Photosynthesis Notes

Photosynthesis Overview

Light Reactions

• Occur in the thylakoid membranes.
• Convert solar energy to chemical energy.
• Net products: NADPH, ATP, and oxygen.
• Primary events:
  • Light energy absorbed by chlorophyll drives electron transfer from water to NADP+, forming NADPH.
  • Water is split, releasing $O_2$.
  • ATP generated via chemiosmosis (photophosphorylation).

Calvin Cycle

• Occurs in the stroma.
• $CO_2$ from the air is incorporated into organic molecules (carbon fixation).
• Uses fixed carbon, NADPH, and ATP from light reactions to form new sugars ($[CH_2O]$).

Chloroplast Structure

• Chloroplasts are the sites of photosynthesis in plant cells.
• Enclosed by two membranes, forming an envelope.
• Stroma: Fluid-filled area within the chloroplast.
• Thylakoids: interconnected membranous sacs within the stroma, segregating the stroma from the thylakoid space.
• Chlorophyll is located in the thylakoid membranes.

Overall Reaction of Photosynthesis

• $6 CO<em>2 + 6 H</em>2O + \text{Light energy} \rightarrow C<em>6H</em>{12}O<em>6 + 6 O</em>2$
• Reverse of cellular respiration.
• Oxygen is formed when a water molecule is split.
• Water is split for its electrons, which reduce carbon dioxide to sugar.

Light and Pigments

• Light is electromagnetic energy that travels in waves; the visible spectrum includes ROY G BIV.
• Light behaves as discrete particles called photons.
• Pigments absorb light of different wavelengths.
• Chlorophyll absorbs violet-blue and red light, reflecting green light.
• Absorption spectrum: a graph plotting a pigment's light absorption as a function of wavelength.
• Action spectrum: graphs the effectiveness of different wavelengths in driving photosynthesis.

Photosystems

• Photosystems are groups of pigment molecules in the thylakoid membrane.
• Composed of a light-harvesting complex and a reaction center.
• Light-harvesting complex: chlorophyll and carotenoid molecules gather light.
  *Energy is transferred to the reaction center where chlorophyll a molecules donate electrons to a primary electron acceptor.
• Splitting of water replaces donated electrons.

Photosystems I & II

• Thylakoid membranes contain two photosystems: PS I and PS II.
• PS I (P700) absorbs red light best at 700 nm; PS II (P680) absorbs red light best at 680 nm.

Light Reactions Steps

1. PS II absorbs light, exciting electrons in the P680 reaction center.
2. Electrons are transferred to the primary electron acceptor; chlorophyll is oxidized.
3. An enzyme splits water into two hydrogen ions ($H^+$), two electrons, and an oxygen atom ($O$).
4. Excited electron passes from PS II to PS I through an electron transport chain.
5. $H^+$ accumulates in the thylakoid space, creating a gradient used in chemiosmosis to produce ATP.
6. Light activates PS I, donating an electron to its primary electron acceptor; electrons are replaced by those from PS II.
7. The primary electron acceptor of PS I passes electrons along another electron transport chain.
8. Excited electrons are transmitted to $NADP^+$, reducing it to NADPH.

Chemiosmosis

• Electron transport chain pumps $H^+$ across the thylakoid membrane from the stroma into the thylakoid space.
• Electrochemical gradient is created.
• ATP synthase phosphorylates ADP to ATP as $H^+$ flows out of the thylakoid space into the stroma.
• Proton-motive force generated by:
  • Hydrogen ions from splitting water.
  • Hydrogen ions pumped by the cytochrome complex.
  • Removal of hydrogen ions from the stroma when $NADP^+$ is reduced to NADPH.
• Chloroplasts transform light energy into the chemical energy of ATP.

Calvin Cycle Steps

1. Three $CO_2$ molecules attach to three RuBP molecules, catalyzed by rubisco, producing an unstable product that splits into 3-phosphoglycerate.
2. 3-phosphoglycerate molecules are phosphorylated (using ATP) to become 1, 3-bisphosphoglycerate.
3. Six NADPH molecules reduce 1, 3-bisphosphoglycerate molecules to glyceraldehyde 3-phosphate (G3P) molecules.
4. One G3P molecule leaves the cycle.
5. The remaining five G3P molecules are rearranged to generate RuBP molecules.

C4 Photosynthesis Adaptation

• Structural strategy: Light reactions and Calvin cycle occur in different cells.
• Biochemical strategy: PEP carboxylase fixes carbon, reducing photorespiration.

CAM Photosynthesis Adaptation

• Stomata close during the day, preventing water loss, and open at night to fix $CO_2$ in organic acids.
• Two stages of photosynthesis are separated temporally.