Photosynthesis High-Yield Flashcards

Overview of Photosynthesis

• Definition: Photosynthesis is the biological process where light energy from the sun is captured and used to synthesize glucose ($C_6H_{12}O_6$).
• Light Absorption:
  • Pigments involved include Chlorophyll A, Chlorophyll B, and carotenoids, which absorb energy from sunlight.
  • Light particles (photons) hit these pigments, which excite electrons ($e^-$).
  • Excited electrons are in an unstable state and consequently re-emit energy.
• Reaction Centers:
  • The energy is eventually absorbed by specific Chlorophyll A molecules known as reaction centers.
  • $P_{680}$ is the reaction center for Photosystem II (PSII).
  • $P_{700}$ is the reaction center for Photosystem I (PSI).
• Molecular Structure: Chlorophyll A is characterized by a porphyrin ring containing a central magnesium ($Mg^{2+}+$) atom.
• Overall Chemical Equation:
  • $6CO_2 + 6H_2O + \text{Sunlight} \xrightarrow{\text{photosynthesis/chlorophyll}} C_6H_{12}O_6 + 6O_2$
  • Reactants: 6 molecules of Carbon Dioxide ($CO_2$) and 6 molecules of Water ($H_2O$).
  • Products: 1 molecule of Glucose ($C_6H_{12}O_6$) and 6 molecules of Oxygen ($6O_2$).
• Biological Connection: Note that glucose serves as a product in photosynthesis and as a reactant in cellular respiration.

Chloroplast Anatomy

• General Structure: Photosynthesis occurs within the chloroplast, a double-membraned organelle found in plant cells.
• Membrane Systems:
  • Outer Membrane: The outermost phospholipid bilayer.
  • Intermembrane Space: The area located between the outer and inner membranes.
  • Inner Membrane: The second membrane layer beneath the outer membrane.
• Internal Structures:
  • Thylakoids: Individual flattened sac-like structures.
  • Granum: A stack of thylakoids (plural: grana).
  • Thylakoid Membrane: The specific structure that absorbs light; it is the site of the Electron Transport Chain (ETC) and Non-Cyclic Photophosphorylation.
  • Thylakoid Lumen: The interior space of the thylakoid. It is the location of photolysis and the site where hydrogen ions ($H^+$) accumulate due to the ETC.
  • Stroma Lamellae: The structures connecting various grana; these are the locations of Cyclic Photophosphorylation.
  • Stroma: The fluid-filled space surrounding the thylakoids; it is the location of the Calvin Cycle (Dark Reaction).
• Evolutionary Context: The chloroplast's green color and its role in photosynthesis provide support for the Endosymbiotic Theory.

Steps of Photosynthesis: The Light Reaction

• Process Initiation: Electrons ($e^-$) generated from photolysis are trapped by Photosystem II (PSII) and energized by light.
• Electron Transport:
  • Two excited electrons ($2e^-$) are passed to a primary electron acceptor.
  • These electrons move through the Electron Transport Chain (ETC).
  • As these electrons lose energy during transport, that energy is utilized to form approximately $1.5\,ATP$.
• Photosystem I (PSI): The ETC terminates at PSI. Electrons are re-energized by light and passed to a different primary electron acceptor.
• Electron Pathways: From PSI, electrons can take one of two paths:
  • Cyclic Phosphorylation:
    • Two electrons ($2e^-$) from PSI return to the first ETC.
    • This cycle generates $1\,ATP$.
    • The electrons are recycled back into PSI and can subsequently enter either the cyclic or non-cyclic pathway again.
  • Non-Cyclic Phosphorylation:
    • Two electrons ($2e^-$) proceed through a different ETC.
    • PSI reduces $NADP^+$ and $H^+$ to $NADPH$ using these two electrons.
    • This $NADPH$ is then utilized in the Calvin Cycle to synthesize glucose.
• Key Products of the Light Reaction: $NADPH$, $O_2$, and $ATP$.

Photolysis and Chemiosmosis

• Photolysis:
  • Occurs at Photosystem II (PSII).
  • Reaction: $H_2O \rightarrow 2H^+ + 2e^- + \frac{1}{2}O_2$.
  • The $2H^+$ (protons) are used for the formation of $NADPH$.
  • The $2e^-$ replenish the electrons lost by PSII during excitation.
  • Oxygen ($O_2$) is released into the atmosphere as a gas byproduct.
• Chemiosmosis:
  • Mechanism: Uses a hydrogen ion ($H^+$) gradient to generate $ATP$. This occurs across the thylakoid membrane.
  • Proton Accumulation in the Thylakoid Lumen:
    1. $H^+$ is released directly into the lumen through the photolysis of water.
    2. Between PSII and PSI, cytochromes pump $H^+$ from the stroma into the lumen.
  • Gradient Formation: This creates both a pH gradient and an electrical gradient.
  • ATP Production: ATP Synthase utilizes the $H^+$ gradient and the resulting proton motive force to phosphorylate $ADP$ into $ATP$.
  • Purpose: The $ATP$ generated here is used in the Calvin Cycle for glucose synthesis.

The Dark Reaction (Calvin Cycle)

• Purpose: To fix inorganic Carbon Dioxide ($CO_2$) into an organic form, specifically glucose ($2\,G3P$).
• Dependency: Although it is the "light-independent" reaction, it requires the high-energy molecules $NADPH$ and $ATP$ produced during the light reaction.
• Stages of the Calvin Cycle:
  1. Carboxylation:
     • $6CO_2 + 6RuBP \rightarrow 12PGA$
     • This reaction is catalyzed by the enzyme RuBisCo.
  2. Reduction:
     • $12ATP + 12NADPH$ are used to convert the $12PGA$ into $12G3P$ (also known as $12PGAL$).
     • The resulting byproducts, $NADP^+$ and $ADP$, return to the non-cyclic photophosphorylation pathway.
  3. Regeneration:
     • $6ATP$ are used to convert $10G3P$ back into $6RuBP$, allowing the cycle to continue.
  4. Carbohydrate Synthesis:
     • The remaining $2G3P$ molecules are extracted from the cycle to form one molecule of glucose ($C_6H_{12}O_6$).

Action Spectrum of Photosynthesis

• Absorption Efficiency: Chloroplasts are most effective at absorbing light in the red and blue wavelength regions.
• Ineffective Wavelengths: Green light wavelengths are reflected rather than absorbed, making green the least effective color for driving photosynthesis.
• Graphic Representation: The absorption peaks for Chlorophyll A and Chlorophyll B align with the blue ($\sim 430-450\,nm$) and red ($\sim 640-680\,nm$) ends of the visible spectrum.

Alternatives to C3 Photosynthesis

• C4 Photosynthesis:
  • Strategy: Alters the physical location of photosynthesis.
  • Mechanism: Carbon dioxide is moved to bundle sheath cells.
  • Benefit: This minimizes photorespiration and water ($H_2O$) loss.
  • Cost: It produces an intermediary 4-carbon compound and requires the expenditure of $1$ extra $ATP$.
• CAM Photosynthesis:
  • Strategy: Alters the timing of photosynthesis.
  • Mechanism: Fixes $CO_2$ during the night rather than the day.
  • Benefit: This temporal separation effectively minimizes photorespiration and water ($H_2O$) loss in arid environments.