5 Photosynthesis and its Relationship to Net Primary Production

Overview of Photosynthesis and Environmental Interactions

• Objective: To understand how the biochemistry of photosynthesis relates to ecosystem-level processes such as Net Primary Production (NPP). This involves analyzing how environmental conditions influence biochemical pathways to change NPP.
• Simplistic versus Complex View:
  • Simplistic Equation: $CO_2 + H_2O + \text{energy (sunlight)} \rightarrow \text{carbohydrates} + O_2$.
  • Comprehensive View: Understanding complex biochemical interactions is essential to explaining why environmental factors like precipitation, nitrogen availability, and light intensity affect ecosystem productivity.

Photosynthetic Pigments and Energy Absorption

• Mechanism of Energy Intake: Organisms utilize sunlight through specialized pigments that absorb energy at specific wavelengths.
• Primary Pigment Types and Absorption Spectrum:
  • Chlorophyll a: The essential molecule for oxygen-generating photosynthesis; found in all photosynthetic eukaryotes and cyanobacteria.
  • Chlorophyll b: Found in many organisms; it broadens the range of light wavelengths absorbed and transfers acquired energy to Chlorophyll a. It is not directly involved in the energy transduction process itself.
  • Carotenoids: Function similarly to Chlorophyll b by absorbing energy and transferring it to Chlorophyll a, but they also play a critical role in antioxidation within the chloroplast.
  • Chlorophyll c: Replaces Chlorophyll b in certain algae types.
• Absorption Specifics: Chlorophyll a and b have distinct peaks, particularly in the range of $600$ to $700\,nm$.
• Pigment Excitation and Fates of Energy: When a pigment absorbs light, it enters an excited state (a rapid process). The energy then follows one of three paths:
  1. Fluorescence: Energy is lost as heat, or a combination of heat and light. This occurs primarily when chlorophyll molecules are isolated from one another.
  2. Resonance Energy Transfer: Energy (but not the electron itself) is transferred to an adjacent chlorophyll molecule. This can occur across multiple molecules in a sequence.
  3. Electron Transfer: A high-energy electron is physically transferred to a neighboring molecule, marking the start of the Electron Transfer Chain. This leaves behind an "electron hole" in the original pigment molecule.

Chloroplast Structure and Organization

• Stroma: The aqueous fluid-filled area within the chloroplast that contains the pigments and is the site for carbon fixation reactions.
• Thylakoids: Sac-like membranes situated within the stroma that contain the photosynthetic pigments.
• Granum: Stacks of thylakoids acting as organized units for energy transduction.

Energy Transduction Reactions (Light-Dependent Reactions)

• Location: Occurs within the thylakoid membranes.
• Photosystems: Functional units containing $200$ to $400$ pigment molecules arranged in a "funnel" shape.
  • Reaction Center: The core of the photosystem containing a "special pair" of Chlorophyll a molecules. This is the site where high-energy electrons are passed to an electron acceptor.
  • Surrounding Molecules: Other chlorophyll and pigment molecules absorb light and bounce the energy toward the reaction center.
• Photosystem Types:
  • Photosystem II ($P680$): Named for its peak light absorbance at $680\,nm$. Contains roughly equal amounts of Chlorophyll a and b. It is the site of Photolysis, where water ($H_2O$) is split into hydrogen ions ($H^+$), electrons, and oxygen ($O_2$). The electrons fill the "electron hole" in the reaction center, and the hydrogen ions create a proton gradient to generate ATP.
  • Photosystem I ($P700$): Named for its peak light absorbance at $700\,nm$. It has a low abundance of Chlorophyll b. It primarily produces NADPH.
• Operation: The two systems typically work together to produce ATP and NADPH. However, Photosystem I can operate independently (Cyclic Electron Flow), which is considered an ancient form of photosynthesis. This pathway produces ATP only, without generating NADPH or oxygen, helping plants balance higher demands for ATP.

Carbon Fixation Reactions (The Calvin Cycle)

• Location: Occurs in the stroma of the chloroplast.
• Nomenclature: Often called the "dark reactions" or "light-independent reactions," though they require the products of the light-dependent reactions (ATP and NADPH).
• The Calvin Cycle Steps:
  1. Carbon Fixation: Three molecules of Ribulose 1,5-bisphosphate ($RuBP$, a $5$-carbon molecule) bind with three molecules of $CO_2$ (totaling $18$ carbons). This is facilitated by the enzyme Rubisco. The result is six molecules of $3-PGA$ (a $3$-carbon molecule), giving the C3 pathway its name.
  2. Reduction: ATP and NADPH are used to reduce the six molecules of $3-PGA$ into six molecules of $G3P$. One molecule of $G3P$ (representing half a glucose molecule) leaves the cycle for use in sucrose or starch production.
  3. Regeneration: The remaining five molecules of $G3P$ (totaling $15$ carbons) are reorganized using ATP back into three molecules of $RuBP$ ($5$-carbons each) to restart the cycle.
• Storage and Transport: Plants rarely store carbon as glucose; they typically transport it as sucrose or store it long-term as starch.

Rubisco and the Problem of Photorespiration

• Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase):
  • The most prevalent enzyme on Earth, comprising approximately $40\%$ of all leaf protein.
  • It is non-specific; it can bind both $CO_2$ and $O_2$.
• Evolutionary Context: Rubisco evolved when the atmosphere was rich in $CO_2$ and nearly devoid of $O_2$, making selectivity unnecessary at that time.
• Photorespiration: Occurs when Rubisco binds $O_2$ instead of $CO_2$.
  • Results in one molecule of $3-BGA$ and one molecule of phosphoglycolate (a $2$-carbon molecule).
  • Recovery of the $2$-carbon molecule is an energy-demanding process utilizing ATP and NADPH, and it actually releases $CO_2$.
• Environmental Drivers of Photorespiration:
  • Oxygen/CO2 Ratio: Atmospheric oxygen is ~$20\%$, while $CO_2$ is <$0.05\%$.
  • Water Limitation: Stomata close to conserve water, leading to internal oxygen buildup from photosynthesis and a lack of incoming $CO_2$.
  • Closed Canopies: Poor air movement allows oxygen to accumulate and $CO_2$ to deplete within the leaf canopy.

Alternative Photosynthetic Pathways: C4 and CAM

• Distribution: $83\%$ of plants are C3; $3\%$ are C4; the remaining $14\%$ are CAM.

C4 Photosynthesis (Spatial Separation)

• Mechanism: Separates carbon fixation steps into two cell types to minimize photorespiration.
  • Mesophyll Cells: $CO_2$ is bound to Phosphoenolpyruvate ($PEP$) by PEP Carboxylase (which cannot bind oxygen) to form Oxaloacetate (a $4$-carbon molecule, hence C4). This is converted to Malate.
  • Bundle Sheath Cells: Malate is transferred here and decarboxylated to release $CO_2$ directly to Rubisco, ensuring a high $CO_2:O_2$ ratio for the Calvin Cycle.
• Efficiency: Requires more ATP ($PEP$ regeneration) but is much more efficient in high light, high temperatures, and dry environments. C4 grasses are $2$ to $3$ times more efficient than C3 grasses.
• Examples: Maize, sorghum, sugarcane.
• Advantages: Use $3$ to $6$ times less Rubisco (higher nitrogen efficiency) and have smaller stomatal openings (higher water efficiency).

CAM Photosynthesis (Temporal Separation)

• Mechanism: Separates steps by time of day; found in succulents and stone crops.
  • Night: Stomata open. $CO_2$ is fixed into Malate and stored as Malic Acid in large vacuoles.
  • Day: Stomata close to prevent water loss. Malic Acid is released from the vacuole, converted back to Malate, and decarboxylated to provide $CO_2$ for the Calvin Cycle.
• Examples: Agave (used for tequila), pineapple.
• CAM Idling: In extremely arid conditions, stomata stay closed day and night; the plant uses $CO_2$ produced by its own respiration for photosynthesis.

Environmental Constraints on NPP

• Resource Efficiency Table (Water Loss per gram of $CO_2$ gained):
  • C3: $400$ to $500\,g$ water.
  • C4: ~$300\,g$ water.
  • CAM: As low as $50\,g$ water.
• Environmental Links to Biochemistry:
  • Precipitation: Low water causes stomatal closure, limiting $CO_2$ and thus limiting the Calvin Cycle (C3/C4 response).
  • Light: Low light limits the production of ATP and NADPH in the energy transduction phase.
  • Nitrogen: Low nitrogen limits the production of Rubisco, the primary enzyme for carbon fixation.
  • Phosphorus: Required for the structures of ATP and NADPH; deficiency limits energy availability for carbon fixation.
• Competitive Advantage: There is no "best" system. C3 plants outcompete CAM and C4 plants in temperate, water-rich environments where the energy costs of C4/CAM offer no benefit.