Light-Dependent Reactions of Photosynthesis [detailed]

Light-Dependent Reactions of Photosynthesis

Photosynthesis locations:

• Light reactions occur in the thylakoid membrane.
• A stack of thykaloids is called grana.
• Dark Reactions (Calvin Cycle) occur in the stroma (fluid between grana).

Light absorbing pigments

• Chlorophylls a and b
• Absorb 70% of red and blue wavelength; highest O2 production at these wavelengths.
• Only present in green plants and green algae.
• Carotenoids
• Absorb violet and blue-green light.
• Phycobilins – present in algae.

Photochemistry Components

• Chlorophyll contains a porphyrin ring
• Light absorbing head containing magnesium and many double bonds that can be excited.
• Light harvesting complexes (aka antenna complex) – link pigments together and are connected to reaction centers.
• Electrons transferred to primary electron acceptor.
• Photosystem (PS) = light harvesting complexes + reaction center embedded in thylakoid membrane.

Photosynthesis Steps

• Photosystem II (P680): Obtains an electron by splitting water (H2O) and releasing oxygen (O2) as a waste product.
• Note that electron energy is also used to pump H+ from the stromal side to the lumen of the thykaloid.
• These hydrogen atoms will be used to produce ATP later.
• Since the traveling electrons loose energy, they must be "reenergized" by photosystem I.

• Electron Transport Chain (ETC): Passing of electrons between PS II and PS I.

• Photosystem I (P700): Similar to PS II, it absorbs a photon from light, which passes through pigments, to chlorophyll a and then the reaction center.
• In this case, an electron came from PS II to replace that one given away by chlorophyll a.
• PS I becomes oxidized and sends an electron to NADP+, reducing it to NADPH.
• ATP Synthase:
• The buildup of H+ in the thykaloid lumen creates a gradient that drives the production of ATP as the ions rush through ATP synthase.
• This is called chemiosmosis.

Non-Cyclic and Cyclic Versions of Light-Dependent Reactions

1. Non-Cyclic Photophosphorylation
2. What was discussed so far was the non-cyclic version of the light-dependent reactions, also known as Z-scheme. It is called that because the energy of the electrons involved goes up and down, in a pattern that resembles a "Z".

1. Cyclic Photophosphorylation
2. If ATP and NADPH are needed at different ratios, electrons can be cycled through photosystem I only.
3. Instead of going to reduce NADP+, the electron goes through the ETC, driving H+ into the lumen.
4. This gradient is used to produce ATP from ADP.