Photosynthesis
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Photosynthesis. The carbon reactions (Dark Reactions). Overall Perspective. Dark Reactions : Expend chemical energy Fix Carbon [convert CO2 to organic form]. Light reactions : Harvest light energy Convert light energy to chemical energy. At the end of the light reactions.

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Photosynthesis

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Photosynthesis

The carbon reactions

(Dark Reactions)


Overall Perspective

  • Dark Reactions:

    • Expend chemical energy

    • Fix Carbon [convert CO2 to organic form]

  • Light reactions:

    • Harvest light energy

    • Convert light energy to chemical energy


At the end of the light reactions

  • The 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 year

    • 40 % of this is from marine phytoplankton

    • The bulk of the carbon is incorporated into organic compounds by the carbon reducing reactions (dark reactions) of photosynthesis


At the end of the light reactions

  • The reactions catalyzing the reduction of CO2 to carbohydrates are coupled to the consumption of NADPH and ATP by enzymes found in the stroma

    • fluid environment

  • These reactions were thought to be independent of the light reactions

    • So the name “dark reactions”stuck

  • However, these chemical reactions are regulated by light

    • So are called the “carbon reactions” of photosynthesis


Overview of the carbon reactions

  • The Calvin cycle:

  • Stage 1:

    • CO2 accepted by Ribulose-1,5-bisphosphate.

    • This undergoes carboxylation

      • Has 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 important

  • Step 1: The enzyme RUBISCO (Ribulose bis-phosphate carboxylase oxygenase) carries out this conversion

  • Rubisco accounts for 40% of the protein content of chloroplasts

    • is likely the most abundant protein on Earth

  • Rubisco is, in fact, very inefficient, and that a mechanism has evolved to deal with this handicap


Overview of the carbon reactions

  • The 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 carbohydrate

      • glyceraldehyde-3-phosphate


3-phosphycerate molecules are altered

  • First phosphorylated through the use of the 3 ATP molecules generated during the light reaction

    • Forms 1,3-bisphosphoglycerate

  • Then reduced through the use of the 2 NADPH molecules generated during the light reaction

    • Forms glyceraldehyde-3-phosphate

  • Note the formation of triose phosphate


Overview of the carbon reactions

  • The Calvin cycle:

  • Stage 3:

    • Regeneration of Ribulose-1,5-bisphosphate.

    • This requires the coordinated action of eight reaction steps

      • And 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-bisphosphate

  • The Calvin cycle reactions regenerate the biochemical intermediates needed for operation

  • More importantly, the cycle is Autocatalytic

    • Rate of operation can be enhanced by increasing the concentration of the intermediates in the cycle

  • So, Calvin cycle has the metabolically desirable of producing more substrate than is consumed

    • Works as long as the produced triose phosphate is NOT diverted elsewhere (as in times of stress or disease)


Overview of the carbon reactions

  • The 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 production

  • Synthesis of polysaccharides, such as starch and sucrose, provide a sink

    • Ensures an adequate flow of carbon atoms through the cycleIFCO2 is constantly available

  • During a steady rate of photosynthesis 5/6 of the triose phosphates are used for the regeneration of Ribulose-1,5-bisphosphate

  • 1/6 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 cycle

  • The high energy efficiency of the Calvin cycle indicates that some form of regulation ensures that all intermediates in cycle:

    • Are present at adequate concentrations

    • The cycle is turned off when it is not needed in the dark

  • Remember:

    • These are the “carbon reactions”, NOT the “dark reactions”

  • Many factors regulate the Calvin cycle


Regulation of the Calvin cycle

  • 1: 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 substrate

    • as photosynthesis stops, there is no more ATP or NADPH in the stroma for the dark reactions to take place.


Regulation of the Calvin cycle

  • 3: 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 closed

    • CO2 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


Photorespiration

  • Instead of producing 2 3C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called phosphoglycolateis produced

  • The plant must get rid of the phosphoglycolate

  • The plant immediately gets rid of the phosphate group

    • converting the molecule to glycolic acid


Photorespiration

  • Theglycolic acid is then transported to the peroxisome and there converted to glycine

    • Peroxisomes 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


Photorespiration

  • The 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 Cycle

  • The C4 carbon Cycle occurs in 16 families of both monocots and dicots.

    • Corn

    • Millet

    • Sugarcane

    • Maize

  • There are three variations of the basic C4 carbon Cycle

    • Due to the different four carbon molecule used


The C4 carbon Cycle

  • This is a biochemical pathway that prevents photorespiration

  • C4 leaves have TWO chloroplast containing cells

    • Mesophyll cells

    • Bundle sheath (deep in the leaf so atmospheric oxygen cannot diffuse easily to them)

      • C3 plants only have Mesophyll cells

  • Operation of the C4 cycle requires the coordinated effort of both cell types

    • No mesophyll cells is more than three cells away from a bundle sheath cells

      • Many plasmodesmata for communication


The C4 carbon Cycle

  • Four stages:

  • Stage 1:

  • In Mesophyll cell

    • Fixation of CO2 by the carboxylation of phosphenol-pyruvate (primary acceptor molecule)

    • forms a C4 acid molecule

    • Malic acid and/or aspartate

  • Stage 2:

    • Transport of the C4 acid molecule to the bundle sheath cell


The C4 carbon Cycle

  • Stage 3:

    • Decarboxylationof theC4 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 cells

    • Regeneration of phosphenol-pyruvate


The C4 carbon Cycle

  • Regeneration of phosphenol-pyruvate consumestwo high energy bonds from ATP

  • Movement 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 atmosphere

    • Prevents photorespiration!!!!!!!!!!


The C4 carbon Cycle

  • The 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 cells

    • This 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 Plants

  • The CAM mechanism enables plants to improve water efficiency

    • CAM plant

      • Loses 50 – 100 g water for every gram of CO2 gained

    • C4 plant

      • Loses 250 – 300 g water for every gram of CO2 gained

    • C3 plant

      • Loses 400 – 500 g water for every gram of CO2 gained

  • Similar to C4 cycle

    • In CAM plants formation of the C4 acid is both temporally and spatially separated


CAM Plants

  • At 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 vacuole

    • Amount of malic acid formed is equal to the amount of CO2 taken in


CAM Plants

  • During 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 leaf

    • Pyruvate is converted to starch in the chloroplast – regenerates carbon acceptor


Phosphorylation regulates phosphenol-pyruvate (PEP) carboxylase

  • CAM and C4 plants require a separation of the initial carboxylation from the following de-carboxylation

  • Diuranal regulation is used

  • IN CAM PLANTS:-

  • Phosphorylation of the serine residue of phosphenol-pyruvate (PEP) carboxylase (Ser-OP) yields a form of the enzyme which is active at night

    • This is relatively insensitive to malic acid


Photophorylation regulates phosphenol-pyruvate (PEP)carboxylase

  • During 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!


Summary

  • The reduction of CO2 to carbohydrate via photosynthesis is coupled to the consumption of ATP and NADPH

  • CO2 is reduced via the Calvin cycle

    • Takes place in the stroma (soluble phase)

  • CO2 and water combine with Ribulose-1,5-bisphosphatein the following reaction

    • CO2 +H2O (CH2O) + O2

  • Regeneration of the carrier is required for the cycle to continue


Summary

  • The Calvin cycle requires the joint action of several light-dependant systems

    • Changes in ions (Mg+ and H+)

    • Changes in effector metabolites (enzyme substrates)

    • Changes in protein-mediated systems (rubisco activase)

  • Rubisco can also act as an oxygenase

    • The carboxylation & oxygenation reactions take place at the active sites of rubisco.


Summary

  • C4 and CAM plants Prevent photorespiration!!!!!

  • C4 leaves have TWO chloroplast containing cells

    • Mesophyll cells

    • Bundle sheath

  • CAM Plants drastically reduce water lass

    • CAM plant

      • Loses 50 – 100 g water for every gram of CO2 gained

    • C4 plant

      • Loses 250 – 300 g water for every gram of CO2 gained

    • C3 plant

      • Loses 400 – 500 g water for every gram of CO2 gained


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