Plant Physiology Ch. 8: The Carbon Reactions
RubisCO catalyzes: (Careful!! Count your carbons and remember your carbon-redox states)
Ribulose 1,5-bisphosphate + O2 --> 3-phosphoglycerate + 2-phosphoglycolate
What is the most abundant enzyme in the biosphere?
Rubisco
In which natural biomes (i.e., not agricultural lands) would C4 plants to be most successful (and thus most prevalent) AND, beside each possible answer, explain why each is or is not your choice
Semi-arid tropical areas (e.g., savannah grasslands)
As temperature increases, the quantum yield of photosynthesis is directly affected by
The increasing ratio of CO2 to O2 in the gas phase
What type of environment do CAM plants live in?
arid environments with seasonal water availability ex: pineapple, agave, cacti, orchids
Where does the Calvin cycle and starch production take place in C4 metabolism?
bundle sheath
What are the catalytic activities of rubisco?
carboxylation and oxygenation
The Calvin-Benson cycle proceeds has three phases:
carboxylation, reduction, and regeneration
Light-actuated ion fluxes _________ the stromal concentration of protons (pH increases from 7 to 8) and ____________ that of Mg^2+ by 2 to 5 mM
decrease; increase
When does CAM take place?
during the night. CO2 loss is restricted during the day
At night, the starch content of chloroplasts ________ to provide carbon skeletons for sucrose synthesis in the cytosol to nourish _____________
falls; heterotrophic tissues
Light regulates the Calvin cycle via the..
ferredoxin-thioredoxin system
The final product of the Calvin Cycle (the product that EXITS the Calvin Cycle, assuming successful carboxylation) is
glyceraldehyde-3-phosphate
During the Calvin-Benson cycle, the rate of CO2 fixation increases with time in the first few minutes after the onset of illumination -- a time lag called the..
induction period
What type of movements modulate enzymes of the Calvin cycle?
light-dependent movements
C4 photosynthesis transports different four-carbon acids from ____________ to ________________
mesophyll; bundle sheath cells
What traits do the leaves of CAM plants have?
minimize water loss - thick cuticles - large vacuoles - stomata with small apertures
Oxygenation produces _______ molecule each of ___________ and __________
one; 3-phosphoglycerate (PGA) and 2-phosphoglycolate (glycolate-P) - Glycolate (2C intermediate) cannot be used to regenerate Calvin Cycle intermediates - wasting C!
What is the flaw of rubisco?
oxygenation - predates O2 in the atmosphere
In hot, dry climates, the C4 cycle ___________ photorespiration
reduces
The FULL name of the "first" enzyme of the Calvin Cycle is (hint: it is probably the most abundant protein in the biosphere;
ribulose 1,5-biphosphate carboxylase/oxygenase (rubisCO)
During the day, sucrose flows from the leaf cytosol to ___________, while starch accumulates as ___________ in chloroplasts
sink tissues; granules
The first "product" of the Calvin Cycle that is not recycled to make more substrate is a phosphorylated 3-carbon sugar. If that sugar stays in the chloroplast, it will most likely be converted to ________________; if it is transported to the cytoplasm it will most likely be converted to ________________; and the protein that controls the distribution of that 3-carbon sugar is ____________________________
starch; sucrose; triose-p
Where does the Calvin Cycle occur?
stroma
The Calvin-Benson cycle takes place predominantly in the
stroma of the chloroplast
The photosynthetic assimilation of CO2 by most leaves yields _________ in the cytosol and _________ in the chloroplasts
sucrose; starch
The Calvin-Benson cycle uses how many molecules of NADPH and ATP to assimilate a single molecule of CO2?
two molecules of NADPH and three molecules of ATP
Carboxylation yields ______ molecules of __________
two; 3-phosphoglycerate (PGA)
Land plants evolved two carbon-concentrating mechanisms for increasing the concentration of CO2 at the rubisco carboxylation site:
- C4 photosynthetic carbon fixation (C4) - Crassulacean acid metabolism (CAM)
Light regulates the activity of key C4 cycle enzymes:
- NADP -malate dehydrogenase - PEPCase - pyruvate-phosphate dikinase
What are the regulated enzymes in the Calvin cycle?
- Rubisco - FBPase - SBPase - R5P kinase - triose-P dehydrogenase
What is photorespiration?
- mechanism for recovering 75% of otherwise lost Carbon - evolved mechanisms to cope with oxygenation
How is the Calvin cycle very efficient?
- no wasteful cycling - exergonic reactions drive reactions forward - autocatalytic - creates its own substrate
What are the enzymes controlled by in the Calvin cycle?
- stroma pH - [Mg^2+] - ATP/ADP ratio - redox potential - carbamolyation (CO2)
A popular quote incorrectly attributed to Mark Twain is "The coldest winter I ever spent was a summer in San Francisco". Based solely on this quote, what type of photosynthetic chemistry would you expect to find among the majority of plants growing in the area? Why, specifically, would your chosen chemistry be better than the other possibilities
-C3 - due to its efficient oxygenation in low temperatures
Describe how CAM (crassulacean acid metabolism) works and how/why it benefits some plants
-Stomata close during the day to prevent evapotranspiration and open at night to collect CO2. - This pathway is an advantage to plants in dry climates that cannot afford to lose any water.
Explain the three phases of the Calvin-Benson cycle.
1. Carboxylation of the CO2 acceptor molecule. The first committed enzymatic step in the cycle is the reaction of CO2 and water with a five-carbon acceptor molecule (ribulose 1,5-biphosphate) to generate two molecules of a three-carbon intermediate (3-phosphoglycerate) 2. Reduction of 3-phosphoglycerate. The 3-phosphoglycerate is converted to three-carbon carbohydrates (triose phosphates) by enzymatic reactions driven by photochemically generated ATP and NADPH. 3. Regeneration of the CO2 acceptor ribulose 1,5-biphosphate. The cycle is completed by regeneration of ribulose 1,5-biphosphate through a series of ten enzyme-catalyzed reactions, one requiring ATP
What are the evolved mechanisms to deal with oxygenation?
1. photorespiration 2. C4 metabolism
Rank the following in order of water use efficiency (1 = most efficient [least water used per gram of biomass], 3 = least efficient [most water used per gram of biomass]) (1 pt)___________ C3 plants, ___________C4 plants, ___________Crassulalean acid metabolism plants
3; 2; 1
To be in an active state, several enzymes of the Calvin Benson cycle require
A reducing environment to break disulfide bridge (-SH, -SH)
What is the C4 cycle?
Bicarbonate anion in mesophyl is fixed to pyruvate to create a C4 carboxylic acid, which then diffuses to the bundle sheath where CO2 is released and enters the Calvin Cycle
The _____________ reduces photorespiration and water loss in hot, dry climates
C4 cycle
________________ captures atmospheric CO2 and scavenges respiratory CO2 in arid environments
CAM photosynthesis
The light compensation point is a measure of the irradiance at which
CO2 uptake for photosynthesis equals CO2 evolution from respiration
The dry-weight mass of a plant (the weight of plant solids after water is removed) is from
Carbon dioxide absorbed from the air
What happens during the three stages of the Calvin-Benson Cycle (short answer)
Carboxylation Reduction Regeneration
During the Calvin-Benson Cycle..
Chemical energy of ATP and NADPH is used to reduce carbon dioxide to sugars
It is July in Texas and day-time high temperatures have exceeded 100°F for 40 days straight without any rain. The sun has not been blocked by any clouds and the raspberry plants in Dr. Ayre's backyard are thriving from all the photosynthesis. TRUE or FALSE.EXPLAIN your answer
FALSE because raspberry plants are C3 plants and these plants have a tendency to lose water in hot conditions, thus would not survive at high temperature - any temperature near 40 degrees Celsius and in the sun all day denatures enzymes and membranes.
What is Rubisco?
It is a very large and very slow (1-12 CO2/sec) enzyme that accumulates to high concentration; 8L8S subunits, 8 active sites, 0.5 mM in stroma, 4 mM active sites! ~40% of total leaf protein
What is the distinctive morphology that C4 plants have?
Krantz anatomy - inner ring of bundle sheath cells around vascular tissues and an outer layer of mesophyll cells
The C4 photosynthetic carbon cycle fixes atmospheric CO2 via ____________ into carbon skeletons in one compartment
PEPCase
____________ minimizes the loss of fixed CO2 by the oxygenase activity of rubisco
Photorespiration
