Photosynthesis Flashcards
(23 cards)
chemical reaction for photosyntehsis
6 CO2 + 6 H2O –> c^H12O6 + 6 O2
what do light abosorbing pigments do
absorb energy from light which is incorported into electrons to excite them - excited e- are unstable and re-emit the absorbed energy as they fall back down, that energy is reabsorbed by e- of nearby pigment molecules; this process ends when energy is abosorbed by P680 (PSII) or P700 (PSI)
chlorophyll a, b and carotenoids
antenna pigments
chlorophyll b, carotenoids, phycobillins (red algae) - capture wavelengths that chlorophyll a does nt and passes energy to chlorophyll a, where the direct light reaction takes place
chorophyll a
poryphrin ring (alternating double and single bonds; pi bonds critical for light reactions) complexed with Mg atom
steps of photosynthesis
- photophosphorylation - light dependent, produce ATP for light independent reactions to use to produce glucose
a. cyclic - PSI uses ETC between PSII and PSI to produce ATP and cycle e- back to PSI
b. non cyclic - PSII –> PSI ETC produces ATP (h2o splits w/light energy to replenish e- of PSII) and then PSI, P700 converts NADP+ to NADPH - light indpendent - calvin cycle, produce carbohydrates by CO2 fixation (turn RuBP to G3P, 2 G3P –> glucose)
a. alternatives = CAM or C4
mesophyll cell
cells in middle of leaf specialized for photosynthesis
why are leaves green
green is reflected, not absorbed
light reaction - what does the light do?
The light that is absorbed splits water into hydrogen and oxygen:
H2O + light energy —> ½ O2 + 2H+ + 2 electrons
electrons used in ETC to produce ATP via chemiosmosis - create H+ gradient between thylakoid space and stroma (inner part)
steps of light reaction
H2O + ADP + Pi + NADP+ + light –> ATP + NADPH + O2 + H+
- PSII: electrons trapped by P680 are excited with light energy
- primary e- acceptor: e- pass to the first in chain of acceptors
- ETC: 2 e- move down chain adn lose energy that is used to phosphorylate ATP
- phosphorylation: ATP synthase, H+ gradient used
- PSI: ETC terminates here at P700
- –noncyclic: - NADPH: 2e- pass down short ETC with ferrodoxin enzyme (fd) to convert NADP+ + H+ + 2e- –> NADPH (coenzyme)
- photolysis: h2o splits to regenerate 2e- in PSII
locations–
- noncyclic photophosphorylation
- cyclic photophosphorylation
- photolysis
- calvin cycle
- chemiosmosis
- thylakoid membrane - noncyclic
- stroma lamellae (pieces connecting to thylakoid) - cyclic
- thylakoid lumen - photolysis
- stroma - calvin
- thylakoid membrane - chemiosmosis
where are photosystems? how many?
few hundred photosystems in each thylakoid; rxn center containing chlorophyll a surrounded by antenna pigments taht funnel energy to it
cyclic photophosphorylation
replenish ATP when calvin cycle consumes it; excited 2e- from PSI go through ETC to generate 1 ATP, these 2 e- recycled into PSI and either take cyclic or noncyclic path from there
calvin cycle - dark, light independent
6 CO2 + 18ATP + 12NADPH + H+ –> 18ADP + 18Pi + 12NADP+ + 2G3P (1 glucose)
- carboxylation - 6CO2 + 6RuBP –> 12PGA
rubisco = catalyst - reduction - 12ATP + 12NADPH convert 12PGA –> 12G3P (PGAL), byproducts NADP+ and ADP used in photophosphorylation
- regeneration - 6ATP convert 10 G3P to 6 RuBP
- carb synthesis - remaining 2 G3P used to make glucose
plants make glucose for their own mitochondria to use for energy; ATP in photosynthesis comes from photophosphorylation, NOT mitchondria
photophosphorylation ATP is not used for general cell fixation
cholorplast structure
- double membrane (like mito and nucleus)
- outer membrane = plasma
- intermembrane space
- inner membrane
- stroma = inside of inner membrane; where calvin cycle occurs
- thylakoid = layer in granum stacks, inside stroma – membrane is where PSI, PSII, cytochromes are; lumen is the interior of thylakoid where H+ accumulates
thylakoid
a single layer of a granum (thyla = pancake; granum = stack); suspended in stroma of chloroplast - thyla membrane is where PS I adn PS II are, where ETC is
chemiosmosis
use H+ gradient to generate ATP;
- H+ accumulation in thylakoid lumen
- pH and elec gradient (pH~5)
- ATP synthase generates ATP; 3H+ diffuse for 1 ATP
- calvin cycle uses NADPH and ATP and CO2 to produce G3P
photorespiration
fixation of O2 by rubsico; no ATP/glucose – inefficient, competes with CO2; arose when low levels of O2 — peroxisomes breakdown products of it
peroxisomes in plants
break down products of photorespiration
alternative pathwayd
- C4: bundle sheath instead of mesophyll to limit photorespiration
- CAM: stroma closed during the day
c4 photosynthesis - hot, dry climate - minimize water loss from stomata (leaf pores) and photorespiration
purpose: move CO2 from mesophyll cells to bundle sheath cells; kranz anatomy; hatch-slack process; overcome tendency of rubsico to waste O2 in photorespiration; bundle sheath cells isolate rubsico from atmospheric O2 and saturate with CO2 by decarboxylation of malate
- all requires more ATP (1 extra, it becomes AMP)
steps of C4
CO2 absorbed by mesophyll cells
CO2 + PEP –> OAA (PEP carboxylase catalyst)
*CO2 is not fixed by rubisco in mesophyll
OAA–>malate; malate through plasmodesmata into bundle sheath cells
malate–> pyruvate and CO2 (pyruvate back to mesophyll, CO2 used in calvin)
CAM photosynthesis - reduce water loss by closing stomata during the day
- crassulacean acid metabolism
1. PEP and PEP carboxylase turn CO2–>OAA
2. OAA–>malic acid
3. malic acid to vacuole of cell
4. at night, stomata open (opposite normal) and PEP carboxylase is active and malic acid accumulates in vacuole
4. during the day, stomata are closed; malic acid moves out of vacuole and converted back to OAA (use 1 ATP) and back to CO2 (goes to calvin) and PEP
