Photosynthesis Flashcards

Overview

• Photosynthesis converts light energy from the sun into glucose.
• Chlorophyll A, B, and carotenoids absorb energy from sunlight.
• Photons excite electrons.
• Excited electrons are unstable and re-emit energy.
• Energy is absorbed by Chlorophyll A molecules, specifically P680 (PSII) and P700 (PSI).
• Chlorophyll A has a porphyrin ring with a $Mg^{2+}$ atom inside.

Chloroplast Anatomy

Photosynthesis takes place in the chloroplast, a double-membraned organelle in plant cells. Key structures include:

1. Outer Membrane
2. Granum: Stacks of thylakoids
3. Thylakoid
4. Thylakoid Membrane
   • Location of the Electron Transport Chain (ETC) involved in Non-Cyclic Photophosphorylation.
   • The structure absorbs light.
5. Thylakoid lumen
   • Location of photolysis.
   • H+ ions accumulate here due to the ETC. This creates a proton gradient to drive ATP synthesis by chemiosmosis.
6. Intermembrane Space
7. Inner Membrane
8. Stroma
   • Location of the Calvin Cycle.
9. Stroma Lamellae
   • Location of Cyclic Photophosphorylation

Steps of Photosynthesis

Photosynthesis is generally divided into two main stages:

• Light Reaction (Light-Dependent Reactions)
• Dark Reaction (Light-Independent Reactions or Calvin Cycle)

Light Reaction (Photophosphorylation)

• The electrons ($e^{-}$-) trapped by PSII are energized by light.
• Two excited $e^{-}$- are passed to a primary $e^{-}$- acceptor and move through the Electron Transport Chain (ETC).
• These two $e^{-}$- lose their energy, which is used to form approximately 1.5 ATP via chemiosmosis.
• The ETC ends at PSI where $e^{-}$- are re-energized and passed to a different primary $e^{-}$- acceptor. At this point, electrons can either go through the cyclic or non-cyclic path.

Cyclic Phosphorylation

• Two $e^{-}$- from PSI go back through the first ETC and generate 1 ATP.
• These two $e^{-}$- are recycled into PSI and can either go through the cyclic or non-cyclic pathway again.

Non-Cyclic Phosphorylation

• Two $e^{-}$- go through an ETC and combine with $NADP^+$ & $H^+$ to form NADPH.
• This NADPH is used in the Calvin Cycle to create glucose.

Photolysis

• $H<em>2O$ splits into $2H^+$, $2e^-$, and $\frac{1}{2} O</em>2$.
  • $2H^+$ is used for NADPH formation.
  • The two $e^{-}$- lost in PSII are replenished through this process.
  • $\frac{1}{2} O_2$ is released as gas.
• Photolysis occurs at PSII.

Chemiosmosis

• Uses the $H^+$ gradient to create ATP.
• $H^+$ accumulates in the thylakoid lumen:
  1. $H^+$ is released into the lumen through photolysis.
  2. Between PSII and PSI, cytochromes bring $H^+$ into the lumen from the stroma creating both a pH and an electrical gradient.
• ATP Synthase uses this gradient to turn ADP into ATP.
• This ATP is used in the Calvin Cycle to create glucose.
• Chemiosmosis occurs across the Thylakoid membrane.

Dark Reaction (Calvin Cycle)

• Purpose: Fixes Carbon Dioxide ($CO_2$) into glucose (2 G3P).

1. Carboxylation:
   • $6 CO_2 + 6 RuBP \rightarrow 12 PGA$
   • This reaction is catalyzed by the enzyme RuBisCo.
2. Reduction:
   • $12 ATP + 12 NADPH$ converts 12 PGA to 12 G3P or 12 PGAL.
   • The byproducts, $NADP^+$ & ADP, go into non-cyclic photophosphorylation.
3. Regeneration:
   • 6 ATP convert 10 G3P to 6 RuBP.
4. Carbohydrate Synthesis:
   • The remaining 2 G3P are used to form glucose.

• The Calvin cycle is light-independent; however, it requires the high-energy molecules, NADPH & ATP, produced in the light reaction.

Action Spectrum

• Chloroplasts are highly effective at absorbing red and blue light.
• Green wavelengths are reflected, making green light the least effective for photosynthesis.

Alternatives to C3 Photosynthesis

C4 Photosynthesis

• Alters the location of photosynthesis.
• $CO<em>2$ is moved to bundle sheath cells to minimize photorespiration & $H</em>2O$ loss.
• Produces an intermediary 4-carbon compound and uses 1 extra ATP.

CAM Photosynthesis

• Alters the timing of photosynthesis.
• Fixes $CO<em>2$ at night instead of during the day to minimize photorespiration & $H</em>2O$ loss.