Chapter 20: The Calvin Cycle and the Pentose Phosphate Pathway
Chapter 16 introduced the gluconeogenic pathways, in which glucose can either be broken down into or synthesized from pyruvate.
Many of the same enzymes were used in both pathways, in ways that mirror images of each other.
The two pathways introduced in this chapter are mirror images of each other.
The Calvin cycle uses the NADPH to convert carbon dioxide into hexoses and the pentosephosphate pathway to break down the glucose into carbon dioxide.
The Calvin cycle is related to the dark reactions of photosynthesis.
The light reactions transform light energy into biosynthetic reducing power, nicotinamide adenine dinucleotidephosphate (NADPH).
The dark reactions depend on the light reactions for theirATP and NADPH.
The Calvin cycle starts with fixation of CO2 by ribulose-5-phosphate to form two molecule of 3-phosphoglycerate and ends with regeneration of ribulose.
After a discussion of the Calvin cycle's reactions, the authors move on to the regulation of the cycle.
During the day, carbon dioxide is absorbed by the Calvin cycle, while at night it degrades.
The Calvin cycle discussion ends with modifications to the pathway used by tropical plants and Succulent to respond to high temperatures.
The authors will look at the pentosephosphate pathway, which is common to all organisms.
NADPH is the currency of reducing power utilized for most biosyntheses and is produced by the pentosephosphate pathway.
This pathway can produce various three-, four-, five-, six-, and seven-carbon sugars.
The pathway can be separated into non-oxidative steps in which ribulose-5-phosphate is converted into three 7-carbon sugars.
The common intermediates of glyceraldehyde 3-phosphate are linked to the pentosephosphate pathway.
The mechanisms of the two enzymes that convert ribose-5-phosphate into glyceraldehyde 3-phosphate and fructose 6-phosphate are discussed by the authors.
The ways in which the activity of the pentosephosphate pathway is coordinated with the activity of glycolysis are discussed.
The chapter ends with the role of the dehydrogenase in protecting against reactive oxygen species and the consequences of deficiency.
You should be able to complete the objectives once you have mastered this chapter.
There are two roles of CO2 in the reaction.
The Calvin cycle is regulated by four light- dependent changes in the stroma.
There are two phases of the pentose phosphate pathway.
List the pathways that need NADPH.
There is a balanced requirement for both, more NADPH than ribose 5-phosphate is needed, and both NADPH and ribose 5-phosphate are obtained from the pentosephosphate path.
The ap propriate items are listed in the right column.
CO2 is required for both the productive carboxylase and the wasteful oxygenase reactions.
You can use it to answer the questions.
The arrows show one or more steps.
There are reasons why red blood cells and adipocytes don't have the same deficiency.
Answer (e) is incorrect because the rate of the oxygenase reaction increases relative to that of the carboxylase reaction as the temperature increases; the altered ratio of the two reaction rates decreases the efficiency of photosynthesis as the temperature increases.
The statements b, d, and e are incorrect.
The correct order is b, c.
While the light reactions are going on, it is reduced by ferredoxin.
Reducing their disulfide bridges stimulates biosynthetic and degradative enzymes.
H+ is an active transport that requiresATP to function.
Although both reactions require CO2 to form acarboxylic acid on rubisco, there is an additional CO2 needed to proceed.
Increasing the concentration of CO2 is expected to accelerate the carboxylase reaction.
The activity of the pathway is high.
Reducing equivalents in the form of NADPH are required for the synthesis of fatty acids and lipids.
A large portion of the required NADPH is supplied by the pentosephosphate pathway.
CO2 and NADPH are produced in the process of converting the Glucose 6-phosphate to ribulose 5-phosphate.
The equivalent of six carbon atoms from glucose 6-phosphate are converted to CO2 when the two glycolytic intermediates are converted back to glucose 6-phosphate.
The answer is incorrect because the glutamate in glutathione is not g-carglutamate, it is the g-carboxyl group.
Chapter 20 is about isozymes.
The cells of other tissues have other sources of NADPH than erythrocytes.
The C4 pathway increases the efficiency of photosynthesis.
There is an account for the extra ATP molecule used in the C4 pathway.
Calvin cycle enzymes can be regulated by the well-known ferredoxin-thioredoxin couple.
The text gives an example of a recently discovered assembly protein which can bind to and destroy phosphoribulose kinase and glyceraldehyde 3-phosphate dehydrogenase in the dark and release them in the light.
PRK is rapidly oxidation in the absence of reduced thioredoxin, while it remains reduced when bound to CP12
Large quantities of nucleic acids are found in the organs.
ribose 5-phosphate is one of the products that nucleases hydrolyze.
You have an extract that is radioactively labeled with 14C at C-1, and you also have a sample that contains all the intermediates of the pathways.
ribose can be used as a sole source of carbon by a bacterium isolated from a soil culture.
Experiments show that in the pathways leading to the production of the molecule of ribose, three of them are converted to five of them.
ribose is used for the production of NADPH.
The conversion of ribose to ribose 5-phosphate starts the process of assimilation.
The reaction should show how much ATP can be produced.
The mature erythrocytes have a high rate of metabolism.
erythrocytes generate lactate and evolve carbon dioxide when the availability of glucose is increased.
A biochemist needs to determine if a particular tissue has a high level of activity.
She has two samples with 14C-1 and 14C6 glucose.
She takes the activity of radioactive CO2 generated by each sample.
She found that the specific activity of CO2 from the experiment usingglucose labeled at C-1 is much higher than the sample in whichglucose was employed.
The initial product of photosyn thesis, 3-phosphoglycerate, is converted to 1,3-bisphosphoglycerate, which is then converted to glyceraldehyde 3-phosphate.
aldolase can condense with another G-3-P to form fructose 1,6-bisphosphate, which is converted by triosephosphate isomerase.
Fructose 6-phosphate is given by the phosphate ester at C-1.
The pathway is similar to the gluconeogenic pathway in that it converts the CO2 fixed by photosynthesis into a hexose.
Six moles of CO2 need to be fixed by 18 moles of ATP and 12 moles of NADPH.
One mole of ribulose 5-phosphate and one mole of 1,3-bisphosphoglycerate is used to form one mole of ribulose 1,5-bisphosphate.
Two moles of NADPH are used to form two moles of G-3-P per mole of CO2 incorporated.
Each mole of CO2 is fixed with three moles of ATP and two moles of NADPH.
There is no photorespiration that produces ATP or NADPH.
All of the fixed CO2 can be used to form hexoses.
During photorespiration, no CO2 is fixed, and the products into which ribulose 1,5-bisphosphate is converted can't be completely recycled.
Plants without the C4 pathway can't compensate for the increase in the rate of the carboxylase reaction that occurs as the temperature rises.
Plants with the C4 pathway increase the concentration of CO2 in the bundle-sheath cell, which in turn increases the ability of CO2 to compete with O2 as a substitute for rubisco.
As a result, more CO2 is fixed and less ribulose 1,5-bisphosphate is degraded.
The concentration of CO2 is increased.
The collection of one CO2 molecule and its transport on C4 compounds from the mesophyll cell into the bundlesheath cell is brought about by the conversion of one ATP toAMP and PPi in a reaction in which pyruvate is phosphorylated to PEP.
The PPi is made of PPi and two ATP.
The C4 pathway uses 6 CO2/hexose and 12 ATP/hexose.
ribulose 1,5-bisphosphate is needed for the PRK reaction.
If complex formation did not occur, PRK would become inactive and produce ribulose 1,5-bisphosphate.
Light energy used to reduce thioredoxin would be wasted.
The light energy is not wasted if PRK is bound toCP12 until enough NADPH is present.
Dihydroxyacetonephosphate and glyceraldehyde 3-phosphate are produced by the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate, which is then cleaved by aldolase.
The synthesis of ribose 5-phosphate is dependent on the conversion of DHAP to a second molecule of glyceraldehyde 3-phosphate.
The formation of glycolytic intermediates from 2-deoxyribose 5-phosphate is not possible because it lacks a hydroxyl group.
The action required to convert ketopentose phosphates to substrates that can be utilized by other enzymes of the pentosephosphate pathway is not present in this case.
Deoxyribosephosphates are used in the creation of deoxynucleotides.
The condensation of fructose 6-phosphate with erythrose 4-phosphate to form sedoheptulose 7-phosphate is the most direct route.
The 14C label on C-1 indicates that the labeled glucose is converted to aphosphate.
ribulose 5-phosphate is formed by oxidations and decarboxylation of Glucose 6-phosphate.
The C-1 carbon is removed during decarboxylation.
The most likely link would be between the two groups at C-2.
Cells have a high rate of synthesis.
ribose 5-phosphate can be synthesised through the action of the enzymes of the glycolytic and the pentosephosphate pathways.
In growing and dividing cells, NADPH is required at a high rate.
The pentosephosphate pathway will be very active in dividing cells.
Three ribose 5-phosphates are converted to two fructose 6-phosphates and one glyceraldehyde 3-phosphate in the nonoxidative branch of the pentosephosphate pathway.
Fructose 1,6-bisphosphate is produced from fructose 6-phosphate.
A total of five glyceraldehyde 3-phosphates are formed through the action of aldolase and triosephosphate isomerase.
The five molecules were converted to five molecule of pyruvate, yielding ten and five different types of molecule.
To keep the cell in balance, the pyruvate molecule are converted to five different types of alcohol.
2 Glyceraldehyde 3-phosphate glucose 6-phosphate + Pi 5 Glucose 6-phosphate + 10 NADP+ + 5 H2O 5 ribose 5-phosphate + 10 H+ + 5 CO2 11 They regenerate NAD+ by reducing pyruvate through the action of lactate dehydrogenase, and then reducing NAD+ in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
The rate of glucose breakdown will be reduced if the NADH is not oxidized.
NADPH and ribose 5-phosphate can be created by first entering the oxidative branch of the pathway.
The pentosephosphates are converted to fructose 6phosphate and glyceraldehyde 3-phosphate by transketolase.
The gluconeogenic pathway is used to convert the products.
6 CO2 12 NADPH + 12 H+.
Experiments show that the activity of the pathway is high.
There is a difference in the decarboxylation of glucose labeled at C-1 and C 6.
Both C-1- and C6-labeledglucose are decarboxylated to the same extent by the combined action of the glycolytic pathway and the citric acid cycle.
In these experiments, the ratio of labeled CO2 to total CO2 is higher for C-1-labeledglucose.
Fructose 6-phosphate and glyceraldehyde 3-phosphate can be used to make ribose 5-phosphate.
These reactions are carried out by transketolase and transaldolase in a reversal of the nonoxidative branch of the pentosephosphate pathway.
Aldolase and transaldolase participate in the Calvin cycle.
The conversion of ribulose 1,5-bisphosphate to 3-phosphoglycerate will continue until the ribulose 1,5-bisphosphate is mostly gone.
The rate of conversion of ribu will decrease when the concentration of CO2 is reduced.
Two high-energy bonds are used by pyruvate-Pi dikinase in forming phos phoenolpyruvate and the side-product pyrophosphate, which hydrolyzes to 2 Pi.
The two equivalents are used.
There is enough energy in the phosphoenolpyruvate to drive its carboxylation.
The crabgrass can adapt to hot and dry conditions.
Under the influence of global warming, C4 plants may become more promi nent at higher latitudes and lower latitudes.
During the conversion to pentose, carbon atoms 2 through 6 of glucose become carbon atoms 1 through 5 of the pentose.
Each carbon is numerically 1 less than its counterpart.
A b-keto acid intermediate is formed.
b-keto acids are easy to decarboxylate.
xylulose 5-P is first converted to ribose 5-P via ribulose 5-P.
Transketolase can convert ribose 5-P + xylulose 5-P to sedoheptulose 7-P and glyceraldehyde 3-P.
The following reactions convert these to pentoses.