Photosynthesis • Photosynthesis is carried out by a number of different organisms • All contain chlorophyll • Absorbs light energy and begins process of photosynthesis • Chlorophyll a (blue green) • Chlorophyll b (yellow green) • They absorb different wavelengths of light
Prokaryotic Autotrophs: Cyanobacteria • Make up largest group of photosynthesizing prokaryotes • First organisms to harness the sun’s energy • Unicellular, but may grow in colonies • Live in many different environments • Grow rapidly in nutrient rich water (known to cause algal blooms) • can be toxic to fish, birds, humans, and other mammals
Eukaryotic Autotrophs: Algae, Photosynthetic Protists, and Plants • Unlike cyanobacteria, algae, some protists, and plant cells contain chlorophyll within the photosynthetic membranes of discrete organelles called chloroplasts • gives leaves, stems and un-ripened fruit their characteristic green colour!
Plant Parts...Review! • Colour Code! • Leaves • The Factory! • Transpiration • Cooling and Transport • Stomata • The Gate Keepers
Chloroplasts • Typical plant cell chloroplast approx. 3um to 8um in length and 2 um to 3 um in diameter • Have two limiting membranes (outer and inner) enclosing an interior space filled with a liquid stroma • Within the stroma, a system of membrane bound sacs called thylakoids stack on top of one another to form characteristic columns called grana • typical chloroplast has 60 grana each consisting of 30-50 thylakoids • Adjacent grana connected to one another by unstacked thylakoids called lamellae • Inside the thylakoids is a water filled space called the Lumen • Chloroplasts have there own DNA and ribosomes, reproduce via fission
Photosynthesis • Converts light energy into the chemical bonds of glucose (plants are Autotrophs). • It is an endergonic rxn that happens in two stages: The Light Dependant Rxn and the Light-Independent Rxn. • The overall rxn is: 6CO2 + 6H2O + Light Energy C6H12O6 + 6O2 Glucose
The Light Dependent Reaction • Occurs in the Thylakoid Membrane • The chlorophyll captures light energy and uses it to break down water molecules and create ATP and NADPH. • The oxygen is released as a product. • The ATP and NADPH are carried over to the Light Independent Rxn.
The Light Independent Reaction • Occurs in the stroma • CO2 from the air is added to the H+ ions (carried by NADPH) the glucose is formed
A General Overview • http://www.youtube.com/watch?v=cX3Iev-JDgM
Tracking Atoms! The sugars that result from photosynthesis are produced by adding the hydrogen ions and electrons from water to carbon dioxide, NOT by splitting CO2 and adding water! oxidized H+ and e- 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O reduced
Light Dep. Rxn - In Depth • Involves the splitting of water (photolysis) • 12 H2O + Energy 6 O2 + 24 H+ + 24e- • Requires light for the energy to split the water molecule • Also forms a highly energetic compound, adenosine triphosphate (ATP) • 3 Parts: • 1. PHOTOEXCITATION • 2. ELECTRON TRANSPORT • 3. CHEMIOSMOSIS
A Virtual Tour • http://vcell.ndsu.nodak.edu/animations/photosynthesis/movie-flash.htm
Pigments • Chlorophyll A is the most important photosynthetic pigment. • Other pigments called antenna or accessory pigments are also present in the leaf. • Chlorophyll B • Carotenoids (orange / red) • Xanthophylls (yellow / brown) • These pigments are embedded in the membranes of the chloroplast in groups called photosystems. • Each pigment absorbs a particular wavelength of light in the visible spectrum
What wavelengths of light do you think plants use the least in photosynthesis?
Photoexcitation • Photon = Packets of electromagnetic radiation • Occurs in Photosystems (clusters of photosynthetic pigments embedded in the thylakoid membranes • Photosystems absorb photons of particular wavelengths • Consist of an antenna complex and rxn centre • Two types: 700, 680 refers to wavelength absorbed (nm) • PS I (P700) • PS II (P680)
Photoexcitation • Antenna complex absorbs a photon of light • Transfers the energy from pigment to pigment until it reaches a chlorophyll a molecule in the centre of the rxn centre • The electron of this chlorophyll moves from ground state to a higher potential energy level (excitation)
Non-Cyclic Electron Flow – Pg 158 • ETC! 1. Photon strikes PS II, excites electron • Usually, excited electron is captured by the primary electron acceptor (REDOX rxn) • Electron then transferred to Plastoquinone (PQ) aka Q cycle • Then to ETC similar to Cell Resp • Process occurs twice, so 2 electrons 2. A Z protein associated with PS II splits water into oxygen, protons, electrons • Oxygen leaves chloroplast as a by-product • 2 electrons used to replace those that have left PS II • Protons remain in Thylakoid space, contributing to H+ gradient that drives chemosmosis
Non-Cyclic Electron Flow – Pg 158 3. Electrons pass through the Q cycle which transports protons into lumen from stroma, adding to H+ gradient (4 protons for every 2 electrons), then through the b6 –f complex • Electrons then move through Plastocyanin (Pc), eventually replacing two electrons lost from PS I when it was struck by photons. 4. Electrons from PS I pass through another ETC containing ferredoxin (Fd) • They then move to the enzyme NADP reductase that uses the two electrons and protons from the stroma to reduce NADP+ to NADPH
Non-Cyclic Electron Flow 3. Chemiosmosis – Protons that have accumulated in the lumen cause electrochemical gradient that drives phosphorylation of ADP to ATP • Called photophosphorylation, since light is required • Page 166 #2,3,4,6
Light Independent Rxn • Calvin Cycle • Also called C3 photosynthesis (since first compound formed is a 3C molecule • Occurs in stroma of chloroplasts • Cyclic series of reactions • Three phases: carbon fixation, REDOX reactions, RuBP (ribulose 1,5-bisphosphate) regeneration
Calvin Cycle • Phase 1: Carbon Fixation • CO2 are added to RuBP, forming 3 unstable 6C intermediate molecules • Each of these instantly splits into two 3C molecules called PGA (3 phosphoglycerate) for a total of six • The enzyme that catalyses these reactions is called Rubisco, a very large enzyme that works very slowly, most abundant protein on earth Phase 2: REDOX • Each of the six PGA is phosphorylated by 6 ATP to form six 1,3bisphosphoglycerate • 6 NADPH molecules each use a pair of electrons to produce six G3P (glyceraldehyde 3 – phosphate • One molecule of G3P exits as a final product
Calvin Cycle • Phase 3: RuBP regeneration • The five G3P are rearranged to regenerate 3 RuBP • 3 ATP are used! http://www.youtube.com/watch?v=_NIhg1qa_L0 • The Bottom Line • For the net synthesis of one G3P, 9 ATP and 6 NADPH are used! • Page 167, #9,10,11
G3P • Is the primary end product of photosynthesis • It may be converted into glucose and polymerized into starch w/in the stroma • Or it may be transported to the cytoplasm and used to produce glucose and sucrose. • Sucrose is the main carb transported from mesophyll cells of the leaf to other cells of the plant
Alternative Methods of Carbon Fixation • Rubisco – the double agent! • Normally rubisco adds CO2 to RuBP but when O2 is very plentiful it adds it to RuBP • This is called Photorespiration – it occurs in light! • It removes PGA molecules from the Calvin cycle • Produces phosphoglycolate • Decrease CO2 fixation and less sugar formed • Carbon fixation 4X that of oxygen fixation (20% loss of C) • Rubisco is an evolutionary remnant (from a time in earth’s history when O2 was not a prevalent • Some plants have adapted strategies to work around this!
Rubisco - The Double Agent! HELPFUL! ANNOYING!!
C4 Plants • Hot, dry environments • C4 photosynthesis – a pathway of carbon fixation that reduces the amount of photorespiration that takes place by continually pumping CO2 molecules from mesophyll cells into bundle sheath cells, where rubisco brings them into the C3 Calvin cycle • C4 refers to the four carbon oxaloacetate that is formed in the mesophyll • CO2 level in leaf is increased, no photoresp.
CAM Plants • Crassulacean Acid Metabolism • Plants in dry, desert environments • Stomata open at night to take in CO2 and incorporate it into organic acids, and close during the day to allow the acids to release CO2 into the Calvin Cycle.
Photosynthesis and the Environment • Light Intensity (P173) • Temperature (P174) • O2 concentration (P175) • Efficiency (P175) • Page 178 1-5