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(Click the red button to start animation) The overall equation
for photosynthesis is deceptively simple. In fact, a complex set
of physical and chemical reactions must occur in a coordinated manner
for plants to synthesis carbohydrates. To produce a sugar such as
glucose, plants require nearly 30 distinct proteins, many of which
that work within the complicated membrane structure of the Chloroplast.
The overall reaction for Photosynthesis is as follows: These two sets of reactions not only occur in different places within the Chloroplast, but they are also biochemically different. We will look at them separately, but they are both part of the same process.
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- Assignment
3.2
Part 1 - The Z - Scheme
The Z - scheme, as shown below, represents the movement of an electron during photosynthesis, both in space and with respect to the free Energy of the electron. Electrons are removed from water (Water is oxidized) raised in energy (twice), passed from molecule to molecule until the electron (and its Energy) are used to reduce NADP+ to from NADPH. The image below is of the Thylakoid membrane, the Z - scheme is in red (it actually looks more like an N - scheme to me).
The components
of the Photosynthetic apparatus: To understand the movement of electrons
during the Z-scheme, we need to examine the various pigments, metals,
and protein molecules that pass the electron around (Mouse
over the terms below to highlight each component).
Antenna
complexes -The two antenna complexes (one for each Photosystem)
contain Chlorophyll, accessory pigments, and proteins. They collect radiant
Energy to excite rxn center chlorophylls.
Photosystem
I (PS I) - PS I a complex of molecules, with an Antenna complex,
Proteins, Ions, a molecule called phylloquinone, a reaction center chlorophyll
(called P700 
), and Ferredoxin.
Ferredoxin is an iron-containing molecule that passes an excited electron
to NADP+.
Photosystem
I I (PSII) - PS II is a lot like PSI. It contains proteins, pigments,
metal and other ions, Plastoquinones, Pheophytin, and a special reaction
center chlorophyll molecule, called P680 .
The
Cytochrome B6/F Complex - The cyt b6-f complex
contains proteins, metal ions and a special iron-sulfur protein. It also
translocates protons across the Thylakoid membrane, much like the etc.
The
Oxygen Evolving Complex (OEC) - The OEC is part of PS II. It contains
several Mn and Fe containing proteins which oxidize water (a tough thing
to do) and generate the O2 we breathe.
ATPase
- ATPase is an Enzyme that generates ATP from ADP + Pi using chemiosmotic
energy from the Proton gradient created by spitting water and the translocation
of Protons.
The path of electrons in the Z-scheme: Starts with Water
then to PS
II to the Cytochrome
B6/F complex to
PS
I and finally to
NADP to make the reduced Energy carrier, NADPH.
To see an animation of the the process, click here. As you can see from the animation, there is a lot going on. Check out the summary below, then take another look at the components and the path of electrons before you try the quiz.
Summary - At Photosystem System II, radiant Energy from the sun is transferred to a reaction center electron, raising its Energy level. As excited electrons move down an Energy gradient from PS II to the Cytochrome cytb6-f complex (a series of re/dox reactions, like the electron transport chain in Mitochondria), Protons (H+) are transferred across the Thylakoid membrane by the cytochrome complex. These protons can be used to generate ATP, as they move thru an ATPase back out into the stroma of the Chloroplast. When the Photosynthetic apparatus generates ATP in this way it is called Photophosphorylation (our next topic). The electrons then travel from the cytochrome complex to PS I, where they are again "excited" by Sunlight. The High Energy electrons are then transferred to NADP+, forming NADPH (this is a reduction reaction, a net gain of chemical Energy). |
Questions