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Photosynthesis Light-dependent Reactions video • http://www.youtube.com/watch?v=hj_WKgnL 6MI&feature=related Overview • Photosynthesis transforms the radiant energy from the sun into the chemical energy of highenergy compounds • Much of the glucose produced by plants is used to make cellulose, other sugars and starches as well as other essential cellular components such as amino acids • Other organisms use the substances made by plants for their own use Overview • The process of photosynthesis can be summarized by the following equation: 6CO2(g) + 6H2O(l) + energy C6H12O6(s) + 6O2(g) • There are more than 100 reactions that lead to the end products Photo (light) Synthesis (the reactions that produce the carbohydrate) • There are two distinct sets of reactions: – The light–dependent reactions – The light–independent reactions • In the light-dependent reactions light energy is trapped to generate ATP and NADPH (similar to NADH) • The light-independent reactions use the energy of ATP and the reducing power of NADPH to make glucose Cross-section of a dicot leaf http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookps.html Chloroplasts • Photosynthesis occurs in the 40 – 200 chloroplasts that are contained in each photosynthetic cell • Thylakoids are interconnected disks that are formed by a membrane system within the chloroplast • Thylakoids are stacked to form grana (singular, granum) Chloroplasts • Molecules that absorb the sun’s energy are embedded in the thylakoid membranes • Surrounding the grana is a fluid filled interior called the stroma • The stroma contains the enzymes that catalyse the conversion of CO2 into carbohydrates Chloroplasts Absorption of Light energy • When any matter absorbs light energy, the light is absorbed in packets of energy called photons • Photons carry specific amounts of energy • Each wavelength of light is associated with photons of one distinct amount of energy • Longer-wavelength photons have smaller amounts of energy Light wavelengths http://www.elixa.com/light/healing.htm Light absorption • The wavelength of the photon and thus the colour of the light, that an atom or molecule absorbs is determined by the energy levels of the electrons in the atom or molecule. • An atom or molecule can absorb a photon only if they have an amount of energy exactly equal to the difference between two energy levels http://www.energygroove.net/atoms.php Photosynthetic pigments • Pigments are compounds that absorb certain wavelengths • Chlorophyll is the main photosynthetic pigment • Chloroplasts contain two types: chlorophyll a and chlorophyll b • They absorb photons from different wavelengths http://ghadvisor.blogspot.com/2011/03/plasma-emitting-diode-lights-for.html Photosystems capture energy • In the thylakoid, pigments are arranged into clusters called photosystems. • Each photosystem has about 250-400 pigment molecules • Energy absorbed by a pigment is passed to a neighbouring pigment until it reaches the reaction centre (chlorophyll a) • All the surrounding pigment molecules that gather the light energy are called the antenna complex Image from http://kvhs.nbed.nb.ca/gallant/biology/photosystem.jpg Photosystems • In green plants and algae, there are 2 photosystems (I and II) named in order of their discovery • Photosystem I: P700 is the chlorophyll a that is associated with its reaction centre • Photosystem II: P680 is the chlorophyll a that is associated with its reaction centre. • The names are based on the wavelength of light that these molecules absorb • There are thousands of photosystems in the thylakoid membranes of just one chloroplast How does photosynthetic pigment bind light energy? • Pigment molecules have conjugated double bonds, this means that every second bond is a double bond • The electrons in these bonds are easily "detachable", so when a light photon excites them, the energy level of one of the electrons in the bond increases • The energy is quickly given off as heat, light, phosphorescence or it can be transferred to another pigment molecule. Image from http://www.lycocard.com/images/main/chem_structure.gif Light-dependent Reactions Image from http://www.biology.arizona.edu/biochemistry/problem_sets/photosynthesis_1/graphics/z-scheme.GIF Light-dependent Reactions Step 1 • The P680 molecule in the reaction centre of photosystem II absorbs a photon, exciting an electron • The excited electron is picked up by an electron carrier leaving a “hole” in the P680 molecule • The P680+ pulls an electron from a water molecule Light-dependent Reactions Step 2 • The energized electrons are passed from the electron acceptor along an electron transport system • With each transfer, a small amount of energy is released and used by the b6f complex to pump hydrogen ions from the stroma into the thylakoid space • This, along with the water splitting, creates a hydrogen ion gradient Light-dependent Reactions Step 3 • While steps 1 and 2 are happening, light energy is absorbed by photosystem I and is transferred to the reactions centre P700 molecule • Excited electrons are passed to an electron acceptor • Lost electrons are replaced by those that have reached the end of the transport system Light-dependent Reactions Step 4 • Electrons are used by enzyme NADP reductase to reduce NADP+ to NADPH • The reducing power of NADPH will be used in the light-independent reactions Making ATP • ATP is formed using the energy of chemiosmosis • The H+ ion gradient formed by the b6-f complex of the electron transport chain is used by the ATP synthase molecule to make ATP from ADP and Pi • This is photophosphorylation Cyclic and noncyclic Photophosphorylation • The production of ATP by the passing of electrons through the Z–scheme is often called noncyclic photophosphorylation • The flow of electrons is unidirectional • The passage of one electron pair produces one NADPH and slightly more than one ATP • Light-independent reactions require two NADPH and three ATP Cyclic and noncyclic Photophosphorylation • Cyclic photophosphorylation produces more ATP • Excited electrons leave photosystem I and are passed to a electron acceptor, then to b6-f complex and back to photosystem I • ATP is formed by chemiosmosis as the proton gradient is generated • No NADPH is made Cyclic Photophosphorylation http://hyperphysics.phy-astr.gsu.edu/hbase/biology/etcyc.html http://bioweb.wku.edu/courses/Biol120/images/Photophosphryl.htm animations • http://www.stolaf.edu/people/giannini/flashanim at/metabolism/photosynthesis.swf • http://dendro.cnre.vt.edu/forestbiology/photosy nthesis.swf • http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::5 35::/sites/dl/free/0072437316/120072/bio12.swf ::Cyclic and Noncyclic Photophosphorylation • http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapt er10/animations.html# • The end of part 1 • The light-independent reactions are next
