2007-2008

Photosynthesis: Life from Light , Air, and Water 2007-2008

ALL LIFE needs CONSTANT input of ENERGY Heterotrophs (Animals) Eat food (that is, organic molecules) Convert food to energy via respiration Autotrophs (Plants) produce their own food Convert energy of sunlight to food That is…use the energy of sunlight to build organic molecules (CHO) from CO2 That’s called…photosynthesis THEN convert food to energy via respiration consumers producers

making energy from ingesting organic molecules How are they connected? Heterotrophs making energy from ingesting organic molecules glucose + oxygen  carbon + water + energy dioxide C6H12O6 6O2 6CO2 6H2O ATP  + oxidation = exergonic Different Forms of Energy Though.. Autotrophs making organic molecules from energy So, in effect, photosynthesis is respiration run backwards powered by light. Cellular Respiration oxidize C6H12O6  CO2 & produce H2O fall of electrons downhill to O2 exergonic Photosynthesis reduce CO2  C6H12O6 & produce O2 boost electrons uphill by splitting H2O endergonic + water + energy  glucose + oxygen carbon dioxide 6CO2 6H2O C6H12O6 6O2 light energy  + reduction = endergonic

Plants are NOT the only autotrophs… Certain bacteria cyanobacteria Also Protists Algae

What does it mean to be a plant Need to… collect light energy transform it into chemical energy store chemical energy in a more STABLE form that can be moved around the plant or stored for later need to get building block atoms from the environment C,H,O,N,P,K,S,Mg produce all organic molecules needed for growth carbohydrates, proteins, lipids, nucleic acids ATP glucose CO2 H2O N P K …

Plant structure Obtaining raw materials sunlight CO2 H2O nutrients leaves = solar collectors CO2 stomates = gas exchange H2O uptake from roots nutrients N, P, K, S, Mg, Fe…

Leaf Structure Mesophyll – where most of the photosynthetic action is Palasaide layer photosynthesis Spongy layer Gas exchange Lower epidermis Stomate Lets gas in and out

stomate transpiration gas exchange

chloroplasts in plant cell chloroplasts contain chlorophyll absorb sunlight & CO2 cross section of leaf leaves CO2 chloroplasts in plant cell chloroplasts contain chlorophyll chloroplast make energy & sugar

Plant structure Chloroplasts Thylakoid membrane contains compartment Plant structure Chloroplasts double membrane stroma fluid-filled interior Stacks of thylakoid sacs Surrounded by stroma granum = stack Thylakoid membrane contains chlorophyll molecules Where the light trapping action happens outer membrane inner membrane granum thylakoid stroma A typical mesophyll cell has 30-40 chloroplasts, each about 2-4 microns by 4-7 microns long. Each chloroplast has two membranes around a central aqueous space, the stroma. In the stroma are membranous sacs, the thylakoids. These have an internal aqueous space, the thylakoid lumen or thylakoid space. Thylakoids may be stacked into columns called grana.

Chlorophyll Found embedded in the membranes of the thylakoids Absorbs light energy which is then used by the plant to make sugar

Basics about Light and Chlorophyll LIGHT behaves both as a wave and a particle When light is behaving as a particle = a photon This is a fixed quantity of energy Chlorophyll is best at absorbing red and blue light in the visible light portion of the electromagnetic spectrum Chlorophyll does NOT absorb green light – REFLECTS it. Hence, leaves appear green.

Types of Chlorophyll and other Pigments Only pigment that DIRECTLY participates in the light reactions Blue green in color Accessory pigments Absorb light and transfer its energy TO chlorophyll a Chlorophyll b Yellow green in color carotenoids Yellow/orange in color Also serve in photoprotection Absorb and dissipate excessive light energy that might damage chloropyll

A Look at Light The spectrum of color V I B G Y O R

Light: absorption spectra Photosynthesis gets energy by absorbing wavelengths of light chlorophyll a absorbs best in red & blue wavelengths & least in green accessory pigments with different structures absorb light of different wavelengths chlorophyll b, carotenoids, xanthophylls Why are plants green?

Molecules and absorption of light energy When a molecule absorbs a photon, one of the molecule’s electrons is elevated to an orbital where it has MORE potential energy. Normal orbital = ground state; Higher orbital = excited state Particular compounds absorb photons at very particular wavelengths Chlorophyll absorbs photons at wavelengths of 680 nm and 700 nm Once a molecule’s electron is excited to a higher energy level, it will normally fall back down to its original level unless something “catches” the electron and holds it at the elevated state.

The Photosynthetic Reactions It takes two sets of chemical reactions (two biochemical pathways) to make a sugar molecule The two sets of reactions are called: Light Reactions and Dark Reactions OR Light Dependent Reactions and Light Independent Reactions OR Light Reactions and Calvin Cycle

Light Reactions vs Dark (light independent / Calvin Cycle) Reactions Fundamentally, Light Reactions MUST have light in order to proceed Dark (light independent / Calving Cycle) reactions can occur with or without light being present, HOWEVER… Dark reactions ARE dependent on the PRODUCTS of the LIGHT reactions in order to occur.

It’s not the Dark Reactions! Photosynthesis Light reactions light-dependent reactions energy conversion reactions convert solar energy to chemical energy ATP & NADPH Calvin cycle light-independent reactions sugar building reactions uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize (build) C6H12O6 ATP It’s not the Dark Reactions!

The Light Reactions Begin With Photosystems Complexes containing chlorophyll, proteins and other pigments and organic molecules Embedded in thylakoid membrane Photosystems contain a special region called the antenna complex Light gathering structure Cluster of chlorophyll a and other pigment molecules Acts like a “catcher’s mitt” to catch photons from the sun Energy from these photons is bounced around in the antenna complex until it “lands on” a particular chlorphyll a molecule

Photosystem: Antenna Complex Diagram

Pigments of photosynthesis How does this molecular structure fit its function? Chlorophylls & other pigments embedded in thylakoid membrane arranged in a “photosystem” collection of molecules structure-function relationship

Antenna Complex – The Reaction Center Contains the particular chlorophyll a molecule upon which light’s energy ultimately lands. Also contains a compound called the primary electron acceptor

Excited Electrons and the Primary Electron Acceptor Energy landing on the chlorophyll a in the reaction center elevates the electrons of this chlorophyll a to a higher energy level. Before these electrons can lose this energy and return to their original energy level, the electrons are “caught” by the primary electron acceptor which keeps them at this elevated state POTENTIAL ENERGY provided by LIGHT is now STORED in CHEMICAL FORM.

Photosystems of photosynthesis 2 photosystems in thylakoid membrane collections of chlorophyll molecules act as light-gathering molecules Photosystem II chlorophyll a P680 = absorbs 680nm wavelength red light Photosystem I chlorophyll b P700 = absorbs 700nm wavelength red light reaction center Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane. When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center. At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a. This starts the light reactions. Don’t compete with each other, work synergistically using different wavelengths. antenna pigments

The LIGHT REACTIONS – Step by Step There are TWO possible paths electrons can take in the light reactions Noncyclic electron flow The one we will focus on NOW Cyclic electron flow We’ll focus on this one LATER

The LIGHT REACTIONS – Step by Step Also…Remember that electrons are going to be MOVING in these reactions. Just like in cell respiration, we depend on REDOX (oxidation-reduction) reactions to pass electrons from one compound to another As electrons move from one compound to the next they are moving from a compound that is less electronegative to one that is more electronegative

Light Reactions: Non Cyclic Electron Flow – Step 1 Photon strikes antenna complex of Photosystem II (P680) Energy is bounced on molecules in antenna complex until they land on chlorophyll a in the reaction center Electrons of reaction center chlorophyll a are excited to a higher energy level and CAUGHT by the primary electron acceptor

Light Reactions: Non Cyclic Electron Flow – Step 2 An electron “hole” is left behind in the chlorophyll a in the reaction center of Photosystem II An enzyme extracts electrons from water found in the chloroplast (stroma) and supplies these electrons to the chlorophyll a molecule so that it can work again. This enzyme splits a water molecule into TWO H+ ions and an oxygen atom that will combine with another oxygen atom to form O2. THIS IS WHY WATER IS NEED FOR PHOTOSYNTHESIS THIS IS WHERE THE OXYGEN GAS COMES FROM IN PHOTOSYNTHESIS

Light Reactions: Non Cyclic Electron Flow – Step 3 Electrons pass from the primary electron acceptor of Photosystem II to Photosystem I by way of an electron transport chain This is VERY much like the ETC found in the mitochondrion As electrons fall down the chain their energy is used to PUMP PROTONS from the stroma INTO the thylakoid space This creates a PROTON GRADIENT HIGH inside the thylakoid; LOW outside in the stroma ATP is made as protons flow WITH their concentration gradient THROUGH a molecule of ATP synthase ADP and Pi are combined at ATP Synthase ATP

ATP

Light Reactions: Non Cyclic Electron Flow – Step 3, cont. Remember: ATP synthesis using a proton gradient and ATP synthase is called CHEMIOSMOSIS Using Light Energy to drive this type of ATP synthesis is called PHOTOPHOSPHORYLATION Remember substrate level phosphorylation and oxidative phosphorlyation? This is the same type of thing. The ATP generated by this method will be sent on to provide energy to the “DARK” REACTIONS to provide energy for making glucose.

Light Reactions: Non Cyclic Electron Flow – Step 4 When electrons reach the bottom of the ETC, they are passed to chlorophyll a of the reaction center of photosystem I (P700) Here, these electrons fill an electron hole left in this reaction center when electrons of this reaction center’s chlorophyll a are excited by their own photon of light. Just as in Photosystem II, the electrons here are also caught and passed on by the primary electron acceptor of photosystem I. This, again, leaves an electron hole to fill.

Light Reactions: Non Cyclic Electron Flow – Step 5 Electrons from Photosystem I are passed from its primary electron acceptor to a compound called NADP+ Reductase This enzyme passes electron to a coenzyme called NADP+ NADP+ will become NADPH when it picks up the electrons. (reduced form of NADP+) NADPH takes the electrons (and their ENERGY!) to the Calvin Cycle (Dark Reactions) where this energy will be used to make sugar.

Light Reactions: BOTTOM LINE The light reactions use LIGHT power to generate ATP and NADPH ATP and NADPH provide CHEMICAL ENERGYand REDUCING POWER to the GLUCOSE-MAKING reactions of the CALVIN CYCLE. WITHOUT THE CHEMICAL ENERGY (ATP/NADPH) PROVIDED BY the LIGHT REACTIONS, the “DARK” REACTIONS CAN’T OCCUR.

Cyclic photophosphorylation If PS I can’t pass electron to NADP…it cycles back to PS II & makes more ATP, but no NADPH coordinates light reactions to Calvin cycle Calvin cycle uses more ATP than NADPH  ATP 18 ATP + 12 NADPH  1 C6H12O6

Photophosphorylation cyclic photophosphorylation NADP NONcyclic photophosphorylation ATP

Photosynthesis summary Where did the energy come from? Where did the electrons come from? Where did the H2O come from? Where did the O2 come from? Where did the O2 go? Where did the H+ come from? Where did the ATP come from? What will the ATP be used for? Where did the NADPH come from? What will the NADPH be used for? Where did the energy come from? The Sun Where did the electrons come from? Chlorophyll Where did the H2O come from? The ground through the roots/xylem Where did the O2 come from? The splitting of water Where did the O2 go? Out stomates to air Where did the H+ come from? The slitting of water Where did the ATP come from? Photosystem 2: Chemiosmosis (H+ gradient) What will the ATP be used for? The work of plant life! Building sugars Where did the NADPH come from? Reduction of NADP (Photosystem 1) What will the NADPH be used for? Calvin cycle / Carbon fixation …stay tuned for the Calvin cycle

You can grow if you Ask Questions! 2007-2008

The ATP that “Jack” built photosynthesis respiration sunlight breakdown of C6H12O6 H+ ADP + Pi moves the electrons runs the pump pumps the protons builds the gradient drives the flow of protons through ATP synthase bonds Pi to ADP generates the ATP … that evolution built ATP

ETC of Photosynthesis Photosystem II Photosystem I chlorophyll a chlorophyll b Photosystem I Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide

Photosystem II P680 chlorophyll a ETC of Photosynthesis sun 1 e PS II absorbs light Excited electron passes from chlorophyll to the primary electron acceptor Need to replace electron in chlorophyll An enzyme extracts electrons from H2O & supplies them to the chlorophyll This reaction splits H2O into 2 H+ & O- which combines with another O- to form O2 O2 released to atmosphere Chlorophyll absorbs light energy (photon) and this moves an electron to a higher energy state Electron is handed off down chain from electron acceptor to electron acceptor In process has collected H+ ions from H2O & also pumped by Plastoquinone within thylakoid sac. Flow back through ATP synthase to generate ATP. Photosystem II P680 chlorophyll a

Photosystem II P680 chlorophyll a Inhale, baby! ETC of Photosynthesis thylakoid chloroplast H+ H+ ATP Plants SPLIT water! 1 O H 2 e O O H e- H+ +H e e fill the e– vacancy Photosystem II P680 chlorophyll a

energy to build carbohydrates Photosystem II P680 chlorophyll a ETC of Photosynthesis thylakoid chloroplast H+ H+ ATP e H+ 3 1 2 e H+ ATP 4 to Calvin Cycle H+ ADP + Pi energy to build carbohydrates Photosystem II P680 chlorophyll a ATP

Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a ETC of Photosynthesis e e sun fill the e– vacancy e 5 Need a 2nd photon -- shot of light energy to excite electron back up to high energy state. 2nd ETC drives reduction of NADP to NADPH. Light comes in at 2 points. Produce ATP & NADPH e e Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a

Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a ETC of Photosynthesis electron carrier e 6 e 5 sun NADPH to Calvin Cycle Need a 2nd photon -- shot of light energy to excite electron back up to high energy state. 2nd ETC drives reduction of NADP to NADPH. Light comes in at 2 points. Produce ATP & NADPH Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a We’re goin’ for another joy ride!

ETC of Photosynthesis e e O split H2O H+ sun sun to Calvin Cycle ATP Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide O to Calvin Cycle split H2O ATP

ETC of Photosynthesis ETC uses light energy to produce ATP & NADPH go to Calvin cycle PS II absorbs light excited electron passes from chlorophyll to “primary electron acceptor” need to replace electron in chlorophyll enzyme extracts electrons from H2O & supplies them to chlorophyll splits H2O O combines with another O to form O2 O2 released to atmosphere and we breathe easier!

Experimental evidence Where did the O2 come from? radioactive tracer = O18 6CO2 6H2O C6H12O6 6O2 light energy  + Experiment 1 6CO2 6H2O C6H12O6 6O2 light energy  + 6CO2 6H2O C6H12O6 6O2 light energy  + Experiment 2 Proved O2 came from H2O not CO2 = plants split H2O!

Noncyclic Photophosphorylation Light reactions elevate electrons in 2 steps (PS II & PS I) PS II generates energy as ATP PS I generates reducing power as NADPH 1 photosystem is not enough. Have to lift electron in 2 stages to a higher energy level. Does work as it falls. First, produce ATP -- but producing ATP is not enough. Second, need to produce organic molecules for other uses & also need to produce a stable storage molecule for a rainy day (sugars). This is done in Calvin Cycle! ATP

Light reactions Electron Transport Chain thylakoid chloroplast H+ Light reactions H+ ATP Electron Transport Chain like in cellular respiration proteins in organelle membrane electron acceptors NADPH proton (H+) gradient across inner membrane find the double membrane! ATP synthase enzyme Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.

ETC of Respiration Mitochondria transfer chemical energy from food molecules into chemical energy of ATP use electron carrier NADH NADH is an input ATP is an output O2 combines with H+’s to form water Electron is being handed off from one electron carrier to another -- proteins, cytochrome proteins & quinone lipids So what if it runs in reverse?? generates H2O

ETC of Photosynthesis Chloroplasts transform light energy into chemical energy of ATP use electron carrier NADPH Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide generates O2

Ghosts of Lectures Past (storage) 2007-2008

Stomates