DISCUSSION AND ESSAY QUESTIONS—SET 3
Photosynthetic carbon metabolism
1. Compare and contrast C3, C4, and CAM photosynthesis in terms of their efficiency (carbon
fixed per amount of light absorbed) under optimum conditions (e.g. 16 h light/8 h dark, 25oC, daily
water to soil capacity), hot conditions (as above, but 35oC), and hot, dry conditions (as above, but
35oC and water only at the beginning of the season).
C3 photosynthesis requires, for one CO2 fixed, 2 NADPH and 3 ATP. The number of photons
required depends on the number of H+ translocated by the photosynthetic ETC and the number of
subunits in the Fo subunit of the ATP synthase. The best estimate from actual measurements is 910 photons/CO2.
C4 photosynthesis requires additional ATP -> AMP (the equivalent of 2 ATP -> 2 ADP) for the
formation of one PEP per CO2 fixed. To produce 2 ATP by cyclic photophosphorylation requires
a little more than 4 additional photons for a total of 13-14 photons/CO2.
CAM photosynthesis is more complicated (see the Wikipedia diagram below). The conversion of
starch to PEP at night may provide some ATP, but this may be reversed by the re-synthesis of
starch from PEP in the day (and one extra ATP may be needed). The storage of malate in the
vacuole at night uses one ATP and one NADPH. In the day, the release of CO2 may recover the
NADPH but use 2 additional ATP (through ME, malic enzyme) or it may produce NADH and use
1 additional ATP (through PEPCK, PEP carboxykinase). At minimum, CAM requires 2-3
additional ATP by cyclic photophosphorylastion, and thus 9-14 additional photons.
2. Predict how C3, C4, and CAM plants will each respond to a
doubling of the concentration of CO2 in the atmosphere.
C3: photorespiration down by ~50%, increase in net photosynthesis
C4: little change
CAM: little change
3. Comment on this hypothesis: the formation of P-glycolate by
rubisco is an “evolutionary mistake,” which occurred because the
primordial atmosphere had a low Ov concentration.
This is likely true; the glycolate pathway releases CO2 to the mitochondrion, and steps in the
pathway are energetically expensive, making photorespiration a highly wasteful process relative
to photosynthesis. However, 3-phosphoglycerate from the glycolate pathway can re-enter the
Calvin cycle in the chloroplast, so some recycling of photorespiration products does occur.
Under climate scenarios (both primordial and future) with elevated CO2, the risk of
photorespiration is decreased.
4. Describe photorespiration, and then discuss why photorespiration occurs. What would be the
physiological consequences of engineering plants that could not complete photorespiration?
5. Explain how photorespiration reduces the quantum efficiency of
photosynthesis as measured by (a) O2 production and (b) CO2
uptake. Include both events at Rubisco and those outside of the
chloroplast.
6. Name three environmental factors that affect the rate of
photorespiration and explain how they do it.
Temperature – at high temps, O2 is more soluble relative to CO2. (Solubility decreases more slowly
as temperature increases). This increases the substrate for photorespiration. O2/CO2 - Similar to
above. At high O2 concentrations photorespiration increases. Light availability / water availability –
if conditions are unfavorable for photosynthesis, less photorespiration will occur as well.
7. The atmospheric CO2 concentration is predicted to double within
our lifetime. How do you think this will impact the distribution of
plant species throughout the world?
There are at least two important issues with regard to photosynthesis. One is the effect of increased
CO2 on the climate; for instance, average temperature might be expected to increase due to a
stronger greenhouse effect. Higher temperatures might favor C4 plants. Higher temperatures might
also increase evaporation from the ocean and consequent rainfall on land. This might reduce the
area that favors drought-tolerant CAM plants.
The second issue is the effect of increased CO2 directly on photosynthesis. At an increase
concentration CO2 will compete better with O2 at the Rubisco active site, suggesting by analogy
with C4 plants a higher temperature optimum for C3 plants, which would then be favored over C4
plants because of their lower energy (ATP) requirement for photosynthesis. At the present time, no
one knows the eventual effect on plant distribution.
8. Explain how the following reactions are important in plant
physiology:
PEP + CO2 --> OAA
OAA + NADPH --> malate + NADP+
Describe how PEP carboxylase might be regulated to maintain the
proper level of malate. How will this regulation be different in C4
and CAM plants? Would these activities occur in C3 plants? If so,
where and when?
OAA is the first product of CO2 incorporation for C4 and CAM plants. Malate is important for C
transport and as a precursor to starch synthesis. PEP carboxylase activity likely increases when
malate is low, and decreases when malate is high. Localization of malate (and therefore malate
signaling to regulate PEP carboxylase) differs between C4 and CAM plants. C4 plants produce
malate in the chloroplast, while PEP carboxylase is active in the cytosol. In CAM plants, malate
is produced in the cytosol. In C3 plants, PEP carboxylase is used only in regulation of the TCA
cycle.
9. Consider the fixation of carbon dioxide leading to the production of an amylose chain 30 glucose
subunits long. Calculate the following: (a) the number of CO2 molecules fixed; (b) the number of
ATP molecules consumed; (c) the number of NADPH molecules oxidized.
From the Calvin cycle:
WITHOUT REGENERATION
3 H2O + 3 CO2 + 6 ATP + 6 H+ + 6 NADPH  6 G3P + 6 ADP + 6 NADP+ + 6 Pi
WITH REGENERATION
3 H2O + 3 CO2 + 9 ATP + 6 H+ + 6 NADPH  1 G3P + 9 ADP + 6 NADP+ + 9 Pi (+ 5 G3Ps
recycled into RuBisCO)
2 G3P  1 glucose
So,
360 NADPH + 180 CO2 + 540 ATP  60 G3P  30 glucose
Plus 1 ATP for each monomer of glucose added to the growing amylose chain = 29 additional.
a) 180 CO2 fixed
b) 569 ATP
c) 360 NADPH oxidized
10. List the two enzymes that are involved in sucrose synthesis regulation, and explain one way that
each is inhibited and one way that each is promoted.
Sucrose-6-P synthase: activated by sucrose-P-synthase phosphatase, inhibited by SnRK1 (kinase)
Sucrose-6-P-synthase phosphatase: stimulated by glucose-6-P, inhibited by phosphate
SnRK1 activated by ATP (substrate), inhibited by glucose-6-P
11. A plant cell uses ATP to make starch, UTP to make sucrose, and GTP to make cellulose.
Suggest a reason why it is useful to employ different nucleotides in the synthesis of these different
polysaccharides.
Different nucleotides allow for independent control of the biosynthetic pathways. If all precursors
were entering the same pathway, regulation would be difficult. Can also allow for localized pools
and therefore localized pathway activity.
12. The textbook points out that the pressure-flow hypothesis has not been confirmed for
gymnosperms, which have sieve cells, but not sieve tubes. How would you test the hypothesis in
gymnosperms (e.g. a pine tree sapling)?
Take pressure measurements within phloem at 2 heights. Sample and flash-freeze tissue. Extract
water to calculate psi p. Calculate psi w. Is phloem water transport due to differences in psi p or psi
w? And replicate the study!
13. What is the difference between symplastic and apoplastic unloading of phloem in sink tissue?
Will flux in the sieve tubes be facilitated or inhibited by an apoplastic step? Which type would you
expect between phloem and a growing tomato fruit?
Symplastic unloading proceeds through plasmodesmata. Inapoplastic unloading, sucrose crosses
plasma membranes, facilitated by protein carriers and possibly enhanced by co-transport with H+
ions. Apoplastic unloading is expected to reduce the concentration of sucrose in the sieve tubes of
the sink more effectively, which by itself would increase the pressure gradient and the rate of flux.
On the other hand, the enlargement of some fruits and the cells in the fruits is so rapid that by itself
enlargement could reduce the concentration of sucrose in cells connected symplastically to the
phloem.
14. Explain how starch synthesis is controlled and why it isimportant. Under what conditions
would starch synthesis be higher than starch breakdown? Under what conditions would the reverse
occur?
a) Control by allosteric effectors and protein phosphorylation leads to net synthesis during the
day, and net breakdown at night. Indirectly via sucrose synthesis, triose phosphate, FBP, PFK,
F2;6-bisphosphate all contribute to regulation of starch synthesis.
b) This allows temporal control of carbohydrate storage and sucrose availability (and therefore
chemical energy availability). It also contributes to control of sucrose localization.
c) Synthesis > breakdown when sucrose concentrations are high. This is controlled by low triose
phosphate  elevated F2;6-bisphosphate  FBP off.
d) Breakdown > synthesis when CO2 assimilation is low. And the reverse chemical regulation
Respiration and lipid metabolism
15. One definition of the “Pasteur Effect” is “a faster uptake of glucose by the reactions of
glycolysis under anaerobic conditions relative to the rate in aerobic conditions. I just read another
definition, which is more quantitative and experimental: “The Pasteur Effect occurs when the
QCO2(N2)/QO2(air) is greater than 0.33.” The QCO2(N2) is the rate of evolution of CO2 when
the tissue is in N2. The QO2(air) is the rate of uptake of O2 when the tissue is in air. Are these
two definitions the same? Under what conditions?
Assume an input of x mol/s of glucose [C6(H2O)6] into aerobic respiration. The process also
requires the input of 6x mol/s of O2, so the QO2(air) = 6x. (This is true even if the input is x
mol/s of glucose-P from phosphorolysis of starch. The output will be 6x mol/s of CO2, and the
QCO2(air)/QO2(air) = 1.) Now assume the input of x mol/s of glucose in N2 (no O2). If the
product is ethanol plus CO2, the QCO2(N2) will be 2x. The ratio QCO2(N2)/QO2(air) = 2x/6x =
0.33. This calculation assumes the same input of glucose in N2 and air. If the ratio is greater than
0.33, it indicates that the uptake of glucose is faster in N2 than in air. Thus the definitions are the
same assuming that anerobic metabolism of glucose proceeds by ethanolic fermentation.
16. Explain why glycolysis might be faster under anaerobic
conditions than aerobic conditions.
The production of ATP is less efficient, relative to glucose used. With unbalanced hydrolysis of
ATP, [Pi] rises, stimulating PFK, a control point. Also increased [Pi] and decreased [ATP]
stimulate PEP pyruvate, lowering [PEP], which also stimulates PFK.
17. Here is a mystery: one fall, a silo filled with wheat exploded
and was completely destroyed. The company that handled the
silo’s insurance looked for accelerants and other signs of sabotage, but without success. The
weather was cool, and it had rained for two days before. What was the cause of the explosion?
A plant physiologist explained that the moisture had most likely stimulated germination of the
wheat. Germinating seeds respire, and the respiration releases heat: deltaGo’ = -2840 kJ/mol.
The heat from the complete respiration of just 1 kg of starch is (1000/168)(2840) = 17,000 kJ.
The heat build-up was sufficient to explode the wheat and the structure.
18. A germinating rice seed derives most of its energy from the
respiration of starch. A germinating oil-palm seed derives its
energy from the respiration of lipids. For each case, what ratio
QCO2/QO2 (rate of CO2 evolved to rate of O2 taken up) do you
expect during the early stages of germination?
As described for problem 16, the QCO2(air)/QO2(air) = 1 for the respiration of carbohydrate,
and this is what you would expect for the germination of rice. For the respiration of fatty acids,
we can write (as a limiting case), -(CH2)n- + (3/2)n O2 --> n CO2 + n H2O. Thus the
QCO2(air)/QO2(air) = n/(3/2)n = 2/3. Of course, not all the carbons in storage lipids are as
reduced as hydrocarbons, so the ratio would be somewhat greater than 2/3.
19. Describe the synthesis of lipids. Where are they produced, and what roles do they play in the
plant? Give examples of lipids for each role.