Bio 110 Lec Chapter 10 Photosynthesis

- solar light energy trapped by chloroplasts -> chemical bond energy stored in sugar and other organic molecules

- CO2: carbon source; light: energy source

- 6CO2 + 6H2O + light -> C6H12O6 + 6O2 (reverse of cellular respiration), primary product is other carbohydrates

- Van Niel: discovered that plants split water as hydrogen source, then release oxygen as a byproduct

- endergonic redoxprocess: requires energy (hydrogens associated w/ water have less PE than those associated w/ sugars) -> light boosts PE

- synthesize organic molecules from inorganic raw materials (e.g. plants only require CO2, H2O, and minerals)

- require energy source: light (photoautotroph) or inorganic substances (chemoautotroph)

- Photoautotrophs: plants, algae, some prokaryotes

- Chemoautotrophs: oxidize sulfur/ammonia, unique to some bacteria

- acquire organic molecules from compounds produced by other organisms

- decomposers: decompose and feed on organic litter (e.g. fungi, bacteria)

- green pigment in chloroplasts that gives leaf color

- absorbs light energy uised to drive photosynthesis

- built into plasma membrane or membranes of numerous vesicles within cell of photosynthetic prokaryotes; usually arranged in parallel stacks of flattened sacs

- absorbed photon boosts electrons from ground state to higher orbital (excited state) -> photon energy absorbed converted to -> potential energy of elevated electron- excited state is unstable, so electrons fall back; energy either dissipated as heat or fluorescence (red part of spectrum)

- cells contain chloroplasts, major sites of photosynthesis

- microscopic pores through which CO2 enters and O2 exits leaf

(1) intermembrane space: separates double membrane that partitions chloroplast contents from cytosol

(2) thylakoid space: segregates interior of chjloroplast into thylakoid spoace and stroma

- thylakoids: flattened membranous sacs in hlorpolast, where chlorophyll is found; function in initial conversion of light -> chemical energy

- grana: stacks of thylakoid in chloroplast

(3) stroma: viscous fluid outside thylakoids; where reactions converting CO2 -> sugar (Calvin cycle) occur

- light energy -> chemical bond energy in ATP and NADPH, in thylakoid membranes

- light absorbed by chlorophyll -> gives energy to add pair of electrons and hydrogen to NADP+ -> NADPH, O2 as by-product

- generates ATP through photophosphorylation of ADP

- assimilate atmospheric CO2 -> reduce to carbohydrate

- requires products of light reactions: NADPH for reducing power; ATP to energy

- nicotinamide adenine dinucleotide phosphate

- coenzyme similar to NAD+ in respiration

- when ATP is generated as light reactions power addition of phosphte group to ADP

- process of incorporating CO2 into existing carbon molecules

- first step of Calvin cycle

- distance between crests of electromagnetic waves

- fully radiated by sun

- atmosphere screens out radiation

- light detectable by human eye

- 380 to 750 nm

- wavelengths most impt for photosynthesis within this range (blue and red)

- discrete particles or quanta of light

- quantity of photon energy inversely proportional to wavelength (violet light: more energy than red)

- determines absorption spectrum

- measures what proportion of wavelengths is absorbed/transmitted by pigment

- characteristic of each pigment (absorbs visible light)

- absorption vs. wavelength

- light-absorbing pigment that participates directly in light reactions

- absorption spectrum underestimates effectiveness of some wavelengths

- other accessory pigments absorb light and transfer to chlorophyll a

- graph of wavelength vs. rate of photosynthesis

- profiles relative effectiveness of different wavelengths of visible light

- blue and red most effective, green light least

- yellow-green accessory pigment

- yellow and orange hydrocarbons

- built into thylakoid membrane with two types of chlorophyll

- chlorophyll a, b, and carotenoids assebled here, located in thylakoid membrane

- composed of:

(1) antenna complex: Ca, Cb, and carotenoids are light-gathering antenna that absorb photons and transfer resonant energy -> inductive resonance; different pigments have slightly different spectra, so can absorb wider range of light

(2) reaction-center chlorophyll: specialized chlorophyll-a molecule that can transfer excited electron

(3) primary electron acceptor

- where specialized chlorophyll a that can transfer excited electron is located

- molecules that trap excited state molecules that have absorbed protons

- why pigment molecules do not fluoresce in thylakoid membranes

- energy stored in trapped electrons powers ATP and NADPH synthesis

- reaction center has P700, which absorbs 700 nm (far red)

- excited state electrons from P700 stored in NADPH

- excited state electrons movement: (1) P700 -> (2) primary electron acceptor -> (3) ferrodoxin (Fd), an iron-containing protein -> (4) redox reaction to NADP+, catalyzed by NADP+ reductase -> (5) NADPH

- photosystem II supplies electrons to fill oxidized (loss electrons) P700 holes

- reaction center has P680, which absorbs 680 nm

- (1) P680 -> (2) primary electron acceptor -> (3) electron transport chain! (i) plastoquinone (Pq) (ii) two cytochromes (iii) plastocyanin (Pc) -> P700 of photosystem I

- gradually lose energy through ETC, in ground state in photosystem I

- so P680 has the holes now -> water-spitting enzyme steals e- from water, splitting it -> releases O2 and 2H+

- creates proton-motive force: proton gradient across thylakoid membrane where electron flow stores energy

- chemiosmosis: thylakoid membrane couples exergonic flow of electrons to phosphorylation of ADP to ATP

- transforms light energy to chemical energy stored in bonds of NADPH and ATP

- passes electrons from water -> NADP+

- produces ATP, NADPH, O2

- process by which ATP is produced in noncyclic flow

- Where ATP synthase enzyme uses proton-motive force to make ATP

- ATP synthase complex has catalytic heads on stroma side; force goes thylakoid compartment -> stroma

- H+ accumulates in thylakoid -> pH gradient

- generates ATP without producing NADPH or evolving oxygen

- (1) P700 -> (2) primary electron acceptor -> (3) ferrodoxin -> (4) same ETC as noncyclic! -> (5) provides energy for ATP synthesis

- ATP powered by electron flow, as opposed to proton-motive force in noncyclic

- supplements ATP supply, because noncyclic pathway produces ATP = NADPH -> not enough to meet demand!

- 3C sugar produced by Calvin cycle (CO2 -> sugar)

- 18 ATP and 12 NADPH -> one glucose molecule

(1) Carbon fixation: each molecule of CO2 attached to ribulose biphosphate (RuBP), a five -carbon sugar -> six carbon intermeidate -> splits into two 3-phosphoglycerate

- 3CO2 -> 3 6C carboxylated RuBP -> 6 3-phosphoglycerate

(2) Reduction: 3-phosphoglycerate phosphorylated -> 1,3-biphosphoglycerate; carboxyl group of 1,3-biphosphoglycerate reduced by electrons from NADPH -> G3P

- only one net gain G3P exits cycle, rest are recycled

(3) Regeneration: G30 rearranged into RuBP

- RuBP carboxylase

- catalyzes carbon fixation

- fiirst stable intermediate is 3-phosphoglycerate

- reduces yield of photosynthesis

- metabolic pathway that consumes oxygen, evolves CO2, produces no ATP, and decreases photosynthetic output

- Occurs when O2 conc. in leaf air spaces > CO2 conc. -> rubisco transfers O2 to RuBP -> 2C glycolate and 3C 3-phosphoglycerate

- glycolate goes to peroxisome, where broken down into CO2

- 3-phosphoglycerate stays in Calvin cycle

- may be evolutionary relic when atmosphere had less O2

- fostered by hot, dry, bright days

- minimized by C4 and CAM

- incorporate CO2 into four-carbon compounds

- e.g. corn, sugarcane

- enhances carbon fixation under conditions that favor photorespiration

- anatomy: spatially segregates Calvin cycle from initial incorporation of CO2; two types of photosynthetic cells

- Calvin cycle

(1) CO2 added to phosphoenolpyruvate (PEP) by PEP carboxylase -> 4C oxaloacetate

- unlike rubisco, has no affinity to O2!

(2) Oxaloacetate converted by mesophyll cells -> 4C compound, usually malate

(3) Mesophyll cells export through plasmodesmata to bundle-sheath cells

(4) 4C compounds release CO2 -> fixed by rubisco

(5) Mesophyll cells pump CO2 back in, maintaining conc. sufficient for rubisco to accept CO2

- tightly-packed sheaths around veins of leaves in C4 plants

- thylakoids not stacked into grana

- Calvin cycle confined to chloroplasts in bundle sheath

- loosely-arranged in area between bundle sheath and leaf surface in C4 plants

- enzyme that catalyzes CO2 addition to PEP

- mode of carbon fixation in CAM plants

(1) CO2 taken up at night and incorporated into oreganic acids

(2) organic acids stored in vacuoles of mesophyll cells until morning

(3) CO2 released from organic acids in morning for light reactions

- succulent plants adapted to very arid climates, open stomata at night instead of day

- temporally separate carbon fixation from Calvin cycle (as opposed to C4, which separate spatially)