Introduction to Photosynthesis and Light Reactions

Introduction: Light Reactions in Chloroplasts

Photosynthesis Definition:
- Photosynthesis is a biological process that utilizes light energy to transform carbon dioxide (CO2) and water (H2O) into organic compounds such as glucose (C6H12O6) and oxygen (O2).
- Chemical Equation:
  6CO2 + 6H2O + light ext{ energy}
  ightarrow C6H{12}O6 + 6O2
Wavelengths of Visible Light:
- Visible light consists of various wavelengths perceived as different colors.
- Short wavelengths include violet/blue light (380-500 nm), whereas long wavelengths include orange/red light (600-700 nm).
- Photosynthetic pigments, primarily chlorophyll a and chlorophyll b, have their peak absorbance in these regions:
- Chlorophyll a and b have peak absorbance around these wavelengths and reflect green light (500-600 nm), leading to the general green appearance of photosynthetic organisms. Some organisms, like brown and red algae, may appear non-green due to accessory pigments that reflect different wavelengths.

Types of Reactions in Photosynthesis:
- Photosynthesis occurs in two distinct phases:
1. Light-Dependent Reactions:
  - Require a light source.
  - Capture sunlight energy and convert it into chemical energy (ATP and NADPH).
2. Light-Independent Reactions (Calvin Cycle):
  - Do not require light directly.
  - Use ATP and NADPH to synthesize organic compounds (e.g., glucose).
Components of Light-Dependent Reactions:
- Involve two photosystems:
1. Photosystem II (PS II):
  - Absorbs light, excites electrons in chlorophyll a (P680), and captures the energy.
  - Primary outputs include ATP production through the electron transport chain (ETC).
  - Water (H2O) is split here, releasing O2 as a by-product.
2. Photosystem I (PS I):
  - Absorbs light, further excites electrons in chlorophyll a (P700) and transfers them to NADP+ to form NADPH.
- Electron Transport Chain (ETC):
- Includes proteins like plastoquinone, cytochrome complex, and ferredoxin, which facilitate electron transfer, leading to the production of energy-rich compounds.
Primary Outputs of Light-Dependent Reactions:
- ATP (Adenosine Triphosphate): Produced through chemiosmotic synthesis as electrons move through the ETC.
- NADPH: Formed as electrons pass through the entire pathway, utilized in light-independent reactions.
- Oxygen (O2): Evolved as a by-product from water splitting.

The Hill Reaction:
- Defined as the reduction of an electron acceptor (A) using electrons and protons derived from water, leading to oxygen generation under light exposure:
- Hill Reaction Equation:
  H2O + A + light ext{ energy} ightarrow AH2 + rac{1}{2} O_2
- Significance: Robert Hill's discovery in 1937 confirmed that:
- The source of electrons in light reactions is water.
- Evolved oxygen comes from water, not carbon dioxide.
- Light reactions can be studied experimentally in isolated chloroplasts.

Study Using Artificial Electron Acceptors:
- In vitro, various artificial electron acceptors can be employed, such as 2,6-dichlorophenol-indophenol (DPIP).
- Functionality of DPIP:
- DPIP appears blue when oxidized and colorless when reduced.
- By adding DPIP to isolated chloroplast solutions, scientists gauge the rate of photosynthesis based on the solution color change from blue to green as DPIP is reduced.
- Utilize a spectrophotometer to measure these color changes, thus helping quantify photosynthetic rates.