Photosynthesis pt 2

This set of flashcards covers key concepts from Chapters 1 and 2 focusing on high energy electrons and the processes of photosynthesis.

Describe the general process of photophosphorylation.

The synthesis of ATP from ADP and inorganic phosphate (P_i) using the energy derived from light by chlorophyll and other pigments during photosynthesis. This process primarily occurs in the thylakoid membranes of chloroplasts.

What are the two main types of photophosphorylation in photosynthesis, and how do they differ in their electron flow?

1. Non-cyclic photophosphorylation: Involves both Photosystem II (PSII) and Photosystem I (PSI), with electrons flowing unidirectionally from water to NADP^+; it produces both ATP and NADPH.
2. Cyclic photophosphorylation: Involves only Photosystem I (PSI) where electrons cycle back to the cytochrome complex, producing only ATP, without forming NADPH or releasing oxygen.

What is the primary function of Photosystem II (PSII) in the light-dependent reactions?

PSII is responsible for absorbing light energy to energize electrons, which are then passed to the electron transport chain. Critically, it also splits water (H2O) into electrons, protons (H^+), and molecular oxygen (O2) to replace the lost electrons via a process called photolysis.

What are the main energy-carrying products generated by the electron transport chain during the light-dependent reactions of non-cyclic photophosphorylation?

The electron transport chain, linking PSII and PSI, facilitates the creation of a proton gradient used to synthesize ATP via chemiosmosis (ATP synthase), and ultimately allows Photosystem I to energize electrons that reduce NADP^+ to NADPH. Therefore, the main products are ATP and NADPH.

How does Photosystem I (PSI) contribute to the overall energy capture in photosynthesis?

PSI re-energizes electrons that have passed through the first electron transport chain. These high-energy electrons are then used by the enzyme NADP^+ reductase to reduce NADP^+ to NADPH, an important reducing agent for the Calvin Cycle.

What is the overarching goal of the Calvin Cycle, and where does it occur?

The Calvin Cycle (light-independent reactions) aims to convert atmospheric carbon dioxide (CO_2) into glucose (sugar) using the ATP and NADPH produced during the light-dependent reactions. It takes place in the stroma of the chloroplast.

Describe the carbon fixation step of the Calvin Cycle, including the key enzyme and substrate.

In carbon fixation, an enzyme called RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the attachment of CO_2 to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), forming an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).

What is the dual fate of glyceraldehyde-3-phosphate (G3P) produced during the reduction phase of the Calvin Cycle?

For every six G3P molecules produced (from 3 CO_2 fixed), one molecule exits the cycle to synthesize glucose and other organic compounds. The remaining five G3P molecules are used to regenerate the starting molecule, ribulose-1,5-bisphosphate (RuBP), ensuring the continuous operation of the cycle.