Chapter 10 Photosynthesis BIOL 3 Photosynthesis and Energy Photosynthesis Making food from light energy Photoautotrophs Use CO and water to make sugars Made life possible as we know it Provides carbohydrates for all higher levels of food chain produceon Liberates atmospheric oxygen O waste product Photosynthesis and Energy Carbon dioxide converted to sugar Process called carbon fixaeon Series of redox reaceons Water (hydrogen atoms) loses electrons Oxidized (OIL) Carbon dioxide gains electrons Reduced (RIG) 1
Photosynthesis and Energy Summary equaeon for photosynthesis CO + H O + photons (CH O) n + H O + O carbon dioxide + water + light energy carbohydrate + water + oxygen Photosynthesis and Energy Somewhat opposite of cellular respiraeon Photosynthesis CO + H O + photons (CH O) n + H O + O carbon dioxide + water + light energy carbohydrate + oxygen + water Cellular RespiraEon (CH O) C H 1 O + O CO + H O + energy Three Types of Photosynthesizers Domain: Eukaryota Kingdom: Plantae Domain: Eukaryota Kingdom: Protista Domain: Prokaryota Kingdom: Eubacteria
The Components of Photosynthesis Chloroplasts Organelles in plants and algae, where photosynthesis takes place captured here Exist in great abundance in the mesophyll cells of plant leaves Double membrane bound like mitochondria Contain thylakoids Stacked as grana In the liquid filled stroma Photosynthesis Captured energy comes mostly from blue and red wavelengths of visible sunlight Absorbed by pigments in the thylakoids Chlorophyll a Accessory pigments Why plants look green high energy short wavelength gamma rays low energy long wavelength x-rays ultraviolet infrared microwaves radiowaves visible light Site of Photosynthesis 1. Leaf The primary site of photosynthesis in plants, leaves have a two-part structure: a petiole (or stalk) and a blade (normally thought of as the leaf). petiole. Leaf cross section blade In cross section, leaves have a sandwichlike structure, with epidermal layers at top and bottom and mesophyll cells in between. Most photosynthesis is performed within mesophyll cells. Leaf epidermis is pocked with a large number of microscopic openings, called stomata, that allow carbon dioxide to pass in and water vapor to pass out. epidermis mesophyll cells epidermis stomata 3. Mesophyll cell nucleus A single mesophyll cell within a leaf contains all the component parts of plant cells in general, including the organelles called chloroplasts that are the actual sites of photosynthesis. chloroplast cell wall vacuole 4. Chloroplast thylakoids stroma granum inner membrane outer membrane Energy from sunlight is absorbed by pigments in the thylakoid membrane. thylakoid thylakoid membrane thylakoid compartment Each chloroplast has an outer membrane at its periphery; then an inner membrane; then a liquid material, called the stroma, that has immersed within it a network of membranes, the thylakoids. These thylakoids sometimes stack on one another to create... 5. A Granum Electrons used in photosynthesis will come from water contained in the thylakoid compartment, and all the steps of photosynthesis will take place either within the thylakoid membrane, or in the stroma that surrounds the thylakoids. 3
Stages of Photosynthesis Two primary stages reaceons Strips electrons from water oxidaeon Boosts these electrons to higher energy levels Makes and NADPH Calvin cycle Makes carbohydrates from high energy electrons and atmospheric CO The ReacEons Electrons derived from water energeecally boosted by photons Electrons physically transferred passed along through a series of electron carriers NADP + NADPH carries them to second stage Stages of Photosynthesis Two primary stages (conenued) reaceons Strips electrons from water Boosts these electrons to higher energy levels Calvin cycle independent reaceons Makes carbohydrates from high energy electrons and atmospheric CO reduceon 4
The Calvin Cycle Electrons carried by NADPH combined with carbon dioxide forms high-energy sugar Glyceraldehyde 3-Phosphate (G3P) combined into complex carbohydrates powered by from light reaceons Occurs in the stroma of the chloroplast First described by Melvin Calvin Photosynthesis works through two molecular complexes Photosystems II and I On thylakoid membranes composed partly of antennae chlorophyll and some accessory absorb and transmit solar energy ReacEons ExcitaEon pigment absorbs light goes from a ground state to excited state which is unstable excited electrons fall back to the ground state photons are given off fluorescence Excited state Energy of electron Photon Chlorophyll molecule Ground state Heat Photon (fluorescence) (a) Excitation of isolated chlorophyll molecule (b) Fluorescence 5
Photosystem antennae A few hundred chlorophyll a and some accessory (pigments) Pigments Photosystem Structure Carotene - an orange pigment Xanthophyll - a yellow pigment PhaeophyEn a[1] - a graybrown pigment PhaeophyEn b[1] - a yellowbrown pigment Chlorophyll a - a blue-green pigment Chlorophyll b - a yellow-green pigment transmit solar energy to reaceon centers ReacEon center Photosystem Structure Pair of modified chlorophyll a and PhaeophyEn Receive light energy Photon -harvesting complexes Photosystem Reaction-center complex STROMA electron transform it to chemical energy splits water and strips electrons off liberated hydrogen atoms Thylakoid membrane also energizes stripped electrons Transfer of energy Special pair of chlorophyll a Pigment Includes primary electron s THYLAKOID SPACE (INTERIOR OF THYLAKOID) Receive these high energy electrons Electron Flow two possible routes for electron flow cyclic and linear P80 Linear electron flow primary pathway involves both photosystems produces and NADPH using light energy 1 photon hits pigment molecule Photosystem II (PS II) Pigment energy passed among pigment unel it excites P80 excited electron from P80 Passed to primary electron -phaeophyen
Linear Electron Flow (PS I) H + H O + 1 / O 3 1 P80 P80 + (missing an electron) very strong oxidizing agent H O is split by enzymes electrons transferred from hydrogen atoms to P80 + Photosystem II (PS II) Pigment O is released reducing it to P80 Linear Electron Flow (PS II) Electrons passed down electron transport chain H + H O + 1 / O 3 1 P80 Photosystem II (PS II) Pq Electron transport chain Pigment 4 Cytochrome complex 5 Pc from primary electron (phaeophyen) of PS II to PS I Energy released by fall drives the creaeon of a proton gradient Diffusion of H + into thylakoid space across membrane drives synthase Linear Electron Flow (PS I) In PS I (like PS II) H H O + + 1 / O 3 P80 Pq Electron transport chain 4 Cytochrome complex 5 Pc P700 transferred light energy excites P700 loses an electron to 1 P700 + Photosystem II (PS II) Pigment Photosystem I (PS I) accepts electron from PS II via the electron transport chain 7
H H O + + 1 / O 3 1 P80 Photosystem II (PS II) Pq Pigment Linear Electron Flow (PS I) Electron transport chain 4 Cytochrome complex 5 Pc P700 Photosystem I (PS I) Electron transport chain Fd 7 8 NADP + reductase Each electron falls down electron transport chain NADP + + H + NADPH from primary electron of PS I to ferredoxin (Fd) electrons then transferred to NADP + reduced to NADPH goes to Calvin cycle Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces but not NADPH no oxygen released generates surplus meeeng demand for Calvin cycle Some organisms such as purple sulfur bacteria have PS I but not PS II thought to have evolved before linear electron flow may protect cells from light-induced damage Fig. 10-15 Pq Fd Cytochrome complex Fd NADP + reductase NADP + + H + NADPH Pc Photosystem II Photosystem I 8
What Makes the ReacEons So Important? Two aceons of great consequence take place in the light reaceons 1. Water is split, yielding both electrons and oxygen. The electrons move through the light reaceons. The oxygen is what organisms such as ourselves breathe in.. The electrons that are derived from the water and then given an energy boost by the sun s rays are transferred to a different molecule: the inieal electron This is the means through which the sun s energy is transferred into the living world ReacEons PLAY Reactions STROMA (low H + concentration) Photosystem II 4 H + Cytochrome Photosystem I complex Fd NADP + reductase 3 NADP + + H + Pq NADPH THYLAKOID SPACE (high H + concentration) H O 1 1 / O + H + 4 H + Pc To Calvin Cycle STROMA (low H + concentration) Thylakoid membrane synthase ADP + P i H + The Calvin Cycle Carbon dioxide from the atmosphere Combined with a sugar RuBP (Ribulose-1,5-bisphosphate) resuleng compound RuBisCo (Ribulose-1,5-bisphosphate carboxylase/ oxygenase) Enzyme that catalyzes RuBP and CO energized with addieon of electrons supplied by light reaceons Makes 3-carbon sugars Later combined into carbohydrates 9
The Calvin Cycle G3P Glyceraldehyde 3-phosphate High-energy sugar The result of the Calvin cycle Final product of photosynthesis Can be used for energy or plant structure The Calvin Cycle Input 3 CO (Entering one at a time) Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate P 3 P P P Ribulose bisphosphate 3-Phosphoglycerate (RuBP) ADP 3 3 ADP Calvin Cycle P P 1,3-Bisphosphoglycerate Phase 3: Regeneration of the CO (RuBP) 5 P P i NADP + NADPH G3P Glyceraldehyde-3-phosphate (G3P) P Phase : Reduction Output 1 P G3P (a sugar) Glucose and other organic compounds The Calvin Cycle All these steps are powered by produced in the light reaceons 10
Summary In plants PhotorespiraEon and the C4 Pathway the enzyme rubisco frequently binds with oxygen rather than with carbon dioxide a process called photorespiraeon that undercuts photosynthesis. PhotorespiraEon problem increases as temperature rises because as plants close their stomata to keep in moisture they also keep out CO increasing likelihood that rubisco will bind with oxygen Rubisco-oxygen then metabolized Releasing carbon dioxide EvoluEonary relic? Atmospheric O low when rubisco evolved May be proteceve Against damage from buildup of light rxn products 11
PhotorespiraEon Some warm-climate plants Have evolved a means of dealing with photorespiraeon C 4 photosynthesis Most plants C 3 PEP carboxylase C 4 Photosynthesis enzyme that binds with carbon dioxide but not with oxygen Forms a 4-carbon compound (Hence C 4 ) OxaloaceEc acid From combinaeon of CO with a 3-carbon molecule phosphoenolpyruvate (PEP) Occurs in the mesophyll cells OxaloaceEc acid then shuiled to bundle sheath cells to Calvin cycle where CO is released to bind with rubisco C 4 Photosynthesis C 4 leaf anatomy The C 4 pathway Photosynthetic cells of C 4 plant leaf Mesophyll cell Bundlesheath cell Vein (vascular tissue) Mesophyll cell CO PEP carboxylase Oxaloacetate (4C) Malate (4C) PEP (3C) ADP Stoma Bundlesheath cell Pyruvate (3C) CO Calvin Cycle Sugar Vascular tissue 1
CAM Plants Crassulacean Acid Metabolism Dry-weather plants such as cace (Crassulacea) employ another form of photosynthesis CAM photosynthesis CAM Photosynthesis CAM photosynthesis stomata open only at night lejng in and fixing carbon dioxide Carbon dioxide is banked unel sunrise when photons supply energy for Calvin cycle. 13
You should now be able to: 1. Describe the structure of a chloroplast. Describe the relaeonship between an aceon spectrum and an absorpeon spectrum 3. Trace the movement of electrons in linear electron flow 4. Trace the movement of electrons in cyclic electron flow 5. Describe the similariees and differences between oxidaeve phosphorylaeon in mitochondria and photophosphorylaeon in chloroplasts. Describe the role of and NADPH in the Calvin cycle 7. Describe the major consequences of photorespiraeon 8. Describe two important photosyntheec adaptaeons that minimize photorespiraeon 5. Describe the similariees and differences between oxidaeve phosphorylaeon in mitochondria and photophosphorylaeon in chloroplasts. Describe the role of and NADPH in the Calvin cycle 7. Describe the major consequences of photorespiraeon 8. Describe two important photosyntheec adaptaeons that minimize photorespiraeon 14