Recall the First Law of Thermodynamics: Energy cannot be created or destroyed, but it can be changed in form.

and the Second Law of Thermodynamics: All systems tend to go from a state of greater organization to a state of lesser organization.

In terms of biological reactions in general, and photosynthesis in particular...

  • SOLAR energy radiated from the sun is captured by plants, and instantaneously changed into ELECTRICAL energy (kinetic energy of the flow of electrons), then packaged as CHEMICAL energy (potential energy in the covalent bonds of sugar molecules)

    This is a direct example of the First Law!

  • No energy transformation is 100% efficient. Not *all* then solar energy captured is converted to electrical and then chemical energy. Some of it gets lost as heat or other forms of randomizing energy (e.g., light)

    This is a direct example of the Second Law.

    The Laws of Thermodynamics are both reflected in the two types of chemical reactions: EXERGONIC and ENDERGONIC

    An EXERGONIC reaction is one which releases energy as it proceeds.

    (Example: burning fuel in your car)

    An ENDERGONIC reaction is one which stores energy as it proceeds.

    (Example: those chemical "cold packs" that you get in the emergency room!)

    Not all chemical reactions happen spontaneously, simply because two molecules come into proximity (and that's a good thing!). Especially in biological systems, a chemical reaction between two substances is more likely to take place (and at a faster rate) if it is mediated by a CATALYST.

  • A CATALYST is a substance which causes a chemical reaction to occur, but it is not permanently changed during the course of that reaction. In biological systems, the most common catalysts are large, three-dimensional proteins called ENZYMES.

  • A SUBSTRATE is the particular molecule on which a specific enzyme can act. (Enzymes are very specific: each one can operate on only one or very few particular molecules! The substrates fit into special "pockets" on the enzymes called ACTIVE SITES where the substrate is poised for reaction.)

    Note that a substrate can be either a component of something the enzyme is building--or it can be a large substance that is broken into smaller parts by the enzyme.

    (Thus, enzymes can promote either endergonic or exergonic reactions, depending on their specific nature.)

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    Many chemical reactions involve the transfer of electrons, which also means the transfer of the energy contained in those electrons. When we speak of electron transfer reactions, we are speaking of OXIDATION/REDUCTION REACTIONS.

  • Oxidation of a molecule occurs when it loses one or more electrons. (i.e., it becomes more positively charged)

  • Reduction of a molecule occurs when it gains one or more electrons. (i.e., it becomes more negatively charged; its electrical charge is "reduced")

    In its most stable condition, an atom (or molecule) will have the same number of protons in its nucleus as electrons in its orbitals. The charges thus balance each other, and the electrical charge of the entire atom/molecule is ZERO.

    An atom/molecule which has lost one or more electrons has become more positive in charge. It is said to be a CATION.

    (An aqueous solution that has more cations than anions tends to be ACIDIC)

    An atom/molecule which has gained one or more electrons has become more negative in charge. It is said to be an ANION.

    (An aqueous solution that has more anions than cations tends to be BASIC)

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    ATP: ADENOSINE TRIPHOSPHATE (See diagram we drew in class) is the "pocket change" of the cell. Think of fat and starch (stored energy) as money in the bank and ATP as quick, spendable cash.

    ATP can move anywhere in the cell, break off a phosphate and release energy to drive any number of ENDERGONIC reactions in the cell, from building sugars to driving an enzymatic reaction. ATP can be thought of as the "energy currency" of the cell.

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    Plants are autotrophs which capture the energy randomizing from the sun and temporarily store it as sugar.

    This takes place in the chloroplasts, and is a multi-step process

    governed by enzymes located in the membranes inside the chloroplast.

    Here's a gigantic OVERVIEW of the process, and where it occurs inside the chloroplast.

    Before we can fully understand photosynthesis, we must understant the nature of LIGHT.

  • A QUANTUM is the smallest indivisible unit of electromagnetic energy.
  • A PHOTON is the smallest indivisible unit of VISIBLE LIGHT energy.

    (Visible light is defined as the region of the electromagnetic spectrum that humans can visually detect.)

    The units of the photon are usually expressed as their wavelength (Let's draw a photon moving through space to see what that means...). Photons range in wavelength from about 380 nm (violet) to about 770nm (red).

    (Note: longer wavelength means lower frequency (fewer waves per unit length) and lower energy. Shorter wavelength means higher frequency (more waves per unit length) and higher energy.)

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    Different wavelengths of photons correspond to the different colors visible to humans (at least the ones with normal color vision).

    We humans see colors because we have pigments in our retinal cells that absorb photons of very specific wavelengths.

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    When a photon hits matter (e.g., a molecule of some kind), one of three things can happen:

  • It can be REFLECTED (bounced back into space)

  • It can be TRANSMITTED (passed through the molecule)

  • It can be ABSORBED (the photon changes from light energy into some other kind of energy--either electrical (i.e., it boosts an electron of the molecule to a higher energy state) or entropy (heat; increased molecular motion).

    You can see evidence of this everywhere around you. But what you can't see is that it's happening even at the level of the chloroplast. *******

    News Flash: Plants, too, have pigments. And that is what's absorbing and reflecting the light.