[AP Bio] TEST FIVE: Photosynthesis
*Autotrophs will be focused on, particularly Photoautotrophs. They use sunlight/light energy, and fix CO2 to organic molecules to produce organic compounds.
-80% of atmospheric O2 comes from UNDERWATER plants!!!
*Although leaves are the major location of photosynthesis, Chloroplasts will be focused on, as they are essential to the process of Photosynthesis & we are looking at the process on a cellular level
-> Chloroplasts’ Thylakoids transform light E to the chemical E of ATP & NADPH
Photosynthesis is, simply, the conversion of light energy into the chemical energy of food.
6 CO2 + 12 H2O + Light Energy -> C6H12O6 + 6 O2 + 6 H2O
Understanding Light Energy
-light energy is known as “electromagnetic” energy
-> travels in rhythmic waves (disturbances of electrical and magnetic fields)
Wavelength = distance b/t CRESTS of E.M. waves
-> range from < 1 nm - >1 km
Range of Radiation = ELECTROMAGNETIC SPECTRUM
-small portion most important to life -> narrow band from 380 nm - 750 nm -> “VISIBLE LIGHT”
Photons = light behaving as though it consists of these “discrete particles” -> not tangible but have a fixed quantity of energy
-relationship b/t wavelength & amount of energy is INVERSE
Ex: photon of violet light has almost 2x as much energy as photon of red light
-atmosphere acts as “selective window”, letting visible light in, but not a lot of other radiation
-VISIBLE LIGHT drives photosynthesis!!!!!
-when light meets matter, it is either: 1) REFLECTED, 2) TRANSMITTED, or 3) ABSORBED
Pigments = substances that absorb visible light
-> diff. pigments absorb diff. wavelengths
-wavelengths that are absorbed, disappear!!!
-when a pigment is illuminated w/ white light, we see the color that is most reflected or transmitted by it!
Ex: leaf is green b/c chlorophyll absorbs violet-blue & red light, but TRANSMITS & REFLECTS green light
Ex: if absorbs all wavelengths -> appears black
Spectrophotometer = instrument that measures ability of pigment to absorb various wavelengths of light
-> machine directs beams of light at diff. wavelengths through solution of pigment & measures fraction of light transmitted @ each W.L.
Absorption Spectrum = graph plotting three diff. types of chloroplast pigments’ light absorption vs. wavelength
-> helps us determine effectiveness of diff. wavelengths for driving photosynthesis
*light can perform work in chloroplasts ONLY if it is absorbed!
Action Spectrum = plots the rate of photosynthesis vs. wavelength and profiles the relative effectiveness of diff. W.L.’s of radiation in driving photosynthesis
-> prepared by illuminating chloroplasts w/ light of diff. colors & plotting W.L. against some measure of photosynthetic rate (such as CO2 consumption & O2 release)
-> resembles the action spectrum for chlorophyll a (but not EXACTLY -> this is partly due to the absorption of light by “accessory” pigments such as chlorophyll b & carotenoids)
-> first demonstrated in Englemann’s experiment
Englemann’s Experiment = In 1883, Theodor W. Englemann illuminated a filamentous alga w/ light that had been passed thru a prism, exposing diff. segments of the alga to diff. W.L.’s. He used aerobic bacteria, which concentrate near an O2 source, to determine which segments of the alga were releasing the most O2 & thus photosynthesizing the most. Bacteria congregates in greatest numbers around the parts of the alga illuminated w/ violet-blue or red light. *There is a close match of the bacterial distribution to the Action Spectrum graph.
-> CONCLUSION: violet-blue & red light are the most effective in driving photosynthesis! green light is the least effective color, as it is reflected/transmitted, not absorbed.
*absorption spectra & action spectra do not exactly match for chlorophyll a
-> this is b/c “accessory” pigments w/ diff. absorption spectra are ALSO photosynthetically important in chloroplasts & broaden the spectrum of colors that can be used for photosynthesis (Ex: chlorophyll b & carotenoids)
*chlorophyll b & chlorophyll a are almost identical, but have a slight structural difference
-> as a result, they have different colors (chlorophyll a is blue-green, chlorophyll b is yellow-green)
-both molecules consist of:
Porphyrin Ring = light-absorbing “head” of molecule; w/ magnesium atom @ center
Hydrocarbon Tail = interacts w/ hydrophobic regions of proteins inside thylakoid membranes of chloroplasts
*the ONLY difference between chlorophyll a & chlorophyll b is the functional group bonded to the porphyrin ring
Carotenoids = hydrocarbons that are various shades of yellow & oranges (b/c they absorb violet-blue & green light) -> broaden spectrum of colors that can drive photosynthesis
Photoprotection = important function of some carotenoids
-> compounds absorb & dissipate excessive light energy that would otherwise damage chlorophyll or interact w/ O2, forming dangerous (to the cell), reactive oxidative molecules
-carotenoids similar to photoprotective ones in chloroplasts have photoprotective role in human eye! Highlighted in health food products as “phytochemicals” -> have antioxidant powers! Plants synthesize all the antioxidants they need, where humans & other animals must obtain from their diets
-when light is absorbed, W.L.’s disappear from spectrum, but energy CANNOT disappear!
-> when molecule absorbs photon of light, 1 of the molecule’s e’s is elevated to an orbital of higher potential energy/“excited state” (when e in normal orbital -> pigment molecule said to be in ground state)
-only photons absorbed are those whose energy is exactly equal to energy diff. b/t ground & excited states (energy diff. varies from one kind of molecule/atom to another)
-> particular compound absorbs only photons corresponding to specific W.L.’s, which is why each pigment has a unique absorption spectrum!
-once raised to excited state, electron cannot remain there long
-> high energy state, therefore unstable
-when isolated pigment molecules absorb light, e’s drop back down to ground state in billionth of a sec. -> RELEASE excess energy as heat!
Ex: why top of car is so hot on sunny day!
-white cars are coolest b/c paint reflects all W.L. of visible light!
-in isolation some pigments emit light, also
-> “afterglow” = FLUORESCENCE
Ex: if solution of chlorophyll is isolated from chloroplasts & illuminated, will fluoresce in red-orange & give off heat
-leaves get their green color from chlorophyll, the green pigment w/i chloroplasts, as it reflects green light
Stomata = The space where CO2 enters and O2 exits
Chloroplasts = found mainly in cells of the Mesophyll
Mesophyll = The interior tissue of the leaf
Guard Cells = Special cells (containing structural proteins) that will close (flaccid-> hypo inside cells & hyper outside cells) to conserve water if needed. When open (turgid-> hyper inside cells & hyper outside cells), they leave the space known as the Stomata.
-> potassium plays a role in the opening & closing of the stomata. when proton pumps transport H+ ions, changing the membrane charge, the K+ “gates” open and K+ diffusion occurs. H2O will follow K+!!!
Stroma = a dense fluid w/i chloroplasts, similar to the cytosol of an animal cell
-chlorophyll molecules excited & in intact chloroplast produce diff. results
-> *in native environment of thylakoid membrane, chlorophyll molecules organized w/ other small organic molecules & proteins into PHOTOSYSTEMS
Photosystem = reaction center surrounded by # of light-harvesting complexes
Light-Harvesting Complexes = pigment molecules bound to particular proteins
-# & variety of pigment molecules enable a photosystem to harvest light over a larger surface & a larger portion of the spectrum than 1 single pigment alone
-together, light-harvesting complexes act as “antenna” for reaction center
-when pigment molecule absorbs photon, energy transferred from pigment molecule to pigment molecule w/i L-H C until funneled into Rx center
Reaction Center = protein complex -> includes molecules & molecule called “primary electron acceptor”
-> includes “special” chlorophyll a molecules; these are special b/c their molecular environment (location to other associated molecules) allows them to use energy from light to boost one of their e’s to a higher E level to be captured by the primary electron acceptor
-(solar-powered) transfer of e from special chlorophyll a -> primary electron acceptor
-chlorophyll e is excited & PEA catches it -> REDOX RX
(isolated chlorophyll would fluoresce b/c no e acceptor)
-> drop right back down to ground state
*in chloroplast, immediate plunge back to ground state is prevented
-each photosystem (Rx center surrounded by L-H C’s) -> functions as a “unit” w/i chloroplast
-> converts light E to chemical E (used for the synthesis of sugar)
-thylakoid membrane has 2 types of photosystems
Photosystem II (PS II) = Rx center chlorophyll a = “p680” (pigment best @ absorb light w/ W.L. of 680 nm/red)
Photosystem I (PS I) = Rx center chlorophyll a = “p700” (pigment best @ absorbing light w/ W.L. of 700 nm/“far” red)
*IMPORTANT: NAMED IN ORDER OF THEIR DISCOVERIES. PS II ACTUALLY OCCURS FIRST!!!!
-they both actually have identical chlorophyll a molecules, BUT they are associated w/ diff. proteins in the thylakoid membrane -> this affects the e distribution in chlorophyll molecules & accounts for their slight difference in light-absorbing properties
*WORK TOGETHER to use light E to create ATP & NADPH!!!!
*IMPORTANT CONCEPT SUMMARY: light drives the synthesis of NADPH & ATP by energizing 2 photosystems (embedded in the thylakoid membranes)
-> the key to E transformation is the FLOW OF E’S thru the PS’s (& other molecular components built into the thylakoid membrane)
*during light Rx’s, there are 2 possible routes for e flow: “cyclic” & “noncyclic” (the dominant route)
Noncyclic Electron Flow
1) Photon of light strikes a pigment molecule in L-H C & is bounced to other pigment molecules until it reaches 1 of 2 p680 chlorophyll a molecules in the PS II Rx center.
2) The electron is boosted up and captured by the Primary Electron Acceptor!
3) An enzyme splits H2O up into: 2 e’s, 2 H+’s, & 1 O. The e’s are supplied one by one to the p680 molecules -> replace the e lost to the PEA. (*As p680 is missing an e, it is the strongest biological oxidizing agent & the e hole must be filled.) The O atom combines w/ another O atom & forms O2.
4) Each photoexcited e passes from the PEA of PS II to PS I via an Electron Transport Chain. The ETC b/t PS II & PS I is made up of “electron carrier plastoquinone” (Pq), a cytochrome complex, & a protein called “plastocyanin” (Pc).
5) The exergonic “fall” of e’s to a lower E level provides the E to synthesize ATP.
6) Meanwhile, light E is transferred via L-H C to PS I’s Rx center. An e of 1 of 2 p700 chlorophyll a molecules is excited. The photoexcited e is captured by PS I’s PEA, creating an “e hole” in p700. This hole is “filled” by the e that reaches the bottom of the ETC from PS II.
7) Photoexcited e’s are passed from PS I’s PEA down a 2nd ETC thru a protein called “ferredoxin” (Fd).
8) The enzyme NADP+ Reductase transfers e’s from Fd to NADP+. 2 e’s are required for NADP+ to be reduced to NADPH.
Cyclic Electron Flow
-under certain conditions, photoexcited e’s take an “alternative path”, called Cyclic Electron Flow
-> USES PS I but NOT PS II!!!!!
-SHORT circuit: e’s cycle back from ferredoxin (Fd) to cytochrome complex, then continue on to the p700 chlorophyll in PS I’s Rx Center
-> NO production of NADPH & no release of O -> BUT DOES generate ATP
*IMPORTANT CONCEPT: The function of CEF is to produce more ATP to make up for the difference b/c the Calvin Cycle consumes more ATP than NADPH
*The concentration of NADPH in chloroplast helps regulate which pathway e’s take thru light Rx’s!!!!
-photosynthesis generates ATP by the same basic mechanism as cellular respiration: chemiosmosis!
-the thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space (which functions as an H+/proton reservoir)
-the thylakoid membrane makes ATP as H+’s/protons diffuse down the concentration gradient from [HIGH] in the thylakoid space back to [LOW] in the stroma thru ATP Synthase (like in cell resp!) complexes (the knobs of ATP Synthase are on the Stroma side)
-thus -> ATP forms in the Stroma, which is used to help drive sugar synthesis during the Calvin Cycle
*when chloroplasts are illuminated: pH in the thylakoid space drop to about 5 ([H+] INC.), & pH in the Stroma inc. to about 8 ([H+] DEC.)!
Noncyclic Electron Flow pushes e’s from H2O (low state of PE) to NADPH (stored @ high state of PE). The light-driven electron current also produces ATP. The thylakoid membrane converts light E to chemical E (which is stored in NADPH & ATP). Also, O2 is a by-product.
-similar to the citric acid cycle in cellular respiration, a starting molecule is regenerated after molecules enter & leave
-> HOWEVER: calvin cycle is ANABOLIC (builds + consumes)
-carbon enters as CO2 & leaves as sugar
-spends ATP as energy source & consumes NADPH as “reducing power” for adding high energy e’s to make sugar
-carbohydrate produced-G3P (glyceraldehyde 3-phosphate)
-net synthesis of 1 molecule of sugar -> CC must take place 3 TIMES!!!!!! (fixes 3 molecules of CO2, 1 time for each one that goes)
1) “Carbon Fixation” = incorporates each CO2 molecule (1 at a time) by attaching to 5-C sugar RuBP (ribulose biphosphate)
a. -> enzyme that catalyzes 1st step = RuBP Carboxylase (“RUBISCO”)
b. -> most abundant protein on earth
c. PRODUCT = 6-C intermediate -> SO UNSTABLE it immediately splits in half -> becomes 2 molecules of 3-phosphoglycerate (for EACH CO2!!)
2) “Reduction” = each molecule of 3-phosphoglycerate receives an additional phosphate from ATP -> becomes 1, 3-biphosphoglycerate
a. -> pair of e’s donated from NADPH
b. -> reduces 1 , 3-biphosphoglycerate to G3P
c. -> e’s from NADPH reduce carboxyl group 3-phosphoflycerate to aldehyde group of G3P (stores more ATP)
d. ->*for every 3 CO2, there are 6 C3P
e. ->*only 1 molecule of G3P can be counted as net gain! -> 1 molecule exits the calvin cycle to be used by plant cell -> other 5 recycled to regenerate the 3 molecules of RuBP!
3) “Regeneration of the CO2 Acceptor (RuBP)” = in a complex series of Rx’s, the carbon skeletons of 5 G3P’s are rearranged by the last steps of the calvin cycle into 3 RuBP
-> to work, the calvin cycle needs to spend 3 molecules of ATP
-> RuBP is now prepared to receive CO2 again!!!!
-> for the net synthesis of 1 G3P, the calvin cycle consumes a total of 9 molecules of ATP & 6 molecules of NADPH!!!!!
Conservation of Water
-plants have anatomical & metabolic adaptations to help them conserve water (water is super duper important to plants, as it is like “feeding them electrons”!!!!)
-photorespiration (photo = light, respiration = the consumption of O2 & production of CO2) is a wasteful metabolic process & may kill plans if it continues for too long.
-“closed stomata” conditions for photorespiration
C3 Plants = 1st organic product of carbon fixation is 3-C “3-phosphoglycerate”
-> *most plants: initial fixation of C occurs via Rubisco, a calvin cycle enzyme that adds CO2 to RuBP
Ex: rice, wheat, & soy beans
-> stomata partially closed on hot, dry days & produce less sugar b/c less CO2 can get in -> also, Rubisco can bind O2 in place of CO2!! -> product splits, 2-C compound leaves chloroplast -> peroxisomes & mitochondria rearrange & split compound -> RELEASES CO2
-> called “photorespiration” (occurs in light, & consumes O2 while producing CO2!!)
-> does not generate ATP, CONSUMES IT
-> does not produce sugar
-> decreases photosynthetic output (siphons organic material from the calvin cycle)
-> modern Rubisco retains some chance affinity for O2 -> certain amount of photorespiration is inevitable now
C4 Plants = an alternate mode of carbon fixation that forms 4-C as 1st product
Ex: sugar cane & corn
-> unique leaf anatomy: 1) Bundle-Sheath Cells; tightly-packed sheaths around veins of leaf, Calvin Cycle takes place here in these plants & 2) Mesophyll Cells; more loosely arranged
-> cycle preceded by incorporation of CO2 into organic compounds in the Mesophyll
-> 1st step: the enzyme PEP Carboxylase adds CO2 to phosphoenolpyruvate (PEP)
-> forms 4-C oxaloacetate
-> *PEP Carboxylase can fix carbon when Rubisco can’t b/c of a high affinity for CO2
-> Mesophyll then “exports”/pumps 4-C product to bundle-sheath cells thru Plasmodesmata (*SPATIAL ADAPTATION)
-> w/i cells -> 4-C’s RELEASE CO2
CAM Plants = “crassulacean acid metabolism” = open stomata @ night & close during day (TEMPORAL/time relationship)
Ex: succulents, pineapples, etc.
-> allows desert plants to conserve H2O
-> *Mesophyll cells store organic acids made @ night in vacuoles until morning (stomata close)