Photosynthesis in a nutshell
Redox reactions = chemical reactions that transfer electrons from one molecule to
another. Oxidation = lose electrons; reduction=gain electrons
Follow the atoms to demonstrate the law of conservation of matter. Energy has
been used to rearrange atoms and molecules to produce the high energy sugar
molecule.
Photosynthesis consists of 3 main ‘parts’:
The first two occur in the thylakoid membrane, and are light dependent.
Photosystem II, which uses water as a source or electrons. Known of as the water
splitting reaction (photolysis). Ultimately produces ATP for the Calvin cycle. This is
noncyclic photophosphorylation: formation of ATP by non-cyclic electron flow
Photosystem I, uses electron transport molecules to reduce NADP+ (Nicotinamide
adenine dinucleotide phosphate) to NADPH. This high energy molecule goes into
the Calvin cycle.
The Calvin Cycle (light independent and occurs in the stroma of the chloroplast)
which fixes CO2 ultimately into G3P. It uses a total of 3 molecules of CO2 (per
round) 9 ATP and 6 NADPH. It produces 6 G3P molecules. 1 is exported and 5 are
recycled to reform ribulose biphosphate.
Photosystem II:
 Location: Thylakoid membrane of the chloroplast
 Chlorophyll a is the primary pigment involved directly in light reactions.
Chlorophyll b is an important accessory pigment
 Light harvesting complex and reaction center at P680
 Water molecule is split to yield O2, electrons and H+ (protons)
 The electrons are passed down an electron transport chain through
plastoquinone (which is reduced), and through the cytochrome complex
 As the electrons move through the electron transport chain, their energy is
used to pump protons from the stroma across the thylakoid membrane into
the thylakoid lumen. This contributes to the proton differential across the
membrane which is used to make ATP
 Plastocyanin carries the electron to photosystem I
 The electrons are replaced by splitting the water molecule
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Meanwhile the buildup of protons are used in a proton pump across the
thylakoid membrane, where ATP synthase generates ATP – a high energy
molecule that then goes to ‘fuel’ the Calvin cycle
So…PSII passes the electrons ultimately used to reduce NADP+ to NADPH in PSI, and
it uses energy generated in the electron transport chain to synthesize ATP for the
Calvin cycle
Photosystem I:
 Location: Thylakoid membrane of the chloroplast
 Chlorophyll a is the primary pigment involved directly in light reactions.
Chlorophyll b is an important accessory pigment
 Light harvesting complex and reaction center at P700
 Plastocyanin carries the electron to the P700 center (same electrons
generated in the water splitting of PSII)
 Light is again used to excite the electron
 Ferredoxin is reduced (gains the electron), and carries it to NADP+ - reducing
it to NADPH. This high energy molecule goes to ‘fuel’ the Calvin cycle too.
So….results of PSII and PSI are ATP and NADPH for the light independent reactions
of the Calvin cycle
Calvin Cycle:
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Occurs in the stroma of the chloroplast
Also known of as Carbon fixation: fixing CO2 into organic molecules,
specifically sugar (G3P or Glyceraldehyde 3-phosphate)
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Faciiltated by the large enzyme Rubisco
In sum: 3 CO2 + 6 NADPH + 9 ATP→ (G3P) Glyceraldehyde 3-phosphate (1
will be ‘shipped out’, 5 will be recycled)
Step I – Rubisco catalyzes the carboxylation of ribulose 1,5 biphosphate
(RuBP) by carbon dioxide. This produces 3-phosphoglycerate (3phosphoglyceric acid, or 3PGA)
3PGA is phosphorylated into 1-3-bisphosphoglycerate 1-3BPGA (also
glycerate-1,3-bisphosphate) using ATP.
1,3 BPGA is reduced into G3P by NADPH (which is oxidized into NADP+)
1 G3P exits, and 5 are recycled
The 5 are used to regenerate RuBP (Ribulose biphosphate), which uses 3
more ATPs.
Ultimately 3 CO2, 6 NADPH, and 9ATP are used for ‘one cycle’.