Detailed Study Notes on Photosynthesis

Introduction to Photosynthesis

Essential inputs for plant growth: carbon dioxide (CO2), water (H2O), and energy (sunlight).

The process through which plants manufacture glucose, which serves as the building blocks of plants.
Oxygen gas (O2) is produced as a byproduct of photosynthesis.
Energy source for photosynthesis: sunlight.
Properties of sunlight:
- Exhibits wave and particle nature (wave-particle duality).
- Composed of particles called photons.
- Photons oscillate along a path measured as wavelengths.

Sunlight contains photons across a range of wavelengths known as the electromagnetic spectrum.
Photosynthetic organisms utilize only a small portion of this spectrum—specifically, visible light.
Role of pigments in photosynthetic organisms:
- Capture visible light wavelengths.
- Color of pigments corresponds to wavelengths of light reflected.
Example: Plants appear green because they reflect yellow and green light while absorbing red and blue light, which provides energy for photosynthesis.

Chloroplasts contain:
- Thylakoids: small, disk-like structures where light-dependent reactions occur.
- Stroma: fluid-filled space where the Calvin cycle occurs.

Photosynthesis consists of two main sets of reactions:
- Light-dependent reactions
- Calvin cycle

Function: Convert light energy into chemical energy.
Location: Thylakoids of the chloroplasts.
Photosystems involved:
- Photosystem I (PSI)
- Photosystem II (PSII)
Structure of Photosystems:
- Composed of accessory pigment molecules and chlorophyll.
Mechanism:
- Absorbed light energy excites electrons to a higher energy state.
- Excitation energy is channeled to a reaction center chlorophyll molecule.
- Electrons are passed to a series of proteins on the thylakoid membrane.
Photon interactions:
- Photons strike PSII and PSI simultaneously.
- In PSII, energized electrons are transferred to an electron transport chain (ETC).
Photolysis:
- Water molecules are oxidized to replace lost electrons in PSII, producing free electrons and oxygen gas (O2).
Importance of oxygen gas:
- Serves as an input for cellular respiration pathways.
Function of electron transport:
- Electrons moving through the ETC generate energy to pump hydrogen ions (H+) from stroma into thylakoid lumen, creating a concentration gradient.
- This gradient powers ATP synthase, which phosphorylates ADP to produce ATP.
PSI Contributions:
- Low-energy electrons from PSII enter PSI, become re-energized, and reduce NADP+ to NADPH through another ETC.

Function: Utilize ATP and NADPH produced from light-dependent reactions to synthesize glucose.
Location: Stroma of the chloroplast.
Steps of the Calvin Cycle:
1. Carbon Fixation:
- Carbon dioxide (CO2) is attached to ribulose 1,5-bisphosphate (RuBP).
- Forms a six-carbon molecule that splits into two three-carbon molecules (3-phosphoglycerate, 3-PGA).
1. Reduction Phase:
- Uses electrons from NADPH and ATP to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).
1. Regeneration of RuBP:
- Five G3P molecules are utilized to reform three molecules of RuBP for every three turns of the cycle.
  - Production of glucose:
- It takes two G3P to synthesize one glucose phosphate molecule.
- Hence, the Calvin cycle must complete six turns to generate one glucose molecule.

G3P can be utilized for:
- Producing glucose and fructose (which combine to form sucrose).
- Synthesizing starch and cellulose, which are essential storage and structural molecules, respectively.
- Plant benefits from produced sugars as storage molecules and structural components.

Photosynthetic organisms (plants) serve as the primary producers of glucose on Earth.
They produce oxygen gas as a byproduct, which is essential for life.
Photosynthesis is foundational for ecosystems, supporting food webs on land and in oceans.