Photosynthesis The carbon reactions (Dark Reactions)

Overall PerspectiveLight reactions:Harvest light energyConvert light energy to chemical energyDark Reactions:Expend chemical energyFix Carbon [convert CO2 to organic form]

At the end of the light reactionsThe reaction of the light reaction is:CO2 +H2O (CH2O) + O2 Recent estimates indicate that about 200 billion tones of CO2 (Mr = 44) are converted to biomass each year40 % of this is from marine phytoplanktonThe bulk of the carbon is incorporated into organic compounds by the carbon reducing reactions (dark reactions) of photosynthesis

At the end of the light reactionsThe reactions catalyzing the reduction of CO2 to carbohydrates are coupled to the consumption of NADPH and ATP by enzymes found in the stromafluid environmentThese reactions were thought to be independent of the light reactionsSo the name dark reactions stuckHowever, these chemical reactions are regulated by lightSo are called the carbon reactions of photosynthesis

Overview of the carbon reactionsThe Calvin cycle:Stage 1:CO2 accepted by Ribulose-1,5-bisphosphate.

This undergoes carboxylationHas a carboxyl group (-COOH) attached to it

At the end of stage 1, CO2 covalently linked to a carbon skeleton forming two 3-phosphycerate molecules.

Carboxylation: The first step is the most importantStep 1: The enzyme RUBISCO (Ribulose bis-phosphate carboxylase oxygenase) carries out this conversionRubisco accounts for 40% of the protein content of chloroplasts is likely the most abundant protein on EarthRubisco is, in fact, very inefficient, and that a mechanism has evolved to deal with this handicap

Overview of the carbon reactionsThe Calvin cycle:Stage 2:Each of the two 3-phosphycerate molecules are altered.

First phosphorylated through the use of the 3 ATPs generated during the light reaction.Then reduced through the use of the 2 NADPHs generated during the light reaction.

Forms a carbohydrateglyceraldehyde-3-phosphate

3-phosphycerate molecules are alteredFirst phosphorylated through the use of the 3 ATP molecules generated during the light reactionForms 1,3-bisphosphoglycerateThen reduced through the use of the 2 NADPH molecules generated during the light reactionForms glyceraldehyde-3-phosphateNote the formation of triose phosphate

Overview of the carbon reactionsThe Calvin cycle:Stage 3:Regeneration of Ribulose-1,5-bisphosphate.

This requires the coordinated action of eight reaction stepsAnd thus eight specific enzymes

Three molecules of Ribulose-1,5-bisphosphate are formed from the reshuffling of carbon atoms from triose phosphate.

Regeneration of Ribulose-1,5-bisphosphateThe Calvin cycle reactions regenerate the biochemical intermediates needed for operationMore importantly, the cycle is AutocatalyticRate of operation can be enhanced by increasing the concentration of the intermediates in the cycleSo, Calvin cycle has the metabolically desirable of producing more substrate than is consumedWorks as long as the produced triose phosphate is NOT diverted elsewhere (as in times of stress or disease)

Overview of the carbon reactionsThe Calvin cycle:The cycle runs six times:Each time incorporating a new carbon . Those six carbon dioxides are reduced to glucose:

Glucose can now serve as a building block to make:polysaccharides other monosaccharides fats amino acids nucleotides

Only one-sixth of the triose phosphate is used for polysaccharide productionSynthesis of polysaccharides, such as starch and sucrose, provide a sink Ensures an adequate flow of carbon atoms through the cycle IF CO2 is constantly availableDuring a steady rate of photosynthesis 5/6 of the triose phosphates are used for the regeneration of Ribulose-1,5-bisphosphate1/6 is transported to the cytosol for the synthesis of sucrose or other metabolites that are converted to starch in the chloroplast

Regulation of the Calvin cycleThe high energy efficiency of the Calvin cycle indicates that some form of regulation ensures that all intermediates in cycle:Are present at adequate concentrationsThe cycle is turned off when it is not needed in the darkRemember:These are the carbon reactions, NOT the dark reactions

Many factors regulate the Calvin cycle

Regulation of the Calvin cycle1: The pH of the stroma increases as protons are pumped out of it through the membrane assembly of the light reactions. The enzymes of the Calvin Cycle function better at this higher pH. 2: The reactions of the Calvin cycle have to stop when they run out of substrateas photosynthesis stops, there is no more ATP or NADPH in the stroma for the dark reactions to take place.

Regulation of the Calvin cycle3: The light reactions increase the permeability of the stromal membrane to required cofactors Mg ions are required for the Calvin Cycle.

4: Several enzymes of the Calvin Cycle are activated by the breaking of disulphide bridges of enzymes involved in the working of the cycle. the activity of the light reactions is communicated to the dark reactions by an enzyme intermediate

When conditions are not optimum

Photorespiration Occurs when the CO2 levels inside a leaf become low This happens on hot dry days when a plant is forced to close its stomata to prevent excess water loss If the plant continues to attempt to fix CO2 when its stomata are closedCO2 will get used up and the O2 ratio in the leaf will increase relative to CO2 concentrations When the CO2 levels inside the leaf drop to around 50 ppm, Rubisco starts to combine O2 with Ribulose-1,5-bisphosphate instead of CO2

PhotorespirationInstead of producing 2 3C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called phosphoglycolate is produced

The plant must get rid of the phosphoglycolate

The plant immediately gets rid of the phosphate group converting the molecule to glycolic acid

PhotorespirationThe glycolic acid is then transported to the peroxisome and there converted to glycinePeroxisomes are ubiquitous organelles that function to rid cells of toxic substances

The glycine (4 carbons) is then transported into a mitochondria where it is converted into serine (3 carbons)Releases CO2

PhotorespirationThe serine is then used to make other organic molecules

All these conversions cost the plant energy and results in the net lost of CO2 from the plant

75% of the carbon lost during the oxygenation of Rubisco is recovered during photorespiration and is returned to the Calvin cycle

The C4 Carbon cycle

The C4 carbon CycleThe C4 carbon Cycle occurs in 16 families of both monocots and dicots. CornMilletSugarcaneMaizeThere are three variations of the basic C4 carbon Cycle Due to the different four carbon molecule used

The C4 carbon CycleThis is a biochemical pathway that prevents photorespiration C4 leaves have TWO chloroplast containing cellsMesophyll cellsBundle sheath (deep in the leaf so atmospheric oxygen cannot diffuse easily to them)C3 plants only have Mesophyll cellsOperation of the C4 cycle requires the coordinated effort of both cell typesNo mesophyll cells is more than three cells away from a bundle sheath cellsMany plasmodesmata for communication

The C4 carbon CycleFour stages:Stage 1:In Mesophyll cellFixation of CO2 by the carboxylation of phosphenol-pyruvate (primary acceptor molecule) forms a C4 acid moleculeMalic acid and/or aspartateStage 2:Transport of the C4 acid molecule to the bundle sheath cell

The C4 carbon CycleStage 3:Decarboxylation of the C4 acid molecule (in bundle sheath)Makes a C3 acid molecule This generates CO2 This CO2 is reduced to carbohydrate by the Calvin cycle

Stage 4:The C3 acid molecule (pryuvate) is transported back to mesophyll cellsRegeneration of phosphenol-pyruvate

The C4 carbon CycleRegeneration of phosphenol-pyruvate consumes two high energy bonds from ATPMovement between cells is by diffusion via plasmodesmata Movement within cells is regulated by concentration gradients This system generates a higher CO2 conc in bundle sheath cells than would occur by equilibrium with the atmospherePrevents photorespiration!!!!!!!!!!

The C4 carbon CycleThe net effect of the C4 carbon Cycle is to convert a dilute CO2 solution in the mesophyll into a concentrated solution in the bundle sheath cellsThis requires more energy than C3 carbon plants

BUT This energy requirement is constant no matter what the environmental conditions

Allows more efficient photosynthesis in hotter conditions

Crassulacean Acid Metabolism (CAM Plants)

CAM PlantsThe CAM mechanism enables plants to improve water efficiencyCAM plantLoses 50 100 g water for every gram of CO2 gainedC4 plantLoses 250 300 g water for every gram of CO2 gainedC3 plantLoses 400 500 g water for every gram of CO2 gainedSimilar to C4 cycleIn CAM plants formation of the C4 acid is both temporally and spatially separated

CAM PlantsAt night:Stomata only open at night when it is cool

CO2 is captured by phosphenol-pyruvate carboxylase in the cytosol leaves become acidic

The malic acid formed is stored in the vacuoleAmount of malic acid formed is equal to the amount of CO2 taken in

CAM PlantsDuring the day:Stomata close, preventing water loss, and further uptake of CO2

Malic acid is transported to the chloroplast and decarboxylated to release CO2

This enters the Calvin cycle as it can not escape the leafPyruvate is converted to starch in the chloroplast regenerates carbon acceptor

Phosphorylation regulates phosphenol-pyruvate (PEP) carboxylaseCAM and C4 plants require a separation of the initial carboxylation from the following de-carboxylation

Diuranal regulation is usedIN CAM PLANTS:-Phosphorylation of the serine residue of phosphenol-pyruvate (PEP) carboxylase (Ser-OP) yields a form of the enzyme which is active at nightThis is relatively insensitive to malic acid

Photophorylation regulates phosphenol-pyruvate (PEP)carboxylaseDuring the day:

De-Phosphorylation of the serine (ser-OH) gives a form of the enzyme which is inhibited by malic acid

THIS IS THE OPPOSITE WAY AROUND FOR C4 PLANTS!

SummaryThe reduction of CO2 to carbohydrate via photosynthesis is coupled to the consumption of ATP and NADPHCO2 is reduced via the Calvin cycle Takes place in the stroma (soluble phase)CO2 and water combine with Ribulose-1,5-bisphosphate in the following reaction CO2 +H2O (CH2O) + O2Regeneration of the carrier is required for the cycle to continue

SummaryThe Calvin cycle requires the joint action of several light-dependant systemsChanges in ions (Mg+ and H+)Changes in effector metabolites (enzyme substrates)Changes in protein-mediated systems (rubisco activase)

Rubisco can also act as an oxygenaseThe carboxylation & oxygenation reactions take place at the active sites of rubisco.

SummaryC4 and CAM plants Prevent photorespiration!!!!!C4 leaves have TWO chloroplast containing cellsMesophyll cellsBundle sheathCAM Plants drastically reduce water lassCAM plantLoses 50 100 g water for every gram of CO2 gainedC4 plantLoses 250 300 g water for every gram of CO2 gainedC3 plantLoses 400 500 g water for every gram of CO2 gained

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