Photosynthesis Question 1986: Describe the light reactions of photosynthesis and for both a C₃ and C₄ plant, trace the path of a CO₂ molecule from the point at which it enters a plant to its incorporation in to the glucose molecule.  Include leaf anatomy and biochemical pathways in your discussion of each type of plant.  (Below is not a great response, but decent)

 

Light reactions of photosynthesis require quantities of light, water, and ATP or NADP+ to function.  These light reactions start when light energy from the sun gets absorbed by the antennae pigments of the leaf and these reactions take place in the thylakoid membrane.  In the thylakoid membrane, there are six different complexes of integral membrane proteins that are embedded.  However, there are two common complexes known as Photosystem I and Photosystem II, each which promote the process of cyclic and noncyclic phosphorylation to produce ATP.  These systems use chlorophyll in the leaf that absorbs a specific amount of light/color in a certain wavelength.  For noncyclic phosphorylation, light is absorbed by the antenna pigments of Photosystems II and I, and the photons in the pigments become excited/boosted to higher orbit.  Then the absorbed energy is transferred to the reaction center pigment P680 (Photosystem II) and P700 (Photosystem I).  The activation of P680 removes an electron from it; thus with its resulting positive charge, P680 is sufficiently electronegative that it can remove electrons from water.  These electrons are transferred through carrier molecules by way of the plastiquinone to the cytochrome b6/f complex where they provide the energy for chemiosmosis in the chloroplast.  The activation of P700 enables it to pick up electrons from cytochrome f from plastocyanin and raise them to a high redox potential so that it can reduce NADP+ to NADPH by passing ferrodoxin.  Cyclic phosphorylation is basically without the addition of Photosystem I, but this also suggests that noncyclic phosphorylation is much more efficient in producing energy to synthesize ATP and to reduce NADP+.  In the end, the byproduct becomes the production of oxygen that is released into the air. 

The logic of a leaf design, however, can change the processes of the light reactions of photosynthesis.  Because of their leaf anatomy and their biochemical pathways, the differences could create changes or rates of photosynthesis.  Nevertheless, most plants have similar biochemical pathways in the beginning with CO₂.  After the CO₂ enters the stomata of the plant, it diffuses from the intercellular space in the mesophyll.  In order to transform CO₂ in to higher energy substances like glucose, it needs to use dark reactions of carbon fixation in the stroma fluid of the plant.  The series of dark reactions is also known as the Calvin cycle.  Each CO₂ molecule in the cycle combines with a five carbon sugar called ribulosee bisphosphate (RuBP) to form a six-carbon intermediate to two three carbon-PGA.  The cycle uses ATP and reduces PGA with NADPH to form PGAL, which is for the use of glycolysis in forming glucose.  Because of a non-existent result of synthesis of ATP, the use of the enzyme RuBP carboxylase in the carboxylation of high-energy RuBP leads to a wasteful regeneration of low-energy CO₂ known as photorespiration.

How would then some other plants in horrible climates survive if photorespiration occurred?  Thus a C₃ plant would differ from a C₄ plant in the Calvin cycle process, leaf anatomy, and biochemical pathways.  C₃ plants would mostly be located in favorable places that allow the carbohydrate synthesis of the Calvin cycle while the C₄ would somehow be able to avoid the process of photorespiration.  The anatomy of C₃ plants includes having palisade and spongy mesophyll that C₄s do not have.  Instead the C₄ plants have functional mesophyll around bundle sheaths and they have chloroplasts located in the bundle sheath cells.  This arrangement is known as Kranz anatomy.  The biochemical pathway of C₃ plants combines CO₂ with RuBP and C₄ plants combine CO₂ with PEP to the Calvin cycle.  The Hath-Slack pathway of C₄ photosynthesis involves the combination of CO₂ and PEP to from a C₄ compound.  C₄ becomes reduced by NADPH and oxidized by NADP+ to split into a three-carbon compound (converts with ATP back to PEP) and CO₂ (fed into Calvin cycle).  Finally the C₄ plants also use the CAM (crassulacean acid metabolism) pathway to also avoid photorespiration.  This is different from regular C₄ photosynthesis, however, because it acts on a day-night basis.  At night it captures CO₂ and then the CO₂ is used during the day for use in the Calvin cycle.

 

 

RUBRIC: (Put A Check Next To Each Element Answered)

 

  LIGHT REACTIONS:                          Max. = 9 points/checks

  __  Light required

  __  Light reactions on thylakoids (in grana)

  __  Cyclic and non-cyclic photophosphorylation/photosystems I & II

  __  H2O used/split/required

  __  Chlorophyll absorbs light

  __  Quality of light/color of light/wavelength of light must be correct

  __  Chlorophyll e-s excited/boosted to higher orbit

  __  Energy of e-s used to make ATP

  __  Chemiosmotic photophosphorylation/Mitchell hypothesis (proton gradient)

  __  Energy of e-s used to make NADPH+

  __  ADP or NADP used

  __  Carrier molecules for ATP or NADPH+ production

  __  Electrons of P700 replaced <- P680 (PSI <- PSII)

  __  Electrons of P680 replaced <- H2O   (PSII <- H2O)

  __  O2 comes from H2O

  __  O2 released to air

  __  Redox mentioned

 

  C3 AND C4 ANATOMICAL AND BIOCHEMICAL PATHWAYS:  Max. = 9 points/checks

  __  CO2 enters through stomates

  __  CO2 diffuses from intercellular spaced into mesophyll

  __  Dark reaction/CO2 fixation in stroma (fluid)

  __  CO2 used/fixed/required

  __  Energy of ATP used

  __  Energy (e-s/Hs) of NADP reused

  __  C3 plants have palisade and spongy mesophyll

  __  C4 plants have functional mesophyll around bundle sheaths

  __  C4 plants have Kranz anatomy

  __  C4 plants have chloroplasts in bundle sheath cells

  __  C3 plants combine CO2 with RuBP

  __  C4 plants combine CO2 with PEP

  __  C4 plants use CAM pathway

  __  Steps in PEP -> 4-C -> PEP

  __  C4 plants send 4-C to bundle sheath

  __  C3 and C4 plants put CO2 into Calvin cycle -> glucose

  __  Steps in Calvin cycle

  __  Mentioning of C3, C4 pathway enzymes