Presentation on theme: "Photosynthesis: Energy from the Sun. Identifying Photosynthetic Reactants and Products Reactants needed for photosynthesis: H 2 O, & CO 2, Products."— Presentation transcript:
Identifying Photosynthetic Reactants and Products Reactants needed for photosynthesis: H 2 O, & CO 2, Products of photosynthesis: carbohydrates and O 2 Energy driving reaction: Light 6 CO 2 + 12 H 2 O C 6 H 12 O 6 + 6 O 2 + 6 H 2 O
The Two Pathways of Photosynthesis: An Overview Photosynthesis occurs in the chloroplasts of plant cells Photosynthesis can be divided into two pathways: The light reaction is driven by light energy captured by chlorophyll Light energy transformed to chemical energy ATP and NADPH + H +. The Calvin–Benson cycle uses ATP, NADPH + H +, and CO 2 to produce sugars. Carbon fixation
The Electromagnetic Radiation: Wave-Particle Duality Electromagnetic radiation comes in discrete packets called photons Photons behave as particles and as waves Particles – mass and impart energy through collisions Waves – interfere positively and negatively with each other Photonic energy Wavelength ( ) 1/energy Frequency 1/ Frequency energy
The Interactions of Photons and Molecules Transmission Photon passes through molecule without interacting Absorption Photonic energy transferred to molecule Molecules absorb photons of discrete energies (wavelengths) and transmit photons of other energies Molecules that absorb visible wavelengths are called pigments or chromophores
The Interactions of Light and Pigments Plotting the absorption by the compound as a function of wavelength results in an absorption spectrum. If absorption results in a measurable activity, plotting the effectiveness of the light as a function of wavelength is called an action spectrum.
Absorption of Photonic Energy Electrons in high enough exited states can move from molecule to molecule Essentially an electric current
Light Absorbing Pigments for Photosynthesis Primary chromophores chlorophyll a and chlorophyll b. Absorption max in blue and red wavelengths Accessory pigments Carotenoids (xanthophylls) & phycobillins Absorption maxima between the red and blue wavelengths
Figure 8.7 The Molecular Structure of Chlorophyll
The Interactions of Light and Pigments molecule enters an excited state when it absorbs a photon. excited state is unstable, and the molecule may return to the ground state. When this happens, some of the absorbed energy is given off as heat and the rest is given off as light energy, or fluorescence. molecule may pass some of the absorbed energy to other molecules
The Interactions of Light and Pigments Pigments in photosynthetic organisms are arranged into antenna systems. The excitation energy is passed to the reaction center of the antenna complex. In plants, the pigment molecule in the reaction center is always a molecule of chlorophyll a.
Figure 8.8 Energy Transfer and Electron Transport
The Light Reactions: Photophosphorylation Excited chlorophyll (Chl*) in the reaction center acts as a reducing agent and participates in a redox reaction Chl* can react with an oxidizing agent in a reaction such as: Chl* + PQ Chl + + PQ – PQ - passes the e - to a series of carriers in the thylakoid membrane The e - carriers pump H + into the thylakoid space The e - is ultimately donated to NADP to generate NADPH + H + The H + gradient is used to synthesize ATP by ATPases in the thylakoid membrane and is called photophosphorylation
Electron Transport, Reductions, and Photophosphorylation There are two different systems for transport of electrons in photosynthesis. Noncyclic electron transport produces NADPH + H + and ATP and O 2 e - come in from H 2 O and leave on NADPH Cyclic electron transport produces only ATP e - come from chl and are returned to chl
The Light Reactions: Photophosphorylation Photosystems light-driven molecular units consisting of chlorophylls and accessory pigments bound to proteins in energy-absorbing antenna systems Photosystem I (PS I) Alone carries out cyclic electron transport In combo with PS II, - non-cyclic transport reaction center chlorophyll a is P 700 ( max = 700nm) Photosystem II (PS II) Initiates non-cyclic e - transport Splits H 2 O to produce e -, H +, and O 2. reaction center chlorophyll a is P 680 ( max = 680nm) To keep noncyclic electron transport going, both photosystems must constantly be absorbing light
Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems Coupled PS II and PS I is the arrangement found in all most all photosynthetic organisms – cyanobacteria to redwoods
The Calvin–Benson Cycle: When carbon breaks, we fix it Calvin-Benson cycle reactions occur in the stroma Requires the ATP and NADPH + H + produced in the light reactions and these can not be “stockpiled”. Thus, the Calvin-Benson reactions require light indirectly but take place only in the presence of light.
Figure 8.12 Tracing the Pathway of CO 2 3 sec reaction 30 sec reaction
The Calvin–Benson Cycle: A fixation with carbon Initial reaction adds one CO 2 to ribulose 1,5-bisphosphate (RuBP; a pentose) The intermediate hexose is unstable and breaks down to form two molecules of 3-phosphoglycerate (a triose) fixation of CO 2 is catalyzed by ribulose bisphosphate carboxylase/oxygenase - a.k.a. rubisco. Rubisco is the most abundant protein in the world.
The Calvin–Benson Cycle: Fixation of CO 2, Conversion of fixed CO 2 into Gyceraldehyde-3P Uses ATP and NADPH Regeneration of the CO 2 acceptor RuBP Uses ATP
Regeneration of RuBP in the Calvin-Bensen Cycle
The Calvin–Benson Cycle The end product of the cycle is glyceraldehyde 3- phosphate, G3P. There are two fates for the G3P: One-third ends up as starch, which is stored in the chloroplast and serves as a source of glucose. Two-thirds is converted to the disaccharide sucrose, which is transported to other organs.
Importance of The Calvin–Benson Cycle The products are the energy yield from sunlight converted to carbohydrates Most of the energy is released by glycolysis and cellular respiration by the plant itself. Some of the carbon of glucose becomes part of amino acids, lipids, and nucleic acids. Some of the stored energy is consumed by heterotrophs, where glycolysis and respiration release the stored energy.
Photorespiration Rubisco as a carboxylase, adds CO 2 to RuBP. Rubisco as an oxygenase Adds O 2 to RuBP. These two reactions compete with each other. Reaction with O 2, reduces the rate of CO 2 fixation Oxygenase reaction occurs when CO 2 levels are very low and the O 2 levels are very high Rubisco binds CO 2 with a O 2 levels become very high when stomata are closed to prevent water loss (when the weather is hot and dry).
Reaction Pathways Compensating for Photorespiration RuBP + O 2 phosphoglycolate + 3PG glycolate transported into glycolate converted to glycine in peroxisome glycine converted to serine in mitochondria serine converted to glycerate in peroxisome glycerate reenters C-B cycle in chloroplast
Figure 8.15 Organelles of Photorespiration C M P
Overcoming Photorespiration C 3 plants have a layer of mesophyll cells below the leaf surface. Mesophyll cells are full of chloroplasts and rubisco. On hot days the stomata close, O 2 builds up, and photorespiration occurs.
Overcoming Photorespiration C 4 plants have two enzymes for CO 2 fixation in different chloroplasts, in different locations in the leaf. PEP carboxylase is present in the mesophyll cells. It fixes CO 2 to 3-C phosphoenolpyruvate (PEP) to form 4-C oxaloacetate. PEP carboxylase does not have oxygenase activity. It fixes CO 2 even when the level of CO 2 is extremely low. The oxaloacetate diffuses into the bundle sheath cells in the interior of the leaf which contain abundant rubisco. The oxaloacetate loses one C, forming CO 2 and regenerating the PEP. The process pumps up the concentration around rubisco to start the Calvin-Benson cycle.
Figure 8.17 (b) The Anatomy and Biochemistry of C 4 Carbon Fixation OAAPyruvate
Figure 8.17 (a) The Anatomy and Biochemistry of C 4 Carbon Fixation
Photorespiration and Its Consequences CAM plants use PEP carboxylase to fix and accumulate CO 2 while their stomata are closed. These plants conserve water by keeping stomata closed during the daylight hours and opening them at night. In CAM plants, CO 2 is fixed in the mesophyll cells to form oxaloacetate, which is then converted to malic acid. The fixation occurs during the night, when less water is lost through the open stomata. During the day, the malic acid moves to the chloroplast, where decarboxylation supplies CO 2 for the Calvin–Benson cycle.
Figure 8.18 Metabolic Interactions in a Plant Cell