photosynthesis hl Flashcards
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Photosystems as arrays of pigment molecules that can generate and emit excited electrons
- Students should know that photosystems are always located in membranes and that they occur in cyanobacteria and in the chloroplasts of photosynthetic eukaryotes.
- Photosystems should be described as….
- A photosystem is a protein complex which is found in the thylakoid membrane inside chloroplasts of plants, algae or cyanobacteria.
- A photosystem harvests light by passing on the energy from photons of the sun from molecule to molecule until it reaches the reaction center chlorophyl.
- Photosystems = molecular arrays of chlorophyll and accessory pigments with a special chlorophyll as the reaction centre from which an excited electron is emitted.
What does a Photosystems consist of?
Reaction center – two special chlorophyll a molecules
Light-harvesting antenna pigments (Accessory pigments such as chlorophyll b, carotenoids, xantophylls, pheophytins bound to proteins.)
*How do ELECTRONS travel?
- When accessory pigments in a photosystem absorb light, electrons get excited. When the electron drops back down to its original state it releases energy.
- This energy is absorbed by an adjacent pigment molecule, becoming excited in turn. This process of excitation energy transfer is repeated across the antenna pigments.
- This way the energy is transferred from pigment to pigment until the reaction center chlorophyll is reached where electrons are transferred to an electron acceptor.
Why do they do this though?
- Energy absorbed from the accessory pigments eventually reaches the reaction center chlorophyll molecules, which donate a pair of electrons to electron acceptors. These excited electrons carry energy for later stages of photosynthesis.
- Electrons are raised from a ground state to an excited state, and the energy stored in the electrons is subsequently used to produced ATP.
photosystems
- PSI (P700) – has its wavelength absorption peak at 700nm and is the strongest biological RA.
- PSII (P680) – has its wavelength absorption peak at 680nm and is the strongest biological OA
- The two photosystems are connected to each other via an electron transport chain, which is passing on the excited electron from the primary acceptor (chlorophyll a) of the PSII along several electron acceptors of the electron transport chain all the way to the primary acceptor of PSI. Along this process, an electrochemical gradient is established, which helps to produce ATP.
Advantages of the structured array of different types of pigment molecules in a photosystem
- Students should appreciate that a single molecule of chlorophyll or any other pigment would not be able to perform any part of photosynthesis.
- on HW sheet!
- The different pigment molecules can absorb light in different ranges of wavelength. How is this useful?
- Antenna pigment molecules are in close and precise orientation to each other. How is this useful?
- More pigments to intercept randomly scattered photons have a higher chance of absorption. How is this useful?
Generation of oxygen by the photolysis of water in photosystem II
- Emphasize that the protons and electrons generated by photolysis are used in photosynthesis, but oxygen is a waste product.
- The advent of oxygen generation by photolysis had immense consequences for living organisms and geological processes on Earth.
- Light energy is used to split water, releasing H+ and O2 (photolysis). Released electrons and H+ are used by ATP synthase to produce ATP.
- NADP+ is reduced to NADPH and H+.
- ATP and the electrons from NADPH are used in the light independent reactions.
- Oxygen is a waste product.
- Photoactivation: A photon of light is absorbed by a pigment in PSII and transferred to other pigments until it reaches chlorophyll A at the reaction center. The energy excites an electron to a higher energy.
- Chlorophyll a is oxidized (loss of electrons). The primary electron acceptor in the thylakoid membrane (Pheophytin) is reduced.
- Photolysis: H2O is split by the energy of light & enzyme to produce electrons, protons and oxygen. The electrons replenish the chlorophyll A pigment to return it to its ground state by filling the “holes”.
ATP production by chemiosmosis in thylakoids
- Include the proton gradient, ATP synthase, proton pumping by the chain of electron carriers and also the electrons sourced from photosystem I in cyclic photophosphorylation or photosystem II in non-cyclic photophosphorylation.
- The excited electron pass from the primary acceptor down an electron transport chain losing energy at each transfer.
- With energy from the electron transport chain H+ ions from the stroma are pumped into the thylakoid space. This generates a concentration gradient.
- Chemiosmosis: Accumulating H+ ions cause the pH to drop, and the establishment of a proton gradient occurs. The flow of protons through ATP synthase allows phosphorylation of ADP + Pi 🡪 ATP
- The electrons keep moving down to the next electron carrier (PC, Plastocyan)
Reduction of NADP by photosystem I
- Students should appreciate that NADP is reduced by accepting two electrons that have come from photosystem I. It also accepts a hydrogen ion that has come from the stroma. The paired terms “NADP and reduced NADP” or “NADP+ and NADPH” should be paired consistently.
Thylakoids as systems for performing the light-dependent reactions of photosynthesis
- Students should appreciate where photolysis of water, synthesis of ATP by chemiosmosis and reduction of NADP occur in a thylakoid.
Light-dependent rxns
- is carried out in the thylakoid membrane, converting light energy to chemical energy in form of ATP and NADPH with the production of O2. It consists of photoactivation and photolysis (hydrolysis of H2O).
Light-independent rxns
- takes place in the stroma of the chloroplasts and converts CO2 into sugars with the help of the previously synthesized energy carriers ATP and NADPH.
Carbon fixation by Rubisco
- Students should know the names of the substrates RuBP and CO2 and the product glycerate 3-phosphate. They should also know that Rubisco is the most abundant enzyme on Earth and that high concentrations of it are needed in the stroma of chloroplasts because it works relatively slowly and is not effective in low carbon dioxide concentrations.
Synthesis of triose phosphate using reduced NADP and ATP
- Reduced NADP supplies hydrogen for reducing NADP, and ATP supplies the necessary energy.
Regeneration of RuBP in the Calvin cycle using ATP
- Students are not required to know details of the individual reactions, but students should understand that five molecules of triose phosphate are converted to three molecules of RuBP, allowing the Calvin cycle to continue.
- If glucose is the product of photosynthesis, five-sixths of all the triose phosphate produced must be converted back to RuBP.
Interdependence of the light dependent and light-independent reactions
- Students are not required to know details of metabolic pathways, but students should understand that all of the carbon in compounds in photosynthesizing organisms is fixed in the Calvin cycle and that carbon compounds other than glucose are made by metabolic pathways that can be traced back to an intermediate in the cycle.
Interdependence of the light dependent and light-independent reactions
- Students should understand how a lack of light stops light-independent reactions and how a lack of CO2 prevents photosystem II from functioning.
