Plants Lecture 2 - The Carbon Assimilation Reactions of Photosynthesis Flashcards
What is a photoautotroph? What is an example of it?
Organisms that carry out photosynthesis e.g. plants
What is a chemoheterotroph? What is an example of it?
Organisms which derives its energy from chemicals, and needs to consume other organisms in order to live e.g. humans
Plants reduce atmospheric CO2 to _____, which then acts as a precursor for synthesis of ____, ____, ____, _____ and ____.
trioses
cellulose, starch, lipids, proteins and other organic molecules.
What are the 3 steps to the Calvin cycle?
- Fixation of CO2 - by RuBisCO
- Reduction of 3-phosphoglycerate to a triose - in this stage synthesis of a carbohydrate occurs
- Regeneration of the carbon acceptor, ribulose 1,5-bisphosphate (RuBP) - involves interconversions of triose and pentose phosphates
Of the triose molecules produced in the Calvin cycle, how many are put towards regenerating RuBP? How many are put towards actually growing the plant?
5/6 –> RuBP
1/6 –> growing plant e.g. make sucroses, hexoses, starch
What is the name of the enzyme that catalyses the fixation of CO2 in the cycle (basically what enzyme lets you go from step 3 to step 1)?
RuBisCO
How many forms of RuBisCO are there? Where is each form found?
Form I: has 2 subunits - found in vascular plants, algae, cyanobacteria
Form II: more complicated - only found in certain photosynthetic bacteria
What are C3 plants?
Plants with 3- phosphosphoglycerate as the first stable product of photosynthesis
Plant RuBisCO has ___ identical large subunits encoded by ____ genome, and ___ identical small subunits encoded in genome.
8 large, chloroplast DNA
8 small, nuclear DNA
RuBisCO has an extremely low/high Kcat (turnover rate). Therefore you need a lot/not much of it.
Low, need lots
For RuBisCO to work, ___ and ___ need to be _____ in its active site.
Lys and Mg2+ need to be carbamyolated
NADPH and ATP are produced in the light reactions in about the SAME/DIFFERENT ratio as they are consumed in the Calvin cycle.
Same
Can production of ATP and NADPH occur in the dark.
No
Can the reactions that occur in the Calvin Cycle occur in animals? can they make glucose from CO2?
Yes these reactions happen in animals except animals are missing 3 key enzymes (which plants have) meaning they can’t do the last step which is converting CO2 to glucose.
What is photorespiration?
A process in plant metabolism where the enzyme RuBisCO oxygenates RuBP (instead of assimilating the CO2 it assimilates the O2 in step 1 instead), causing some of the energy produced by photosynthesis to be wasted. Because the molecule created by adding the O2 can’t be used for anything.
What does photorespiration consume? What does it produce? What is it dependent on? Does it use a lot of energy?
Consumes O2
Produces CO2
Dependent on light
Yes - substantial metabolic cost!
RuBisCO has a Km for CO2 of ~9μM while the Km for O2 is ~350 μM – so the enzyme is quite specific for CO2. However there are 3 things that can occur to make the affinity for CO2 less allowing more O2 to bind therefore allowing photorespiration to occur. What are these 3 things?
- Atmosphere has a greater concentration of O2 compared to CO2
- At higher temps, there is more O2 and the affinity of RuBisCo for CO2 lowers
- As CO2 is consumed in the carbon assimilation reactions the greater the proportion of O2 to CO2 becomes.
What is the name of the cycle that photorespiration occurs through? What 3 organelles are involved?
Oxidative phosphosynthetic carbon cycle.
Chloroplast, peroxisome, mitochondrion.
What is the name of the protein that makes up half the total protein in the mitochondrial matrix?
Glycine decarboxylase
What is so special about C4 plants? How does this occur?
They concentrate CO2 around RuBisCO, thus minimizing photorespiration.
PEP carboxylase is an enzyme used in these plants and they cannot use O2 as a substrate.
What are plasmodesmata?
Special plant-specific cell junctions. Protein lined channels that take specific molecules from cell to cell. Animals don’t have these.
C4 plants require more energy than C3 plants and therefore photosynthesis has a greater metabolic cost. Is it worth it just to reduce photorespiration by a bit?
Yes! Easily worth it especially at temps. at or above 28 degrees.
Noncyclic photophosphorylation (from the light reaction) can be represented by the following overall equation:
2H2O + 8 photons + 2NADP+ + ~3ADP+ + ~3Pi —>
O2 + ~3ATP + 2NADPH
Bacteria have __ membrane. They pump protons into the ____.
Mitochondria have __ membrane. They pump protons into the ____.
Chloroplast have __ membrane. They pump protons into the ____.
1, outside the cell.
2, intermembrane space.
3, stroma.
