Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing organic compounds from carbon dioxide and water. It occurs in two main stages: the light reactions and the Calvin cycle.

• Light reactions occur in the thylakoid membrane and convert solar energy into chemical energy (ATP and NADPH).

• Calvin cycle occurs in the stroma of the chloroplast and uses ATP and NADPH to fix carbon dioxide into sugars.

Light Reactions

Location and Function

The light reactions take place in the thylakoid membranes of chloroplasts. Their main function is to capture light energy and convert it into chemical energy in the form of ATP and NADPH, which are then used in the Calvin cycle.

• Photosystem II (PSII) absorbs light and uses it to split water molecules, releasing oxygen (O2) as a byproduct and generating a proton (H+) gradient.

• Electron transport chain moves electrons from PSII to Photosystem I (PSI), pumping protons into the thylakoid lumen to create a gradient.

• ATP synthase uses the H+ gradient to synthesize ATP from ADP and inorganic phosphate ().

• Photosystem I (PSI) absorbs light and transfers electrons to NADP+, reducing it to NADPH ().

Key products: ATP, NADPH, and O2 (byproduct).

Summary Table: Light Reactions

Calvin Cycle

Overview and Phases

The Calvin cycle is the set of biochemical reactions that take place in the stroma of chloroplasts. It uses ATP and NADPH produced in the light reactions to fix carbon dioxide and synthesize sugars. The cycle must turn three times to produce one molecule of glyceraldehyde 3-phosphate (G3P).

• Phase 1: Carbon Fixation – CO2 is attached to ribulose bisphosphate (RuBP) by the enzyme RuBisCO, forming 3-phosphoglycerate (3PG).

• Phase 2: Reduction – 3PG is phosphorylated by ATP and reduced by NADPH to form G3P.

• Phase 3: Regeneration of RuBP – Some G3P is used to regenerate RuBP, enabling the cycle to continue.

Phase 1: Carbon Fixation

Carbon fixation is the process of incorporating CO2 into an organic molecule. In C3 plants, CO2 combines with RuBP (a 5-carbon sugar) to form a short-lived 6-carbon intermediate, which quickly splits into two molecules of 3-phosphoglycerate (3PG).

• Enzyme: RuBP Carboxylase-Oxygenase (RuBisCO).

• Product: 3PG, a 3-carbon molecule.

• Significance: C3 plants are named for this 3-carbon product.

Phase 2: Reduction

During the reduction phase, 3PG is converted into G3P using ATP and NADPH.

• Step 1: 3PG is phosphorylated by ATP to form 1,3-bisphosphoglycerate (BPG).

• Step 2: BPG is reduced by NADPH to form G3P.

Equation:

Phase 3: Regeneration of RuBP

To continue the cycle, RuBP must be regenerated. Five out of six G3P molecules produced are used to regenerate three RuBP molecules, using ATP.

• Input: 5 G3P

• Output: 3 RuBP

• ATP is required for this process.

Net Output of Calvin Cycle

• For every three turns of the cycle: 3 CO2 + 9 ATP + 6 NADPH → 1 G3P + 9 ADP + 8 Pi + 6 NADP+

• G3P can be used to synthesize glucose, fructose, sucrose, starch, and cellulose.

Summary Table: Calvin Cycle

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The main energy currency of the cell.

• NADPH: An electron carrier that provides reducing power for biosynthetic reactions.

• RuBisCO: The enzyme that catalyzes the first step of carbon fixation in the Calvin cycle.

• G3P (Glyceraldehyde 3-phosphate): A 3-carbon sugar produced in the Calvin cycle, used to make glucose and other organic molecules.

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

• Stroma: The fluid-filled space in chloroplasts where the Calvin cycle takes place.

Example: Photosynthesis in C3 Plants

Most plants are C3 plants, meaning the first product of carbon fixation is a 3-carbon molecule (3PG). These plants use the Calvin cycle as described above.

Additional info:

• RuBisCO is the most abundant protein on Earth and is responsible for fixing atmospheric CO2 in plants.

• ATP and NADPH produced in the light reactions are consumed in the Calvin cycle to drive the synthesis of sugars.

• Oxygen produced in the light reactions is released as a byproduct and is essential for aerobic life.