Bio 09-17

Photosynthesis: Light Reactions, Calvin Cycle, and Plant Adaptations

• Overview

  • Photosynthesis uses light energy to build chemical energy (ATP and NADPH) and then uses that energy to convert CO₂ into sugars for the plant and, ultimately, for the food chain.
  • The chloroplast contains thylakoid membranes densely packed with photosystem II (PSII), photosystem I (PSI), and electron transport proteins. The thylakoid lumen (inside) accumulates protons to drive ATP synthase.
  • Two main phases: the light reactions (capture light energy) and the Calvin cycle (use energy to fix carbon and build sugar).

• Light Reactions: What happens in the thylakoid membrane

  • Photosystem II (PSII) and Photosystem I (PSI) each have a pair of specially located chlorophylls that capture light energy.
  • PSII is associated with P680 (reaction center); PSI with P700. The energy from light excites these chlorophylls and drives electron flow.
  • Water splitting at PSII releases electrons to replace those lost by PSII, releasing O₂ as a byproduct: $2\,\mathrm{H<em>2O} \rightarrow \mathrm{O</em>2} + 4\,\mathrm{H^+} + 4\,e^-.$
  • The electrons travel through an electron transport chain (ETC) in the thylakoid membrane, releasing energy that is used to pump protons from the stroma into the thylakoid space, creating a proton gradient (proton motive force).
  • ATP synthase uses this proton gradient to convert ADP and inorganic phosphate into ATP: $\mathrm{ADP} + \mathrm{P_i} \rightarrow \mathrm{ATP}$ via downhill proton flow.
  • Electrons reach PSI, where light energy re-energizes them. These high-energy electrons are transferred to a primary electron acceptor and then to NADP⁺ to form NADPH: $\mathrm{NADP^+} + 2e^- + H^+ \rightarrow \mathrm{NADPH}.$
  • Linear (non-cyclic) electron flow (the “in a line” flow) moves electrons from water through PSII, the ETC, PSI, and finally to NADP⁺, creating both ATP and NADPH and releasing O₂.
  • NADPH and ATP produced in the light reactions are then used in the Calvin cycle to fix CO₂ and synthesize sugars.

• ATP/NADPH balance and the idea of linear flow

  • The linear flow produces both ATP and NADPH in roughly fixed proportions, and this energy wealth is used to drive carbon fixation.
  • The system can adjust via cyclical electron flow to meet ATP demand without producing NADPH or O₂; this is described next.

• Cyclic electron flow: making more ATP without NADPH or oxygen

  • In cyclic electron flow, electrons from PSI can be redirected back into the ETC rather than being sent to NADP⁺, so they pump protons and drive more ATP production.
  • This means ATP can be increased without making NADPH or splitting water, and without producing additional CO₂ or O₂.
  • Both linear and cyclic flows can operate simultaneously in the thylakoid membranes, balancing ATP and NADPH production to meet Calvin cycle needs.
  • The term “extant” (stochastic) is used to describe how ATP and NADPH production are regulated by cellular demand: some portion of the light energy goes toward linear flow (for NADPH and ATP) and some toward cyclic flow (for extra ATP).

• The big picture: moving from light energy to chemical energy to sugar

  • Light reactions accumulate energy in ATP and NADPH, ready to be used in carbon fixation.
  • The Calvin cycle uses ATP and NADPH to reduce CO₂ and build sugars, starting from a simple CO₂ molecule.
  • The overall aim is to convert solar energy into chemical energy stored in carbon compounds.

• The Calvin cycle: carbon fixation and sugar synthesis

  • The Calvin cycle uses ATP and NADPH produced in the light reactions to reduce CO₂ and build sugars (anabolic process).
  • It fixes carbon as follows: Rubisco (ribulose-1,5-bisphosphate carboxylase) catalyzes the addition of CO₂ to RuBP (a 5-carbon sugar phosphate), forming a six-carbon intermediate that is split into two 3-carbon molecules.
  • The cycle proceeds through a series of enzymatic steps that convert these 3-carbon compounds using ATP and NADPH, ultimately yielding glyceraldehyde-3-phosphate (G3P).
  • Net result per fixed CO₂: energy from ATP and NADPH reduces CO₂ to G3P, which can then be used to make sugars.
  • In this lecture, specific named products and intermediates were highlighted:
  • The Calvin cycle begins with RuBP (a 5-carbon compound) and CO₂, catalyzed by Rubisco, producing a 6-carbon intermediate that splits into two 3-carbon compounds.
  • The 3-carbon compounds are further energized by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate (G3P).
  • G3P (glyceraldehyde-3-phosphate) is a key building block for sugars.
  • Rubisco is described as the most important enzyme in this pathway and is defined as rubisco (ribulose-1,5-bisphosphate carboxylase).
  • The transcript emphasizes that this is a cycle, regenerating the CO₂ acceptor RuBP so the cycle can continue.

• The carbon accounting in the Calvin cycle (as presented in the transcript)

  • The cycle is often explained using three CO₂ molecules at a time.
  • Start with three RuBP (each C₅). Add three CO₂, yielding three six-carbon intermediates.
  • Each six-carbon intermediate is split to yield six 3-carbon compounds.
  • The six 3-carbon compounds are processed to form six molecules of glyceraldehyde-3-phosphate (G3P).
  • Of these six G3P, one is exported to synthesize sugar (e.g., glucose, sucrose, starch). The remaining five G3P are left to regenerate RuBP.
  • The five G3P (5 × 3 = 15 carbons) are used to regenerate the initial three RuBP (15 carbons total).
  • Thus, after three turns of the cycle (fixing three CO₂), one G3P is produced for sugar synthesis, and the cycle is reset to continue.
  • This is the classic bookkeeping in the transcript: three CO₂ → six G3P → 1 G3P exported → 5 G3P regenerated RuBP (15 C used to rebuild RuBP).
  • The “rubisco bisphosphate” term appears in the transcript as rubisco (RuBP) binding to CO₂, emphasizing the initial carboxylation step.

• Important terminology and molecules to know from this section

  • Rubisco: ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase).
  • RuBP: ribulose-1,5-bisphosphate, a five-carbon sugar phosphate that acts as the CO₂ acceptor.
  • 3-Phosphoglycerate (3-PGA) and glyceraldehyde-3-phosphate (G3P): key 3-carbon intermediates/products in the Calvin cycle.
  • NADPH and ATP: the reducing power and energy carriers used by the Calvin cycle.
  • G3P export and RuBP regeneration: the balance that sustains the cycle and produces sugar.

• Photorespiration and plant physiology: Rubisco’s oxygenase activity and adaptations

  • Rubisco can also bind O₂ instead of CO₂ when CO₂ is scarce and O₂ is relatively high, leading to a process called photorespiration, which is energetically wasteful for the plant.
  • In low CO₂/high O₂ conditions (e.g., air with hot, dry conditions or desert environments), photorespiration can consume ATP and reduce the yield of sugars because CO₂ is not fixed into sugars efficiently.
  • Agricultural and ecological implications: photorespiration affects crop yields; plants have evolved strategies to minimize it.

• Plant adaptations to reduce photorespiration (two major strategies mentioned)

  • C4 photosynthesis: spatial separation of the Calvin cycle from the initial CO₂ fixation.
  • In C4 plants, CO₂ is first fixed in the mesophyll cells by PEP carboxylase (phosphoenolpyruvate carboxylase) to form oxaloacetate, which is quickly converted to malate (a four-carbon compound with three carbons plus one extra carbon).
  • Malate is transported via plasmodesmata to neighboring bundle sheath cells where CO₂ is released for fixation by Rubisco, greatly increasing CO₂ concentration around Rubisco and reducing O₂ competition.
  • The initial CO₂ fixation step uses a different enzyme (PEP carboxylase) and circumvents the Rubisco oxygenase problem by concentrating CO₂ where the Calvin cycle proceeds.
  • The anatomy includes mesophyll cells and bundle-sheath cells; the vascular tissue (xylem and ph