E. Photosynthesis
➢ $6CO_2 + H_2O → C_6H_12O_6 + 6O_2$
➢ Chloroplast structure
- Stroma=inner fluid-filled region
- grana=structures inside stroma that look like stacks of coins
- thylakoids=”coins” of grana
■ Contain chlorophyll, a light-absorbing pigment that drives photosynthesis
● Chlorophyll a
● Chlorophyll b
● Carotenoids
● Pigments gather light, but are not able to excite electrons, only one molecule in the reaction center can
■ Contains enzymes involved in photosynthesis
➢ 2 reaction centers:
- Photosystem I (PS I)
■ $p700$ - Photosystem II (PS II)
■ $P680$ - Both comprised of a Light harvesting complex, where a photon of light is passed like a wave between pigments and a Reaction center complex, which contains chlorophyll-a and uses light energy to “boost” and electrons and pass onto primary electron acceptor
➢ Absorption spectrum measures how well a certain pigment absorbs electromagnetic radiation
- Opposite of emission spectrum
- Chlorophyll a and b absorb blue and red light but reflect green (reason why plants are usually green)
- Carotenoids absorb light at blue-green end, and reflect red light
➢ Light reactions
- When a leaf captures sunlight, the energy is sent to$ p680$ of photosystem II
■ Sidenote: it may seem weird that the light reaction starts off in PSII and not PS I but its only called PS I because it was discovered first - Activated electrons trapped by p680 and passed down to molecule called the primary acceptor, and then they are passed down to carriers in the electron transport chain
- Photolysis
■ To replenish electrons in the thylakoid, water is split into $O^{-}$ , $2H^{+}$ , and electrons
● Water is split again into hydrogen ions (used for ETC) and Oxygen (released) - As the energized electrons from PSII travel down the ETC, they pump H+ into the thylakoid lumen
■ Proton gradient is created
● As hydrogen ions move back into the stroma through ATP synthase along their concentration gradient, ATP is created - After the electrons leave PS II, they enter PSI, where they are passed through a second ETC until they reach the final electron acceptor$ NADP^{+}$ to make $NADPH$
