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Photosynthesis watch


    Can anyone explain photosynthesis (for A2) to me in a very nice easy way?



    1) Light hits photosystem 2, which causes 2 electrons to become excited (i.e. jump up energy levels) in the chlorophyll molecule. These electrons pass to an 'electron acceptor', and the chlorophyll gets a positive charge (as losing electrons causes something to get a positive charge).

    2) Water is split in photolysis. It gives 2 hydrogen ions, 2 electrons and oxygen. The oxygen is given off as waste gas, and the electrons replace those lost from photosystem 2.

    3) The electrons excited from photosystem 2 pass down electron carriers. The energy they 'lose' on the way down them is used to produce ATP molecules.

    4) Light is also hitting photosystem 1. The same series of events occurs as did in part 1. The electrons from photosystem 2 replace those now lost from photosystem 1.

    Is this OK so far?

    Two stages light dependent stage and light independent stage.

    -light dependent stage in thylakoid membranes of chloroplast (includes lamellae)

    -This stage is carried out by photosystems, PI (P700) and PII (P680) The numbers 700 and 680 correspond to the wavelength of light they absorb best

    -Light (usually sunlight) is absorbed by accessory cells and through resonating they transfer this energy to the primary control centre of the photosytem thereby promoting an electron from the magnesium atom. This accumulation of energy to promote the electron through resonance energy is photophosphorylation.

    -two types of photophosphorylation, cyclic (P700 only) and non-cylic (Both P680 and P700)

    -cyclic, electron promoted and accepted by an electron carrier. Then passed down a series of electron carriers from a higher energy level to lower energy levels. These redox reaction produce enough energy for the phosphorylation of one ADP into ATP. Electron returns to photosystem reaction centre
    non-cyclic is where two electrons are promoted from P680, are accepted by electron carriers, pass through sequence of electron carriers from high energy levels to low energy levels. Again this sequence of redox reaction yields one ATP. This also happens at P700 except ATP is not generated but rather the energy required to reduce NADP to reduced NADP. The electrons for P680 replace electrons lost in P700. The electrons lost from P680 are replaced by electrons gained from the photolysis of water:
    H20----light----> 2H+ +2e- +0.5O2
    The hydrogen ions from this photolysis couple with electrons at the end of non-cylic photophosphorylation to produce hydrogen which reduces the NADP.

    Overall: cyclic photophosphorylation produces one ATP
    non-cyclic produces one ATP, one reduced NADP and oxygen (0.5O2)

    Anyways, light-independent stage:
    -Ribulose bisphosphate carboxylase (Rubisco ) fixes carbon dioxide onto ribulose bisphosphate in the calvin cycle in the stroma of the chloroplast

    -The 6 carbon intermediate quickly breaks down into two molecules of glycerate-3-phosphate

    -this is converted to triose phosphate (glyceraldehyde-3-phosphate) using hydrogen carried by reduced NADP from light dependent stage and energy is provided by ATP

    - From this a hexose sugar (glucose) is made, or can contribute to making other organic molecules such as amino acids

    -The rest of the glyceraldehyde-3-phosphate molecules go towards regenerating RuBP. This requires ATP as it provides energy and phosphate.

    all good!
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