Biochem II- Chapter 20: The Calvin Cycle and the Pentose Phosphate Pathway
(_____________), catalyzes an aldol condensation between dihydroxyacetone phosphate (DHAP) and an aldehyde.
(Aldolase)
Photosynthetic organisms are called (_____________)-(literally, self-feeders ) because they can convert sunlight into chemical energy, which they subsequently use to power their biosynthetic processes.
(Autotrophs)
A specific phosphatase converts phosphoglycolate into (___________), which enters (_________________) (also called microbodies)
(Glycolate) (Peroxisomes)
Organisms that obtain energy from chemical fuels only are called (____________), and such organisms ultimately depend on autotrophs for their fuel.
(Heterotrophs)
The mixture of the three phosphorylated hexoses is called the......
(Hexose monophosphate pool)
Pathway common to all organisms, known variously (........)
(Pentose phosphate pathway The hexosemonophosphate pathway The phosphogluconate pathway or the pentose shunt)
Finally, ribose 5-phosphate is converted into ribulose 5-phosphate by (____________ ___________), whereas xylulose 5-phosphate is converted into ribulose 5-phosphate by (___________ ___________).
(Phosphopentose isomerase) (Phosphopentose epimerase)
Ribulose 5-phosphate is converted into ribulose 1 ,5 -bisphosphate through the action of (______________)
(Phosphoribulokinase)
The (_____________), requires the coenzyme thiamine pyrophosphate (TPP) to transfer a two-carbon unit (CO — CH2OH) from a ketose to an aldose .
(Transketolase)
The reactive enediolate intermediate generated on the Mg 2+ ion sometimes reacts with O 2 instead of CO 2 ; t hus, rubisco also catalyzes a deleterious oxygenase reaction. The products of this reaction are (_______________) and (_______________).
(phosphoglycolate) (3-phosphoglycerate).
What are the fates of the members of the hexose monophosphate pool? These molecules are used in a variety of ways, but there are two primary roles. Plants contain two major storage forms of sugar: (__________ and ________)
(starch and sucrose)
The pathway provides a means by which glucose can be oxidized to generate NADPH, (.................)
(the currency of readily available reducing power in cells)
The Calvin cycle comprises three stages: 1. 2. 3.
1. The fixation of CO 2 by ribulose 1 ,5 -bisphosphate to form two molecules of 3-phosphoglycerate. 2. The reduction of 3 phosphoglycerate to form hexose sugars. 3. The regeneration of ribulose 1 ,5 -bisphosphate so that more CO 2 can be fixed.
20.1: The Calvin Cycle Synthesizes Hexosesfrom Carbon Dioxide and Water
20.1: The Calvin Cycle Synthesizes Hexosesfrom Carbon Dioxide and Water
20.2 :The Activity of the Calvin Cycle Depends on Environmental Conditions
20.2 :The Activity of the Calvin Cycle Depends on Environmental Conditions
Fructose 6-phosphate + 2 glyceraldehyde 3-phosphate + dihydroxyacetone phosphate + 3 ATP ---------------->
3 ribulose 1,5-bisphosphate + 3 ADP
Another difference is that ___________-___________, not UDP-glucose, is the activated precursor. Starch is synthesized and stored in chloroplasts.
ADP-glucose
The light reactions transform light energy into ________ and _____________ _________ _________, _________________.
ATP Biosynthetic reducing power NADPH
The dark reactions use the _________and _________ produced by the light reactions to reduce carbon atoms from their fully oxidized state as carbon dioxide to a more reduced state as a ___________.
ATP NADPH Hexose
Photorespiration is wasteful because organic carbon is converted into CO 2 without the production of ______, ___________, or another _________-_________ ___________.
ATP, NADPH, or another energy-rich metabolite.
Like the zinc ion in the active site of carbonic anhydrase, this metal ion serves to.......
Activate a bound substrate molecule by stabilizing a negative charge
The _____________ ______, used in the synthesis of nitrogen-containing compounds, is salvaged by a glu tamine synthetase reaction
Ammonium ion
Fixation of CO2 begins with...
Begins with the conversion of ribulose 1 ,5 -bisphosphate into a highly reactive enediol ate intermediate. The CO 2 molecule condenses with the enediol intermediate to form an unstable six-carbon compound, which is rapidly hydrolyzed to two molecules of 3-phosphoglycerate.
write a balanced equation for the net reaction of the Calvin cycle: 6 CO2 + 18 ATP + 12 NADPH + 12 H2O ----------->
C6H12O6 + 18 ADP + 18 Pi + 12 NADP+ 1 6 H+
The_________ _________ introduces into life all the carbon atoms that will be used as fuel and as the carbon backbones of biomolecules.
Calvin cycle
The dark reactions are called either the ( _________ -_________ _______) , after Melvin Calvin and Andrew Benson , the biochemist who elucidated the pathway , or simply the Calvin cycle .
Calvin- Benson cycle
(The activity of rubisco increases markedly on illumination because light facilitates the _____________ formation necessary for enzyme activity)
Carbamate
This CO 2 molecule adds to the uncharged e´ -amino group of lysine 201 to form a _______________.
Carbamate
Carbon dioxide reacts with ribulose1,5 -bisphosphate to form two molecules of 3-phosphoglycerate
Carbon dioxide reacts with ribulose1,5 -bisphosphate to form two molecules of 3-phosphoglycerate
(the light reactions and dark reactions of photosynthesis cooperate to transform light energy into ________ _______)
Carbon fuel
The pentose phosphate pathway can also be used for the _________ ___ _________sugars from the diet, the synthesis of pentose sugars for nucleotide biosynthesis, and the catabolism and synthesis of less common four- and seven-carbon sugars.
Catabolism of pentose
Mg 2+ ions from the thylakoid space are released into the stroma to.....
Compensate for the influx of protons.
The problem is to........
Construct a five-carbon sugar from six-carbon and three-carbon sugars.
Triose phosphate intermediates such as glyceraldehyde 3-phosphate cross into the _________ in exchange for phosphate through the action of an abundant triose phosphate-phosphate antiporter.
Cytoplasm
Two molecules of __________ are required for the synthesis of a member of the hexose monophosphate pool. Consequently, the cycle as presented must take place twice to yield a hexose monophosphate .
DHAP
The components of the Calvin cycle are called the (_______ _________) because, in contrast with the light reactions, these reactions do not directly depend on the presence of ________.
Dark reactions Light
The first step in the Calvin cycle is the.......
Fixation of CO 2 .
Fructose 6-phosphate formed from triose phosphates joins the glucose unit of UDP-glucose to form sucrose 6-phosphate
Fructose 6-phosphate
The 3-phosphoglycerate product of rubisco is next converted into ____________ __-____________, which readily isomerizes to glucose 1-phosphate and ___________ ___-___________.
Fructose 6-phosphate Glucose 6-phosphate.
Carbon dioxide can be used as......
Fuel
Transamination of glyoxylate then yields (_________), which then enters the mitochondria
Glycine
Glycolate is then oxidized to (______________) by glycolate oxidase , an enzyme with a flavin mononucleotide prosthetic group.
Glyoxylate
Hexosephosphates are made from phosphoglycerate , and ribulose1,5 -bisphosphate is regenerated
Hexosephosphates are made from phosphoglycerate , and ribulose1,5 -bisphosphate is regenerated
In the absence of CO 2 , rubisco binds to its substrate, ribulose 1 ,5 -bisphosphate, so tightly that its enzyme activity is __________.
Inhibited
The ATP requirement of rubisco activase coordinates rubisco activity with the __________ ________.
Light reactions.
Role of the magnesium ion in the rubisco mechanism: Ribulose 1 ,5 -bisphosphate binds to ______ through its keto group and an adjacent hydroxyl group
Mg2+
The light reactions lead to an increase in stromal pH (a decrease in the H+ concentration) , as well as an increase in the stromal concentrations of _______ ___________, and ___________ __________— changes that contribute to the activation of certain Calvin-cycle enzymes , located in the stroma
Mg2+ NADPH, and reduced ferredoxin
(There is a fundamental distinction between NADPH and NADH in biochemistry: _______- is oxidized by the respiratory chain to generate ATP, whereas _________ serves as a reductant in biosynthetic processes)
NADH NADPH
One carbon atom is lost as CO2. This process is called _________________ because O 2 is consumed and CO 2 is released.
Photorespiration
__________ lack the ability to transport hexose phosphates across the chloroplast membrane, but they are able to transport triose phosphates from chloroplasts to the cytoplasm.
Plants
The metal center plays a key role in binding ________ __,___-____________ and activating it so that it react s with CO2
Ribulose 1 ,5 -bisphosphate CO2
This highly exergonic reaction is catalyzed by ________ ___,___-_____________ ___________/___________) (usually called ____________ ), an enzyme located on the stromal surface of the thylakoid membranes of chloroplasts.
Ribulose 1,5-bisphosphate carboxylase/oxygenase (usually called rubisco
__________ requires a bound divalent metal ion for activity, usually magnesium ion.
Rubisco
The enzyme (_________ __________) uses ATP to induce structural changes in rubisco that allow release of the bound substrate and formation of the required carbamate .
Rubisco activase
Rubiscoactivaseis essential for rubiscoactivity
Rubiscoactivaseis essential for rubiscoactivity
Rubiscoactivity depends on magnesium and carbamate
Rubiscoactivity depends on magnesium and carbamate
Rubiscoalso catalyzes a wasteful oxygenasereaction: Catalytic imperfection
Rubiscoalso catalyzes a wasteful oxygenasereaction: Catalytic imperfection
Rubiscois activated by light-driven changes in proton and magnesium ion concentrations
Rubiscois activated by light-driven changes in proton and magnesium ion concentrations
__________ _________ of the Calvin cycle are required, because one carbon atom is reduced in each round.
Six rounds
__________ is synthesized and stored in chloroplasts.
Starch
_________, like its animal counterpart glycogen, is a polymer of glucose residues, but it is less branched than ____________ because it contains a smaller proportion of alpha -1 ,6 -glycosidic linkages
Starch Glycogen
Starch and sucrose are the major carbohydrate stores in plants
Starch and sucrose are the major carbohydrate stores in plants
In the __________ , the pH increases from 7 to 8, and the level of Mg 2+Mg 21 ions from the thylakoid space are released into the rises. Both effects are consequences of the light-driven pumping of protons into the thylakoid space.
Stroma
(The principal means of regulation is alteration of the __________ environment by the light reactions)
Stromal
__________ (common table sugar), a disaccharide, is synthesized in the cytoplasm.
Sucrose
Photosynthesis proceeds in two parts: ____________ and ___________.
The light reactions and the dark reaction
Thioredoxinplays a key role in regulating the Calvin cycle
Thioredoxinplays a key role in regulating the Calvin cycle
________ molecules of ATP and _____ molecules of NADPH are consumed in incorporating a single CO 2 molecule into a hexose such as glucose or fructose.
Three Two
Three ATP and two NADPH molecules are used to bring carbon dioxide to the level of a hexose
Three ATP and two NADPH molecules are used to bring carbon dioxide to the level of a hexose
A ___________ and an ____________ play the major role in the rearrangement of the carbon atoms.
Transketolase and an aldolase
____________ ___________ of ATP are expended in phosphorylating 12 molecules of 3-phosphoglycerate to 1 ,3 -bisphosphoglycerate, and 12 molecules of _________ are consumed in reducing 12 molecules of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate.
Twelve molecules NADPH
