Photosynthesis

Overview of Photosynthesis

Connection to Cellular Respiration:
- Photosynthesis and cellular respiration share themes of energy and matter, specifically focusing on carbon, oxidation, and reduction.

Cellular Respiration: Both plants and animals perform cellular respiration to generate ATP from sugars.
Sugar Acquisition:
- Animals consume sugars from food.
- Plants produce their own sugars through photosynthesis using CO2 and sunlight.

CO2 Input: Plants convert atmospheric CO2 into sugars.
- Requires energy from sunlight, thus representing a positive change in free energy (positive delta G).
- Plants utilize CO2, which is seen as waste by animals, to synthesize sugars.

Stomata:
- Small openings on the underside of leaves for gas exchange (CO2 and O2).
Chloroplasts:
- Organelles where photosynthesis occurs.
- Double membrane with an internal structure called thylakoids, which contain chlorophyll (green pigment).
- Mesophyll: Leaf tissue packed with chloroplasts.
Thylakoid Structures:
- Individual thylakoids represent 'pancakes', and a stack is called a granum.
- The stroma is the fluid surrounding the thylakoids.

Overall Reaction Comparison:
- Photosynthesis involves reducing CO2 to form sugar, while cellular respiration involves oxidizing glucose to CO2.
- Oxygen produced in photosynthesis comes from water, not directly from CO2.

Two Major Phases:
1. Light Reactions (Light-Dependent Reactions):
  - Occur in thylakoids and depend on sunlight.
  - Convert light energy into chemical energy (ATP and NADPH).
  - Involve the splitting of water, releasing O2 as a byproduct.
  - Chlorophyll and photosystems play critical roles in capturing sunlight.
2. Calvin Cycle (Light-Independent Reactions):
  - Occur in the stroma.
  - Utilize ATP and NADPH to convert CO2 into sugars (G3P).
  - Carbon Fixation: CO2 is fixed into a 5-carbon sugar (RuBP) by the enzyme Ribulose bisphosphate carboxylase/oxygenase (Rubisco).
  - Energy Investment: ATP and NADPH are used to convert 3-phosphoglycerate into G3P.
  - Regeneration: Remaining G3P is used to regenerate RuBP, facilitating continuous cycling.

Nature of Light:
- Electromagnetic radiation traveling in waves with different wavelengths.
- Shorter wavelengths (e.g., gamma rays) carry higher energy, while longer wavelengths (e.g., radio waves) carry lower energy.
- Visible light (400-700 nm) is crucial for photosynthesis; plants primarily absorb red and blue wavelengths, reflecting green.
Pigment Molecules:
- Chlorophyll a and b absorb light, and carotenoids provide protective functions.
- Different pigments in plants lead to color changes in autumn as chlorophyll degrades.

Photosystem II and I:
- Photosystem II (PS II) captures light and donates energized electrons to an electron transport chain, producing ATP.
- Photosystem I (PS I) receives low-energy electrons and reenergizes them to generate NADPH.
Electron Transport Chain (ETC):
- Similar to cellular respiration; utilizes a proton gradient to produce ATP (via ATP synthase) and reduces NADP+ to NADPH (using NADP+ reductase).

Components:
- Carbon Fixation: Rubisco catalyzes the incorporation of CO2 into RuBP, yielding 3-phosphoglycerate (3-PGA).
- Reduction Phase: ATP and NADPH fuel the conversion of 3-PGA to G3P.
- Regeneration Phase: Some G3P is converted back to RuBP using ATP, allowing the cycle to continue.
Output:
- For every 3 CO2 molecules fixed, 1 G3P is produced, which can be rearranged to form glucose or other carbohydrates.
- The Calvin cycle must turn 6 times to produce one glucose molecule.

Commonality Between Processes: Both utilize electron transport chains and ATP synthases to create ATP, despite operating in different organelles (mitochondria vs. chloroplasts).
Cyclic vs. Linear Electron Flow:
- Cyclic flow primarily generates ATP, ensuring a sufficient supply relative to NADPH for the Calvin cycle.