Photosynthesis Reactions 10/15

Light Dependent Reactions

Occur in the thylakoid membrane, require light, and produce ATP and NADPH.

Calvin Cycle (Light Independent)

Occurs in the stroma and processes where enzymes reduce CO₂ into carbohydrates, requiring ATP, NADPH, and CO₂.

NADP+

Molecule that accepts and delivers 2 electrons and 1 hydrogen ion (H⁺).

Reduction

The process of gaining electrons.

Oxidation

The process of losing electrons.

ATP (Adenosine Triphosphate)

High energy molecule with 3 phosphate groups.

ADP (Adenosine Diphosphate)

Lower energy molecule with 2 phosphate groups.

Water Splitting (Photolysis)

Water is split at PSII, donating electrons to PSII, releasing oxygen atoms as O₂ and building up hydrogen ions (H⁺) concentration in the lumen.

Light Absorption

Light is absorbed by pigments in PSII, transferring energy to electrons in the reaction center, energizing them.

Electron Transport Chain (ETC)

Electrons move down the ETC.

Photosystem I (PSI)

PSI accepts electrons from the ETC and absorbs light by pigments, reenergizing the electrons.

Electron Transport Chain

A series of protein complexes and other molecules that transfer electrons through a membrane within mitochondria or thylakoids, critical for generating ATP.

NADP Reductase

Electrons pass through the membrane protein called NADP reductase.

Lumen Function

The lumen acts as a hydrogen ion reservoir, with hydrogen ions (H⁺) moving through ATP synthase.

ATP Synthase

ATP synthase catalyzes the bonding of ADP and an inorganic phosphate (Pi) to produce ATP.

Chemiosmosis

Movement of ions down a chemical gradient.

Final Products

The main products of the light-dependent reactions are ATP and NADPH.

RuBisCO

RuBisCO is the enzyme that fixes carbon into ribulose bisphosphate (RuBP).

Carbon Reduction

ATP and NADPH, produced from the light-dependent reactions, are used in the Calvin Cycle; 3PGA undergoes reduction to form PGAL and G3P.

Organic Molecule Production

The 3PGA molecules are starting materials for many organic molecules, helping produce starch, cellulose, sucrose, and plant oils.

Photorespiration

Occurs when Rubisco fixes O₂ instead of CO₂, leading to a loss of CO₂, more likely when CO₂ levels are low and O₂ levels are high.

RUBP

Ribulose bisphosphate, the substrate for Rubisco.

3-PGA

3-Phosphoglycerate, a product formed in the Calvin Cycle.

Triose Phosphate

Includes molecules like G3P (Glyceraldehyde-3-phosphate).

2-PG

2-Phosphoglycolate, a byproduct of photorespiration.

Calvin Cycle Overview

CO₂ is fixed by Rubisco into RUBP, forming 3-PGA, which is converted into triose phosphates using ATP and NADPH, resulting in sugars and other organic molecules.

Evolutionary Context

Photorespiration evolved when atmospheric O₂ levels were lower and became problematic as O₂ levels increased.

Significance of Oxygen Increase

The increase in O₂ allowed for the evolution of complex life forms and aerobic organisms, while anaerobic life forms declined.