Photosynthesis.

Why Study Photosynthesis? This is the subject of a very interesting essay you can read on the web

I. Photosynthesis.

Photosynthesis is the means by which the energy that keeps life out of equilibrium with its physical environment is captured from sunlight. Photosynthesis:

II. The History of the Discovery of Photosynthesis.

  1. Jan Baptista van Helmont, (1577-1644)
    Wanted to test the assumption that plants grew from the soil. So in what is heralded as one of the first carefully designed biological experiments ever conducted, he grew a willow tree in an earthen pot, giving it only H2O. After 5 years, he found that the plant had gained 74.4 kg (163 lbs) whereas the soil lost only 57 g (2.5 ounces), proving that the mass of the tree did not come from the soil. However he incorrectly assumed all of the weight gain came from H2O. (read excerpts of his classic work.)
  2. Joseph Priestly, (1733-1804)
    Used the classic bell jar experiments to show that plants can restore air in which candles had been burned. He did similar experiments that demonstrated that plants could keep a small mouse alive. However he had some difficulty reproducing his results (a requirement of the Scientific Method), leading to a great deal of controversy (excerpts of his classic works 1, 2, 3).

     


  3. Jan Ingenhousz (1730-1799)
    not only confirmed Priestley's work but added that the "purification" of air only takes place in the presence of sunlight, and that it was the green part of the plant that had this restorative power (read excerpts his classic work).

  4. Antoine Lavoisier (1743-1794)
    demonstrated that the exchange of CO2 and O2 are linked. During this time it was believed that O2 came from CO2 when the C was used to make sugar. (read from his classic works)

  5. Nicholas Theodore de Saussure (1767-1845) showed equal volumes of CO2 and O2 are exchanged during photosynthesis and that plants gained more weight than could be explained by the exchange of CO2 and O2, suggesting that the rest of the dry mass could be accounted for by H2O and minerals.
  6. Blackman, 90 yr. ago
    1. showed that photosynthesis was a 2 stage process
    2. light dependent
    3. light independent, enzymatic

  7. van Niel, 60 yr. ago
    1. speculated that O2 came from H2O
    2. it was subsequently proven by using light 18O in H2O in the general formula:

      CO2 + 2H218O ---> CH2O + 18O2 + H2O

Today, we express this combined knowledge as the photosynthetic equation:

6CO2 + 12H2O + energy ---> C6H12O6 + 6O2 + 6H2O

III. Photosynthetic Overview.

There are three basic steps in photosynthesis:

  1. Light Reactions - energy capture

    chemiosmosis generation of ATP (adenosine triphosphate) from harvested sunlight

  2. Dark Reactions - fixation of carbon

    enzyme catalyzed reactions using the energy formed (ATP) in the light reactions to fix atmospherically derived carbon (CO2) into sugars (CH2O)

  3. Pigment Regeneration - electron replacement

    from the splitting of H2O in oxygenic photosynthesis.

IV. Chloroplast

Many types of bacteria, algae and higher plants photosynthesize. The reactions of photosynthesis take place within the thylakoid membranes within chloroplasts in leaf cells.

The diagram above is from a previous edition of Raven and Johnson but is similar to Figure 10.2

Within the chloroplast there is a spatial separation of the chemical reactions of photosynthesis (below, R & J, Figure 10.2).

V. Light Absorption & Photosynthetic Pigments:

To understand this conversion of sunlight to chemical energy we need to consider energetics once more, specifically the nature of the light energy absorbed by plants to initiate the conversion to chemical energy in the form of reduced sugars.

The light energy that reaches the surface of the Earth, makes up of a continuous spectrum of various sized wavelengths and photons known as the electromagnetic spectrum, the energy in each being inversely proportional to its' wavelength.

speed of light (3x108 ms-1) = wavelength x frequency

c = λn

photon energy = planks constant x frequency

E = hν

(Planck's constant = 6.63x10-34 Js-1)

The wavelengths vary from nanometers (X-rays) to kilometers (radio waves)

The electrons orbiting the nucleus of any atom or molecule occupy discrete energy levels and therefore to boost a an electron to an higher energy level requires a very specific energy addition. This is known as the photoelectric effect, and only photons of a certain critical wavelength can dislodge electrons from an object. This is the basis of the light-dependent reaction of photosynthesis. Photosynthetic pigments are used to absorb light of precisely the right energy to facilitate electron transfer via the photoelectric effect.

Pigments

When pigments absorb light, electrons are temporarily boosted to a higher energy level (the photoelectric effect. When the e- returns to a lower energy level the energy may be:

  1. dissipated as heat
  2. re-emitted as a longer wavelength of light - fluorescence
  3. captured in a chemical bond (carbon gain!)

Photosynthetic pigments include the chlorophylls, carotenoids and phycobilins embedded in the thylakoid membranes. Chlorophyllb, can act as an accessory pigment passing its energy to Chlorophylla, broadening the range of light that can be used for photosynthesis.

VI. Photosystems.

The photosynthetic pigments in the thylakoid membrane are arranged into photosystems that work to increase the efficiency of light capture. Each photosystem contains 250-400 pigment molecules (chlorophyll and accessory pigments) but only one pair of Chlorophylla molecule in each is situated in the reaction center. Ultimately the absorbed photon energy will be passed as an e- from pigment to pigment until it reaches one of the two types of reaction centers, PSI or PSII.

PSI contains Chla molecules that absorb optimally at 700 nm (P700)

PSII contains chla molecules that absorb optimally at 680 nm (P680)

VII. The Light Reactions: Absorbing Light Energy.

  1. Primary Photo event
    • photon capture by a chlorophyll or accessory pigment in the antenna complex
    • transfer to a specific Chla molecule of the reaction center - P700 (in PSI)
  2. Electron Transfer

  3. Figure 10.12 from your textbook

  4. Chemiosmotic synthesis of ATP

  5. Figure 10.17 from your textbook

In plants and algae a two-stage photocenter is used. First photosystem II absorbs a photon and passes and electron to photosystem I, driving the proton pump to generate ATP. When a second photon is adsorbed by photosystem I and eventually passed along to nicotine adenine dinucleotide phosphate (NADP+) to generate NADPH (a two electron transfer). This two stage process is irreversible and non-cyclic and the two stages are needed to provide charge separation while gaining sufficient energy to drive the reduction of ferredoxin. Ultimately electron replacement comes from water, thereby producing oxygen.


Figure 10.15 from your textbook


Figure 10.16 from your textbook

Summary:

Electrons flow from H2O --> PSII --> PSI --> NADP+ generating O2, ATP and NADPH

This is called non-cyclic photophosphorylation and requires 4 photons for each pair of electrons that reach NADP+. A mole of photons contains 40 kcal of energy and thus the free energy change from H2O and NADP+ to NADPH and 1/2 O2 is 51 kcal mol-1. Ultimately 32% of the energy in the photons is harvested!

PSI can work independently of PSII generating no O2 but generating AT. This happens when the electrons are shunted from ferredoxin back to the reaction center to generate another ATP and is referred to as cyclic photophosphorylation. In photosynthetic purple sulfur bacteria the electron ejected from the pigment (P870) by a photon of light cycles back, driving the proton pump to form ATP (energy capture) and then returns to the photocenter via cytochromes. Notice that this scheme with a single reaction center (thought to be more primitive) does not result in the evolution of oxygen!


Figure 10.14 from your textbook

VIII. The Dark (or light independent) Reactions

The cellular energy source gained from the light reactions, ATP and NADPH is subsequently used for the metabolism of carbon to make the basic building blocks of organic molecules (plants are 45% Carbon by dry mass). The steps that comprise the enzymatic fixation of carbon are known as the dark reactions since they can readily proceed in the absence of light, however don't be confused, these processes can just as easily proceed (and commonly do) in the light. The enzymes responsible for the uptake of atmospheric CO2 and formation of simple sugars are present in the chloroplast of plant cells.

Organic molecules are highly reduced compared to CO2.

To build organic molecules plants use the two products of the light reactions:

Key is to use these products to attach a C, from a CO2 molecule, to an organic molecule. The organic molecule use is an energy-rich five-carbon sugar, ribulose1,5-bisphosphate (RuBP).

This is accomplished by the enzyme, ribulose1,5-bisphosphate carboxylase - Rubisco

The Calvin Cycle (named for the scientist, Melvin Calvin, who first worked out the steps involved) uses the energy (ATP and NADPH) generated in the chloroplast during the light reactions. In this reaction, CO2 is covalently linked to the five carbon sugar RuBP and immediately splits into two three carbon molecules, 3PGA, and thus is known as C3 photosynthesis (since the first product is a three carbon sugar). The carboxylation of RuBP is catalyzed by Rubisco which is thought to be the most abundant protein in the world. From there the three carbon molecules go through a number of modifications consuming reducing power and ATP but ultimately RuBP is regenerated at the end of each round. The regeneration of RuBP is essential if the processes is to remain active. One molecule of CO2 is fixed at a time, but it takes six carbons to make a complete molecule of glucose and thus they dark reactions require six turns of the Calvin cycle to produce 1 molecule of glucose.

3 molecules of CO2 are fixed by rubisco to produce 6 molecules of PGA = 18C.

The 3 molecules of RuBP used in the reaction are reformed consuming 15C.

The net gain then is one 3C molecule - glyceraldehyde 3-phosphate (G3P).

3 CO2 + 9 ATP + 6 NADPH + H2O -> G3P + 8 Pi + 9 ADP + 6 NADP+

XI. Alternative Photosynthetic Pathways

In addition to the C3 pathway described above in which a 3C molecule is the first stable product there are also C4 and CAM (crassulacean acid metabolism) pathways for C fixation

  1. C4 Photosynthesis
  2. In C3 photosynthesis Rubisco can combine either O2 or CO2 with RuBP. (the inability to completely discriminate against O2 is most likely the consequence of the conditions present during the evolution of the Calvin Cycle and the similarity between the size and shape of the two gases). In the case of O2 one two carbon molecule of phosphoglycolate and one G3P are generated. Phosphoglycolate is converted to glycolate and in the peroxiosomes glycolate is oxidized during photorespiration to release CO2 Photorespiration amounts to using O2 to generate CO2, a very wasteful process. This can occur whenever O2 levels are high and CO2 levels are low. Remember the atmosphere is nearly 21% O2 and only 0.036% CO2, so the chance of an O2 molecule finding its way to the active site of Rubisco is quite high! C4 photosynthesis is an adaptation to limit photorespiration by increasing the chance that a CO2 molecule will enter the active site rather than and O2 molecule.

    The basic steps include:

    1. A three carbon substrate phosphoenolpyruvate (PEP) is carboxylated to form a four carbon molecule, oxyaloacetic acid (OAA) by PEP carboxylase in the mesophyll cells.
    2. The OAA is converted to malate or aspartate and transferred from the mesophyll cells to the bundle sheath cells. These specialized cell known as Kranz anatomy (see figure) are relatively impermeable to CO2.
    3. In the bundle sheath cells malate is decarboxylated yielding pyruvate and releasing CO2. By doing so the atmosphere within the bundle sheath cell is elevated in CO2 and reduced in O2, and thus greatly favors carboxylation over oxygenation of RuBP by Rubisco.
    4. At this point, CO2 enters the normal C3 pathway and pyruvate is transported back to the mesophyll cells where it is converted to PEP and requires the input of ATP. Accordingly there is a greater energy cost to fixing carbon this way. C4 photosynthesis requires 5 ATP's to fix one molecule of CO2, whereas in C3 photosynthesis requires only 3. At this rate the cost of producing a molecule of glucose is almost doubled from 18 to 30 ATP.

    As stated above C4 photosynthesis thought to confer benefit to plants growing in low CO2 conditions, it also provides a water savings. The key to this water savings is that while CO2 enters the leaves through the stomata water is lost by the same route. C3 plants must balance water loss with carbon gain. C4 plants have a slight advantage, using the carbon concentrating mechanisms to get more CO2 uptake per unit water loss. Accordingly you would expect, and do find, that C4 plants are more common to warm dry environments. Furthermore, photorespiration itself is sensitive to temperature. Under warmer conditions oxygenation is favored over carboxylation and C3 photosynthesis becomes even less efficient. Again C4 confers a competitive advantage by limiting photorespiration.

  3. Crassulacean Acid Metabolism (CAM)
  4. Like C4 photosynthesis, CAM is an adaptation to hot dry environments. CAM plants (many succulents and cacti) utilize both the C3 and the C4 pathways of photosynthesis. Like C4 plants, the need is to maintain carbon uptake while limiting water loss. Where as C4 plants accomplish this by a physical/spatial separation of the initial carbon fixation by PEP carboxylase from the ultimate fixation by Rubisco, CAM plants use a temporal separation. In CAM photosynthesis, CO2 is fixed by PEP carboxylase at night to form malic acid which is then is stored in the vacuole of the cell. By completing this step at night water loss is minimized. During the following day, the malic acid is transported out of the vacuole to the cytosol and decarboxylated. The released CO2 is used by Rubisco and enters the Calvin cycle as in any other C3 plant.

Want to learn more or do you just need a fresh approach? Check out this excellent multimedia photosynthesis tutorial.

Links

MIT's Biology Hypertextbook (Chapter 7 Photosynthesis)

Online Biology Book from MaricopaCommunity College

ASU's Photosynthesis Center

Lecture by Professor Kevin Griffin.

Updated April 20, 2005
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