PHOTOSYNTHESIS

First, how important is it?

“This process -- photosynthesis -- is the route by which virtually all energy enters our biosphere. Each year, more than 250
billion tons of sugar are produced worldwide by photosynthetic organisms. The importance of photosynthesis, however, extends
far beyond the sheer weight of this product. Without this flow of energy from the sun, channeled largely through the chloroplasts
of eukaryotic cells, the pace of life on this planet would swiftly dininish and then would virtually cease altogether, as dictated by
the inexorable second law of thermodynamics.”   Raven et al., 6th ed.

This is what photosynthesis is about:

        3 CO2 +6H2O ---->  C3H6O3 + 3O2 + 3 H2O

The 3 carbon compound is a simple sugar called a triose.
 Trioses are then used to make more complex sugars, including glucose, fructose and starch.

Can express this more simply as:

CO2 + H2O  ------> (CH2O) + O2
    (...) -- just a symbolic term for carbohydrate
            It takes the energy of light to accomplish this process

Carbon dioxide is reduced to produce carbohydrate (CH2O).
 That requires a source of energy and reductant, or high energy electrons.
 The energy ends up as chemical potential energy stored in carbohydrate and other complex molecules.

Photosynthesis consists of two sets of reactions.
 One set is responsible for supplying energy and energized electrons.
 In a second set, the energy and electrons are used to fix carbon dioxide and create  carbohydrate.

These two sets of reactions occur in different parts of the chloroplast.
    Figs. 10.5, 10.9
 The first set occurs on chloroplast membranes, the thylakoids.
 The second set occurs in the water soluble matrix, or stroma, that bathes the thylakoids.
 

Let’s look at the first set of reactions.

First, what is light?

 Visible light is a narrow segment of the electromagnetic spectrum.
    Fig. 10.2

 Light behaves as both a wave and a particle.
  Shorter the wavelength, the more energetic the radiation.
 Particles are called photons, or quanta.
  Photons have discrete energy content.
  Shorter wavelength, the photons are more energetic.

 Pigments are compounds that absorb light of SPECIFIC wavelengths or  photon energies.  Fig. 10.7
   Electrons are 'excited' as a result.
        When they absorb photons, electrons are energized -- excited to a higher energy state.

 Chlorophylls are pigments. So are carotenoids.
  So, photosynthesis uses specific wavelengths, or photons.

What is the significance of alternating double bonds in biological pigments????

  In biological systems, pigments are complexed with specific proteins.
 

The first set of photosynthetic reactions, capturing light energy, is carried out on internal chloroplast membranes,
        the thylakoids.
 Membranes provide a hydrophobic environment for photosynthetic pigments. Fig. 10.9
  Again, the pigments are hydrophobic.
 Membranes provide a surface on which electron carriers and other components are ordered as arrays.  Fig. 10.9
  Allow complexes to interact in an orderly way, for efficient energy and electron transfer.

Thylakoids contain lots of chlorophyll and associated pigments. These  pigments aren’t free, but are associated with proteins.
These pigment/protein complexes are organized into PHOTOSYSTEMS,  I and II.  Fig. 10.8
 Each photosystem consists of around 250-400 pigment molecules

Pigments in each photosystem include chlorophylls a and b, and other pigments like carotenoids.
 
 Light energy absorbed by the pigments is funneled to a 'reaction center', where electrons from a pair of chlorophylls is excited.
     TWO energized electrons are generated
Excited electrons are then donated to protein electron carriers.
 
The photosystems work together to energize electrons to reduce carbon dioxide.     Figs. 10.9

Excitation of photosystem II raises energy level of electrons partway.
 The electrons are then passed down a transport chain feeding into photosystem I.
 The electrons fill ‘holes’ created when photosystem I chlorophylls are excited and lose electrons to other carriers.

Electrons from PSI are ultimately used to reduce carbon dioxide.

Holes in PSII are filled by electrons from water.
        Sooooo, the ultimate source of electrons, or reductant, for plant photosynthesis is water.
Water is split to provide electrons.  Called PHOTOLYSIS.   Oxygen is a biproduct.

In other words, it's as if electrons move on a continuous electrical 'circuit' from water to carbon dioxide, and in the process are energized.
 
Also, it takes two light absorbtion events to energize each electron enough so that it can reduce carbon dioxide.
    One excitation isn’t enough.
  That is, 2 photons are absorbed per electron.

At the end of the line, electrons are delivered to carriers that eventually reduce carbon dioxide to sugar.

The process also generates ATP, which is also used to make sugar. Fig. 10.8
    NOTE: there's more than enough energy in the two photons to reduce carbon dioxide. Some of the excess energy becomes heart, which ultimately drives the reaction according to the Second Law of Thermodynamics.

Other excess energy is converted to ATP.

What do proton (H+) gradients have to do with the production of ATP?

Soooo, the energized electrons and ATP now enter the water soluble stroma and are used in the second set of photosynthetic reactions, to convert carbon dioxide into sugar.

Key photosynthetic enzyme in this process is called ribulose bisphosphate carboxylase.  Rubisco.
 Initiates THE CALVIN CYCLE.   Fig. 10.10