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Chapter 6 Energy for Life: Photosynthesis Plants do photosynthesis for us, right??? NOPE! Plants do photosynthesis because they are solar powered—for their own benefit It is not some sort of altruistic gesture on their part for our benefit We and other heterotrophs benefit from their leftovers Overview Solar (light) energy chemical energy Organisms: ◦ Plants ◦ Algae ◦ Cyanobacteria If all of the carbohydrate energy made by autotrophs was instantly converted into coal, it would fill 100 cars/second! Flowering Plants Photosynthesis carried out by green parts, mainly leaves CO2 in through stomata ◦ Delivered to mesophyll cells: photosynthesis specialists H2O in through the root system ◦ Delivered through veins (xylem) Combine in chloroplasts The Chloroplast Space inside=stroma Disks=thylakoids Stacks of disks=grana (sing. granum) All thylakoids are probably linked together=thylakoid space Chlorophyll is found embedded in the membranes of the thylakoid disks ◦ Other pigments too Carbohydrates Sole source of energy for most of life on earth CO2 carbohydrate CO2 etc… The Process Carbohydrate has more energy than just the CO2 and H2O ingredients that go into it. ◦ More than the sum of its parts. Why? Electrons from H2O are energized by photons of light before they are given to CO2 ◦ That extra energy is what makes the difference Redox reaction Redox = oxidation/reduction reaction Involves addition/subtraction of energized electrons Final product of photosynthesis is actually a molecule called G3P ◦ (glyceraldehyde-3-phosphate) Two sets of reactions Light reactions and Calvin cycle Like many metabolic processes, there are many steps to each one Light reactions: in thylakoid membranes ◦ Produces ATP and NADPH using light energy Calvin cycle: in stroma ◦ CO2 taken up and converted to G3P using energy from ATP and NADPH Light Reactions Light-involves a whole spectrum of wavelengths with different amounts of energy Visible light a small part of whole spectrum Violet=most energy, red=least energy Only about 42% of light energy that hits the Earth’s atmosphere reaches the surface Most of that is in the visible spectrum Photosynthetic Pigments Chlorophylls and carotenoids Each type of molecule is only capable of absorbing a small portion of the spectrum Plants have a range of pigments in order to cover absorption of most of the spectrum ◦ Chlorophyll a & b mostly absorb light in violet, blue and red range ◦ Carotenoids can absorb some green, mostly violet and blue The Electron Pathway Plants don’t use photons to energize ATP and NADPH directly Use special collections of pigments and proteins to collect and concentrate the energy from photons ◦ Reaction centers Electron Pathway (cont.) Photosystem II (PSII) (named in order of discovery)—splits water ◦ Absorbs photon energy and uses it to excite electrons ◦ Excited electrons escape to near-by molecule Sent down electron transport chain ◦ Missing electrons are replaced by splitting H2O Releases O and H+ Electron Pathway (cont.) Electron Transport Chain ◦ A series of molecules pass the excited electron from PSII down the line ◦ This energy is used to pull the H+ ions from the split water into the thylakoid space in the disks ◦ Creates a concentration gradient—lots of H+ builds up inside the disks (More on this later…) Electron Pathway (cont.) Photosystem I (PSI)—makes NADPH ◦ Has another set of pigment molecules ◦ Photons energize electrons ◦ They escape and attach to NADP+NADPH Kind of like ATP ◦ Electrons are replaced by those coming off of the electron transport chain What about all of those H+ ions? ATP synthase complex ◦ Adjacent to the photosysem/electron transport chain area of the thylakoid membrane ◦ Remember the Law of Entropy? ◦ H+ have been building up in the thylakoid space, creating a concentration gradient ◦ They diffuse out of the thylakoid through the path of least resistance—the ATP synthase complex ATP synthase H+ ions flowing out of the thylakoid release energy, just like a river or waterfall ATP synthase harnesses that energy much like an old-fashioned mill or a hydroelectric turbine ATP synthase is an enzyme whose job is to hook ADP and free phosphatesATP Light Reactions Summary PSII pigment absorbs light energy ◦ Excites electrons for transport chain H2O is split at PSII to replace lost e◦ Makes H+ and O2 (waste) e- transport chain pulls H+ into thylakoid space ◦ Creates concentration gradient PSI pigment absorbs light energy ◦ Excited e- charge up NADPH H+ diffuses out through ATP synthase ◦ Makes ATP Got it???? Good news: We’re halfway done! Bad news: We’re not done yet! We still don’t have any carbohydrate! The Calvin Cycle Out in the stroma of the chloroplast Use the energy from ATP and NADPH to hook CO2 into chains of G3P Melvin Calvin use radioactive 14C to figure out the steps involved Simplified: 3 Steps ◦ CO2 fixation ◦ CO2 reduction ◦ Regeneration of beginning substrate (RuBP) Step 1: CO2 Fixation Constant supply of RuBP hangs out in the stroma of the chloroplast ◦ Made of 5 carbons In step 1, CO2 is attached (fixed) to RuBP ◦ Enzyme that does this = rubisco Makes a 6C molecule Immediately splits into 2 x 3C molecules Step 2: CO2 reduction More redox reactions! Step 2 is actually a series of reactions Energy (excited electrons) from NADPH & ATP used to rearrange the 3C molecule Ends with G3P ◦ ADP and NADP+ sent back to thylakoid membrane to get recharged Step 3: Regeneration of RuBP Step 1 & 2 change original RuBP molecule to other forms Need to regenerate RuBP for the next CO2 molecules coming in so the cycle can start over Some carbon leaves the cycle as G3P, most of it stays and gets rearranged back into the starting material so the cycle can keep going The Fate of G3P Plant cells are much more versatile than animal cells They can do a lot with the raw ingredient of G3P Mostly they put two G3P’s together to make glucose Also: ◦ Fatty acids & glycerolsoils ◦ Add nitrogenamino acidsprotien ◦ Other sugars (fructose) Glucose + fructosesucrose Glucosestarchcellulose But wait, there’s more! Other types of photosynthesis Adaptations for living in a dry environment C4 ◦ Rearranges location of cells where photosynthesis happens ◦ Corn, sugarcane, Bermuda grass, crabgrass CAM ◦ Rearranges time when parts of photosynthesis happen—CO2 only fixed at night ◦ Most succulents in deserts, including cactuses. Pineapple. ◦ More adaptive and widespread than C4
