Photosynthesis: Light Reactions Slide 2 The term photosynthesis normally has students thinking about the green plant tissue that is present all around us. It should also be remembered that at least fifty percent of global carbon dioxide is fixed into organic molecules by trillions upon trillions of microscopic autotrophic organisms which form part of the plankton community found in the surface waters of the ocean and freshwater! Autotrophs (phototrophs) are also found in extreme environments such as hot springs. All photosynthetic tissue converts light energy from the sun to chemical energy in the form of bonds between atoms, as in ATP and NADPH formed during the light reactions of photosynthesis. Photosynthetic tissue also reduces carbon dioxide from the atmosphere into carbohydrates during the carbon fixation reactions. These organic molecules are the building blocks for all organisms. Because of this unique ability to produce all the molecules necessary for life, photosynthetic organisms are termed autotrophs, or self feeders. Heterotrophs, on the other hand, are organisms that are dependent on the carbon products produced by autotrophs. Thylakoid membranes contain the pigments associated with photosynthesis and may be distributed throughout the cell, as in the prokaryotic cyanobacteria (blue-green algae) and purple and green sulfur bacteria. In eukaryotic organisms the thylakoid membranes and associated pigments are bound into a chloroplast. Organisms that evolve oxygen contain chlorophyll a but may differ in the types of accessory pigments that are present Slide 3 When plant cells are observed with a light microscope the most obvious structure is the green disc shaped chloroplast. At high magnification the most obvious structure inside the chloroplast is the elaborate arrangement of thylakoid membranes and the matrix that surrounds it, called the stroma. The thylakoid membrane forms stacks of disc like structures called grana (singular: granum) which are all interconnected by ribbons of thylakoid membrane, the stroma lamellae. All of the pigments associated with photosynthesis are anchored in the thylakoid membrane. Now lets look at how pigments that are involved in photosynthesis perform this function. [See Photosynthesis: Properties of light if you havent already done so]. Slide 4 The process of photosynthesis occurs in two stages. The first stage is referred to as the light or energy transduction reactions. During this stage, photons of light energy are absorbed by chlorophyll molecules and used indirectly in the production of ATP and NADPH. During the second stage of photosynthesis, the carbon fixation reactions, carbon dioxide is reduced to sugars during the Calvin cycle. The organic molecules produced also form carbon skeletons for other macromolecules, such as lipids and proteins, which are synthesized in an organism. ATP and NADPH produced during the light reactions are used to power the carbon fixation reactions. The light reactions are associated with the thylakoid membranes; the carbon fixation reactions take place in the chloroplast stroma.
Slide 5 Pigments involved in photosynthesis are embedded in the thylakoid membrane. The main photosynthetic pigments in eukaryotic plant cells are chlorophyll a, chlorophyll b and the carotenoid pigments. These pigments are in close proximity to each other forming individual photosystems. Each photosystem has an antenna complex which gathers light energy and funnels it to the reaction center at the heart of the photosystem, much like a satellite dish focuses transmitted signals to your TV allowing you to watch your favorite show. Carotenoid pigments and both chlorophyll a and b are associated with the antenna complex. When a pigment absorbs a photon of light the pigment acquires the energy of that photon causing the transition of that pigment to an excited, or higher energy state. This energy is transferred to neighboring pigments by resonance energy transfer. No electrons are exchanged causing a chemical change, only the energy is transferred. Think of a resonating tuning fork, when placed close to a second tuning fork it can cause the second fork to resonate. Energy is transferred causing the sound but no chemical reaction takes place. Energy is funneled through the antenna complex this way until it reaches the reaction center of the photosystem. [Pause for a moment and relate this information to the figure]. Two chlorophyll a molecules are associated with the reaction center. When either of these molecules absorbs the energy funneled toward it, one of its electrons is boosted to a higher energy level and the electron is transferred to an electron acceptor that initiates electron flow. A chemical reaction has now taken place; the chlorophyll a molecule of the reaction center has been oxidized. The arrangement of pigments in the photosystem allows a continual harvest of light photons and their energy is continually funneled towards the reaction center allowing uninterrupted electron flow. Hundreds of photosystems embedded in the thylakoid membrane function like this one, to harvest light energy. Slide 6 The electron lost from the chlorophyll a molecule in the reaction center needs to be replenished for the photosystem to continue functioning. These electrons are extracted from water molecules. Water molecules are split to produce oxygen, protons and electrons. Oxygen is released to the atmosphere through the stomata, protons contribute to a proton gradient across the thylakoid membrane facilitating the production of ATP and the electrons replenish those that were lost from chlorophyll a when it was oxidized. This process is not fully understood but involves a water-splitting enzyme complex that is located on the inside of the thylakoid membrane. This is the only place in nature where water molecules can be split to produce oxygen, protons and electrons! Slide 7 The thylakoid membrane houses two types of photosystems, photosystem I (PSI) and photosystem II (PSII). Photosystem II can donate electrons to photosystem I through a series of redox reactions that occurs in the electron transport chain. The electron transport chain contains many electron carriers molecules that will accept and donate electrons- the main ones to consider for this class are plastoquinone, plastocyanin, ferredoxin and the cytochrome complex. The reaction center chlorophyll of photosystem II is known as P680 because it is more efficient at absorbing light of wavelength 680 nm; the chlorophyll in the reaction center of photosystem I is designated P700 since it best absorbs light of wavelength 700nm. This is light in the red and far-red region of the visible spectrum respectively. Now lets follow the path of an electron as it participates in non-cyclic electron flow. As previously discussed, the photons of light energy harvested by the antenna complex cause enough excitement of the chlorophyll a molecule in the reaction center of photosystem II for it to release an electron. This electron is passed to the
primary electron acceptor of the electron transport chain and then on to plastoquinone, through the cytochrome complex and plastocyanin eventually arriving at the reaction center of photosystem I. The electron loses energy with each successive transfer, this energy is utilized to produce ATP at the thylakoid membrane. We will return to the process of ATP production shortly. The antenna complex of photosystem I harvests photons of light energy, funneling the energy to the reaction center P700 where there is sufficient energy to excite the electron in the reaction center to a higher level than in photosystem II. This high energy electron can now be transferred through a second electron transport chain, eventually reducing NADP+ to NADPH. The oxidized chlorophyll a molecule in the reaction center of photosystem I needs an electron to continue functioning. This electron is donated from photosystem II after it has passed down the first electron transport chain. The structure of the photosystems and their arrangement in the thylakoid membrane allows a continual stream of electrons to pass through the process generating ATP and NADPH. These energy producing reactions are complicated but keep in mind the overall function light energy from the sun is used to produce ATP and NADPH, the chemical energy required for the carbon fixation reactions. Slide 8 As we shall see in the next section, the Calvin cycle requires more ATP than NADPH to reduce carbon dioxide to sugars. When ATP is in low supply, more ATP is produced through cyclic electron flow. Now follow the path of the excited electrons produced in photosystem I which are shunted through ferredoxin to the cytochrome complex in the first electron transport chain, associated with photosystem II and photosystem I, and then back to the reaction center of photosystem I. This cyclic electron flow does not produce any NADPH nor does it involve photosystem II, therefore no oxygen is released during this process. ATP is the only product of cyclic electron flow. Slide 9 This figure shows the organization of the different photosystems and electron carriers in the thylakoid membrane. Now, think of each disk in a grana stack as having the same structure as a Reeses peanut butter cup The chocolate is equivalent to the thylakoid membrane of the disk, the peanut butter center is equivalent to the thylakoid space which contains a high concentration of protons resulting from the splitting of water molecules. As electrons move down the electron transport chain that connects photosystem II and photosystem I protons are continually pumped across the membrane into the thylakoid lumen (space) helping maintain the high proton concentration there. The protons naturally want to diffuse down their electrochemical gradient. This is facilitated by the ATP synthase complex which produces ATP as the protons diffuse through the complex from the thylakoid space into the stroma. To see how ATP synthase complex functions, refer to the virtual cell link or the lesson on respiration. Note that molecules of ATP and NADPH are released into the stroma where they will be used as an energy source for the reactions of the Calvin cycle.