Photosynthesis: Light Reaction

• Photosynthesis is divided into two parts: the light reaction and the dark reaction.

Light Reaction

• The light reactions happen in the grana and stroma lamellae of the chloroplast.
• Sunlight is turned into chemical energy as ATP (Adenosine Triphosphate) and NADPH (Nicotinamide adenine dinucleotide hydrogen phosphate).
• ATP and NADPH are used as energy sources for carbohydrate production in the dark reaction.
• Light energy gets absorbed by pigments like chlorophyll a, chlorophyll b, and carotenoids.
• Chlorophyll a is the primary pigment in photosynthesis.
• Chlorophyll b and carotenoids are known as accessory pigments.
• Accessory pigments absorb light energy and pass it to chlorophyll a.
• Chlorophyll a molecules use the energy to start reactions producing ATP, NADPH, and oxygen.
• There are two sites for the light reactions: Photosystem I (PSI or P700) and Photosystem II (PSII or P680).
• PSI and PSII are chlorophyll a molecules that absorb light energy at specific wavelengths.
  • PSI absorbs light at 700nm
  • PSII absorbs light at 680 nm
• PSI and PSII receive energy directly from the sun and from accessory pigments.
• PSI and PSII absorb light at the same time and start the light reactions when sunlight hits the leaves.
• PSII releases electrons that go through an electron transport chain toward PSI after absorbing light energy.
• ATP is produced during the electron movement in the electron chain.
• PSII becomes unstable when it releases electrons.
• PSII receives electrons from the splitting of water in order to continue absorbing light energy and return to a stable state.
• The splitting of water releases oxygen (O2), protons (H+), and electrons (e-).
• Oxygen and protons are released at the end of the light reaction in PSII.
• PSI releases electrons that travel through a different electron transport chain to reduce NADP (Nicotinamide adenine dinucleotide phosphate) to NADPH (Nicotinamide adenine dinucleotide phosphate).
• The light reaction yields ATP, NADPH, and oxygen as by-products

Dark Reaction (Carbon-Fixing Reaction, C3 Cycle, or Calvin Cycle)

• It follows the light reaction and is light-independent.
• It is the carbon-fixing reaction and referenced as C3 or the Calvin cycle.
• The reactions take place in the stroma of the chloroplast.
• Uses ATP and NADPH from the light reaction to produce glucose and other carbohydrates (end products of photosynthesis).
• The first step involves ribulose bisphosphate (RuBP), a five-carbon sugar molecule in the stroma.
• Three RuBP molecules bind to 3 molecules of atmospheric CO2 (that enters the chloroplast) to produce 3 molecules of a 6-carbon intermediate compound.
• Ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO) catalyzes the reaction to form the 6-carbon intermediate compound.
• Each 6-carbon compound splits to give 2 molecules and produces 6 molecules of 3-phosphoglycerate (3PG), or phosphoglyceric acid (PGA),.
• 6 ATPs (Adenosine Triphosphate) initially activate the 6 PGAs to 6 ADPs (Adenosine Diphosphate).
• Six molecules of NADPH (a product of the light reaction) then reduce 6 PGAs to 6 phosphoglyceraldehyde (PGAL) by adding a hydrogen ion.
• One molecule of PGAL is used to synthesize glucose, which gets converted to sucrose and other carbohydrates.
• The other 5 molecules of PGAL regenerate RuBP to continue the cycle.
• The regeneration of RuBP requires three ATP molecules.
• The initial RuBP in the stroma must be replaced because it gets used up during the synthesis of glucose.
• Glucose gets converted into sucrose and starch.
  • Sucrose is transported throughout the plant.
  • Starch is stored in the chloroplast.
• C3 plants: rice, wheat, soya beans, barley and potatoes