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Calvin Cycle TED-ED - Biology

Calvin Cycle TED-ED - Biology


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Nature’s Smallest Factory

https://youtu.be/0UzMaoaXKaM

  1. Energy exists in the form of ____________________ made by the plant.
  2. Plants get their carbon from the __________________.
  3. Photosynthesis occurs in two steps, the second is the _____________________.
  4. RuBP contains __________ carbons.
  5. The enzyme, _________________________ builds an initial 6 carbon sequence.
  6. This sequence split into two short chains called _________.
  7. NADPH transfers a __________________ to those molecules, they become G3P.
    Glucose needs 6 carbons to form, made from two molecules G3P. Sugar has been manufactured, but not quite…..
  8. The original RuBP must be _________________________.
  9. How many production lines are going on at the same time? _____
  10. How many carbons exist from all of these production lines? ______
  11. How many of these are used to make glucose? _____ To make RuBP? ______
  12. The molecular mix and match ensures that _______ RuBP are regenerated.

Summarize the Stage

Stage

Description

Molecules involved

x3

x3

Glucose

x2

(It takes 2 turns to fix 6 carbon atoms from CO2)

reduction
glucose

regeneration of RUBP
carbon fixation

3 molecules of CO2
RuBisCO

6 ATP → 6 ADP
3 ATP → 3 ADP


Calvin Cycle: History and Phases (With Diagram)

The cycle was discovered by Calvin, Benson and their colleagues in California, U.S.A. They fed Chlorella and Scenedesmus with radioactive 14 C in carbon dioxide. Radio­active carbon, 14 C has a half life of 5568 years.

Therefore, the path of CO2 fixation can be easily traced with its help. Algal suspension illuminated and carrying out photosynthesis with normal carbon dioxide was supplied with 14 CO2. The alga was killed at intervals in near boiling methanol.

It immediately stopped photosynthetic activity due to denaturation of enzymes. Alcohol was evaporated and after crushing the alga, the product was made into paste. The paste was placed on paper chromatogram and the different compounds were separated by two dimensional chromatography first developed by Martin and Synge (1941).

An X-ray film was then pressed against the paper chromatogram. The film developed spots where radioactive compounds were present. The process is called autoradiography. The radioactive compounds were identified by comparing their position on the chromatogram with standard chemicals.

Calvin and co-workers found that after three seconds, radioactivity appeared in phosphoglyceric acid or PGA. Phosphoglyceric acid is, therefore, the first stable product of photosynthesis. Radioactivity was also found out to be present in only one carbon of this compound which happened to be the first one.

Apparently only the first carbon group of the chemical came from CO2 while the rest were contributed by some acceptor molecule. A number of other compounds having radioactivity were found after intervals of 5, 10, 15 and 30 seconds.

They included hexoses, tetroses, pentoses, heptoses. After 60 seconds, all the three carbon atoms of PGA were radioactive indicating cycling of reactions. After many painstaking calculations, Calvin worked out the pathway of CO2 fixation. The primary acceptor molecule was found out by Basham to be ribulose-1, 5- bi-phosphate or RuBP.

Phases of Calvin Cycle:

Photosynthetic Carbon Reduction (PCR) Cycle or Calvin cycle occurs in all photo­synthetic plants whether they have C3 or C4 pathways. It is divided into the following three phases— carboxylation, glycolytic reversal and regeneration of RuBP (Fig. 13.21).

Carboxylation is the addition of carbon dioxide to another substance called acceptor. Photosynthetic carboxylation requires ribulose-1, 5-bi-phosphate or RuBP as acceptor of carbon dioxide and RuBP carboxylase-oxygenase or RuBisCo as enzyme. The enzyme was previously called carboxydismutase.

Rubisco is the most abundant protein of the biological world. It constitutes 16% of chloropiast proteins (40% of soluble leaf proteins). However, it is a slowest enzyme with a turnover of 3 carbon dioxide molecules per second. Rubisco is located in the stroma on the outer surface of thylakoid membranes.

Carbon dioxide combines with ribulose-1, 5-bio-phosphate to produce a transient inter­mediate compound called 2-carboxy 3-keto 1, 5-bi-phosphoribotol. The intermediate splits up immediately in the presence of water to form the two molecules of 3-phosphoglyceric acid or PGA. It is the first stable product of photosynthesis.

2. Glycolytic Reversal:

The processes involved in this step or phase are reversal of the processes found during glycolysis part of respiration. Phosphoglyceric acid or PGA is further phosphorylated by ATP with the help of enzyme triose phosphate kinase (phosphoglycerate kinase). It gives rise to 1, 3-biphosphoglyeerie acid.

Biphosphoglyceric acid is reduced by NADPH through the agency of enzyme glyceraldehyde 3-phosphate dehydrogenase (triose phosphate dehydrogenase). It produces glyceraldehyde 3-phosphate or 3-phosphoglyceraldehyde.

Glyceraldehyde-3-phosphate is a key product which is used in synthesis of both carbo­hydrates and fats. For forming carbohydrates, say glucose, a part of it is changed into its isomer called dihydroxyacetone-3-phosphate. The enzyme that catalyses the reaction is phosphotriose isomerase.

The two isomers condense in the presence of enzyme aldolase forming fructose 1,6- bi-phosphate.

Fructose 1,6-bi-phosphate (FBP) loses one phosphate group, forms fructose 6-phosphate (F 6-P) which is then changed to glucose-6- phosphate (G 6-P). The latter can produce glucose or become part of sucrose and polysaccharide.

As glucose is a six carbon compound, six turns of Calvin cycle are required to synthesise its one molecule.

3. Regeneration of RuBP:

Fructose 6-phosphate (F 6-P) and glyceraldehyde 3-phos­phate (GAP) react to form erythrose 4-phosphate (E 4-P) and xylulose 5-phosphate (X 5-P). Erythrose 4-phosphate combines with dihydroxy acetone 3-phosphate to produce sedoheptulose 1: 7 diphosphate (SDP)which loses a molecule of phosphate and gives rise to sedoheptulose 7-phosphate (S 7-P).

Sedoheptulose 7-phosphate reacts with glyceraldehyde 3- phosphate to produce xylulose 5-phosphate (X 5-P) and ribose 5-phosphate. (R 5-P) Both of these are changed to their isomer ribulose 5-phosphate (Ru 5-P). Ribulose 5-phosphate picks up a second phosphate from ATP to become changed into ribulose 1, 5 bi-phosphate (RuBP).


Label the Calvin Cycle

(It takes 6 turns to fix 6 carbon atoms from CO2)

reduction
glucose
regeneration of RUBP
carbon fixation
6 ATP → 6ADP
3 ATP → 3ADP
3 molecules of CO2
RuBisCO

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Stages of the Calvin cycle

Carbon fixation

The Calvin cycle begins when a CO2 molecule is attached to a five-carbon sugar called ribulose biphosphate (RuBP). The enzyme that catalyses this process is called Ribulose biphosphate carboxylase (or rubisco). Perhaps unsurprisingly, rubisco is the most abundant protein on earth.

The product of this reaction is a highly unstable 6 carbon intermediate that immediately splits into two 3 carbon sugars (3-phosphoglycerate, also called 3-PGA). For each molecule of CO2, 2 molecules of 3-PGA are produced. The number of carbon atoms remains the same.

Reduction

A phosphate group from ATP is incorporated into each molecule of 3-PGA, becoming 1,3-biphosphoglycerate. Following this, 1,3-biphosphoglycerate is reduced, and a phosphate is lost, becoming glyceraldehyde-3-phosphate (G3P). The electron pair required for this reduction comes from NADPH. Energy is provided for this process when ATP is converted to ADP, and when NADPH is converted to NADP+. Both of these molecules then return to light-dependent reactions to be reused.

G3P has 3 carbon atoms, therefore it takes 3 rounds of the carbon cycle to obtain enough carbon to export one molecule of G3P. For every molecule of CO2 that enters the cycle, there are 6 molecules of G3P produced. However, this is not a net production, as there are 3 molecules of 5 carbon RuBP required for every molecule of G3P formed. Only 1 molecule of G3P exits the cycle to be used in the plant cell it is the starting material for pathways synthesizing more complex carbohydrates. The other 5 molecules are recycled to regenerate RuBP. These numbers are illustrated more clearly in the below
figure.

Regeneration

In step 3, RuBP is regenerated. This occurs through a complex sequence of reactions that rearranges 5 G3P (5x 3 carbons) molecules into 3 molecules of RuBP (3x 5 carbons). This process takes at least 3 molecules of ATP.

Again, each turn on the Carbon cycle makes 2 G3Ps, so 3 carbon dioxide molecules make 6 G3Ps. Whilst 1 is exported to the cytoplasm, the remaining 5 are used to regenerate RuBP, allowing the cycle to begin again. For the net synthesis of 1 G3P molecule, the Calvin cycle requires a total of 9 molecules of ATP and 6 molecules of NADPH.


The Innerworkings of the Calvin Cycle

Figure 1. Light-dependent reactions harness energy from the sun to produce ATP and NADPH. These energy-carrying molecules travel into the stroma where the Calvin cycle reactions take place.

In plants, carbon dioxide (CO2) enters the chloroplast through the stomata and diffuses into the stroma of the chloroplast—the site of the Calvin cycle reactions where sugar is synthesized. The reactions are named after the scientist who discovered them, and reference the fact that the reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery (Figure 1).

The Calvin cycle reactions (Figure 2) can be organized into three basic stages: fixation, reduction, and regeneration. In the stroma, in addition to CO2, two other chemicals are present to initiate the Calvin cycle: an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP). RuBP has five atoms of carbon and a phosphate group on each end.

RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds. This process is called carbon fixation, because CO2 is “fixed” from its inorganic form into organic molecules.

ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called G3P. This type of reaction is called a reduction reaction, because it involves the gain of electrons. A reduction is the gain of an electron by an atom or molecule. The molecules of ADP and NAD + , resulting from the reduction reaction, return to the light-dependent reactions to be re-energized.

One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose (C6H12O6). Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. ATP is also used in the regeneration of RuBP.

Figure 2. The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule. In stage 2, the organic molecule is reduced. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue.

In summary, it takes six turns of the Calvin cycle to fix six carbon atoms from CO2. These six turns require energy input from 12 ATP molecules and 12 NADPH molecules in the reduction step and 6 ATP molecules in the regeneration step.

Check out this animation of the Calvin cycle. Click Stage 1, Stage 2, and then Stage 3 to see G3P and ATP regenerate to form RuBP.

Evolution in Action: Photosynthesis

Figure 3. Living in the harsh conditions of the desert has led plants like this cactus to evolve variations in reactions outside the Calvin cycle. These variations increase efficiency and help conserve water and energy. (credit: Piotr Wojtkowski)

The shared evolutionary history of all photosynthetic organisms is conspicuous, as the basic process has changed little over eras of time. Even between the giant tropical leaves in the rainforest and tiny cyanobacteria, the process and components of photosynthesis that use water as an electron donor remain largely the same. Photosystems function to absorb light and use electron transport chains to convert energy. The Calvin cycle reactions assemble carbohydrate molecules with this energy.

However, as with all biochemical pathways, a variety of conditions leads to varied adaptations that affect the basic pattern. Photosynthesis in dry-climate plants (Figure 3) has evolved with adaptations that conserve water. In the harsh dry heat, every drop of water and precious energy must be used to survive. Two adaptations have evolved in such plants. In one form, a more efficient use of CO2 allows plants to photosynthesize even when CO2 is in short supply, as when the stomata are closed on hot days. The other adaptation performs preliminary reactions of the Calvin cycle at night, because opening the stomata at this time conserves water due to cooler temperatures. In addition, this adaptation has allowed plants to carry out low levels of photosynthesis without opening stomata at all, an extreme mechanism to face extremely dry periods.


Top 3 Stages of Calvin Cycle (With Diagram)

(c) Formation of hexose sugar and regeneration of RuBP which consumes addi­tional ATP, so that the cycle continues (Fig. 11.18).

Detailed steps of Calvin-cycle (C3-cycle) or PCR-cycle which have also been shown in Fig. 11.18A, are as follows:

(a) Carboxylation:

(i) The CO2 is accepted by ribulose 1, 5-bisphosphate (RuBP) already present in the cells and a 6-carbon addition compound is formed which is unstable. It soon gets hydrolysed into 2 molecules of 3-phosphoglyceric acid (3PGA). Both these reactions take place in the presence of ribulose bisphosphate carboxylase (Rubisco). 3-Phosphoglyceric acid is the first stable prod­uct of dark reaction of photosynthesis.

(b) Reduction:

(ii) 3-Phosphoglyceric acid is reduced to 3-phosphoglyceraldehyde by the assimilatory power (generated in light reaction) in the presence of triose phosphate dehydrogenase.

This reac­tion takes place in two steps:

(c) Formation of Hexose Sugar and Regeneration of RuBP:

(iii) Some of the molecules of 3-phosphoglyceraldehyde isomerise into dihydroxyaeetone phosphate, both of which then unite in the presence of the enzyme aldolase to form fruc­tose 1, 6-bisphophate.

(iv) Fructose 1, 6-bisphosphate is converted into fructose 6-phosphate in the presence of phosphatase.

(v) Some of the fructose-6-phosphate (hexose sugar) is tapped off from the Calvin cycle and is converted into glucose, sucrose, and starch. Sucrose is synthesized in cytosol while starch is synthesized in chloroplast.

(vi) Some of the molecules of 3-phosphoglyceraldehyde produced in step (ii) instead of forming hexose sugars, are diverted to regenerate ribulose 1, 5-bisphosphate in the system as follows:

(vii) 3-Phosphoglyceraldehyde reacts with fructose-6-phosphate in the presence of en­zyme transketolase to form erythrose-4-phosphate (4-C atoms sugar) and xylulose 5-phosphate (5-C atoms sugar).

(viii) Erythrose-4-phosphate combines with dihydroxyaceotone phosphate in the presence of the enzyme aldolase to form sedoheptulose 1, 7-bisphosphate (7-C atoms sugar).

(ix) Sedoheptulose 1, 7-bisphosphate loses one phosphate group in the presence of phosphatase to form sedoheptulose-7-phosphate.

(x) Sedoheptulose-7 phosphate reacts with 3-phosphoglyceraldehyde in the presence of transketolase to form xylulose-5-phosphate and ribose-5-phosphate (both 5-carbon atoms sugars).

(xi) Xylulose-5-phosphate is converted into another 5-C atoms sugar ribulose-5-phosphate in the presence of the enzyme phosphoketopentose epimerase.

(xii) Ribose-5-phosphate is also converted into ribulose-5-phosphate. The reaction is catalysed by phosphopentose isomerase.

(xiii) Ribulose-5-phosphate is finally converted into ribulose 1, 5-bisphosphate in the presence of phosphopentose kinase and ATP, thus completing the Calvin cycle.

Structural formulae of various 4, 5 and 7-C atoms sugars involved in the Calvin cycle are given Fig. 11.19.

Because first visible product of this cycle is 3-phosphoglyceric acid which is a 3-C compound, Calvin cycle is also known as C3-pathway. (Recent studies with algal cells, leaves and isolated chloroplasts have shown that ‘dark reactions’ of photosynthesis are not completely independent of light.

Several critical enzymes in the carbon reduction cycle are light activated in the dark, they are either inactive or exhibit low activity. Activity of the enzyme Rubisco declines rapidly when light is turned off and re­gain slowly when light is turned on. At least four other enzymes of the PCR-cycle are known to be stimulated by light viz., 3-PGAld dehydrogenase (reaction ii), fructose 1, 6-bis phosphatase (reaction iv), Sedoheptulose 1, 7- bisphosphatase, and Ribulose 5-phosphate kinase (reaction xiii). Therefore, the designation “dark reaction” to the photosynthetic carbon reduction reac­tions is now considered as unappropriate).


Products of the Calvin cycle (CIE A-level Biology)

A Science teacher by trade, I've also been known to be found teaching Maths and PE! However, strange as it may seem, my real love is designing resources that can be used by other teachers to maximise the experience of the students. I am constantly thinking of new ways to engage a student with a topic and try to implement that in the design of the lessons.

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This fully-resourced lesson describes the conversion of Calvin cycle intermediates to carbohydrates, lipids and amino acids. The engaging and detailed PowerPoint and accompanying resources have been primarily designed to cover point 13.1 (h) of the CIE A-level Biology specification concerning the uses of GP and TP but as the lesson makes continual references to biological molecules, it can act as a revision tool for a lot of the content of topic 2.

The previous lesson described the three stages of the Calvin cycle and this lesson builds on that understanding to demonstrate how the intermediates of the cycle, GP and TP, are used. The start of the lesson challenges the students to identify two errors in a diagram of the cycle so that they can recall that most of the TP molecules are used in the regeneration of ribulose bisphosphate. A quiz version of Pointless runs throughout the lesson and this is used to challenge the students to recall a biological molecule from its description. Once each molecule has been revealed, time is taken to go through the details of the formation and synthesis of this molecule from TP or from GP in the case of fatty and amino acids. The following molecules are considered in detail during this lesson:

  • glucose (and fructose and galactose)
  • sucrose
  • starch and cellulose
  • glycerol and fatty acids
  • amino acids
  • nucleic acids

A range of activities are used to challenge their prior knowledge of these molecules and mark schemes are always displayed for the exam-style questions to allow the students to assess their understanding.

As detailed above, this lesson has been specifically written to tie in with the earlier lessons in this topic on the structure of the chloroplast, the light-dependent stage of photosynthesis and the Calvin cycle.

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Topics 12 & 13: Respiration and photosynthesis (CIE A-level Biology)

Respiration and photosynthesis are two of the most commonly-assessed topics in the terminal A-level exams but are often poorly understood by students. These 14 lessons have been intricately planned to contain a wide range of activities that will engage and motivate the students whilst covering the key detail to try to deepen their understanding and includes exam-style questions so they are fully prepared for these assessments. The following specification points in topics 12 and 13 of the CIE A-level Biology course are covered by these lessons: * The need for energy in living organisms * The features of ATP * The synthesis of ATP by substrate-level phosphorylation in glycolysis and the Krebs cycle * The roles of the coenzymes in respiration * The synthesis of ATP through the electron transport chain in the mitochondria and chloroplasts * The relative energy values of carbohydrates, lipids and proteins as respiratory substrates * Determining the respiratory quotient from equations for respiration * The four stages of aerobic respiration * An outline of glycolysis * When oxygen is available, pyruvate is converted into acetyl CoA in the link reaction * The steps of the Krebs cycle * Oxidative phosphorylation * The relationship between the structure and function of the mitochondrion * Distinguish between aerobic and anaerobic respiration in mammalian tissue and in yeast cells * Anaerobic respiration generates a small yield of ATP and builds up an oxygen debt * The products of the light-dependent stage are used in the Calvin cycle * The structure of a chloroplast and the sites of the light-dependent and light-independent stages of photosynthesis * The light-dependent stage of photosynthesis * The three stages of the Calvin cycle * The conversion of Calvin cycle intermediates to carbohydrates, lipids and amino acids * Explain the term limiting factor in relation to photosynthesis * Explain the effects of changes in light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis * Explain how an understanding of limiting factors is used to increase crop yields in protected environments Due to the detail of these lessons, it is estimated that it will take up to 2 months of allocated A-level teaching time to cover the detail included in the slides of these lessons If you would like to sample the quality of the lessons, download the roles of the coenzymes, the Krebs cycle and the products of the Calvin cycle lessons as these have been shared for free

Topic 13: Photosynthesis (CIE A-level Biology)

This bundle contains 5 fully-resourced lessons which are highly detailed and will engage and motivate the students whilst the following content that is set out in topic 13 of the CIE A-level Biology specification is covered: Topic 13.1 * Energy transferred as ATP and reduced NADP from the light dependent stage is used during the Calvin cycle to produce complex organic molecules * The sites of the light-dependent and light-independent stages of photosynthesis * The light-dependent stage as the photoactivation of chlorophyll, the photolysis of water and the transfer of energy to ATP and reduced NADP * Cyclic and non-cyclic photophosphorylation * The three main stages of the Calvin cycle * The conversion of Calvin cycle intermediates to carbohydrates, lipids and amino acids Topic 13.2 * Explain the term limiting factor in relation to photosynthesis * Explain the effects of changes in light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis * Explain how an understanding of limiting factors is used to increase crop yields in protected environments The lesson PowerPoints and accompanying resources contain a wide range of tasks which include exam-style questions, whole class discussion periods and quiz competitions which are designed to introduce key terms and values in a memorable way.


The Interworkings of the Calvin Cycle

In plants, carbon dioxide (CO2) enters the chloroplast through the stomata and diffuses into the stroma of the chloroplast—the site of the Calvin cycle reactions where sugar is synthesized. The reactions are named after the scientist who discovered them, and reference the fact that the reactions function as a cycle (Figure 1).

Figure 1 Light-dependent reactions harness energy from the sun to produce ATP and NADPH. These energy-carrying molecules travel into the stroma where the Calvin cycle reactions take place.

RuBisCO is an enzyme that catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds. This process is called carbon fixation, because CO2 is “fixed” from its inorganic form into organic molecules.

ATP and NADPH, which were made during the light dependent reactions, use their stored energy to convert carbon dioxide into a three-carbon compound called G3P. The molecules of ADP and NAD + , which are low-energy molecules, return to the light-dependent reactions to be re-energized.

One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose (C6H12O6). Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. ATP is also used in the regeneration of RuBP.

Figure 2 The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule. In stage 2, the organic molecule is reduced. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue.

In summary, it takes six turns of the Calvin cycle to fix six carbon atoms from CO2. These six turns require energy input from 12 ATP molecules and 12 NADPH molecules in the reduction step and 6 ATP molecules in the regeneration step.


Calvin Cycle TED-ED - Biology

Article Summary:

The Calvin cycle is what is referred to as the dark reactions in photosynthesis. It is divided into three phases. The first phase is carboxylation, where CO2 reacts with 3 molecules of rubisco to carboxylate ribulose-1,5-bisphosphate to yield 6 molecules of 3-phosphoglycerate. The second phase is reduction of 3-phosphoglyceraldehyde to give glyceraldehyde-3-phosphate. Reduction phase is actually divided into two steps where firstly, ATP phosphorylates 3-phosphoglycerate to give 1,3-bisphosphoglycerate which is then reduced to 6 molecules of glyceraldehyde-3-phosphate using energy from NADPH. One molecule of glyceraldehyde-3-phosphate is converted into triose phosphates which are in turn converted to the carbohydrates sucrose and starch The last phase is the regeneration phase, where the CO2 acceptor ribulose-1,5-bisphosphate is regenerated from the other 5 molecules of glyceraldehyde -3-phosphate.These reactions are catalysed by enzymes which will be looked into in this article and how their regulation of the Calvin cycle impact on photosynthesis

Phase 1: Carboxylation

In a chemical reaction catalysed by ribulose-1,5-bisphosphate carboxylase-oxygenase (rubisco), 3 molecules of CO2 binds to 3 molecules of ribulose-1,5-bisphosphate (RuBP) in the presence of three molecules of water to give 6 molecules of 3-phosphoglyceraldehyde and 6H+.
This initial step is very important as it incorporates CO2 and thus initially many studies focused on this reaction in order to try and enhance photosynthesis. This was initially done by trying to reduce or deactivate the oxygenase activity of rubisco, as CO2 and O2 competes for the same site to bind, and thus, if this is manipulated so that only CO2 can be bound, then there will be more CO2 to reduce to carbohydrates. But these manipulations have not successful as yet.

More studies have been done now focused on manipulating levels of rubisco. The studies showed that rubisco itself regulates photosynthesis by being regulated by levels of CO2, light intensity and also nitrogen levels. In a study by Raines (2003) using rubisco antisense plants with reduced rubisco, it was observed that reducing levels of rubisco in plants under the same conditions that the plant was grown under, does not have any significant effect on photosynthesis. However when a tobacco antisense plant grown under ambient CO2 and moderate light was exposed to saturating light and/or saturating CO2,an increase in rubisco control over photosynthesis was observed .This led to the conclusion that rubisco is regulated by availability of CO2 and by light intensity and thus in turn regulated photosynthesis.

a) 6 3-phosphoglyceraldehyde + 6ATP resulting in 6 1,3-bisphosphoglycerate +6ADP
Catalysed by 3-phosphoglycerate kinase

b) 6 1,3-bisphosphoglycerate +6NADPH +H+ producing 6 glyceraldehyde-3-phosphate + 6NADP+ + 6PiCatalysed by GAPDH.

Raines (2003) reported that no effect on photosynthesis was observed when antisense tobacco plants with reduced GAPDH were grown in highlight greenhouse conditions, but some effect was observed when GADPH activity was reduced to 35% below the wild-type plant.

From the 6 molecules of glyceraldehyde-3-phosphate produced in reduction phase, 5 of them goes into the regeneration phase to regenerate the 3 molecules of the CO2 binding RuBP and 1 molecule goes towards carbohydrate synthesis( sugars and other compounds)

This occurs in a lot of reaction steps and each and the enzyme catalysing it are listed below and if studies have been done to see how the particular enzyme regulates photosynthesis, then these studies and their findings are discussed.

a) 2 glyceraldehyde-3-phosphate producing 2 dihydroxyacetone-3-phosphate, catalysed by triose phosphate isomerase

b) Glyceraldehyde-3-phosphate+ dihydroxyacetone-3-phosphate producing Fructose-1,6-bisphosphate, catalysed by aldolase

Aldolase levels in plants have been observed to have significant control on photosynthesis, looking at carbon partitioning (Raines, 2003). Studies have shown that reduced levels of aldolase (in aldolase antisense plants) result in reduced levels of carbon accumulation looking at levels of starch, but was only shown to have an effect on sucrose levels when its activity was reduced to 30% of the wild type levels. This study showed for the first time that, a non-regulate enzyme which catalyses a freely reversible reaction, can have significant effect or control on photosynthetic carbon flux.

c) Fructose-1,6-bisphosphate + H2O producing fructose-6-phosphate +Pi, catalysed by Fructose-1,6-bisphosphate phosphatase (FBPase)

FBPase is a key regulated enzyme and some studies have been done to see if it has any effect on photosynthesis (Raines, 2003). As for GAPDH, it was observed that FBPase does not have a significant effect on photosynthesis in antisense potato plants but some effect was only observed when FBPase activity was reduced to less than 34% of the wild-type.

d) Fructose-6-phosphate + glyceraldehyde-3-phosphate producing erythrose-4-phosphate + xylulose-5-phosphate, catalysed by transketolase

Partitioning of carbon between sucrose and starch is affected by reductions in transketolase. Studies have shown that as light intensity increases, so does the effect of transketolase on carbon partitioning in antisense tobacco plants. The actual observed results were that levels of sucrose decreased as transketolase activity decreases. In regard to starch accumulation, effects were only observed when activity was reduced to below 60% of the wild-type (Raines 2003). Most studies done using antisense plants in the Calvin cycle, showed a trend towards partitioning of carbon towards starch biosynthesis, but these results show carbon partitioning in favour of sucrose instead.

e) Erythrose-4-phosphate + dihydroxyacetone-3-phosphate producing sedoheptulose-1,7-bisphosphate, catalysed by aldolase

f) Sedoheptulose-1,7-bisphosphate + H2O producing seduheptulose-7-phosphate + Pi, catalysed by sedoheptulose-1,7-bisphosphate phosphatase (SBPase).

SBPase is also a key, regulated enzyme and its effect on photosynthesis, have been studied. The studies showed that small decreases in SBPase activity, lead to reduced photosynthetic carbon fixation in SBPase antisense tobacco plants (Raines, 2003).This was shown by observations made that as SBPase activity decreases, so does starch levels and that starch is barely detectable in plants with less than 20% of wild-type SBPase activity.

g) Sedoheptulose-7-phosphate+ glyceraldehyde-3-phosphate producing ribose-5-phosphate + xylulose-5-phosphate, catalysed by transketolase

h) 2 xylulose-5-phosphate producing 2 ribulose-5-phosphate, catalysed by ribose-5-phosphate epimorase

i) Ribose-5-phosphate producing ribulose-5-phosphate, catalysed by ribose-5-phosphate isomerase

Then the last reaction which is catalysed by ribulose-5-phosphate kinase also called phosphoribulokinase PRKase) is:

j) 3 ribulose-5-phosphate + 3ATP producing 3 ribulose-1,5-phosphate +3ADP +3H+

PRKase is also a key, regulated enzyme and like FBPase and GAPDH, has not been observed to have any significant effect on photosynthesis (Raines 2003).It was observed that activity of PRKase have to be reduced to less than 20% than the wild-type plants, in PRKase antisense tobacco plants, before a decrease in photosynthesis can be observed, when the plants were grown in low light or in nitrogen deficient conditions.

The net equation of the Calvin cycle from all the three phases is thus

3CO2 +5H2O +6NADPH +9ATP producing glyceraldehyde-3-phosphate + 6NADP+ + 3H+ + 9ADP +8Pi

The molecule of glyceraldehyde-3-phosphate that goes into the production of carbohydrate is converted via a cascade of reactions which are also catalysed by different enzymes.

Some of the enzymes mentioned above that did not seem to have any regulatory effect on photosynthesis in the Calvin cycle, can have regulatory effect in other pathways of photosynthesis, thus regulating it in a way. The carbon from the Calvin cycle is partitioned inside the cell into either sucrose synthesis, which is the main transportation molecule of sugars in plants or into starch biosynthesis which is the main storage form of carbohydrates is plants. Therefore these two biosynthesis pathways can have a regulatory effect in photosynthesis and thus they can also be looked at in order to see their effect. By genetic manipulation of these pathways the rate of photosynthesis can be regulated by bioengineers like in the case of sugarcane or potato where high photosynthesis rates are needed for sucrose and starch accumulation respectively.

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