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The light reactions of photosynthesis and the calvin cycle (Photosynthesis…
The light reactions of photosynthesis and the calvin cycle
Photosynthesis
the process of converting light energy into chemical energy, which use energy from the sun to convert CO2 and H2O into carbohydrates and O2
kinds of plant
autotroph
photosynthesic organisma that can synthesizes glucose
heterotroph
obtain energy from chemical fuels only
2 parts
the light reaction
transform light energy into reducing power and ATP
the dark reaction
use the product of the light reactions to drive the reduction of CO2 and its conversion into glucose and other sugars
to synthesize glucose from CO2 high energy electrons are required for to provide reducing power to reduce CO2 and generate ATP to power the reduction
takes place in chloroplast
like a MT
chloroplast have three different membrane (outer membrane and thylakoid) and 3 spaces (outer, inner and thylakoid lumen)
chloroplast contain their own DNA and the machinery for replicating and expressing it
they are not autonomous: nuclear DNA encodes many chloroplast
a photosynthetic m/o most likely ancestor of a cyanobacterium was engulfed by a eukaryotic host
transforming light energy into chemical energy
absorption of light by a photoreceptor molecule; photo receptor capable of absorbing the energy of light of a specific wavelength (pigment)
energy from the light excites an electron from its ground energy level to an excited level
the excited electron returns the ground state and the absorbed energy is converted to heat or light
The excited electron itself may move to a nearby molecule with a lower excited state, in a process called electron transfer
this process called fluoresces
this procsess is referred to as photo induces charge separation. the excited electron in its new molecule now has reducing power: it can reduce other molecules to store the E originally obtained from light in chemical forms
the pair of electron carriers at which the charge separation takes place called the special pair and are located the reaction center
chlorophyll is the primary light acceptor in most photosynthetic systems
chlorophyll a is the principal chlorophyll in green plant, with the wavelength of maximum absorbance at 420 nm and 670 nm
the reason that chlorophyll are effective photoreceptors because they contain networks of altering single and double bonds, which display resonance structure
the absorption of E by chlorophyll in the reaction center from electron transfer or directly from light
Light-harvesting complexes enhance the efficiency of photosynthesis
The absorption spectrum is expanded by the use of antennae molecules: chlorophyll a molecule not in a reaction center, chlorophyll b and other accessory pigments such as carotenoids. These pigments absorb light and deliver the energy to the reaction center by resonance energy transfer.
the accessory pigments suppress damaging photochemical reactions from oxidative damage
2 photosystem (PSI , PSII) generate a proton gradient and NADPH
PSI responds to light with wavelength <700 nm and is responsible forproviding electrons to reduce NADP+ to NADPH
PSII; wavelength < 680 nm sending electron through a membrane-bound proton pump and then on to PS I to replace the electrons domated by PSI to NADP+
The electrons in the reaction center of PSII are replaced by oxidation of H2O to O2
Photosystem I
Includes 14 peptide chains and multiple associated proteins and cofactors
When activatedby the light, the reaction center initiates photo induced charge separation that generates high E electrons
The electrons flow down an electron-transport chain to ferredoxin (Fd)
Ferredoxin carries one electron and transfer eletron to NADP+. The reaction is catalyzed by ferredoxin-NADP+ reductase, a flavoprotein
The reaction take place on the stromal side of the thylakoid memb. Using the proton in the stroma makes the stroma more basic than the lumen
Photosystem II
Photosystem II replenishes electrons of PS I and generates a proton gradient
PS II catalyzes the light-driven transfer of electron from water to PS I. In the process, the protons are pumped into the thylakoid lumen to generate a proton-motive force
PS II is a large assembly of more than 20 subunits
Cytochrome b6f links PS II to PS I
The QH2 prpduced by PS II transfer its electrons to plastocyanin, which in turns donates the electrons to PS I, thereby replenishihg the missing electrons in PS I
QH2 releases the protons into the lumen. The enz. pump 2 additional protons from the stroma into the lumen, strengthening the proton-motive force.
The oxidation of water aceives oxidation-reduction balance and contributes protons to the proton gradient
Results of the light reaction
2H2O +2NADP+ + 10H+(stroma)—> O2 + 2NADPH+ 12H+(lumen)
3ADP3- + 3Pi2-+ 3H+ —> 3ATP4- + 3H2O +12H+(stoma)
2NAPH+ + 3ADP3- + 3Pi2- + H+ —> O2 + 2NADPH + 3ATP4- + H2O
8 protons are required to yield 3 molecules of ATP (2.7 proton/ATP)
For cyclic photophosphorylation, 4 photons leads to the release of 8 protons, when can generate 8*3/12=2ATPs
The Calvin cycle
the second pat of photosynthesis uses ATP and NADPH to reduce CO2 to the more reduced state as a hexose sugar
"dark reaction / " light-independent reaction"
CO2 is trapped in an inorganic moleclue,3-phosphoglycerate, which has many biochemical fates
stage 1: fixation of CO2 by ribose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate
CO2 reacts with ribose 1,5-bisphosphate to form two molec. of 3-phosphogycerate. this reaction is catalyzed by ribose 1,5-bisphosphate carboxylase/oxylase (usually called Rubisco) this reaction is the rate-liiting step in the hexose synthesis
only C3 plants use calvin cycle to fix CO2
Rubisco is the most abundant enzyme in plants and probably the most abundant protein in the biosphere because it is an inefficient enzyme
iIn the ansence of CO2, rubisco binds to its subtrate, ribose 1,5-bisphosphate, so tightly that the enz. is inhibited
stage 2: the 3-phosphoglycerate products are converted into a hexose phospahte
same to gluconeogenesis, except that glyceraldehyde 3-phosphate dehydrogenase in plants in specific for NADPH rather than NADH
the hexose phosphate product of the calvin cycle exits in 3 isomers
stage 3: the regeneration of ribose 1,5-bisphosphate, the acceptor of CO2 in the first stage
the challenge is to construct a five-carbon sugar from a six-carbon member of the hexose monophosphate pol and three-molec.
a transketolase, and an aldolase have major roles in the rearrangement of the carbon atoms
fructose 6-phosphate + 2glycealdehyde 3-phosphate + dihydroxy acetone phosphate + 3ATP --> 3ribose 1,5-bisphosphate + 3ADP
what is the energy expenditure for synthesizing a hexose?
six rounds of the calvin cycle are rquired, because one C is reduced each round
12 ATP are spent in phosphorylation 12 3-phosphoglycerate, 12NADPH are concumed in reduction
additional 6ATP are spent in regenerating ribose 1,5-bisphosphate
6CO2 + 18ATP + 12NADPH + 12H2O --> C6H12O6 + 18 ADP + 18Pi + 12NADP+ + 6H+
what are the fates of the C atom fixed and processeed by the enz. of the calvin cycle?
the hexose sugars are used in a variety of ways, but there are 2 prominent fates: the synthesis of starch and sucrose, storage forms of carbohydrate
C4 pathway
CO2(in mesophyll cell) + ATP+ 2Pi + H+
in comparison, only 18 ATP are required per hexose molec. in the ansence of the C4 pathway
tropical plants with a C4 pathway do little photorespiration b/c the high conc. of CO2 in their bundle-sheath cells accelerate the carboxylase reaction relative to the oxygenase reaction