.
Electron micrographs of a chloroplast may differ in appearance depending on where the cross-section occurs
Typically, chloroplast diagrams should display the following features:
Usually round in appearance with a double membrane exterior
Flattened discs (thylakoids) arranged into stacks (grana), connected by lamellae
Internal lumen of thylakoids is very small (allows for a more rapid generation of a proton motive force)
Ribosomes and chloroplast DNA are usually not visible at standard resolutions and magnifications
Starch granules may be visible and will appear as dark spots within the chloroplast
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