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Photosynthesis: The Breath of Life

Recall... Photosynthesis, the conversion of inorganic water (H2O) and carbon dioxide (CO2) into organic sugar is the plant personification of these two Laws. During the process of photosynthesis...

  • randomizing solar energy intercepted by plants, and instantaneously changed (transduced) into electrical energy (the kinetic energy of electron flow). It is then "packaged" as chemical energy (the potential energy stored in the covalent bonds of sugar molecules). (First Law)

  • No energy transformation is 100% efficient. Not all solar energy captured by a plant cell is converted to electrical and then chemical energy. Some of it is lost as heat or other randomizing energy that cannot be used to do work (entropy). (Second Law)


    Like cellular respiration, photosynthesis proceeds via a series of orderly enzymatic reactions.

    Recall Baby Chemistry:

    Keep these familiar terms in mind as we follow the course of photosynthesis in overview.


    The Nature of Light Energy: Physics 101

    By now we should all know that the sun provides most of the earth's energy in the form of electromagnetic radiation.

    (Can you think of any energy on earth that's not solar in origin?)

    The smallest unit of light energy is known as a quantum, which has properties of both a particle (it can be deflected by solid matter) and a wave (it travels through space in an up and down pattern at a specific wavelength).

    Not all quanta are the same. Although they all travel through space at the speed of light (299,792,458 m/sec), they may do so at different wavelengths...

    ...and different frequencies.

    Different wavelengths of electromagnetic energy correspond to different physical entitles which react with matter in different ways.


    Visible Light: Quanta that Stimulate your Central Nervous System

  • The quanta that we can perceive as light are called photons, and photons of different wavelengths comprise the visible spectrum.

    Humans can see photons ranging in wavelength from about 380 nm (violet) to about 700 nm (red), and a photon will be perceived by your brain as a certain color depending on its wavelength when it hits the color-sensing photoreceptors of your retina.
    (Note: A nanometer = 10-9 meters, or 0.000000001 meters.)


    The Wonder of Photosynthesis

    Recall that an autotroph (auto = "self" and troph = "feeding") is an organism that captures energy and stores it in the chemical bonds of organic molecules that it manufactures from inorganic molecules. They are also known as producers or primary producers. The greatest autotroph biomass on earth is comprised of plants.

    (A heterotroph (hetero = "other" and troph = "feeding") is an organism that eats other organisms to obtain energy. They are also known as consumers.)

    The most common means by which autotrophs make organic molecules (sugar) is via photosynthesis.

    (Autotrophs that capture light energy are called photoautotrophs, though there are other kinds of autotrophs.)

    Plants are photoautotrophs that absorb photons only in a specific region of the spectrum.
    A pigment is any substance that absorbs light. The main pigments responsible for the initiation of photosynthesis are chlorophylls and carotenoids which absorb light in different regions of the visible spectrum. And these pigments, as you already know are embedded in the thylakoid membranes of the chloroplasts.

    Photons interact with matter--including plant pigments--in one of three ways. A photon striking matter (liquid, gas or solid) can be

    Only when absorbed can photons initiate biological activity.

  • Plant pigments absorb photons in the violet/blue region and in the red region.
  • Thus, only violet, blue and red light will drive photosynthesis.
  • All other wavelengths are reflected, which is why plants look green. They are reflecting or transmitting the green light, not using it to make sugar.

    You should already know the overall chemical reaction of photosynthesis:

    It takes...

    in the presence of light and the proper enzymes in the cell, to make

    The sugar (glucose) is the storage form for energy in plants, and it's often converted into long chains for long-term storage as carbohydrate, which forms the body of the plant.
    The oxygen and water are side products that are not used by the plant in this reaction.


    Why Photosynthesis?

    What does the plant do with the sugar molecules, once it has them? The latter, of course, is done via cellular respiration, the overall chemical equation for which is exactly the opposite of photosynthesis:

    It takes...

    can be "burned" to release stored energy as well as the "waste" products of


    Chlorophyll

    All photoautotrophs use an isomer of chlorophyll known as chlorophyll a as the primary photosynthetic pigment. This is the only type of chlorophyll that can pass excited electrons to the primary electron acceptor protein in the thylakoid membrane. But there are other isomers of chlorophyll. The one present in all members of the monophyletic taxon known as Viridaeplantae also contain chlorophyll b and carotenoids as accessory pigments.

    The accessory pigments cannot pass excited electrons to the PEA protein, but they can pass them to the "team captain," chlorophyll a. What do you suppose might be the advantage of accessory pigments? Consider...

  • absorption spectrum - range of wavelengths absorbed by a particular pigment
  • action spectrum - range of wavelengths capable of driving a particular biological process

    Isolated chlorophyll molecules in solution and exposed to light will absorb light, resulting in an excitation of the chlorophyll's electrons. In a live cell, the excited electron would be sent along a transport chain, and its energy harvested in a stepwise fashion.

    The isolated chlorophylls, with no place to send their electrons, exhibit a phenomenon known as fluorescence. (And if you didn't come to class for the explanation, you're just going to have to read and figure this out on your own. Hint: Check out your old BIL 151 lab on Photosynthesis pigments!)

  • absorption spectrum - range of wavelengths absorbed by a particular pigment
  • action spectrum - range of wavelengths capable of driving a particular biological process

    The action spectrum of photosynthesis was first reported in 1883 by T. W. Engleman, who did an experiment now considered classic.

    Absorption of photons is the first step in the...

    Light-dependent Reactions of Photosynthesis

    Before we go any further, let's watch a short movie, which we'll see again at the end.

    Overview of Photosynthesis


    The essential steps in the light reactions are...

    Heart of the Light Reactions: The Photosystems

    A photosystem is a light-gathering complex composed of a proteinaceous reaction center complex surrounded by several light-harvesting complexes. These are embedded in the thylakoid membranes.


    Photosystems I and II

    There are two types of photosystems (named in order of their discovery, not in order of their function) in the thylakoid membranes: Here's how it works...
  • A photon is absorbed by a chlorophyll or carotenoid molecule in the thylakoid membrane. As it falls back to its ground state, its energy is transferred to the electron of an adjacent pigment, raising it to an excited state. This continues until the excited electron belongs to the famous P680 chlorophyll of PS II.
  • The excited P680 electron is transferred to a primary electron acceptor protein. The oxidized P680 is now P680.
  • Nearby, an enzyme splits water to yield
  • The excited electrons from PS II travel to PS I via an electron transport chain (similar components as those found in the mitochondrial electron transport chain) consisting of a cytochrome complex known as plastoqinone (Pq) and another protein, plastocyanin (Pc).
  • As electrons "fall" exergonically from one component of the electron-transport chain to the next, our old pal the Second Law of Thermodynamics rears its head: energy is released at each transfer, but quickly captured and packaged in the high-energy phosphate bonds of ATP. (Electrons passign through teh cyctochrome comlex results in the pumping of protons out of the membrane, and the resulting potential difference is used in chemiosmosis.

  • Meanwhile, back at PSI, chlorophylls and carotenoids are behaving in a similar manner, doing the wave, and transferring photon energy (not converted to electrical energy--the flow of electrons) to the pair of P700 chlorophylls at the reaction center.
  • P700 transfers its excited electron to its own primary electron acceptor, and becomes P700+ (redox again!).
  • The electron reaching the "bottom" of the electron transport chain in PSII is shuttled to PSI, where it replaces the lost electron of P700+, restoring it to its original P700 configuration.
  • PSI excited electrons travel along a different electron transport chain via the protein ferredoxin ((Fd). There is no proton pump at the PSI electron transport chain, so no ATP is produced there.
  • A special enzyme, NADP+ reductase, catalyzes the transfer of two electrons from Fd to one NADP+, reducing it to energy-storing NADP-H.

    One picture is worth a lot of words.

    The flow of electrons from PSII to PSI has been called linear electron flow or non-cyclic electron flow to distinguish it from a less common phenomenon...

    Cyclic electron flow

    Once in a while, an excited electron from PSI's primary electron acceptor will "short circuit" and pop over to Fd and back into the electron transport chain between PSII and PSI, instead of to NADP+ reductase.

    This will produce more ATP (and is a good supplement), but no NADP-H.

    It's probably an accident of the proximity of the various molecules, but because it's not deleterious there has been no selection pressure against it. It just happens.


    The Calvin Cycle: An Anabolic Cycle

    During the light-independent reactions of photosynthesis, a.k.a., The Calvin Cycle, the energy stored in ATP and NADP-H during the light-dependent reactions is briefly released and then repackaged into the covalent bonds of sugar.

  • The sugar first produced by the Calvin Cycle is not 6-carbon glucose (this is constructed later), but a 3-carbon sugar known as glyceraldehyde-3-phosphate or G3P.

  • The Calvin Cycle "spins" three times to make this 3-carbon sugar from three molecules of CO2. The conversion of inorganic carbon from CO2 into the carbons of an organic molecule, G3P, is known as carbon fixation.

  • Remember that there are many Calvin Cycles going on in the stroma at any given time, so many atoms of carbon are constantly being fixed quickly into sugar.

  • The Calvin Cycle can be reduce to three phases:

    What's the cost of making carbohydrate?

    To make one molecule (or mole) of G3P, the plant must expend the energy of nine ATP molecules (or moles) (at 7.3kcal/mole) and six molecules of NADP-H. Some of this is packaged in the sugar, of course. But because of our old pal The Second Law of Thermodynamics, some is also lost as entropy.

    (A good question might be...How many CO2 molecules, ATP molecules and NADP-H molecules are needed to make one molecule of glucose?)


    And here again is our nice Overview of Photosynthesis that will tie all this together.

    Plants that produce 3-carbon G3P via the Calvin Cycle are called C3 plants. But there are some plants that have an alternative mechanism for carbon fixation, and this is particularly true in plants that evolved in hot, arid climates.


    The Balancing Act: Water Loss vs. Photosynthesis

    Carbon dioxide for photosynthesis enters via the stomates. But during the hottest part of the day, stomates often close to prevent water loss. Even with stomates partially open, CO2 levels drop rapidly. And this triggers a process known as photorespiration. If the plant continues to attempt to fix CO2 when its stomates are closed, the CO2 will become depleted, causing the cellular [O2] to increase relative to cellular [CO2].

    Photorespiration: a wasteful relic?

  • As CO2 levels drop, the Calvin Cycle becomes "starved" for raw material.

  • Rubisco has some affinity for O2 (albeit lower than for CO2).

  • When CO2 is scarce, rubisco will bind to O2 to its ribulose-biphosphate substrate, instead of the normal CO2.

  • The resulting "deformed" product of this metabolic cycle, like the normal one, is unstable. But instead of splitting into two molecules of 3-phosphoglycerate, (the normal products), it splits into one molecule of 3-phosphoglycerate and one molecule of a 2-carbon compound, 2-phosphoglycolate.

  • The 2-phosphoglycolate diffuses out of the chloroplast.

  • When 2-phosphoglycerate enters the peroxisomes or mitochondria, it is further split to release CO2.

  • This process is called photorespiration because it occurs in the light and generates CO2. But unlike cellular respiration, it generates no ATP. (In fact, it consumes ATP.)

    If photorespiration is wasteful, why does it exist?

    Some suggest it's evolutionary baggage left over from an era when there was far more CO2 in the atmosphere than oxygen, and this reaction was unlikely to happen.

    However, mutant plants that cannot perform photorespiration (they have a mutant form of rubisco) often suffer more damage from intense light than non-mutant plants. Though it's not well understood, it's possible that photorespiration may neutralize potentially damaging by-products of the light reactions.

    Still, in some plants (and notably in many agricultural crops), photorespiration steals away up to 50% of the carbon fixed by the light reactions! Not a big deal for the plants, but potentially a big deal for heterotrophs (us) using those plants for their own energy.


    C4 Plants

    Some plants, across about 19 different families, have a clever trick that allows photosynthesis while stomates are closed while minimizing photorespiration. These plants, known as C4 plants manufacture a 4-carbon compound that serves as a "shuttle" for CO2 to the Calvin Cycle's initial carbon fixation phase when plants can't access atmospheric carbon dioxide.

    A unique leaf anatomy is associated with the C4 pathway: C4 plants have two types of photosynthetic cells, bundle-sheath cells and mesophyll cells, arranged in the leaf as shown here:

    • The Calvin Cycle occurs only in the bundle-sheath cells.
    • The mesophyll cells act as a "carbon dioxide pump," concentrating carbon dioxide by transporting CO2 cleverly bound to other compounds.

    But how?

  • In the mesophyll cells, an enzyme known as PEP carboxylase binds free CO2 to phosphoenolpyruvate (PEP) to form 4-carbon oxaloacetate and other 4-carbon compounds (e.g., malate).

  • PEP carboxylase has a very high affinity for CO2, but NONE for oxygen gas. So when rubisco is bamboozled by stomatal closure and oxygen, PEP carboxylase can still sequester carbon dioxide.

  • Via plasmodesmata, mesophyll cells transport their 4-carbon, CO2-carrying products to the bundle-sheath cells.

  • The 4-carbon compounds release their precious CO2 passengers, which then enter the Calvin Cycle and can be incorporated into G3P and other products in a normal fashion.

  • This CO2 release generates pyruvate, which travels back to the mesophyll cells and is converted to PEP (with energy expended).

  • So this process is not without cost, but it keeps the Calvin Cycle going even when CO2 concentration in the cell is low, while minimizing photorespiration.

    But where do we get that extra ATP?

  • Bundle sheath cells have only Photosystem I, which generates ATP, but no NADP-H. Only cyclic electron flow is used to generate ATP in these cells.

    In essence, C4 plants pump enough extra CO2 into the bundle sheath cells to keep rubico loaded with CO2, at a cost of ATP.

    This mechanism would be particularly advantageous in a hot, arid climate, where stomates must close during the day to prevent desiccation, and it is in these areas that C4 plants first evolved and still thrive.


    CAM Plants

    Many succulent plants have evolved a different way to allow normal photosynthesis even when stomates are closed. The plants in which this pathway was first discovered are members of
    Family Crassulaceae, and the pathway, named for them is known as crassulacean acid metabolism, or CAM.

    During the day, CAM plants' stomates are closed. But at night they open, taking up CO2 and incorporating it into a variety of organic acids for storage. These acids are stored in the vacuoles of leaf mesophyll cells until daybreak.

    As the light reactions start up in response to light, CO2 is released from the organic acids. The energy from ATP and NADP-H from the light reactions can now be used to fix that carbon, even though the stomates are closed.


  • C4 and CAM are similar in that they use organic intermediates to store CO2 for later release, concentrating it when the plant isn't open to the atmosphere.

  • C4 and CAM are different in that C4 plants have two types of cells, and they separate carbon dioxide storage cells (mesophyll) from Calvin Cycle-performing cells (bundle-sheath). CAM plants perform both functions in the same (mesophyll) cells).