Photosynthesis Steps to Know for AP Biology

Photosynthesis is the process plants use to convert light energy into chemical energy. It involves light absorption, electron excitation, and the production of ATP and NADPH, which fuel the Calvin cycle to create glucose. Understanding these steps is key in biology.

1. Light absorption by chlorophyll and other pigments

   • Chlorophyll a and b absorb light primarily in the blue and red wavelengths, reflecting green light.
   • Accessory pigments, such as carotenoids, capture additional light energy and protect against photo-damage.
   • Light absorption occurs in the thylakoid membranes of chloroplasts, where pigments are organized into photosystems.

2. Excitation of electrons in photosystems

   • When chlorophyll absorbs light, it excites electrons to a higher energy state.
   • This process occurs in two main photosystems: Photosystem II (PSII) and Photosystem I (PSI).
   • The excited electrons are transferred to an electron transport chain, initiating the light-dependent reactions.

3. Electron transport chain

   • The electron transport chain consists of a series of proteins embedded in the thylakoid membrane.
   • As electrons move through the chain, they release energy used to pump protons (H+) into the thylakoid lumen, creating a proton gradient.
   • The final electron acceptor is NADP+, which is reduced to NADPH.

4. ATP synthesis through chemiosmosis

   • The proton gradient created by the electron transport chain drives ATP synthesis via ATP synthase.
   • Protons flow back into the stroma through ATP synthase, catalyzing the conversion of ADP and inorganic phosphate (Pi) into ATP.
   • This process is known as photophosphorylation and is essential for energy production in photosynthesis.

5. Light-dependent reactions overview

   • These reactions occur in the thylakoid membranes and require light to produce ATP and NADPH.
   • Water is split (photolysis) to provide electrons, releasing oxygen as a byproduct.
   • The products (ATP and NADPH) are then used in the Calvin cycle for carbon fixation.

6. Carbon fixation in the Calvin cycle

   • The Calvin cycle occurs in the stroma of chloroplasts and does not require light directly.
   • Carbon dioxide (CO2) is fixed into an organic molecule by the enzyme RuBisCO, forming 3-phosphoglycerate (3-PGA).
   • This process is crucial for converting inorganic carbon into organic compounds.

7. Reduction of 3-PGA to G3P

   • ATP and NADPH produced in the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).
   • G3P is a three-carbon sugar that can be used to form glucose and other carbohydrates.
   • This step is essential for synthesizing energy-rich molecules that support plant growth.

8. Regeneration of RuBP

   • Some G3P molecules are used to regenerate ribulose bisphosphate (RuBP), allowing the cycle to continue.
   • This regeneration requires ATP and ensures that the cycle can fix more CO2.
   • The process maintains the cycle's efficiency and supports ongoing photosynthesis.

9. Light-independent reactions overview

   • Also known as the Calvin cycle, these reactions do not require light but depend on ATP and NADPH from the light-dependent reactions.
   • The cycle consists of carbon fixation, reduction, and regeneration phases.
   • It ultimately produces glucose and other carbohydrates, which serve as energy sources for the plant.

10. Factors affecting photosynthesis rate

    • Light intensity: Higher light levels increase the rate of photosynthesis up to a certain point.
    • Carbon dioxide concentration: Increased CO2 levels enhance the rate of carbon fixation.
    • Temperature: Photosynthesis has an optimal temperature range; extreme temperatures can inhibit enzyme activity.
    • Water availability: Insufficient water can limit photosynthesis by causing stomatal closure and reducing CO2 intake.