Photosynthesis

Introduction to Photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water. This process is fundamental to life on Earth as it provides the primary energy source for most organisms.

• Photosynthesis occurs in the chloroplasts of plant cells.

• It consists of two main stages: the light-dependent reactions and the Calvin cycle (light-independent reactions).

Light-Dependent Reactions

The light-dependent reactions take place in the thylakoid membranes of the chloroplasts and require light to produce ATP and NADPH, which are used in the Calvin cycle.

• Photosystem II (PSII): Absorbs light and uses the energy to extract electrons from water, releasing oxygen as a byproduct.

• Electron Transport Chain (ETC): Transfers electrons from PSII to PSI, pumping protons into the thylakoid lumen to generate a proton gradient.

• Photosystem I (PSI): Absorbs light and uses the energy to transfer electrons to NADP+, forming NADPH.

• ATP Synthase: Utilizes the proton gradient to synthesize ATP from ADP and inorganic phosphate (Pi).

Key Products: ATP, NADPH, and O2

Key Components and Terms

• Chlorophyll: The main pigment involved in capturing light energy.

• Thylakoid: Membrane-bound compartments inside chloroplasts where the light reactions occur.

• Stroma: The fluid-filled space surrounding the thylakoids, where the Calvin cycle takes place.

• Plastoquinone (PQ): An electron carrier in the electron transport chain.

• Cytochrome b6f Complex: Transfers electrons from plastoquinone to plastocyanin and pumps protons into the thylakoid lumen.

• Plastocyanin (PC): A copper-containing protein that transfers electrons to PSI.

• Ferredoxin (Fd): Transfers electrons from PSI to NADP+ reductase.

• NADP+ Reductase: Enzyme that catalyzes the formation of NADPH.

Summary Table: Main Components of the Light Reactions

Key Equations

• Overall light reaction:

Additional info:

• The handwritten notes list the sequence of electron carriers and protein complexes involved in the light reactions of photosynthesis, which is a core topic in biochemistry (see Ch. 15 Photosynthesis).