Calvin Cycle

how many CO2 are fixed per cycle?

1

reduction overview

2 3-phosphoglycerate reduced to make fructose 6 phosphate. this requires 2 ATP and 2 NADPH

reduction

2 3-phosphoglycerates are converted to hexoses, which are initially fructose 1,6-bisphosphate. this is interconvertible in the hexose monophosphate pool. fructose 6 phosphate and glucose 1 phosphate can be linked to make sucrose, and 3-phosphoglycerate can be transported to the cytoplasm and used for other processes.

what is the cost of transport in c4 plants

2 high energy bonds (ATP to AMP)

each fixed CO2 costs...

3 ATP and 2 NADPH

what is the overall cost of making 6-carbon sugars in C4 plants

30 ATP

11. in the C4 pathway, one ATP molecule is used in combining the CO2 with phosphenolpyruvate to form oxaloacetate, but, in the computation of energetics bookkeeping, two ATP molecules are said to be consumed. explain.

ATP is converted into AMP. to convert AMP back into ATP, two molecules of ATP are required: one to form ADP and another to form ATP from the ADP

7. explain why the maintenance of a high concentration of CO2 in the bundle-sheath cells of C4 plants is an example of active transport. how much ATP is required per molecule of CO2 to maintain a high concentration of CO2 in the bundle-sheaths of C4?

ATP is required to form phosphoenolpyruvate from pyruvate. PEP combines with CO2 to form oxaloacetate and, subsequently, malate. two ATP molecules are required because a second ATP molecule is required to phosphorylate AMP to ADP.

what type of plants have the calvin cycle?

C3 plants

16. C3 plants require 18 molecules of ATP to synthesize 1 molecule of glucose. C4 plants, on the other hand, require 30 molecules of ATP to synthesis 1 molecule of glucose. why would any plant use C4 metabolism instead of C3, given that C3 is so much more efficient?

C4 metabolism allows rubisco to function efficiently even when the temperatures are high, which favor oxygenase activity. moreover, C4 metabolism allows desert plants to accumulate CO2 at night when the temperatures are cooler and water evaporation is not a problem.

fixation overview

CO2 + ribulose 1,5-bisphosphate --> 2 3-phosphoglycerate

fixation

CO2 is fixed by rubisco. this is highly exergonic. the lysine active site binds CO2 to form carbamate, which is coordinated with magnesium to activate CO2.

sucrose

a disaccharide of glucose and fructose. it is made in the cytoplasm after the transport of 3 carbon products out of chloroplast

starch

a glucose polymer with alpha 1-4 and alpha 1-6 linkages. it is made in chloroplasts

what are starch and sucrose made from?

activated UDP-glucose molecules

8. glyceraldehyde-3-phosphate dehydrogenase in chloroplasts uses NADPH to participate in the synthesis of glucose. in gluconeogenesis in the cytoplasm, the isozyme of the dehydrogenase uses NADH. why is the use of NADPH by the chloroplast enzyme advantageous?

because NADPH is generated in the chloroplasts by the light reactions

6. in an atmosphere devoid of CO2 but rich in O2, the oxygenate activity of rubisco disappears. why?

because the carbamate forms only in the presence of CO2, this property would prevent rubisco from catalyzing the oxygenase reaction when CO2 is absent.

regulation by thioredoxin

both reduced ferredoxin and NADPH regulate the calvin cycle. thioredoxin contains disulfide bonds that are sensitive to redox environment. electrons from the reduced ferredoxin are transferred to thioredoxin which activates multiple proteins

rubisco oxygenase

can react with oxygen in photorespiration. the nonproductive reaction uses 1 3PG and phosphoglycolate. 2 phosphoglycolate are salvaged to 1 3PG and CO2 using 1 ATP.

regeneration

carbon fixation converts 5 carbon sugars to 3 carbon sugars and then to 6 carbon sugars. ribulose 5 phosphate has to be regenerated if the cycle is to continue, so the 6 and 3 carbon sugars are converted into ribose 5 phosphate through 4, 5 and 7 carbon intermediates. most reactions are reversible but certain steps, especially those that hydrolyze high energy phosphate bonds, drive regeneration

regulation of the calvin cycle

conditions in the stroma change when light reactions are active. there is a decrease in H+ concentration, increase in magnesium concentration, increased NADPH and ferredoxin becomes reduced. rubisco is activated by light, which makes pH and magnesium concentrations high which promotes the formation of carbamate needed for activity.

crassulacean acid metabolism

conserves water, which is essential for photosynthesis. these plants conserve water by keeping stomata closed during the day and by using stored CO2 in the calvin cycle. they open stomata at night and absorb CO2. this lowers the rate of water loss and it is used by succulents and plants that grow in arid climates

carbamate function

coordinates essential magnesium

3 stages of the calvin cycle

fixation, reduction, regeneration

complete regeneration reaction

fructose 6 phosphate + 2 glyceraldehyde 3 phosphate + dihydroxyacetone phosphate + 3 ATP --> 3 ribulose 1,5-bisphosphate + 3 ADP

metabolic connections between gluconeogenesis and photorespiration

gluconeogenesis uses oxaloacetate and malate to transport pyruvate from the mitochondria to reaction. the differeces are that C plants with phosphoenolpyruvate so that ATP is not used directly to make oxaloacetate.

hexose monophosphate pool

glucose 1 phosphate, glucose 6 phosphate and fructose 6 phosphate

photorespiration

has the energy cost of the normal calvin cycle plus 1 ATP, but no carbon is fixed. the structures of CO2 and O2 are similar enough that the enzyme can't distinguish it chemically. but, oxygen is more soluble in high temperatures, and photorespiration can predominate under hot conditions.

where do C4 plants predominate

hot climates with lots of sun

where do c3 plants predominate

in temperate climates

where does calvin cycle occur

in the stroma of chloroplasts

the fate of fixed carbon

it is stored in plants as either starch or sugar

aldolase

joins or separates sugars. this occurs is glycolysis, when it converts fructose 1,6-phosphate into GAP and DHP

6 calvin cycles..

makes 1 6-carbon sugar.

13. what is photorespiration, what is its cause, and why is it believed to be wasteful?

photorespiration is the consumption of oxygen by plants with the production of CO2, but it doesn't generate energy. photorespiration is due to the oxygenate activity of rubisco. it is wasteful because, instead of fixing CO2 for conversion into hexoses, rubisco generates CO2.

regeneration overview

regenerates ribulose 1,5-bisphosphate

fixation process

ribulose 1,5-bisphosphate --> enediolate intermediate --> unstable intermediate --> 3-phosphoglycerate

rubisco

ribulose 1,5-bisphosphate carboxylase/oxidase. it is composed of 8 large and 8 small subunits located on the stromal surface of thylakoid. each large subunit has an active site. it is the rate-limiting step for the calvin cycle. it is 16% of total protein in chloroplast.

5. suggest a reason why rubisco might be the most abundant enzyme in the world.

rubisco catalyzes a crucial reaction, but it is highly ineffecient. consequently, it is required in large amounts to overcome its slow catalysis.

3. the calvin cycle can be thought of as taking place in three stages. describe them.

stage 1 is the fixation of CO2 with ribulose 1,5-bisphosphate and the subsequent formation of 3-phosphoglycerate. stage 2 is the conversion of some of the 3-phosphoglycerate into hexose. stage 3 is the regeneration of ribulose 1,5-bisphosphate.

why does bread get stale?

starch is stored in granules in cells that are crystalline. baking breaks the hydrogen bonds allowing starch granules to absorb water, which causes them to be in an amorphous structure, which contributes to taste and texture of fresh bread. as bread sits, water is lost and granules start to recrystallize. water close to the surface is lost before the water thats internal

differences between gluconeogenesis and the calvin cycle

the compartment and NAD form

10. what are the light-dependent changes in the stroma that regulate the calvin cycle?

the light reactions lead to an increase in the stream concentration of NADPH, reduced ferredoxin, and Mg2+, as well as an increase in pH.

9. rubisco requires a molecule of CO2 covalently bound to lysine 201 for catalytic activity. the carboxylation of rubisco is favored by high pH and high Mg2+ concentration in the stroma. why does it make good physiological sense for these conditions to favor rubisco carboxylation?

the stroma will accumulate mg2+ and become alkaline as a consequence of the movement of protons from the stroma to the thylakoid space. thus, rubisco is primed for activity when the light reactions are providing ATP and NADPH required for carbon fixation and glucose synthesis.

transketolase

transfers 2 carbon units

two key enzymes in regeneration

transketolase and aldolase

c4 plants photorespiration

uses compartmentalization. C4 plants separate carbon dioxide uptake in the mesophyll cell and the calvin cycle in the bundle-sheath cell. 4-carbon compounds like oxaloacetate malate transport CO2. it costs one ATP to AMP. C4 plants increase the concentration of CO2 in bundle-sheath cells, which dramatically reduces photorespiration

in 3 calvin cycles...

you use 3 ribulose 1,5-bisphosphates, fix 3 CO2, make 6 3 carbon sugars, 5 3 carbon sugars are used to regenerate 3 ribulose 1,5-phosphates and you yield 1 3-carbon sugar