Calvin cycle/ C4 photosynthesis/Water

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The NADPH and ATP generated by the light reaction of photosynthesis are used to...

fix and reduce carbon and to synthesise simple sugars. In most plants, CO2 reaches the photosynthetic cells through special openings called stomata. In most plant species, the reduction of carbon occurs in the stroma of the chloroplast.


The starting (and ending) compound in the Calvin cycle is a

5-carbon sugar with two phosphate groups, known as RuBP.


There are three stages to the Calvin cycle:

Regeneration of acceptor


Fixation stage of the calvin cycle

  • Three molecules of RuBP (Rubisco) are combined with 3 molecules of CO2. This produced 3 molecules of an unstable 6 carbon molecule which is split immediately, yielding 6 molecules of PGA, a 3-carbon compound.
  • Rubisco is the most abundant enzyme in the world and fixes carbon (coverts atmospheric carbon dioxide into organic compounds)

To get one G3P released from the cycle, you need

3 CO2, 9ATP and 6NADPH – high energy cost.


Photoinhibition is

when there is damage to PSII from too much light levels

  • Under high light levels there is more light absorbed by P680 than can be dissipated “safely”

–P680+ is a very strong oxidant

–Can cause damage

–Reduce rate of photosynthesis

  • à photoinhibition

Photoinhibition is also a protective process

  • Xanthophylls associated with reaction centre pigments
  • Under excess light the xanthophyll cycle is engaged
  • Zeaxanthin absorbs light and disposes of it as heat preventing the “overexcitation” of PSII
  • Reducing rate of photosynthesis
  • à photoinhibition (protective)


  • In the presence of oxygen Rubisco can promote the reaction

–RuBP(5C) + O2 ® Phosphoglycerate (3C) + phosphoglycolate (2C)

  • Result under high oxygen

No carbon fixed

–One carbon lost in regeneration of RuBP (as CO2)

  • Production of CO2 as in respiration hence Photorespiration
  • Trouble is…. Photosynthesis produces oxygen

The problem of photorespiration under hot climates

  • Two options under hot dry conditions

–Stomata close

  • raising the internal concentration of oxygen

–Increase photorespiration

  • Reduced CO2 available

–photosynthesis declines

–Stomata open

  • Dramatic water loss
  • Rubisco is more ‘specific’ for CO2 than O2,

–by about 2000

Rubisco’s preference for CO2 declines as temperature increases

  • But today’s atmosphere is ~20% O2, and <0.04% CO2

–ie CO2 concentration is 500x less than O2

  • Photorespiration is inevitable

–Function of enzyme structure

–AND photosynthesis PRODUCES O2

  • Photorespiration can reduce the rate of net carbon uptake by as much as 30%

–enhanced by high temperature, and water stress


  • Decrease oxygen concentration in the chloroplast

–Probably impossible as PSII produces it!

  • Increase carbon dioxide concentration in the stroma of the chloroplast

–Should increase efficiency of photosynthesis

® C4 photosynthesis


C4 Photosynthesis

  • In many grasses but no trees
  • One of two pathways that have evolved in hot environments
  • Additional series of biochemical steps

–Energetically more expensive

  • A modified leaf anatomy

–C4 plants spatially separate Rubisco from the initial site of CO2 fixation in the mesophyll tissue


C4 pathways seperate the

Light dependent and independent pathways to different parts of the leaf


The cells of C4 plants are not sperated into palisade and spongy mesophyll but into

Bundle sheath cells with mesophyll around it


Kranz Anatomy

  • Mesophyll

–Light dependent reactions (PSII & PSI) ie O2 production

  • Bundle sheath

–Light independent reactions

–ie Rubisco and the rest of the Calvin cycle

–No O2 production

–Bundle sheath cells large and close to vascular tissue

–Bundle sheath usually has an outer impermeable layer, preventing the diffusion of O2

  • Still have to fix CO2

C4 Photosynthesis

  • Mesophyll cells have a different carbon fixation enzyme in place of Rubisco

–PEP carboxylase

  • Substrate PEP (phosphoenol pyruvate)

–3 Carbon molecule

–CO2 + PEP ® oxaloacetate (4C molecule)

  • Product (4C sugar)

–Transferred to Bundle sheath cells

–CO2 released

–Normal Calvin cycle reactions


PEP carboxylase

–High conc of oxygen not a problem

–Enzyme does not have oxygenase activity

–fixes CO2 at very low concentrations


Bundle sheath cells have Rubisco

–High concentration of CO2 (released from oxaloacetate) near Rubisco

–RuBP + CO2 ® 2 PGA

–Normal Calvin cycle reactions generate carbohydrates


C4 photosynthesis - Advantages

  • CO2 concentration in the bundle sheath of C4 leaves is several times greater than atmospheric

–suppresses the oxygenase function of Rubisco

  • PEP carboxylase has greater affinity for CO2 than Rubisco and can fix CO2 at lower concentration

–Stomata can be closed more than in C3 photosynthesis

–Dramatically reduces H2O water loss

  • PEP carboxylase not affected by oxygen

–no photorespiration

–particularly advantageous at higher temperatures

–C4 plants are most common in tropical or subtropical habitats

  • Same photosynthesis as a C3 plant with less N invested in Rubisco:

–this higher nitrogen use efficiency (NUE) enables N to be allocated to other uses, such as reproduction


C4 photosynthesis

  • 6 additional ATP required to produce 1 GPA molecule

–ATP needed in C4 pathway to regenerate PEP

–Cost of producing glucose by C4 photosynthesis nearly twice that of C3 photosynthesis


Crassulacean Acid Metabolism (CAM)

  • Separate uptake and fixation over time

–CAM plants fix C via the C4 acid cycle at night

–run the Calvin cycle by day

  • Stomata open at night

–PEP Carboxylase produces 4-C acids (malic acid)

–stored in the vacuole

–Lower temperatures at night reduce water loss

  • Stomata close during day

–the malic acid is decarboxylated

–released CO2 is fixed by Rubisco

–water loss is minimised

  • CAM plants are extremely good at conserving water
  • Less efficient photosynthesis

CAM Photosynthesis

  • CO2 fixation occurs at night

–light dependent and light independent (dark) reactions of photosynthesis are thus separated in time

  • CAM plants can switch modes

–Important ecologically

–can operate in a conventional C3 mode, with stomata open during the day and the C4 acid cycle suppressed

–Useful if water availability is high, because C3 photosynthesis is more efficient than CAM

  • most CAM plants are slow-growing

Typical C3, C4 and CAM plants

card image
  • C3 - most plants

–first product of Calvin-Benson cycle is a 3-carbon molecule

  • C4 - better adapted to arid climates;

–first product is 4-carbon molecule (oxaloacetate)

  • CAM

–first product is 4-carbon molecule (malic acid)


Why don’t all plants use C4 or CAM photosynthesis?

  • Alternate pathways energetically more costly (requires more ATP) under ‘normal’ conditions
  • Under ‘normal’ conditions, C3 plants have higher photosynthetic rate than C4 or CAM plants
  • Change in biochemical pathway leading to C4 pathway did not arise in all families of plants


  • Net movement of molecules (or ions)
  • From: a region of their high concentration
  • To : a region of their lower concentration

–concentration gradient.

  • Molecules have kinetic energy

–ie move about randomly

  • Molecules reach an equilibrium

–No net movement of molecules


Rates of diffusion vary due to

  • The steepness of the concentration gradient
  • Temperature

–Higher temperatures = more kinetic energy

  • Warmer molecules move around faster, so diffusion is faster
  • The surface area

–The smaller the surface area the faster the diffusion

  • The type of molecule

–Large molecules need more energy to move, diffuse more slowly



The diffusion of water molecules

from an area of high concentration of water

to an area of low concentration of water

across a partially permeable membrane


Water Potential
Greek symbol psi (y)

High water potential = close to zero. Pure water is zero

Low water potential = a negative number.


Transpiration requires a

continuous gradient of decreasing water potential

  • Water moves towards the more negative y
  • High salinity = low to very low y

99% of plants cannot survive in

saline soils


Murray Darling Basin

  • Arid environments are often naturally saline

–Accumulation of salt over millenia

–Some natural salt lake regions

  • Kept in balance by natural flora

–adapted to conditions

–Deep rooted

  • Fully utilise rainwater
  • But 33% of irrigated land on earth is salt-affected

–Human activity greatly exacerbated problem

  • Water now sometimes too salty for irrigation
  • Too salty for domestic supplies

Aquatic Ecosystem effects

  • Low flows in rivers mean sediments and organic matter not flushed

–Less dilution of pollutants

–Less mixing of water in rivers

  • Consequences for fish larvae and aquatic plants
  • Plants on river banks die

–River bank collapses

–Further reduction in water quality

  • Invasions of marine species and cyanobacteria in lower estuaries replace natural species
  • Reduced phytoplankton production in estuaries

–Reduced zooplankton production

–Coastal fisheries effected


Consequences of salty soils

  • If the soil is loaded with salts then the water potential of the soil is lower (more negative)

–Less flow of water into plant

–Extremes may cause water flow out of plant


  • Waterlogging of roots

–Oxygen limitation

  • Salts alter soil structure
  • Reduced capacity to trap nutrients
  • Reduced seed germination
  • Changes to distributions of plant and animal species
  • –Loss of shelter and foraging areas

levels of plant diversity drop

  • Natural replacement of plants in wilderness areas with more salt tolerant species

–Eg natural scrub plants (Eucalyptus viridis) replaced with more salt tolerant blue bush (Maireana spp)

  • Loss of aquatic plants affects animal species breeding efficiency

Effect of increased salinity on plants

  • Physiological stress in low salt tolerant plants

–Na+ most important

–but Cl-, Ca2+, Mg2+, SO42- also significant


  • Water stress and toxic stress from excess ions
  • Na+ ion channels more active

–Higher energy cost to plant

  • High Na+/K+ ratio:

–Inactivates enzymes

–Inhibits photosynthesis and protein synthesis

–Disrupts stomata function


Salt tolerance strategies

  • Salt exclusion from roots

–Requires energy to extrude Na+

  • Absorption from the transpiration stream

–secretory salt glands.

  • Reduce Ψ in cells

–Ie lower water potential in cells promotes water flow from soil

–Accumulate solutes in cytoplasm eg sugars

  • Expensive.

–Accumulate salt in vacuoles

  • Less expensive
  • Reduce leaf area
  • Drop leaves
  • Altered protein synthesis

Components of Water Potential

  • Osmotic or Solute Potential (yS )

–Due to dissolved solutes in the cytoplasm

  • Pressure Potential (yP)

–Plant cells have a cell wall that exerts an inward pressure when the cell is turgid.

  • Matrix Potential (yM)

–Due to the tendency for water to adhere to surfaces

  • Matrix potential dominates soil water

The water potential of a plant cell

y = sum of all 3

y = yS + yP + yM


Comparison: C3, C4, CAM

How efficiently do they use water?

  • C3 plants need 380 to 900 g of H2O
    to make 1 g of biomass
  • C4 plants need 250 to 350 g of H2O
    to make 1 g of biomass
  • CAM plants need around 50 g of H2O
    to make 1 g of biomass

Reduction phase

six molecules of PGA are reduced to six molecules of PGAL/G3P (a 3 carbon carbohydrate). ATP and NADPH from the light-dependent reaction are used to do biochemical modifications of these molecules. So we end up with a different 3-phosphate molecule (G3P).

  • The six PGA are reduced to six G3P. This requires six NADPH and six ATP.

Regeneration of acceptor phase

Regeneration of acceptor: five of the six molecules of PGAL are used to regenerate the original three molecules of RuBP. One molecule of PGAL/G3P will be left over, which can be used to synthesise sugars or other carbohydrates as required by the plant. Requires ATP.

  • Five of the six PGAL/G3P are combined to form 3 RuBP. This requires 3 ATP. The one ‘extra’ molecule is the net gain from the Calvin Cycle.

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