FREE ENERGY (available for work) vs. HEAT (not available for work) THE SUN: MAIN SOURCE OF ENERGY FOR LIFE ON EARTH
THE BASICS OF PHOTOSYNTHESIS • Almost all plants are photosynthetic autotrophs (self producing), as are some bacteria and prtozoas • Autotrophs generate their own organic matter through photosynthesis • Sunlight energy is transformed to energy stored in the form of chemical bonds (a) Mosses, ferns, and flowering plants (c) Euglena (d) Cyanobacteria
Light Energy Harvested by Plants & Other Photosynthetic Autotrophs 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
WHY ARE PLANTS GREEN? Plant Cells have Green Chloroplasts The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).
THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED • Chloroplasts absorb light energy and convert it to chemical energy Reflected light Light Absorbed light Transmitted light Chloroplast
Photosynthesis occurs in chloroplasts • In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts • A chloroplast contains: • stroma, a fluid • grana, stacks of thylakoids • The thylakoids contain chlorophyll • Chlorophyll is the green pigment that captures light for photosynthesis
Chloroplast LEAF CROSS SECTION MESOPHYLL CELL • The location and structure of chloroplasts LEAF Mesophyll Intermembrane space CHLOROPLAST Outer membrane Granum Innermembrane Grana Stroma Thylakoidcompartment Stroma Thylakoid
Thylakoid Membrane Thylakoid Space Granum Thylakoid
Chloroplast Pigments • Chlorophyll a • Chlorophyll b • (Chlorophyll a (alpha) absorbs well at a wavelength of about 450 nm but its primary absorption is at 675nm in the long red wavelengths.Chlorophyll b (beat) absorbs most effectively at blue 470 but also with shorter peaks at 430 and 640nm) • Carotenoids • Xanthophyll • Chloroplasts contain several pigments
Fall Colors • During the fall, the green chlorophyll pigments are greatly reducedrevealing the other pigments. • Carotenoids are pigments that are either red or yellow.
Chlorophyll Molecules • Located in the thylakoid membranes. • Chlorophyll have Mg+in the center. • Chlorophyll pigmentsharvest energy (photons) by absorbing certain wavelengths (blue-420 nmand red-660 nm are most important). • Plants are green because the greenwavelength is reflected, not absorbed.
Porphyrin ring delocalized e- Phytol tail Chlorophyll a & b • Chl a has a methyl group • Chl b has a carbonyl group
violetbluegreenyelloworangered Absorption of Chlorophyll Absorption wavelength
AN OVERVIEW OF PHOTOSYNTHESIS • Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Carbondioxide Water Glucose Oxygengas PHOTOSYNTHESIS
AN OVERVIEW OF PHOTOSYNTHESIS Light • The light reactions convert solar energy to chemical energy • Produce ATP & NADPH Chloroplast NADP ADP + P Calvin cycle • The Calvin cycle makes sugar from carbon dioxide • ATP generated by the light reactions provides the energy for sugar synthesis • The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose Light reactions
Steps of Photosynthesis • Light hits reaction centers of chlorophyll, found in chloroplasts • Chlorophyll vibrates and causes water to break apart. • Oxygen is released into air • Hydrogen remains in chloroplast attached to NADPH • “THE LIGHT REACTION”
Steps of Photosynthesis • The DARK Reactions= Calvin Cycle • CO2 from atmosphere is joined to H from water molecules (NADPH) to form glucose • Glucose can be converted into other molecules with different flavors!
Redox Reaction • Thetransfer of one or more electronsfrom one reactantto another. • Two types: 1. Oxidation 2. Reduction
Oxidation 6CO2 + 6H2O C6H12O6 + 6O2 glucose Oxidation Reaction • The loss of electrons from a substance. • Or the gain of oxygen.
Reduction 6CO2 + 6H2O C6H12O6 + 6O2 glucose Reduction Reaction • The gain of electrons to a substance. • Or the loss of oxygen.
Two types of photosystems cooperate in the light reactions Photon ATP mill Photon Water-splitting photosystem NADPH-producing photosystem
1. Light Reaction (Electron Flow) • Occurs in the Thylakoid membranes • During the light reaction, there are two possibleroutes for electron flow. A. Cyclic Electron Flow B. Noncyclic Electron Flow
P A. Cyclic Electron Flow • Occurs in the thylakoid membrane. • Uses Photosystem II only • P700 reaction center- chlorophyll a • Uses Electron Transport Chain (ETC) • Generates ATP only ADP + ATP
Cyclic Photophosphorylation(addition of phosphate to ADP to make ATP.) • Process for ATP generation associated with some Photosynthetic Bacteria • Reaction Center => 700 nm
Plants produce O2 gas by splitting H2O • The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-)
In the light reactions, electron transport chains generate ATP, NADPH, & O2 • Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons • The excited electrons are passed from the primary electron acceptor to electron transport chains • Their energy ends up in ATP and NADPH
Chemiosmosis powers ATP synthesis in the light reactions • The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane • The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP • In the stroma, the H+ ions combine with NADP+ to form NADPH
SUN (Proton Pumping) H+ H+ Thylakoid E T PS I PS II C high H+ concentration H+ H+ H+ H+ H+ H+ Thylakoid Space H+ ATP Synthase low H+ concentration ADP + P ATP H+ Chemiosmosis
B. Noncyclic Electron Flow • Occurs in the thylakoid membrane • Uses PS II and PS I • P680 rxn center (PSII) - chlorophyll a • P700 rxn center (PS I) - chlorophyll a • Uses Electron Transport Chain (ETC) • Generates O2, ATP and NADPH
Primaryelectron acceptor Electron transport Primaryelectron acceptor Electron transport chain Photons Energy forsynthesis of PHOTOSYSTEM I PHOTOSYSTEM II by chemiosmosis Noncyclic Photophosphorylation • Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product
P (Reduced) B. Noncyclic Electron Flow • ADP + ATP • NADP+ + H NADPH • Oxygen comes from the splitting of H2O, not CO2 H2O 1/2 O2 + 2H+ (Reduced) (Oxidized)
How the Light Reactions Generate ATP and NADPH Primary electron acceptor NADP Energy to make Primary electron acceptor 3 2 Light Electron transport chain Light Primary electron acceptor Reaction- center chlorophyll NADPH-producing photosystem 1 Water-splitting photosystem 2 H + 1/2
Summary—Light Dependent Reactions a.Overall input light energy, H2O. b. Overall output ATP, NADPH, O2.
Light Independent Reactions aka Calvin Cycle Carbon from CO2 is converted to glucose (ATP and NADPH drive the reduction of CO2 to C6H12O6.)
Light Independent Reactions aka Calvin Cycle CO2 is added to the 5-C sugar RuBP by the enzyme rubisco. This unstable 6-C compound splits to two molecules of PGA or 3-phosphoglyceric acid. PGA is converted to Glyceraldehyde 3-phosphate (G3P), two of which bond to form glucose. G3P is the 3-C sugar formed by three turns of the cycle.
Summary—Light Independent Reactions a.Overall input CO2, ATP, NADPH. b. Overall output glucose.
Review: Photosynthesis uses light energy to make food molecules • A summary of the chemical processes of photosynthesis Chloroplast Light Photosystem IIElectron transport chains Photosystem I CALVIN CYCLE Stroma Electrons Cellular respiration Cellulose Starch Other organic compounds LIGHT REACTIONS CALVIN CYCLE
Photorespiration (Competing Reactions) • Occurs under the following conditions: • Intense Light (high O2 concentrations, hot, dry, bright days) • High heat (Stomatas close) • Rubisco grabs CO2, “fixing” it into a carbohydrate in the light independent reactions. • O2 can also react with rubisco, inhibiting its active site • not good for glucose output • wastes time and energy (occupies Rubisco) • So Fixation of O2 instead of CO2. • Produces no sugar molecules or noATP. • Photorespiration is estimated to reduce photosynthetic efficiency by 25%
Types of Photosynthesis C3 C4 CAM Rubisco: the world’s busiest enzyme!
Types of Photosynthesis • Certain plants have developed ways to limit the amount of photorespiration • C3 Pathway • C4 Pathway* • CAM (Crassulacean Acid Metabolism) Pathway* * Both convert CO2 into a 4 carbon intermediate C4 Photosynthesis
C3 Photosynthesis : C3 plants • Called C3 because the CO2 is first incorporated into a 3-carbon compound. • Stomata are open during the day. • RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2. • Photosynthesis takes place throughout the leaf. • Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy).. • Most plants are C3.
C4 Photosynthesis : C4 plants • Called C4 because the CO2 is first incorporated into a 4-carbon compound. • Stomata are open during the day. • Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis. • Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)
C4 Photosynthesis : C4 plants • Adaptive Value: • Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration. • Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis. • C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants.
CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism • Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis. • Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis
CAM Photosynthesis : CAM plants. • Adaptive Value: • Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.). • May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells). • CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads
Mesophyll cells Bundle sheath cells Leaf Anatomy • In C3 plants (those that do C3 photosynthesis), all processes occur in the mesophyll cells.
C4 Pathway • In C4 plants photosynthesis occurs in both the mesophyll and the bundle sheath cells.
C4 Pathway • CO2 is fixed into a 4-carbon intermediate • Has an extra enzyme– PEP Carboxylase (Phosphoenolpyruvate carboxylase) that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate