Light Dependent Reactions

Light dependent reactions take place in the thylakoid membrane

Photosynthesis Begins

• Starts with light being absorbed by a photosystem

  • A large complex of molecules - pigments, proteins, and cofactors embedded in thylakoid membrane

• Reaction Center

  • The core of the photosystem

  • Contains “special pair” a pair of chlorophylls

  • Jointly absorb energy

  • Emits electrons

• Photosystem I (PSI)

  • AKA p700

  • Photons are best by 700 nm wavelengths

• Photosystem II (PSII)

  • AKA p680

  • Photons are best absorbed by 680 nm wavelengths

• Light Harvesting Complexes (LHC)

  • Most energy absorbed by photosynthesis happens here instead of photosystems

  • Circular array of chlorophylls, accessory pigments, lipids, and proteins

  • Part of photosystem

LHC Energy Volleyball

• Pigments in LHC absorbs photon

• Electron moves to a higher energy level (further from nucleus)

• Then quickly drops back down to lower energy level (closer to nucleus)

• Movement of electron emits energy

• LHC holds energy and passees it back and forth to reaction centers of other nearby LHCs

There are 2 types of light dependent reactions based on PSI and II

Cyclic light dependent reactions

Also called: cyclic photophosphorylation

• Only uses photosystem I

  • 1: Starts by special pair in PSI absorbing energy

  • 2: Electrons from light energy enter electron transfer chain in thylakoid membrane = energy

  • 3: use energy to move Hydrogen ions (H+) from stroma to thylakoid compartment (inside thylakoid disk)

  • 4: movement of H ions into thylakoid creates a H ion gradient across thylakoid membrane

  • 5: H ion gradient creates potential energy. Can use potential energy to make ATP. Ions want to diffuse following gradient (high to low) back to stroma but can’t. Only way to leave is by ATP synthases in thylakoid membrane

  • 6: H ions flow through ATP synthase, causing it to spin. Spinning synthase phosphorylates ADP to ATP - forming ATP in stroma. Called electron transfer phosphorylation: any process where flow of electrons through electron transfer chain drives ATP formation

  • 7: need constant supply of electrons for photosystem. Photosystem accepts electrons that reach end of electron transfer chain = cycle.

Noncyclic Light dependent reactions

Main pathway in modern photosynthetic organisms

AKA noncyclic electron transfer phosphorylation

• Uses photosystems I and II (which prefer different wavelengths)

  • 1: Starts when special pair in PSII absorbs energy. Emits electrons

  • 2: PS pulls replacement electrons from water molecules. This is hard to do so it breaks the water molecules in H, H, and O. H ions stay inside thylakoid. O combines with others and is then released as O₂ gas. Breaking apart of molecules by light energy = photolysis.

  • 3: electrons enter electron transfer chain in thylakoid membrane

  • 4: transfer of electrons creates energy to actively transport H ions across stroma to inside thylakoid. Movement of H ions creates ion gradient across thylakoid membrane

  • 5: electrons being passed in transfer chain of PSII is accepted by PSI. PSI reaction center absorbs energy from electron transfer. Starts new electron transfer chain in PSI. electron hot potato (PSII passes electron transfer energy to PSI)

  • 6: At the end of the electron transfer chain is NADP+. Coenzyme NADP+ accepts electrons and H ions = NADPH. NADPH is reducing agent (electron donor) - used in second stage

  • 7: H ions have to actively transport through ATP synthases since they can't diffuse back following the gradient

  • 8: spins generating ADP to ATP (electron transfer phosphorylation). ATP forms in stroma.

  • Noncyclic since uses light energy to drive electron transfer phosphorylation

Main Differences

• Cyclic produces only ATP and electrons come back to be passed again, does not make oxygen

• Noncyclic produces ATP, NADPH, and oxygen. And the electrons do not come back (they’re passed to PSII)

Evolution of the two pathways

• Cyclic evolved first

  • Can produce ATP but not NADPH to make sugar

• Both pathways run on electrons

• Early photosynthesizers got their electrons from limited resources (inorganic compounds) - cyclic pathway (recycle)

• Cyclic became noncyclic

  • As photosynthesizers evolved so did their photosystems

  • PSI became PSII - noncyclic pathway

Advantages to noncyclic pathway

• Organisms didn’t have to rely on scarce inorganic compounds

• Made sugar production more efficient

  • PSII is strong oxidizer (can pull electrons from other molecules) - only one that can pull electrons from water molecules

  • So can get more electrons from environment than PSI

  • More electrons = more ATP = more sugars

Modern Photosynthesizers

• All plants, algae, and cyanobacteria today use noncyclic pathway (primary)

• The noncyclic pathway by itself doesn’t make enough ATP for NADPH

• Cyclic pathway provides bit more ATP (secondary)

When Does Cyclic Pathway Take Over?

• Excess light energy stalls noncyclic pathway

• Too much light can create dangerous free radicals (unstable atoms that can destroy cells)

• PSII minimizes effect - stops releasing electrons to transfer chain

• Emits absorbed energy as heat instead

• Use cyclic pathway during this time