Light Dependent Reactions IB Topic 8.2.3: Explain the light dependent reactions

What is light dependent reactions? Light energy is trapped by the photosynthetic pigment chlorophyll Chlorophyll molecules do not occur haphazardly in the grana Rather, they are grouped together in structures called photosystems, held in the thylakoid membranes of the grana

The Structure of Photosystems Clegg, pg. 278

Photosystems In each photosystem, several hundred chlorophyll molecules and accessory pigments are arranged All these pigments harvest light energy They funnel the energy to a single chlorophyll molecule of the photosystem, known as the reaction center The different pigments around the reaction centers absorb light at slightly different wavelengths

Two types of photosystems Two types present in the thylakoid membranes  Identified by the wavelength of light that the chlorophyll of the reaction center absorbs Photosystem 1: activated by light wavelengths 700nm  P700 Photosystem 2: activated by light wavelengths 680nm  P680

Why two photosystems? Each have a specific and differing role However, they are grouped together in the thylakoid membrane, along with specific proteins  Enzymes catalyzing Splitting of water Formation of ATP from ADP and P  Electron carrier molecules

What happens when light reaches the reaction center? Ground-state electrons of the key chlorophyll molecule are raised to an excited state  Said to be photoactivated As a result, high energy electrons are released (from the chlorophyll molecule) These electrons bring about the biochemical changes of the light dependent reactions The spaces vacated by the high energy electrons are continuously filled by non-excited or ground state electrons

Sequence of reactions First, the excited electrons from photosystem 2 are picked up by plastoquinone and moves away to another position in the membrane As these excited protons are passed, some of the energy causes the pumping of hydrogen ions (protons) from the chloroplast's matrix into the thylakoid spaces They accumulated, causing the pH to drop

Sequence of reactions (cont.) The result? … proton gradient created across the thylakoid membrane This sustains the synthesis of ATP  Called photophosphorylation and chemiosmosis

Photophosphorylation and chemiosmosis The production of ATP using energy derived from light is called photophosphorylation  Carried out in the thylakoids Main type is non-cyclic  Reduced plastoquinone carries the pair of excited electrons from the reaction center of P2  Plastoquinone carries the electrons to the start of the chain of electron carriers and they are passed along  As the electrons are passed, energy is released which is used to pump protons across the thylakoid membrane into the space inside the thylakoids

Next … Concentration gradient of protons develops (stores potential energy)  Photolysis also contributes to the gradient The protons can travel back across the membrane, down the concentration gradient, by passing through the enzyme ATP synthase The energy released by the passage of protons is used to make ATP from ADP and P  This method of producing ATP is also called chemiosmosis Occurs in the mitochondrion (cell respiration; next topic)

Next … When the electron reach the end of the chain of carriers they are passed to plastocyanin  Needed for the next stage of photosynthesis

Completing the light dependent reactions Remaining parts involve photosystem 1  Product: NADPH (needed for light independent reactions)  NADPH carries a pair of electrons Chlorophylls within P1 absorb light energy and pass it to the reaction center  This raises the electron’s energy level; photoactivation  The excited electron passes along a chain of carriers and is eventually passed to ferrodoxin  The reduced ferrodoxin are used to reduce NADP+ to NADPH + H+

Next … The electron that P1 donated is replaced by an electron carried by plastocyanin P1 and P2 are therefore linked:  Electrons excited in P2  plastocyanin  P1  Electrons are re-excited with light energy and are eventually used to reduce NADP+  For reasons later discussed, NADP+ sometimes runs out  When this happens, the electrons returns to the ETC which links the two systems rather than being passed to NADP+  Cyclic photophosphorylation

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