B1115 - SP24 - Stud - Ch 10 - Photosynthesis.pdf

Photosynthesis Vinicius Azevedo Converting Solar Energy into Chemical Energy Earth’s main source of energy is the SUN 1% of the solar energy is captured by life processes Photosynthesis (light) (put together) Formation of molecules using energy from light The process that feeds the biosphere Directl...

Photosynthesis Vinicius Azevedo Converting Solar Energy into Chemical Energy Earth’s main source of energy is the SUN 1% of the solar energy is captured by life processes Photosynthesis (light) (put together) Formation of molecules using energy from light The process that feeds the biosphere Directly or indirectly, photosynthesis nourishes almost the entire living world Autotrophs X Heterotrophs Autotrophs üSustain themselves without eating anything derived from other organisms üThey are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules ü They are divided into photoautotrophs (e.g. plants) and chemoautotrophs (e.g. some archaea) Thiobacillus ferrooxidans (Vrdoljack and Spiller, 2017) Heterotrophs üObtain organic material from other organisms. üAlmost all heterotrophs, including humans, depend on photoautotrophs for food and O2 üThey are the consumers and decomposers of the biosphere. ü They include animals (consumers), fungi (consumers and decomposers), and most bacteria (consumers and decomposers) Photoautotrophs Organisms Use the energy of sunlight to make organic molecules from H2O and CO2 Photosynthesis occurs in plants, algae, certain other protists and some prokaryotes Plants Cyanobacteria Multicellular alga (i.e., kelp) Purple sulfur bacteria Unicellular protist (i.e., euglena) Cyanobacteria, a Photosynthetic Prokaryotes üCyanobacteria have thylakoid membrane with aggregates of light-harvesting proteins attached to it called phycobilisomes üCyanobacteria have photosynthetic pigments such as chlorophyll a Endosymbiosis Hypothesis Review Chloroplast Recent hypotheses believe the ancestral prokaryote was an archaea Chloroplasts Site of conversion of light energy to chemical energy in eukaryotes Major site of photosynthesis Chloroplast structure includes: ü Two membranes around a central aqueous space called the stroma ü In the stroma is an elaborate system of interconnected membranous sacs called thylakoids o The interior of thylakoids forms another compartment, the thylakoid space o Thylakoids may be stacked in columns called grana All green parts of a plant have chloroplasts There are about 500,000 chloroplasts per square millimetre of leaf surface ü Leaves are the major site of photosynthesis for most plants. ü Their green colour is from chlorophyll (green pigment within chloroplasts) ü Chlorophyll plays an important role in the absorption of light energy during photosynthesis. Chloroplasts mainly found in mesophyll (this tissue forms the interior of leaf tissue) – A typical mesophyll cell has about 40-50 chloroplasts CO2 enters, and O2 exits leaf through microscopic pores called stomata Photosynthesis What type of reaction is this? Photosynthesis can be summarized as the following equation: 6 CO2 + 6 H2O + Light energy ® C6H12O6 + 6 O2 Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules. Which reactant is oxidized, which is reduced? Photosynthesis Photosynthesis is a redox reaction in which H2O is oxidized to form oxygen and CO2 is reduced to form carbohydrates. The two stages of photosynthesis: a preview Light Reactions (the photo part) ü Occurs in the thylakoids ü Enzymatic splitting of H2O ü Release of O2 ü Reduction of NADP+ to NADPH ü Generation of ATP Calvin cycle (the synthesis part) ü Occurs in the stroma) ü Forms sugar from CO2, using ATP and NADPH from the light reactions NADPH is an electron carrier that will act as a reducing agent in the Calvin cycle. The two stages of photosynthesis: a preview Light Reactions (the photo part) Calvin cycle (the synthesis part) ü H2O is the reactant ü CO2 is the reactant ü O2 is the product ü Glucose is the product glucose 6 CO2 + 6 H2O + Light energy ® C6H12O6 + 6 O2 The sun emits energy in the form of electromagnetic radiation Electromagnetic radiation travels in rhythmic waves. üThe distance between crests of electromagnetic waves is called the wavelength. The electromagnetic spectrum displays the range of wavelengths of electromagnetic radiation The sun emits energy in the form of electromagnetic radiation The most important segment for life is a narrow band between 380 to 750 nm, the band of visible light § Visible light is the portion that humans can see Light also behaves as though it consists of discrete particles called photons § The amount of energy is inversely related to the wavelength of the light. Thylakoids convert light energy from the visible light spectrum into chemical energy of ATP and NADPH Photosynthetic pigments are the light receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted § Leaves appear green because chlorophyll reflects and transmits green light Absorption Spectrum X Action Spectrum Absorption Spectrum oA graph plotting a pigment’s light absorption versus wavelength. Action Spectrum o A graph plotting the rate of photosynthesis versus wavelength. Absorption Spectrum X Action Spectrum Results of both spectra show that photosynthetic reactions rely on light absorption Pigments Chlorophyll is the main photosynthetic pigment. Other plant pigments, like carotenoids, are called accessory pigments. Allow photosynthetic cells absorb a broader range of visible light. Protect the photosynthetic electron transport chain from damage. Excitation of Chlorophyll When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable. o When excited electrons fall back to the ground state, photons are given off, producing an afterglow called fluorescence. o If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat. Four possible fates of electrons in chlorophyll that are excited by photons…the energy released from these electrons can: be emitted in the form of light via fluorescence; be given off as heat alone; excite an electron in a nearby pigment and induce resonance; or be transferred to an electron acceptor in a redox reaction Photosystems About 200 – 300 chlorophyll molecules and accessory pigment molecules are organized with other organic molecules and proteins into a complex called a photosystem, which is embedded in the thylakoid membrane. A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by antenna chlorophylls. o The antenna chlorophylls funnel the energy of photons to the reaction center (resonance energy transfer – black arrow). o A primary electron acceptor in the reaction center accepts an excited electron from the chlorophyll in the reaction center (yellow arrow). § Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions. Photosystems There are two types of photosystems in the thylakoid membrane: oPhotosystem II (PS II) functions first and is best at absorbing a wavelength of 680 nm. § The reaction-center chlorophyll of PS II is called P680. oPhotosystem I (PS I) is best at absorbing a wavelength of 700 nm. § The reaction-center chlorophyll of PS I is called P700. For each of the following pairs of molecules, indicate which is the reduced form and which is the oxidized form. P680 P680+ P700 P700+ NADP+ NADPH Flow of Electrons During the light reactions, there are two possible routes for electron flow: cyclic and linear. oLinear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy. § Photosystem II is the first step, followed by Photosystem I; numbers are flipped because they represent the order of discovery. oCyclic electron flow uses only photosystem I and produces ATP, but not NADPH. § Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle. Linear Electron Flow Steps 1. A photon strikes a pigment molecule, and its energy is passed among pigment molecules via resonance energy transfer (black arrows) until it excites P680 (reaction-center complex of PSII). 2. The energy excites electrons from P680, which are then transferred to the primary electron acceptor (yellow arrow) – P680 is now in its oxidized state (P680+) Linear Electron Flow Steps Resonance Step 1 P680 becomes oxidized Step 2 P680+ (P680) becomes reduced Linear Electron Flow Steps 3. Breakdown of H2O (water-splitting enzyme catalyzes it) into two electrons, two H+ ions and an oxygen atom (1/2 O2). o electrons are transferred to P680+, thus reducing it to P680. o H+ ions are released into thylakoid space. o Oxygen atoms immediately combine to form O2 gas. Linear Electron Flow Steps b. Reduction of the reaction center Resonance P680 becomes oxidized Step 1 P680+ P680 becomes Reduced Step 2 (P680) P680 becomes reduced Linear Electron Flow Steps 4. The photoexcited electrons move from the primary electron acceptor of PSII to PSI via an electron transport chain. 5. As electrons pass through the electron transport chain, H+ ions are pumped into the thylakoid space, contributing to the proton gradient that is subsequently used in chemiosmosis to make ATP. Linear Electron Flow Steps Energy released by the fall down the electron transport chain drives the generation of a proton gradient across the thylakoid membrane. Diffusion of H+ (protons) across the membrane drives ATP synthesis. What process is this similar to? Linear Electron Flow Steps 6. Meanwhile, light energy from a photon has been transferred between pigment molecules in PSI via resonance energy transfer (black arrows) until it excites P700 (reaction-center complex of PSI)…the photoexcited electrons are transferred to the electron acceptor of PSI o P700 is now in its oxidized state (P700+) o Electron from P680 reduces P700+ to P700 Linear Electron Flow Steps 7. Photoexcited electrons from PSI go down to a second electron transport chain (note – this electron transport chain does not create a proton gradient and thus does not produce ATP) 8. The enzyme NADP + reductase catalyzes the transfer of electrons from the electron transport chain to NADP+. NADP+ is reduced to NADPH. o Two electrons are required for its reduction to NADPH. o This process also removes an H+ atom from the stroma (contributing to the H+ gradient). ATP and NADPH are used for the Calvin cycle Linear Electron Flow Steps Linear Electron Flow Steps Each electron “falls” down a second electron transport chain from the primary electron acceptor of PS I. o The electrons then reduce NADP+ to NADPH. Summary of the Linear Flow of Electrons Summary of the Linear Flow of Electrons Proton (H+) gradient is generated by: 1) water being split by PS II, 2) the cytochrome electron transport chain using energy to translocate H+ across the membrane and 3) removal of one H+ from stroma during the reduction of NADP+ Diffusion of H+ down the concentration gradient through ATP synthase drives ATP synthesis in the stroma ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place Red arrow = path of proton (H+) flow Gold arrow = linear flow of electrons Cyclic Flow of Electrons When light levels are high, the light energy absorbed begins to overwhelm the Calvin cycle’s use of NADPH. o When no NADP+ is returned from the Calvin cycle, the high energy electrons can damage the cell. To keep this from happening, electrons are shunted into an alternate pathway that increases the production of ATP while decreasing the production of NADPH. Light Reactions In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH. Calvin Cycle The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle. The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P). Is the cycle anabolic or catabolic? Calvin Cycle The Calvin cycle has three phases: Fixation Reduction Regeneration The Calvin cycle uses ATP and NADPH to convert CO2 to sugar 3 molecules of Ribulose biphosphate (RuBP) 5 molecules of G3P Unstable intermediate RUBISCO 6 molecules of 3-phosphoglycerate (3-PGA) 6 molecules of Glyceraldehyde-3-phosphate (G3P) 1 molecule of G3P (half-glucose) Later used for the synthesis of glucose and other sugars Carbon Phosphate Fixation CO2 is added to the 5-carbon sugar called ribulose 1,5bisphosphate (RuBP). o This step is catalyzed by the enzyme rubisco. RUBISCO = Ribulose bisphosphate carboxylase/oxygenase RUBISCO is the most abundant protein on Earth! o The product is a 6-carbon molecule that immediately breaks down into two 3-carbon molecules called 3phosphoglycerate. (RuBP) RUBISCO CO2 3 molecules of Ribulose biphosphate (RuBP) Unstable intermediate 6 molecules of 3-phosphoglycerate (3-PGA) RUBISCO Carbon Phosphate Reduction For their energy to increase, the carbon compounds must now be reduced. Carbon Phosphate o The reduction of 3-PGA involves two steps. 6 molecules of 3-phosphoglycerate (3-PGA) § ATP is used to phosphorylate 3-PGA. § NADPH transfers two high-energy electrons to the phosphorylated compound. o The energy transfer steps result in the formation G3P. § For every six triose phosphate molecules that are produced, only one can be transferred out of the chloroplast. 5 molecules of G3P 6 molecules of Glyceraldehyde-3phosphate (G3P) 1 molecule of G3P (half-glucose) Later, it is used for the synthesis of glucose and other sugars Regeneration of RuBP RuBP, the 5-carbon molecule needed for carboxylation, needs to be regenerated. Carbon Phosphate 3 molecules of Ribulose biphosphate (RuBP) Regeneration accounts for 12 of the 15 steps in the Calvin cycle. Three 5-carbon RuBP molecules are produced from the five remaining G3P molecules. ATP is required for this step. 5 molecules of G3P Calvin Cycle For net synthesis of 1 G3P, the cycle must fix 3 molecules of CO2. How many CO2 must be fixed in the Calvin cycle to make a molecule of glucose?