SBI 4U
Photosynthesis
Photosynthesis: The Details
Introduction
 The heart of photosynthesis as it occurs in most autotrophs consists of two key processes:
1. the ____________________________________________________
2. the_____________________________________________________________
________________________________________________________________
 The electrons (e−) and protons (H+) that make up hydrogen atoms are stripped away
separately from water molecules.
 The electrons serve two functions:
o They reduce _____________________ for use in the Calvin Cycle.
o They set up an ______________________that provides the energy for pumping
protons from the stroma of the chloroplast into the interior of the thylakoid.
 The protons also serve two functions:
o They participate in the reduction of _______________________
o As they flow back out from the interior of the _____________________, by
facilitated diffusion, (passing down their ________________________), the
energy they give up is used for the conversion of ADP to ATP.
o Because it is driven by light, this process is called _____________________.
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SBI 4U
Photosynthesis
The Light Reactions
 The light reactions begin when photons strike a ______________________________
Steps in Light Reactions
Steps
1. Photoexcitation
2.
3. Chemiosmosis
Description
Excited electrons are transferred through a series of membrane
bound e- carriers, so a H+ is pumped through a photosynthetic
membrane , which creates an ____________________ which will
reduce an electron acceptor (NADP+)
the phosphorylation of __________________________________
through ATP-ase complexes
Photoexcitation
 When chlorophyll or accessory pigments absorb
protons, the photon boots one of the pigment molecules’
electrons in its lowest energy state (ground state)
to an orbital of higher potential energy (excited state).
 The excited state is unstable, so excited electrons
quickly fall back to the ground state orbital, releasing
excess energy in the process.
Photosystems
 Each photosystem has an antenna composed of pigment molecules (chlorophyll and
accessory molecules.)
 When a photon strikes a pigment molecule, the energy is passed from molecule to
molecule until it reaches the reaction centre where ___________________ is found.
 An excited e- from the reaction-centre chlorophyll is captured by a specialized molecule
called the ___________________________.(Called Pheophyton)
 The energy stored in the trapped electrons powers the synthesis of __________________.
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SBI 4U
Photosynthesis
Types of Photosystems
Photosystem II
Photosystem I
 The reaction centre has a specialized
 The reaction centre has a specialized
chlorophyll a which absorbs best at
____________________________
chlorophyll a which absorbs best at
____________________________
Note: P700 and P680 are identical chlorophyll molecules, but each is associated with a different
___________________.
Noncyclic Electron Flow
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SBI 4U
Photosynthesis
Another
way to
look at it
1. Ps II absorbs light, boosting excited e- to a higher energy levelcaptured by primary
acceptor.(called _______________)
2. The Z protein completes Photolysis of water _________________________, (The
electrons are used to replace the e- lost by chlorophyll P680.)(___________________
chloroplast as byproduct.)
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SBI 4U
Photosynthesis
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3. Photoexcited e pass from the primary electron acceptor of Ps II to Ps I via
____________ [consists of electron carrier ________________(PQ), a complex of two
cytochromes and a protein called ________________________(Pc)]
4. Noncyclic Phosphorylation –chemiosmotic mechanism. As electrons move down the
ETC, the energy released is captured by thylakoid membrane to produce NADPH.
5. When electrons reach the bottom of the ETC, they fill electron “holes” in the
________________________ molecule in the reaction centre of Ps I (replaces light
energy that drives from the chlorophyll to the primary electron acceptor of Ps I).
6. The primary electron acceptor of Ps I passes the excited e- to ferredoxin (Fd). The
enzyme NADP+ reductase then transfer e- from Fd to NADP+ (this is the redox rxn. that
stores the high energy in NADPH-the molecule that will provide reducing power for the
synthesis of sugar in the Calvin Cycle)
Cyclic Electron Flow
 Involves only Ps I and generates ATP without producing ____________or evolving
oxygen.
 When light is not the limiting factor, NADPH tends to accumulate in the stroma and there
is a shortage of NADP+.
 The normal flow of electrons is inhibited b/c NADP+ is needed as a final acceptor of
electrons.
 Cyclic photophosphorylation is required in order to make ATP
When does cyclic phosphorylation occur?
 Cyclic phosphorylation occurs when:
o NADP+ is absent due to __________________________________
o Cells need additional supplies of ___________________________
Chemiosmotic Mechanism of Photophophorylation
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SBI 4U
Photosynthesis
 As electrons pass from carrier to carrier during redox rxn. H+ removed from stroma are
deposited in thylakoid compartment, storing energy as a proton-motive force (H+
gradient)
 Water is split by Ps II, as Pq transfers e- to Cyt. complex, protons are translocated across
membrane, H+ is taken up by NADP+ reducing it to NADPH.
 The diffusion of H+ from thylakoid compartment to the stroma powers the
_____________, and phophorylating ________________________
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