Photosynthesis Overview

Overview

This lecture introduces photosynthesis, its stages, the key organelles and molecules involved, and summarizes the main reactions and products.

Photosynthesis uses light energy to build carbohydrates (mainly glucose) from water and carbon dioxide.
The overall equation: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂.
Water enters plants through roots, CO₂ enters leaves via stomata, and O₂ exits through stomata.

Photosynthesis occurs in chloroplasts; cellular respiration occurs in mitochondria.
Chlorophyll, found in thylakoids, absorbs blue/red light and reflects green.
Thylakoid stacks are called grana (singular: granum); the stroma is chloroplast fluid.

Photosynthesis has two stages: light-dependent reactions (in thylakoids) and light-independent reactions (Calvin cycle, in stroma).
Light-dependent reactions use water, ADP, phosphate, and NADP⁺, producing O₂, ATP, and NADPH.
Light-independent reactions (Calvin cycle) use CO₂, ATP, and NADPH to make glucose, with ADP, phosphate, and NADP⁺ as byproducts.

Begins when light excites chlorophyll in Photosystem II (P680 nm); water is oxidized, releasing O₂, electrons, and H⁺ ions.
Electrons travel via plastoquinone to cytochrome b6f, pumping protons into the thylakoid lumen.
Electrons pass to plastocyanin, then are re-energized in Photosystem I (P700 nm).
Ferrodoxin carries electrons to NADP⁺ reductase, producing NADPH.
ATP synthase uses proton gradient (chemiosmosis) to form ATP from ADP and phosphate.

Has three phases: carbon fixation, reduction, and regeneration of RuBP (ribulose-1,5-bisphosphate).
CO₂ is fixed to RuBP by enzyme rubisco, forming 3-phosphoglycerate (PGA).
ATP phosphorylates PGA to 1,3-bisphosphoglycerate; NADPH reduces it to glyceraldehyde-3-phosphate (G3P).
1 out of 6 G3P molecules forms sugars; the rest regenerate RuBP.
To make one glucose: 6 CO₂, 18 ATP, and 12 NADPH are used.

Photosynthesis — Process converting light energy, water, and CO₂ into glucose and O₂.
Chloroplast — Plant organelle where photosynthesis occurs.
Chlorophyll — Green pigment absorbing light for photosynthesis.
Thylakoid — Membranous sac inside chloroplast, site of light-dependent reactions.
Stroma — Fluid inside chloroplast surrounding thylakoids.
Calvin Cycle — Series of reactions converting CO₂ to glucose, independent of light.
RuBP (Ribulose-1,5-bisphosphate) — Five-carbon compound accepting CO₂ in Calvin cycle.
Rubisco — Key enzyme catalyzing CO₂ fixation in Calvin cycle.
ATP Synthase — Enzyme producing ATP using proton gradient.