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Option 9.9 Biochemistry: 4. The light reaction

Syllabus reference (October 2002 version)
4. By the middle of the 20th century, a description of the light dependent reaction was developing	Students learn to: describe and discuss the importance of Van Neil’s hypothesis that water was the source of oxygen given out in photosynthesis outline the classic experiments of Emerson and Arnold and their interpretation by Gaffron and Wahl that led to the hypothesis of a photosynthetic unit consisting of chlorophyll and photoenzyme molecules identify the light dependent reaction as that which traps light energy and coverts it to chemical energy stored in ATP identify the role of chlorophylls in the light reactions explain the significance of the difference in function of photosystems I and II identify the role of the coenzymes ADP and NADP in the light reactions	Students: present information that describes the repetitive flash technique first used by Emerson and Wahl and account for its subsequent importance in the study of photosynthesis gather and process information from secondary sources to trace the light dependent reaction of photosynthesis on a suitable biochemical pathways flow chart

Extract from Biology Stage 6 Syllabus (Amended October 2002) © Board of Studies, NSW.
[Edit 18 June 09]

Prior learning: Stage 4, Structures and Systems 4.8.2 (c), 4.8.4 (d) Interactions 4.10 (c). Preliminary module 8.2 (subsection 2) module 8.3 (subsection 4), module 8.4 (subsections 2 and 3).

Background: By the middle of the 20th century, a description of the light dependent reaction was developing.

 

describe and discuss the importance of Van Niel’s hypothesis that water was the source of oxygen given out in photosynthesis

• Van Niel studied photosynthesis in bacteria that used H₂S instead of H₂O and produced sulfur solid in globules, rather than O₂. These bacteria still required CO₂ and produced carbohydrates (general formula CH₂O). The equation for photosynthesis by the sulfur producing bacteria is

CO₂ + 2H₂S → CH₂O + H₂O + 2S

• Van Niel reasoned that the bacteria split the H₂S (because the S was a product) and used the H to make sugar. He generalized that all photosynthetic organisms require hydrogen but the source from which they take varies, in green plants the H would come from splitting water. From this generalisation the equation for photosynthesis for green plants becomes

CO₂ + 2H₂O → CH₂O + H₂O + O₂

The importance of van Niel’s hypothesis is that it gave one of the first clues to the mechanism of photosynthesis. Prior to Van Niel’s work, scientists had thought that carbon dioxide was split, the oxygen released and the C added to water to make glucose. Van Niel’s hypothesis that water is split to produce oxygen was later proved using radioactive oxygen – 18. Water labeled with ¹⁸O produced oxygen gas labeled with ¹⁸O. When CO₂ labeled with ¹⁸O was used, the radioactivity was detected in the glucose and water, not the oxygen gas produced.

present information from secondary sources to describe the repetitive flash technique used for the first time by Emerson and Wahl and outline its subsequent importance in the study of photosynthesis

• Present the information you have obtained from secondary sources in a medium that you feel is appropriate. You may decide on an oral presentation with overhead transparencies or you may prefer to do a powerpoint presentation.
• The web sites below will assist with secondary information.

Graph of Emerson's and Arnold's results University of Illinois, Illinois, USA.

Milestones in photosynthesis University of Illinois.

outline the classic experiments of Emerson and Arnold and their interpretation by Gaffron and Wahl that led to the hypothesis of a photosynthetic unit consisting of chlorophyll and photoenzyme molecules

• Emerson and Arnold illuminated algal cells with very brief flashes of light that could excite every chlorophyll molecule at least once. The importance of their work was that it allowed Emerson and Arnold to study the relationship between the amount of incident light energy, the amount of chlorophyll present and the amount of oxygen evolved in algal cells. By exposing algal cells to repetitive flashes of light, they found that only one O₂(g) molecule was given off by the algae for every 2400 chlorophyll molecules. This result implied that not all chlorophyll molecules are photochemically active.

• Gaffron and Wahl interpreted this information to propose that light is absorbed by hundreds of chlorophyll molecules that transfer their energy to a single reaction centre. The hundreds of molecules described by Gaffron and Wahl form the antenna complex of a photosystem. The antenna complex also includes accessory pigments, such as the carotenoids, to collect light of other wavelengths. When a chlorophyll molecule in the antenna complex is excited, the energy is transferred from one molecule to another by resonance transfer energy (kinetic energy in vibrations) until it reaches a pair of chlorophyll-a molecules in the photochemical reaction centre. Each antenna complex effectively acts as a funnel, sending energy to the photochemical reaction centre.

• The two chlorophyll molecules at the heart of the photochemical reaction centre are held in a transmembrane protein-pigment complex next to a chain of electron acceptors. The pair of chlorophyll molecules acts as an irreversible trap by passing their excited electron to the chain of electron acceptors and moving it quickly to a more stable environment where it is used in further reactions.

identify the light dependent reaction as that which traps light energy and coverts it to chemical energy stored in ATP

• The light dependent reaction of photosynthesis is associated with the thylakoid membranes. (See later in this Option 9.9 Biochemistry: 8. The structure of chloroplasts) Oxygen is evolved as a part of the reaction and the light energy captured by chlorophyll and accessory pigments is converted to chemical energy in the form of high energy phosphate bonds in ATP.

identify the role of chlorophylls in the light reactions

• Chlorophylls are excellent light absorbers because they possess delocalised electrons (in π orbitals above and below a planar ring structure). The electrons are “excited” or “jump up” to a higher energy level when light is absorbed by the chlorophyll molecule. The chlorophylls and accessory pigments in the antenna complex pass the energy from these excited electrons by resonance transfer to other molecules in the complex until it reaches the reaction centre.

explain the significance of the difference in function of photosystems I and II

• In the 1950s Emerson showed a sharp decrease in oxygen production in the red part of the spectrum at wavelengths above 700nm when observing the action spectrum of the green alga Chlorella sp. This has been called the “red drop”. When Emerson exposed the alga to wavelengths of light at both 680nm and 700nm, oxygen evolution was greater than the sum of each wavelength, the “Emerson enhancement effect”. Emerson interpreted these results to mean there were two photosystems, one using light of 700nm and the other using light of 680 nm or less.

• Photosystem I is defined as containing reaction centre chlorophylls with red light absorption at 700nm. It is not involved in O₂(g) production but provides NADP⁺ which accepts an electron from water and the hydrogen atom split in Photosystem II. NADPH is formed which is later involved in photophosphorylation. (See later in this Option 9.9 Biochemistry: 6.Using isotopes to prove carbon dioxide is used in the light independent reaction. also 9.9 Biochemistry: 7. Structure and function of ATP.)

• Photosystem II uses reaction centres that absorb red light at 680 nm. It splits water, producing oxygen and feeds the electrons to an electron transport chain that couples photosystem II to photosystem I. Electron transfer between photosystems I and II provides electron flow and a proton gradient for the production of ATP from ADP and inorganic phosphate (P_i). The reaction occurring is

2H₂O + 4 photons of light → 4H⁺ + 4e^- + O₂

identify the role of the coenzymes ADP and NADP in the light reactions

• Coenzymes are compounds that play a role with enzymes to catalyse reactions. Some coenzymes are oxidising and reducing agents.
• The conversion of NADP⁺ to NADPH is a reduction reaction while the splitting of water to release electrons is an oxidation reaction. The half reactions of oxidation and reduction are kept separate in the two photosystems.

Animation of the light reaction Cornell University, USA. [Flash required]

gather and process information from secondary sources to trace the light dependent reaction of photosynthesis on a suitable biochemical pathways flow chart

• Gather information to locate a biochemical flowchart to trace the light dependent reaction of photosynthesis. To do this you will need to extract information from graphs and tables. See for example:

Photosystem I and II and the Light Reaction McDaniel College, Westminster, Maryland, USA

• Process the information to illustrate trends and patterns. To do this use the biochemical flow chart of the light dependent reaction to identify:
  • in photosystem II, a P680 chlorophyll electron excited by light energy
  • manganese atoms that holds two water molecules while electrons are removed one at a time
  • the water molecules split into O₂(g) and 4H⁺. The H⁺ released into the lumen of the thylakoid vesicle
  • the electrons in P680 in an excited state (a higher redox or electrode potential)
  • the electrons passing from photosystem II (PS II) to the electron accepting quinones in PS II and then down the electrode potential gradient to the cytochrome b – cytochrome f complex that links PS II with PS I
  • in the cytochrome b – cytochrome f complex, protons are moved from the stroma to the lumen
  • the electron is passed to PS I, the chlorophyll P700 is excited by light and the energy of the excited electrons is increased
  • the electrons are transferred to the iron-sulfur centre in ferredoxin and then to NADP⁺ to form NADPH with H⁺ from the stroma.

Don’t try to memorise this information but use it to interpret as many diagrams of the light reaction as you can find! You may be interested to know the electrons removed from the water move to fill holes created by light in chlorophyll molecules in the reaction centre of PS II.