Photosynthesis- making Sugar from Sunlight
February 11, 2004

Readings: Ch 6 p 92-95, 98-99, 101,104-105;
Warm Up 5 is due Weds Feb 18 at 9:30, and Good For 5 is due Fri Feb 20 at 11:30. See top of notes page for info on Good For Amnesty!

"Life is woven out of air by light" -- Jacob Moleschott Dutch

We have spent a lot of time talking about carbohydrates ~ sugars ~ but how are those sugars made by cells to become part of the food chain? Plants, protists (like phytoplankton), and cyanobacteria can do what no other organisms on Earth can do: photosynthesize, or make sugar out of water and carbon dioxide, a gas. This sugar gives energy to the organisms that make the sugar, as well as to all other organisms on Earth*. Photosynthesis also gives us the oxygen that we breathe.
(*Except organisms, like those in Domain Archaea, that get their energy from compounds like hydrogen sulfide.)

I. "Photosynthesis
- making Sugar from Sunlight"
- a Play in Two Acts...
The Chloroplast: hundreds of these organelles can be found in every plant cell that is above ground!

Key players in the chloroplast:

Chlorophyll: a pigment that reflects green light, and absorbs red and blue light to start theprocess of photosybthesis. Chlorophyll pigments are clustered in the thylakoid membranes into similar types of pigment complexes: "Photosystems I and II" .
Thylakoid membranes:
membranes within the chloroplast that contain enzymes involved in the Light Reactions of photosyntehsis (Act 1). Stacks of thylakoid membranes are called Granum.
The Stroma:
the liquid-y part of the chloroplast that are involved in the Calvin Cycle (Act 2).

Plot Summary: During today's performance, 6 molecules of the gas Carbon Dioxide (CO2) will be combined with 12 molecules of water (H2O) to produce 1 molecule of glucose (sugar), and some waste products (oxygen gas and water).

  • 6CO2 + 12H2O + solar energy ------------> C6H12O6 + 6O2 + 6H2O
  • (PS. Don't understand what all these letters and numbers could possibly represent? Please look at the CD for Chapter 2, or your book, for an intro to chemical reactions)

    Introducing Today's Supporting Cast:

    CO2, carbon dioxide gas.................A small inorganic gas
    Enzymes ...........................................Break and rearrange chemical bonds
    ...................................................A molecule that stores and releases chemical energy
    NAD+, NADP+, and FADH
    ...........Escort electrons and protons around cells to make energy

    Today's performance was made possible by a Generous Donation of Energy from The Sun, that beautiful burning ball of gas....

    Act 1: The Light Reactions: Goal -Convert solar energy to ATP and NADPH
    (The "photo" part of photosynthesis!)

    The goal: Use the energy of the sun to (1) excite chlorophyll, (2) split water into 2, releasing O2 gas as a waste product, and (3) make Energy (ATP and NADPH) needed for Act 2.

    Scene: These reactions occur in the thylakoid membrane stacks (Chlorophyll photosystems I and II):
    1. Sunlight that strikes the
    chlorophyll pigment excites the molecule so that it loses an electron (part of an atom).
    2. A molecule of water is
    split apart to replace the electron that was lost by the chlorophyll.
    3. When the water is split, a waste product is released from the chloroplast =
    Oxygen gas!!!!!
    4. The electron from chlorophyll is passed around a series of proteins (called the electron transport chain)
    5. In the process,
    chemical energy is made (ATP and NADPH). (It takes a lot of energy to do the next step, which is to make sugar!) the end result of the light reactions? Now the chloroplast has the energy (ATP and NADPH) needed to make sugar. Next step:

    Act II: The Calvin Cycle (Light-independent) Reactions:
    Use CO
    2 and Energy to make Sugar
    (The "synthesis" part of photosynthesis!)

    The goal: Using the energy made in the Light Reactions, add hydrogens (H) from water to CO2 to make glucose (C6H12O6)

    Scene II: These reactions occur in the chloroplast stroma, and are named for Melvin Calvin (Nobel Prize winner). As you will note by the name, the Calvin Cycle is a cycle - it begins and ends with the same starting compound that is regenerated over and over. The cycle has to go around twice to make 1 glucose.

    There are three phases in the Calvin Cycle. You don't need to know the compounds named, just a summary of what happens in that phase as listed:

    1. Carbon Fixation: 6 molecules of Carbon Dioxide (CO2) are combined or "fixed" with 6 molecule, Ribulose Bisphosphate(RuBP) (containing a total of 30 carbons, 5 carbons each), to make 12 molecules of Phosphoglycerate (PGA), each having 3 carbons for a total of 36 carbons.

    6 CO2 + 6 RuBP -------> 12 PGA

    2. Sugar formation: The 12 molecules of Phosphogycerate are rearranged into G3P (Glyceraldehyde-3-phosphate), a sugar. Two of the G3Ps are sent out of the chloroplast and rearranged further into sugars and starches.

    12 PGA ------------> 12 G3P (2 of these G3Ps leave the chloroplast at this point)

    3. Regeneration of RuBP: The other 10 G3Ps (and their 30 carbons) and rearranged to regenerate 6 molecules of the starting molecule, Ribulose Bisphosphate (5 carbons each; a total of 30 carbons).

    10 G3P -------------> 6 RuBP

    In the process, ALL the energy made in the Light Reactions is used to make sugar and re-form RuBP!!!
    But at the end of the performance, the cell has
    made sugar from sunlight, and uses that sugar to grow and make flowers, fruits and seeds. Animals eat plants and use the sugar made by plants for their growth, development, and reproduction! This is the subject of the next discussion... cellular respiration.

    More Nobel Laureates in Photosynthesis...

    Summary (or, Why should YOU care? )

    In today's performance, light energy was used to transfer electrons from H2O to CO2, making sugar, and releasing O2 as a waste product. All of the oxygen that we breathe, and all the oxygen in the Earth's atmosphere, has been generated by photosynthesis. In the ~2.5 billion years since photosynthetic organisms have appeared daily in this performance, enough oxygen has accumulated to account for 21% of the atmosphere... If plant cells, plankton, and photosynthetic bacteria around the world weren't busy photosynthezing and releasing O2 into the - an aerobic organism - would not be here today! Neither would all other animals, fungi, and many bacteria on the earth

    If we're so smart, then why can't we figure out how to use a clean, readily available, constant source of renewable (not to mention free) power? Hint: we CAN! We just haven't converted yet!


    1. Compare and contrast these terms:

    2. Where do the light reactions occur in the chloroplast?
    3. In your own words, describe the end result of the light reactions.
    4. Where do the
    Calvin Cycle reactions occur in the chloroplast?
    5. In your own words, describe the three phases of the Calvin Cycle Reactions. What significant stepin sugar synthesis happens in each phase?
    6. In your own words, describe the end result of the Calvin Cycle Reactions.Why is it called a cycle?