Photosynthesis: Carbon Reactions - 5 Flashcards
What provides energy for the assimilation of inorganic carbon into organic material
sunlight
What is the Calvin-Benson cycle, biochemical pathway, for
the reduction of CO2 to carbohydrates
In what organisms is the Calvin-Benson cycle found in
many prokaryotes and all photosynthetic eukaryotes
What two things drive the energy-consuming fixation of atmospheric CO2 through the Calvin-Benson cycle
NADPH and ATP
Where does the Calvin-Benson cycle occur
chloroplast stroma
What are the three phases of the Calvin-Benson cycle
carboxylation
reduction
regeneration
What is the input into the carboxylation phase
1 CO2, 1 H2O, with 1 molecule of ribulose 1,5- bisphosphate
What is produced in the carboxylation phase
2 molecules of 3-phosphoglycerate
What catalyzes the reaction that occurs in the carboxylation phase
Rubisco (a chloroplast enzyme)
Briefly summarize what occurs in the carboxylation phase
an H+ is removed from the ribulose 1,5-bisphosphate - CO2 added to unstable rubisco bound intermediate - hydration occurs to create 2 molecules of 3-phosphoglycerate
How many successive reactions are involved in the reduction phase of the Calvin-Benson cycle
2
What is the “goal”/ function of the reduction phase
reduce the carbon of the 3-phosphogylcerate (produced from the carboxylation phase)
What occurs in the first successive reaction of the reduction phase
ATP from the light reactions, phosphorylates 3-phosphoglycerate - creating 1,3-biphosphoglycerate, due to a catalyst
What occurs in the second successive reaction of the reduction phase
NADPH from the light reactions, reduces 1,3-biphosphoglycerate to produce glyceraldehyde 3-phosphate, due to a catalyst
How many molecules of ____ are produced from 3 carboxylation and reduction phases
6 molecules of glyceraldehyde 3-phosphate
What is the “goal”/function of the regeneration phase of the Calvin-Benson cycle
regenerates ribulose 1,5-bisphosphate to ensure continuous assimilation of CO2
What happens to each of the 6 molecules of glyceraldehyde -3-phosphate in the regeneration phase
5 molecules reshuffle carbons to restore 3 ribulose 1,5-bisphosphate molecules
1 molecule is the net assimilation of 3 CO2 molecules and is available for carbon metabolism of the plant
Summarize the regeneration phase
3CO2 + 3 ribulose 1,5-bisphosphate + 3H2O + 6NADPH + 6H+ + 6ATP PRODUCE:
6 triose phosphates + 6NADP+ + 6ADP = 6Pi
THEN: 5 triose phosphates + 3ATP +H2O PRODUCE 3 ribulose 1,5-bisphosphate + 3ADP +2Pi
The other triose phosphate is net synthesis of CO2 (for metabolic processes)
What are the catalysts involved in the regeneration phase
triose phosphate isomerase, aldolase, fructose 1,6-bisphosphate, sedoheptulose 1,7-bisphosphatase, ribulose 5-phosphate epimerase or isomerase, phosphoribulokinase
What is the enzyme that transfers molecules in the regeneration phase
transketolase
The Calvin-Benson cycle uses ___ molecules of NADPH and ___ molecules of ATP to assimilate _____ molecules of CO2
Therefore, fixation of 3 CO2 molecules into one triose phosphate uses ____ NADPH and ___ ATP
2
3
1
6
9
In the dark, photosynthetic enzymes and concentration of intermediates of the Calvin-Benson cycle are ____
low
Enzymes in the Calvin-Benson cycle are activated by what
light and the concentration of intermediates increase
Define induction period
the time lag between the onset of illumination (light) and the full activation of the Calvin-Benson cycle
Why do regulatory mechanisms exist in the Calvin-Benson cycle
to ensure all intermediates are present in adequate concentrations in the light, and so that cycle turns off in the dark
How do chloroplasts adjust the rate of Calvin-Benson cycle reactions
through modifications of enzyme levels and catalytic activities
What determine enzyme concentrations in cell compartments
gene expression and protein biosynthesis
Where is the small and large subunits of Rubisco encoded in and what does this require
small = nuclear genome
large = plastid genome
requires coordination between areas
Where are the nucleus-encoded and plastid-encoded genes of Rubisco translated
nucleus = cytosol then transported to plastid
plastid = stroma
how does light modulate the expression of nuclear enzymes
via specific photoreceptors (phytochrome and blue-light receptors)
Define anterograde and retrograde regulation
anterograde = signaling from nucleus to chloroplasts
retrograde = from nucleus to chloroplasts
catalytic rates caused by changes in enzyme concentration are ______ than posttranslational modifications
slower
What are the two general mechanisms of light-mediated modifications of the kinetic properties of stromal enzymes
- changes in covalent bonds that result in chemically modified enzyme
- modification of noncovalent interactions caused by changes in 1. ionic composition 2. binding of enzyme effectors 3. association with regulatory proteins 4. interaction with thylakoid membranes
Rubisco that is inactivate, is reactivated when…
conditions become favourable for photosynthesis
Rubisco activase removes ______ from Rubisco and then rubisco is activated by binding of a ____ molecule
sugar phosphates
CO2
What system is Rubisco regulated by
ferredoxin-thioredoxin system
what is the mechanism used in the ferredoxin-thioredoxin system
uses ferredoxin reduced by the electron transport chain and two chloroplast proteins to regulate enzymes
Explain what happens to ferredoxin
reduced ferredoxin converts protein thioredoxin to reduced state with the ferredoxin-thioredoxin reductase, then reduced thioredoxin cleaves and causes conformational changes to increase catalytic activity
deactivation of thioredoxin takes place when
darkness relieves the electron pressure form the electron transport chain
What 3 organelles does photorespiration occur in
chloroplasts, leaf peroxisomes, and mitochondria
Define photorespiration
cycle that minimizes the loss of fixed CO2 by the oxygenase activity of rubisco
What does Rubisco catalyze
carboxylation and the oxygenation of ribulose 1,5-bisphosphate
oxygenation produces ____ molecule of ____ and ____
1
3-phosphoglycerate and 2-phosphoglycolate
Photorespiration has been shown to be necessary because of what two reasons
salvages part of the assimilated carbon and links to other metabolic pathways
What 3 atoms circulate through photorespiration
carbon, nitrogen, and oxygen
What occurs in the carbon cycle
chloroplasts transfer 2 glycolate to peroxisomes and recover 1 glycerate. mitochondria releases 1 CO2 molecule
What occurs in the nitrogen cycle
chloroplasts transfer 1 glutamate molecule and recover 1 molecule of NH4+
What occurs in the oxygen cycle
Rubisco and glycolate oxidase catalyze incorporation of 2 O2 when 2 ribulose 1,5-bisphosphate enter the photorespiration cycle
How many molecules of O2 are reduced in the photorespiratory cycle
3
What 3 factors control the balance between the Calvin-Benson cycle and the photorespiration
kinetic properties of Rubisco, temperature, and concentrations of atmospheric CO2 and O2
Warmer environments shifts the balance from ____ and toward ____
photosynthesis (carboxylation) and toward photorespiration (oxygenation)
What triggered adaptations to handle promoted photorespiration
pronounced reduction in CO2 concentrations and rise is O2
CO2 must what 4 barriers in order for the diffusion of CO2 into the chloroplasts in photosynthesis
cell wall, plasma membrane, cytoplasm, and chloroplast envelope
What 2 mechanisms did land plants evolve for increasing the concentration of CO2 at Rubisco carboxylation sites
- C4 photosynthetic carbon fixation
- Crassulacean acid metabolism (CAM)
Define C4 photosynthesis
carbon metabolism which the initial fixation of CO2 is catalyzed by phosphoenolpyruvate carboxylase (PEPCase) (NOT Rubisco like is C3 photosynthesis), producing a 4-carbon compound
Examples of C4 plants
corn and sugarcane
What doesnt compete in C4 photosynthesis
O2
What happens to the C4 molecules
formed in mesophyll cells and move to bundle sheath cells, then decarboxylated, release CO2 that is refixed by Rubisco in the Calvin-Benson cycle
____ causes the primary carboxylation of C4 plants
PEPCase
The transportation of CO2 from the external atmosphere to the bundle sheath cells in C4 photosynthesis occur through what 5 successive stages
- fixation (HCO3- by PEPCase)
- Transport (4-carbon acid to the bundle sheath cells)
- decarboxylation (CO2 from the Calvin-Benson cycle)
- Transport (of 3-carbon to mesophyll cells)
- regeneration (of HCO3- acceptor)
Define Kranz Anatomy
particular leaf structure associated with the C4 cycle
wreathlike arrangement of mesophyll cells around layer of bundle sheath cells
C4 cycle may be driven by _____ or ____
diffusion gradients within a single cell
OR
gradients between mesophyll and bundle sheath cells
_____ regulates the activity of the 3 key C4 cycle enzymes
light
NADP-malate dehydrogenase, PEPCase, and pyruvate-phosphate dikinase
In C3 plants ____ transport process is required to export triose phosphates vs. in C4 plants it requires ____ transport processes
1
more
In hot and dry climates, C4 _____ photorespiration
reduces
What two features contribute to overcome the deleterious effects of high temperature
- CO2 enters mesophyll cells and converted into bicarbonate (PEPCase has high affinity for bicarbonate) and enables C4 plants to reduce stomatal aperture and conserve water while fixing CO2
- high CO2 concentrations in bundle sheath minimizes oxygenase activity
C3 and C4 optimal in what temperatures
20-25
25-35
C4 species more abundant in what areas and C3 more abundant in what areas
tropics and subtropics
temperate regions
Explain CAM mechanism
concentrates CO2 around Rubisco in plants that inhabit arid environments and experience seasonal water availability
In CAM, when occurs at night and what occurs during the day
night = uptake and initial carbon fixation
day = decarboxylation and reduction of internally released CO2
What enhances CAM performance
tightly packed mesophyll cells that restrict CO2 loss during the day
CAM pathways are induced by what two factors
water loss or salt stress
what increases the concentration of CO2 near Rubisco and reduces the rate of oxygenase activity and increasing the efficiency of photosynthesis in CAM
temporal separation of nocturnal initial carboxylation from diurnal decarboxylation
CAM is sensitive to what
environmental conditions
What helps conserve water and enhances CO2 concentration near Rubisco during CAM
stomatal closure
CAM are able to adjust their patterns of CO2 uptake in response to what
longer-term variations of environmental conditions
CAM is also used in aquatic habitats of what conditions
high resistance to gas diffusion restricts availability of CO2
photosynthetic assimilation of CO2 yields sucrose and starch where
sucrose = cytosol
starch = chloroplasts
sucrose flows from where to where
and starch flows where to where
sucrose = leaf cytosol to heterotrophic sink tissues
starch = accumulates as granules in the chloroplasts
Transitory starch
when in the dark CO2 assimilation stops and starch in chloroplasts is degraded
What are the 2 functions of transitory starch
- overflow mechanism that stores photosynthate when the synthesis and transport of sucrose are limited during the day
- energy reserve to provide adequate supply of carbs at night when sugars are not formed by photosynthesis
