chapter 10 Flashcards
what is photosynthesis?
1.) Energy from sunlight is captured and converted to chemical energy (ATP & NADH)
1.) The energy is used to convert CO2 to more complex carbon compounds
3.) Organisms that carry out photo synthesis are autotrophs(plants, algae, cyanobacteria), others are heterotrophs
4.) Water is the source of O2 released during photosynthesis
only 5% of sunlight energy is transformed to the energy of chemical bonds
2 pathways of photo synthesis
2 photosynthesis pathways that occur in different [arts of the chloroplast
-Light dependent reaction: converts light energy to chemical energy as ATP & NADPH
-Carbon-fixation reaction (Light independent recation/Calvin cycle/dark reaction): uses the ATP and NADPH plus CO2 to produce carbs
electromagnetic radiation
-light is a form of energy—electromagnetic radiation
-It is propagated as waves— the amount of energy is inversely proportional to its wavelength
-Light also behaves ad particle, called photons
Photons
-Light particles
-Certain molecules absorb photons of light at specific wavelengths
-when a photon hits a molecule, it can
1.) bounce off—scattered or reflected
2.) pass through—transmitted
3.) be absorbed, adding energy to the molecule (excited state)
absorbed energy in molecules
-absorbed energy boosts an electron in the nucleus
-the electron is held less firmly—making the molecule more unstable and reactive
pigments
molecules that absorb specific wavelengths in the visible range
Chlorophyll absorbs blue and red light and scatters green (why plants are green jhii)
the pigment molecules are arranged in light-harvesting complexes, or antenna systems
Chlorophyll a
-major pigment in photosynthesis
-hydrocarbon “tail” that anchors it in a protein complex in the thylakoid membrane called a photosystem
photosystem
-hydrocarbon “tail” that anchors it in a protein complex in the thylakoid membrane called a photosystem
-photosystem also has other pigments: chlorophyll b, carotenoids, and phycobilins
pigment molecule
-the pigment molecules are arranged in light-harvesting complexes, or antenna systems
-several complexes surround a reaction center in the photosystem
-light energy is captured in light harvesting complexes and transferred to reaction centers
-when pigment molecule absorbs a photon, the excited state is unstable and energy is quickly released
-energy absorbed by other pigment molecules and passed to chlorophyll a in a reaction center
electron acceptor
-the excited chlorophyll a molecule (Chl) and oxidized to Chl+; gives up an electron to an acceptor
-the final electron acceptor is NADP+, which gets reduced
NADP+ + H+ + 2e- == NADPH
Noncyclic electron transport photosystems
Photosystem I has P700 chlorophyll( absorbs best at 700 nm)
Photosystem II had P680 chlorophyll (absorbs best at 680 nm)
Photosystem II
-when excited chlorophyll (CHL) gives up its electron, it is unstable, and grabs another electron from water
-the water become oxidized
-H+ from H2O and electron transport capture energy is transported across the thylakoid membrane into the lumen, creating an electrochemical gradient for the chemiosmotic synthesis of ATP by photophosphorylation
photosystem I
-an excited electron from the Chl reduces and acceptor
-The oxidized Chl takes an electron from the last carrier in photosystem II
-The energetic electron is passed through several carriers and reduced NADP+ to HADPH
calvin cycle: CO2 fixation
-CO2 is reduced to carbs and occurs in the stroma and energy in ATP and NADPH is used to reduce CO2
-OC2 binds to 5-C RuBP, catalyzed by ribulose bisphosphate carboxylase/ oxygenase (rubisco)
-The 6-C compound immediately breaks down into 2 molecules of 3PG
1.) fixation of CO2 to 3PG
2.) reduction of 3PG to G3P
3.) Regeneration of RuBP, the CO2 acceptor
For every turn of the cycle, one CO2 is fixed and one RuBP is regenerated
G3P
-glyceraldehyde 3-phosphate
-product of the Calvin cycle
-some is exported to the cytoplasm and converted to glucose and fructose and starch withing the chloroplast
-these can be used in respiration or converted to sucrose and transported to other parts of the plant for energy or build other molecules
-the stored starch is used at night so that photosynthetic tissue can continue to export sucrose to the rest of the plant
calvin cycle is stimulated by light
-induces pH changes in the stroma that favor activation of rubisco
-light-induced electron transport reduces disulfide bonds in Calvin cycle enzyme to activate them, via ferredoxin and thioredoxin
affinity of rubisco for CO2 is much stronger than it is for O2
but is there is relatively high O2 and low Co2 concentrations within the leaf, oxygenase activity is favored
that can occur of hot, dry days — stomata close to prevent water loss and O2 levels increase relative to CO2
Plants differ in how the fix CO2
C3 plants:
-first product of COs fixation is 3PG (3 carbons) On hot days, photorespiration occurs
-Stomata closes to prevent water loss but oxygen accumulates; reaction of RuBP w O2 instead of CO2 reduces net carbon fixed by the Calvin cycle
C4 plants:
-first product of CO2 fixation is oxaloacetate (4 carbons) No photorespiration on hot days
-in mesophyll cells, PEP carboxylase catalyzes the reaction of CO2 and PEP to form oxaloacetate, which is converted to malate
-PEP + CO2 = OA = Malate
-PEP carboxylase has no oxygenase activity and fixes CO2 even when levels are low
Malate
-in mesophyll cells, PEP carboxylase catalyzes the reaction of CO2 and PEP to form oxaloacetate, which is converted to malate
-PEP + CO2 = OA = Malate (c4 plants)
Malate diffuses to bundle sheath cells, which have modified chloroplasts that concentrate CO2 around rubisco
Malate is decarboxylated to pyruvate and CO2. Pyruvate moves back to mesophyll cells to regenerate PEP
The CO2 enters the Calvin cycle
C3 vs. C4 plants
-C4 plants use energy to increase CO2 concentration in bundle sheath cells
-In cool conditions, C3 plants have an advantage—they don’t expend energy concentrating CO2 around rubisco
-in warm dry climates, C4 plants have the advantage—photorespiration doesn’t occur and photosynthesis rates don’t fall
Crassulacean acid metabolism (CAM)
Stomata are open at night and closed during the day to conserve water
Night: CO2 is fixed by PEP carboxylase, and malate is stored in vacuoles
Day: malate moves to chloroplasts and is decarboxylated and the CO2 goes into the Calvin cycle
