Lecture 14 - Photosynthesis PDF

Photosynthesis  2 Quick Review of Cellular Energy  Enzymes Catalyze reactions by binding and stabilizing the transition state  That stabilization reduces the amount of energy necessary to pass through the transition state...

Photosynthesis  2 Quick Review of Cellular Energy  Enzymes Catalyze reactions by binding and stabilizing the transition state  That stabilization reduces the amount of energy necessary to pass through the transition state, thereby reducing the activation energy  Chymotrypsin is a classic example  Some AA’s bind the substrate polypeptide at Phenylalanine residues  Other AA’s then work in concert to push and pull Electrons to pull apart the Peptide bond  3 Quick Review of Cellular Energy, pt II  Chemical Reactions are classified as either:  Exergonic – they release energy  Endergonic – They absorb energy  This is determined by measuring the change in Free Energy, ΔG.  ΔG is related to:  The Heat Released or Absorbed, ΔH  The change in Entropy, ΔS  A measure of the Randomness or Disorder in the system  4 Quick Review of Cellular Energy, pt III  Some examples of Entropy are really easy to see in Biological Systems:  Glycogen is a very ordered structure  Burning Glucose takes a fairly structured molecule, and creates a bunch of smaller molecules, and releases thermal energy  Thermal Energy is the most unstructured form of energy  5 Quick Review of Cellular Energy, pt IV  Redox Chemistry – the movement of electrons from one molecule to another  LEO – Lose Electrons = Oxidized  GER – Gain Electrons = Reduced  6 Quick Review of Cellular Energy, pt V  The Breakdown of Glucose happens in 3 parts:  Glycolysis – the Breakdown of a 6 carbon sugar to 2 x 3 carbon intermediates  2 Pyruvates  2 NADH’s  Net 2 ATP  Investment Phase leading to Fructose-1,6- bisphosphate  Followed by the Energy Recovery Phase  Lots of Regulation around Phosphofructokinase  7 Quick Review of Cellular Energy, pt VI  Citric Acid Cycle  Fully Oxydizing Glucose to CO2  Linked to Glycolysis by Pyruvate Oxidation  Generating lots of NADH’s, 2 FADH2’s and 2 ATP’s  Some Feedback inhibition of Phosphofructokinase  Electron Transport System  Located on the inner membrane of the Mitochondria  Converts the energy from NADH to a H+ gradient  That H+ gradient turns ATP Synthase forming ATP  8 Going back up the Energy Hill  Photosynthesis – Transferring Energy from the Sun to Cellular Molecules  Making Deposits in the Energy Bank  An Endergonic Process  Reducing Entropy  Storing Energy in Chemical Bonds  Going to need a BIG Energy Source  9 Photosynthesis Review  Localized in Chloroplasts in Eukaryotes  Found mostly in the Mesophyll Layer of leaves  Addition structures include the Thylakoids  Stacked in Granum  The Chloroplast “cytosol” is the Stroma  The interior of the Thylakoids is the Thylakoid space or Lumen  10 Photosynthesis in Two Parts  Photosynthesis is Two Parts  The “Light Reaction”  Incident light energy is captured and converted to chemical energy  In the form of ATP and NADPH  The “Dark Reaction”  Not dark at all  Converts Redox chemistry to Covalent Bonds  11 Pigments and Light Absorption  Electromagnetic Radiation covers a wide spectrum of Wavelengths and Energies  Only a narrow band is visible  Plants are selective in the wavelengths and therefore energies they absorb  12 Absorption Spectra  Chloroplast contain multiple light absorbing pigments  Each has wavelengths of Maximum Absorption  How many Absorb strongly in Green light?  13 Which Wavelengths Produce the Most O2  14 Chlorophyll Structure  Two parts to the structure:  Long Hydrocarbon Tail to anchor in membrane  Porphyrin Ring Structure  Very similar to Heme, but  With a Magnesium Ion at it’s core  15 Three Outcomes for Excited e-  Fluorescence – the electron can return to ground state with loss of light (and heat)  Resonance – the energy (not the e-) can be transferred to other pigments and the Reaction Center  Redox – The Reaction center can lose an electron to an electron transport chain  16 The Photosystem  Two parts to each Photosystem  Light Harvesting Complexes  Collect Light energy  Transmit energy through resonance, to  Reaction Center  Centered around two special Chlorophyll-A molecules  17 Energy Flow through Photosystems  The Complete “Light Reaction”  Light energy transferred to PS-II reaction Center, to  An ETC, which pumps H+’s  To make ATP  PS-I receives the e-  PS-I receives a 2nd Photon  Energy is transferred, this time to reduce NADP+  18 Cyclic Electron Flow  Electrons don’t have to flow only forward to NADP+  From PS-I, electrons can flow backwards to form a cycle  Generating more ATP  Not generating NADPH  PS-1 can toggle between both paths  19 Complete Overview of Light Reaction  Lots of surface area in Thylakoids for proteins  Note the sides of the Thylakoid bi-layer that the enzymes face  Protons get pumped into the Lumen  Small Volume  Greater Gradient  ATP and NADPH already in Stroma  20 Calvin Cycle – The “Dark” Reaction  Three Main Parts to Calvin Cycle  Carbon Fixing – by Rubisco  Reduction Phase  Regeneration  This all happens in the Stroma  At the same time as the “Light Reaction”  21 How do Plants deal with Excess Light  Imagine a situation of high light incidence, but low temperatures  Summer in the arctic  High in the mountains  The Calvin Cycle may be slow due to temperature.  NADPH will build up with linear e- flow  Can Damage Cells  Cyclic e- Flow can protect cells from too much sunlight  22 How do Plants deal with Excess Light – pt II  Carotenoids also play a role  In normal Photosynthesis:  Help Resonance transfers toward Reaction Center  Reactions go forward  In Excess Light:  Tend towards Fluorescence  Light energy is dissipated Thank You!