2   Photosynthesis has produced all of the free oxygen (O₂) in the atmosphere
      AND in marine and freshwater habitats
        O₂ = ~21% of Earth's atmosphere by volume
       All organisms with a high metabolic rate require abundant oxygen for aerobic respiration.
       Only after oxygen had become significantly concentrated  in our atmosphere
                could birds and mammals evolve and become successful.
           (For example, the human brain begins to die after being deprived of oxygen
                    for only 3-4 minutes - or less time than that, if you're very young.)

3    Photosynthesis formed and continues to maintain Earth's "ozone layer"
        Ozone = O₃
        Ozone forms high in the atmosphere, when energy from the sun
                and cosmic radiation converts some O₂ to O₃
       Earth's ozone "layer" is only possible because of the O₂
                that plants have produced and continue to produce.
        The ozone layer screens out most of the ultraviolet light that reaches Earth from the Sun.
        Why is that important?  Because ultraviolet light denatures enzymes and is mutagenic.
        Basically,life on land isn't possible without the ozone layer.

4  Photosynthesis produces or has produced all fuels and fossil fuels
        (wood, coal, petroleum, natural gas)
      That means that photosynthesis is responsible for:
       All fuels for heating and cooking - oil, propane, natural gas, wood, biomass
       All fuels for transporation - such as, gasoline, oil and grease
       Electricity (all of it that is produced from burning coal, oil, natural gas or biomass
           (geothermal & hydroelectric aren't linked to photosynthesis)
        (Yes, I know this point is a little anthropocentric! - so is the next point.)

5    And here are a few more things you can thank photosynthesis for --
       (the actual list is practically endless)
    plastics
    paint
    latex & rubber (natural and synthetic)
    tar for paving roads
    lumber for construction
    spices
    cooking oils
    fragrances
    vitamins
    antibiotics
    anti-cancer drugs
    habitats and food for animals
    scenery, flowers, landscape plants
    ecosystems - forests, coral reefs, grasslands
    shady summer places
    pollen allergies & mold spores
    web pages (plastics, remember?)
    the sound of wind in pine needles
    a cool beer on a hot day
           and so on, and so on, and so on . . .

IF ANYTHING ELSE IN YOUR ENVIRONMENT GAVE YOU SO MUCH, WOULDN'T YOU SHOW IT GREAT RESPECT? Well, go thank a tree!!  and be quick about it.

                                        light energy
                                       chlorophyll a
                                                         electron carriers, etc.
6CO₂     +     12H₂O --------------------> C₆H₁₂O₆   +  6H₂O  +  6O₂

carbon dioxide + water ----------------> glucose + some reconstituted water + oxygen

See the PowerPoint supplement for an alternate explanation of photosynthesis .

THE PRIMARY PIGMENT  =  CHLOROPHYLL a   reflects blue-green
All oxygen-liberating (plant) photosynthesis requires chlorophyll a
           Some bacteria can photosynthesize
                but except for Cyanobacteria (= blue-green algae), bacteria don't have chlorophyll a
                and bacteriochlorophyll doesn't liberate O₂

ACCESSORY PHOTOSYNTHETIC PIGMENTS
    chlorophyll b - reflects bright yellow-green
            (some additional chlorophylls occur in various algae)
    carotenes
            reflect orange,reddish-orange
    xanthophylls
            reflect red, yellow, brown

FUNCTION OF ACCESSORY PHOTOSYNTHETIC PIGMENTS
           is to increase the wavelengths of light that can be used for photosynthesis -
                to make photosynthesis more efficient

PLEASE NOTE Fig. 7-5 on p. 119 of your textbook.  The heavy line at the top shows the OVERALL percent of different colors of light absorbed by all of the pigments (the scale is on the right side).  On the left side is the scale showing the approximate percent of light absorbed by the specific pigments.  Note that there are no pigments that absorb strongly in the yellow-green part of the spectrum.

     QUESTIONS
       If the pigments reflect the colors given above, what colors of light are they absorbing?

        How is light absorbed by the pigments?
                (Answer:   It's captured by certain electrons in the pigment molecules.)

Some fluorescent light tubes are especially engineered to emit the wavelengths of light that most useful in photosynthesis.  These "grow-lights"-whose light is pinkish-purple- often are used to grow plants under lights indoors and to boost the growth of aquarium plants.  They work very well.

Please note:
Anthocyanin - the purplish pigment that we've seen in several plants in lab - is a non-plastid pigment that has nothing directly to do with photosynthesis..  It accumulates in vacuoles of plant cells.

THE PROCESS OF PHOTOSYNTHESIS

Please refer to the PowerPoint supplement linked here, for a visual summary. The details of the process of photosynthesis are included in the handout.  Also, review the structure of a chloroplast.

There are two main phases in photosynthesis.  Both of these phases occur in chloroplasts in the tissue (= mesophyll) in the middle of leaves.  Often chloroplasts also occur in cells just under the surface of young, green stems.

PHASE    1
LIGHT DEPENDENT REACTIONS (sometimes just called the Light Reactions)
        Occur in the chloroplast grana
        Light energy is required

PHASE      2
LIGHT INDEPENDENT REACTIONS (sometimes just called the Dark Reactions)
        Occur in the chloroplast stroma
        Light energy is not required

STRUCTURE
The grana membranes are organized into so-called antenna complexes.  Each antenna complex is a group of chloroplast pigments and electron carriers.  Within each antenna complex are two photosystems, called Photosystem I and Photosystem II.  There is a molecule of chlorophyll a at the heart of each photosystem.  (I and II aren't too significant; the photosystems were named for the order in which they were discovered.)

EVENTS
Light of a useful wavelength strikes a pigment molecule and some of the light energy is captured by electrons in the pigment molecule.  As a result of the energy capture, the electrons gain energy. [This is THE crucial, THE all-important step in photosynthesis:  the conversion of radiant energy to chemical energy.]   The energized electrons are transferred through a series of electron carriers to form ATP and to furnish energy for the dark reactions.

Some of the electrons cycle back to the light-trapping pigments, but some electrons don't cycle back.  The electrons that don't return to the pigments are replaced when water molecules are split (= the photolysis of water).  When the water molecules are split, oxygen and hydrogen ions are produced.  The oxygen ions unite with each other to produce O₂, the oxygen we all depend on.  O₂ is a leftover product, produced in order to replace electrons that left the pigments when light energy was captured.  What a great leftover! - without it, we wouldn't be here!

LOCATION
        membranes of chloroplast grana

MAIN PRODUCTS
       oxygen
                is used either for aerobic respiration in the plant,
                or diffuses out into atmosphere
       high energy electrons
                are carried to the Dark Reactions and
                are used to power the Calvin Cycle
        ATPs
                 are used to power the Calvin Cycle

The Dark Reactions are often called the Calvin Cycle, named for the chemist who led the research team to discover what was happening.

STRUCTURE
The Dark Reactions do not occur on the chloroplast membranes.  Instead, they occur throughout the interior of the chloroplast (= chloroplast stroma).

EVENTS
Carbon dioxide (CO₂) diffuses into leaves through stomates or stomata.  When it encounters the wet cell walls of mesophyll cells, the CO₂ goes into solution, forming carbonic acid.  In this form, the CO₂ enters the mesophyll cell and diffuses into a chloroplast.  In the stroma of the chloroplast, the enzyme Rubisco catalyzes a reaction where CO₂ is added to ribulose biphosphate.  Ribulose biphosphate is a 5-carbon molecule, and as it picks up a CO₂ (now becoming a 6-carbon molecule which splits in two), it forms two, 3-carbon molecules called phosphoglyceric acid (= PGA).    These PGA molecules are converted to  PGAL (= phosphoglyceraldehyde). Additional reactions convert PGAL back into ribulose biphosphate, which can pick up more CO₂.  But, because CO₂ is picked up and added to the cycle every time the cycle turns, all of the PGAL is not needed to regenerate enough ribulose biphosphate to keep the Calvin cycle turning.

Aha!! -  The surplus PGAL molecules are the food produced by photosynthesis. (Two PGAL molecules can be stuck together to form glucose - which means that the Calvin Cycle must turn twice to produce a single glucose molecule.)

So, just as the oxygen was a "left-over" of the Light Reactions, PGAL, the food produced by the Calvin Cycle is also a "left-over".  So, you might think of photosynthesis as a process that produces left-overs!  But it's a fortunate organism indeed that contains something like a chloroplast that can produce such valuable left-over materials.  Having chloroplasts in your cells is a lot like having a flock of geese that lay golden eggs every day!

These reactions are also called the Calvin Cycle.

LOCATION
        stroma of chloroplast

MAIN PRODUCTS
       ribulose biphosphate
                regenerate the Calvin cycle
                picks up CO₂ with the help of rubisco, then splits into 2, three-carbon molecules
       PGAL
          = food!

See the PowerPoint supplement for an alternate explanation of photosynthesis .

end of notes for Dec. 2