Photosynthesis pt 2

Photophosphorylation

  • Definition: Photophosphorylation is the process of creating ATP from ADP using the energy derived from light, particularly sunlight.

  • Comparison with Oxidative Phosphorylation: Unlike photophosphorylation, oxidative phosphorylation occurs during cellular respiration, where ATP is generated through the electron transport chain and the reduction of oxygen to water.

Photosystems and Electron Transport Chains

  • Focus: The discussion centers on Photosystem II (PS II) and Photosystem I (PS I), which work in sequence during linear electron flow.

  • Electron Transport Chain (ETC) between PS II and PS I:

    • Process Overview:

    • Light energy is absorbed by pigments in PS II, exciting electrons to a higher energy state.

    • These high-energy electrons are then transferred through a series of electron carrier proteins embedded in the thylakoid membrane (including plastoquinone (Pq), the cytochrome b6f complex, and plastocyanin (P_c)).

    • As electrons move down this chain, their free energy is used to pump protons (H^+) from the stroma into the thylakoid lumen, generating a proton-motive force (electrochemical gradient).

    • This proton gradient drives ATP synthase to produce ATP through chemiosmosis. This ATP, along with NADPH, powers the Calvin Cycle.

    • Comparison of Free Energy Capture (PS II ETC vs. PS I):

    • PS II ETC: Light energy captured by PS II excites electrons. The exergonic cascade of these energized electrons through the ETC from PS II to PS I releases free energy, which is primarily captured by actively transporting protons (H^+) from the stroma into the thylakoid lumen. This process builds an electrochemical potential (proton gradient), which is a form of stored free energy.

    • PS I: Electrons, having lost some energy but still possessing considerable potential by the time they reach PS I, are re-energized by the absorption of more light energy. This additional captured free energy re-excites them to a very high energy level. These re-energized electrons are then used in the reduction of NADP^+ to NADPH, directly storing chemical potential energy in the bonds of NADPH.

  • Electron Transport Chain after PS I (to NADPH):

    • Process Overview:

    • Electrons, re-energized by light absorption in PS I, are passed to ferredoxin (Fd).

    • The enzyme NADP^+ reductase then catalyzes the transfer of these electrons to NADP^+.

    • Simultaneously, a proton (H^+) from the stroma is consumed, reducing NADP^+ to NADPH (NADP^+ + 2e^- + H^+ \rightarrow NADPH).

    • Chemical Energy and Hydrogen in Stroma:

    • The consumption of H^+ from the stroma during NADPH formation, coupled with the active pumping of H^+ into the thylakoid lumen by the PS II ETC, contributes to a lower H^+ concentration (higher pH) in the stroma. The chemical energy is crucially stored in the high-energy electrons of NADPH, making it a powerful reducing agent.

    • Function and Outcome:

    • NADPH is a crucial product of the light reactions, serving as a primary source of reducing power (high-energy electrons) for the Calvin Cycle. It facilitates the conversion of 3-PGA into glyceraldehyde-3-phosphate (G3P).

    • The outcome is the transformation of light energy into chemical energy, stored in the transferrable electrons of NADPH, essential for the synthesis of organic molecules.

Calvin Cycle

  • Location, Inputs, and Outputs:

    • Location: Stroma of the chloroplast.

    • Inputs: CO_2 (carbon dioxide), ATP, NADPH.

    • Outputs: Glyceraldehyde-3-Phosphate (G3P), ADP, inorganic phosphate (P_i), and NADP^+ (which are recycled back to the light reactions).

  • Functions of Phases: The Calvin Cycle proceeds in three main phases, each with a distinct function:

    1. Phase 1: Carbon Fixation

    • Function: To incorporate inorganic carbon (CO2) from the atmosphere into an existing organic molecule (ribulose-1,5-bisphosphate or RuBP). The enzyme RuBisCO catalyzes the attachment of CO2 to RuBP, forming an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).

    1. Phase 2: Reduction

    • Function: To reduce the fixed carbon (3-PGA) into an energy-rich sugar, glyceraldehyde-3-phosphate (G3P). This phase utilizes the chemical energy stored in ATP and the reducing power of NADPH produced during the light reactions.

    1. Phase 3: Regeneration

    • Function: To regenerate the CO2 acceptor molecule, RuBP, from the remaining G3P molecules. This regeneration requires additional ATP, ensuring the cycle can continue to fix more CO2.

  • Energy Transfer during Reduction Phase:

    • Molecules providing energy: ATP and NADPH.

    • Origin of energy: Both ATP and NADPH obtain their chemical potential energy from the light-dependent reactions of photosynthesis (ATP via photophosphorylation, NADPH via linear electron flow in the photosystems).

    • Fate after transferring free energy: After donating their energy, ATP is hydrolyzed to ADP + P_i, and NADPH is oxidized to NADP^+ + H^+. These "spent" molecules are then recycled back to the thylakoid membrane (light reactions) to be re-energized by light.

  • Net G3P Production (Why only one net G3P from 3 CO_2):

    • For every three molecules of CO_2 that enter the Calvin Cycle, three molecules of RuBP are consumed, leading to the formation of six molecules of G3P.

    • However, five of these six G3P molecules are used in the regeneration phase to reform the three molecules of RuBP, which are necessary to keep the cycle going.

    • Therefore, only one net molecule of G3P (a three-carbon sugar) is produced as an output that can be used by the plant to synthesize glucose or other organic compounds.

Glyceraldehyde-3-Phosphate (G3P) and Photorespiration

  • Context: G3P is an important three-carbon molecule generated during the Calvin Cycle, which is a part of photosynthesis.

  • Inefficiency due to Photorespiration:

    • When the enzyme RuBisCO binds O2 instead of CO2, it initiates photorespiration, leading to the formation of a two-carbon compound (phosphoglycolate) instead of two molecules of 3-PGA.

    • This phosphoglycolate is not useful and requires energy (ATP) to convert it back into a molecule somewhat usable by the Calvin Cycle, but with the loss of fixed carbon as CO_2. This represents a significant decrease in the net product and carbon fixation efficiency.

  • Weak Product Formation: A byproduct that is not useful for the plant (phosphoglycolate) is formed, prompting the need for further energy usage to convert it into a usable form, often losing fixed carbon.

  • Carbon Dioxide Release: The conversion process in photorespiration releases CO_2, counteracting the aim of photosynthesis.

Photorespiration

  • Definition: Photorespiration is a process that occurs when the enzyme RuBisCO fixes oxygen (O2) instead of carbon dioxide (CO2) at its active site, leading to a significant reduction in the efficiency of the Calvin Cycle. Unlike carbon fixation, it consumes ATP and NADPH and releases CO_2 without producing sugar.

  • Competitive Inhibitor for RuBisCO (C3 Plants) and Interference:

    • Competitive Inhibitor: Oxygen (O_2) acts as a competitive inhibitor for the active site of RuBisCO.

    • Interference with Net Sugar Production: When O2 binds to RuBisCO, the enzyme catalyzes the oxygenation of RuBP instead of its carboxylation. This reaction produces one molecule of 3-PGA (which can proceed in the Calvin Cycle) and one molecule of phosphoglycolate (a 2-carbon compound). The phosphoglycolate cannot be directly used in the Calvin Cycle and must be recycled through a series of reactions in the peroxisomes and mitochondria, consuming ATP and releasing CO2. This process reduces the net output of G3P and wastes energy that could have been used for sugar production.

  • Implications:

    • The necessity to separate carbon fixation and the Calvin Cycle (as seen in C4 plants) indicates a complex relationship between these processes that helps prevent interference from oxygen, especially in environments where O2 may be abundant and CO2 scarce (e.g., hot, dry conditions when stomata close).

    • Adaptations, such as those found in C4 and CAM plants, have evolved to minimize photorespiration.

  • C4 Plants: Adaptations to Photorespiration:

    • Function of Reaction in the Mesophyll Cell: In C4 plants, initial CO2 fixation occurs in the mesophyll cells. The enzyme PEP carboxylase catalyzes the addition of CO2 to phosphoenolpyruvate (PEP), forming a 4-carbon compound (e.g., oxaloacetate). This acts as a CO2 pump, effectively concentrating CO2 for the Calvin Cycle.

    • PEP Carboxylase's Affinity for CO2: Yes, PEP carboxylase has a much higher affinity for CO2 than RuBisCO and, crucially, does not bind O2. This allows efficient CO2 capture even at low atmospheric CO_2 concentrations.

    • Function of the Bundle Sheath Cell: Bundle sheath cells in C4 plants are specialized anatomical structures where the Calvin Cycle takes place. These cells are typically arranged in a ring around the vascular bundles and are isolated from high O_2 concentrations.

    • Carbon Fixation in Bundle Sheath Cells without O2 Interference: The 4-carbon compound (e.g., malate) formed in the mesophyll cells is transported into the bundle sheath cells. Inside the bundle sheath cells, this 4-carbon compound is decarboxylated, releasing CO2, which then enters the Calvin Cycle. The spatial separation and this concentrated CO2 release create a high CO2 environment in the bundle sheath cells, effectively saturating RuBisCO with CO2 and minimizing its interaction with O2, thereby preventing photorespiration.

  • Impact of Increased Atmospheric CO_2 on C3 vs. C4 Plants:

    • If atmospheric carbon dioxide concentration increased, C3 plants would benefit the most.

    • Explanation: C3 plants are typically limited by the availability of CO2 for RuBisCO. When CO2 levels are low, RuBisCO is more likely to bind O2, leading to photorespiration and reduced photosynthetic output. A higher atmospheric CO2 concentration would increase the ratio of CO2 to O2 at RuBisCO's active site, thereby reducing the incidence of photorespiration and boosting photosynthetic efficiency and sugar production in C3 plants. C4 plants, already efficient at concentrating CO2 internally, are less sensitive to variations in external CO2 levels and would therefore see less significant increase in photosynthetic efficiency.