L14-19 Flashcards
chemical reaction of photosynthesis
6CO2 + 12H@O -> c6h12o6 +6O2 +6H2o
what is photosynthesis?
converting light energy to chemical energy by reducing organic carbon dioxide to organic carbon. generates O2 by oxidation of water
what powers cellular porcesses in the plant?
energy stored in carbohydrates.
where does photosynthesis occur?
chloroplast
two components of photosynthesis
light reactions + carbon fixation reactions
where does energy come from in light reactions?
from absorption of light energy - converted to stable chemical energy.
where does light reaction occur in the plant?
inner thylakoid membrane
chlorophylls.
What occurs in light reaction at chlorophyll?
absorb light energy for oxidation of water + release O2, generating NADPH and ATP
discuss asymmetry of chloroplast membrane
lumen (inside) vs stroma (outside)
grana lamella - stacked vs stroma lamella - flat
what is function of carbox fixation reactions?
NADPH + ATP used to reduce carbon dioxide to carbon skeleton to generate sugar precursors
main components of the light reaction machinery
light energy converts chemical energy at 2 photosystems.
- absorbed light energy transferred through electron carrier proteins to reduce NADP+ to NADPH
electron transport generates PMF = allows synthesis of ATP
properties of sunlight
radiant energy within electromagnetic spectrum.
- wave properties: wavelength and frequency inversely proportional
- particle properties: photon. each photon has amount of energy, some of which absorbed by plants.
what wavelengths of light are strongly absorbed by plant chlorophylls
blue (430 nm)
red (660nm)
green is transmitted/reflected
describe chlorophyll excitation
photon absorbed, electron to shift from inner molecular orbit to outer molecular orbit.
-> to return to ground state, electron must return to its original orbit + the absorbed energy released
higher energy light on chlorophyll?
blue light. shorter wavelength. excites Chl to unstable excited state.
-rapidly gives up energy as heat.
lower energy light excites chlorophyll how?
red light.
stable for a few nanosecs.
- four ways to use potential energy
how excited chlorophyll returns to ground state
- re-emit a photon: fluorescence. lower energy emitted
- thermal deactivation: energy released as heat
- energy transfer: transfer from chl* to neighbouring molecule (pigment, O2 =ROS, photoinhibition)
- photochemistry
2 part molecule - chlorophyll. describe
hydrophilic porphyrin ring.
- Mg2+ cofactor.
- site of photon harvesting
hydrophobic hydrocarbon tail anchors to thylakoid membrane
chl a and chl b - discuss
major types in plants.
differ in substitutions around porphyrin ring.
- slight difference in absorption
carotenoids - discuss
accessory pigment. 400-600nm range. orange/yellow colour. usually closely assoc with Chl. absorbs photons from blue region - help protect photosynthetic machinery from photoinhibition
what happens to energy absorbed in pigments?
stored as chemical energy through formation of chemical bonds.
- > light dependent reactions required.
- > photoynthetic electron transport
central components of photosynthetic electron transport?
multi- molecular chlorophyll-protein complexes
- PS1
- PSII
what’s between the two photosystems?
multiprotein cytochrome complex (Cyt b6f)
what does the system of 3 photosynthetic electron transports do?
raises low energy electrons obtained from oxidation of water to higher energy level needed to synthesize NADPH
what is antenna complex?
many pigment molecules in close association
what is function of antenna complex?
collects energy + transfers to other pigment molecules until received by reaction centrer
- increase e- to outer orbitals.
what is reaction centre?
where oxidation occurs - converts energy to chemical energy. can reduce product once gains e- from antenna complex
-> often reaction centre is chlorophyll.
how is reaction center chlorophyll unique?
Chl a has specific absorbance maxima
PSI: far-red light, >680 nm
PSII: red light, 680
how light drives the reduction of NAD+
1.absorption of red light by PSII = excitation of P680.
2. P680* returns to stable state by transferring electron to acceptor
molecule
3. transfer from P680* produces strong oxidant
4. strong oxidant oxidizes water, returning PSII to initial state.
5. far-red light absorbed by PSI (P700*) = weak oxidant and strong reductant.
6. strong reductantreduces NADP+ to NADPH
. reductant re-reduces weak oxidant produced by PSI
name of light reducing NADP+ pathway?
Z scheme.
what does X scheme generate?
stored chemical energy in NADPH and O2
photosystem separation on thylakoid membrane?
spatially separated
- distributed across the membrane
what is light harvesting complex
- maximizes light absorption
- binds antenna complex pigments.
- contain Chl a and b. some carotenoids. different variations tho
how do LHC’s maximize light absorption?
funnel energy to reaction center.
absorption maxima progressively shift towards longer red wavelengths (lower energy)
- small fraction of energy lost to heat = gets photon to reaction center even if lower energy
-directionality of energy-trapping process is irreversible
what is oxygen-evolving complex
supplies electrons to PSII and produces oxygen.
- obtains 4 electron from Mn2+ to oxidize 2 molecules of water.
process of electron transport in photosynthesis?
- excitationg energy -> chemical energy. as P680 excites, transfers electrons to pheophytin. charge separation stores light energy as redox-potential energy
- physical separation of charge by electron = unidirectional movement, no reversion of charge. pheo- pass e- to Qa (x2)-> PQ (+e- +H+ = coverts to PQH2) PQH2 dissociates from PSII
- charge separationgenerated by photo-oxidation of P680 stabliied by P680+ (strong oxidant - reduced by oxidizing water)
- electron from PQH2 transferred to Cytb6f. PQH2 oxidized releases 2H+ into lumen. one e- from Cytb6f goes to plastocyanin, other e- gets recycled back.
- electron transport through electron carriers of PSI. (excited P700 = photo-oxidized) e- transferred through Fe-S centers to ferredoxin, p700+ -> P700 bc accept e- from plastocyanin. ferredoxin mediates synthesis of NADPH via ferredoxin-NADP+-reductase
chlorophyll is 2 part molecule
hydrophilic prophyrin ring (Mg2+ cofactor, photon harvesting)
hydrophobic hydrocarbon tail - anchor Chl in thylakoid membrane
what does z-scheme describe?
free energy of non-cyclic photosynthetic electron transport
= formation of NADPH conserved 32% of energy required to mediate electron transport.
= remaining energy coupled to transport of protons against their gradient, from stroma to the lumen
herbicides block photosynthetic electron flow
DCMU - block e- flow at plastoquinone acceptor of PSII. compete for binding site
Paraquat accept e- from early PSI. reacts with O2 to form O2- damages chloroplasts
Proton gradient by e- transport
4 cycles of PSII uses 4e- to generate 2 PQH2
4e- replaced by oxidation of H2O (2 rounds)
yielding 4H+ in lumen
movement of 4e- from PQH2 to Cyt b6f = proton movement across thylakoid membrane to lumen. 4H+ via PQH2 from PSII; 2H+ from Cyt b6f Q-cycle
e- recycling in e- transport.
2 continue into cyt b6f to plastocyanin
2 recycled to reduce PQ -> PQH2. but bc 4 e- move through, 4 protons do to. 2 protons recyled. 2 pushed across to luman.
how photosynthetic electron transport establishes electrochemical gradient for H+?
protons only move from lumen to stroma via ATP synthase.
loading into lumen by e- transport allows pmf.
= exergonic process, provides synthesis of ATP
summary of light reactions
- excitation of chlorophyll at PSII and PSI
- convert light energy to chemical by photo-oxidation of P680 + P700 reaction centers
- P680 reduced via electrons obtained from H2O
- P700 reduced via electrons from electron transport chain
- electron transport to ferredoxin used to generate NADPH
- proton gradient used by ATP synthase to generate ATP
function of Carbon fixation reactions
use NADPH and ATP generated in light reactions to drive endergonic reduction of CO2 to carbohydrate.
three phases of Calvin-benson cycle
- carboxylation
- Reduction
- Regeneration
what is purpose of carboxylation step of Calvin Benson Cycle?
covalently links atmospheric CO2 to carbon skeleton.
- RuBP catalyzed by Rubisco to yield two 3C intermediates.
why is Calvin-benson cycle referred to as C3 cycle?
because 1st product is 3C
what is Rubisco
extremely abundant catalyst.
- has high affinity for CO2
does carboxylation step require energy input?
no. change in free energy is -35
what is reduction step of CB cycle?
forms carbohydrate using ATP and NADPH from light reactions
= 3-phosphoglycerates to 3C carbohydrates using ATP and NADPH
What is regeneration step of CB cycle?
restores CO2 acceptor, RuBP using ATP
What are triose phosphates
converted to starch in chloroplast.
- exported to cytosol for formation of sucrose (transported via phloem)
two part reaction of Rubisco
- CO2 combined with 5C RuBP by rubisco.
- 6C intermediate spontaneously hydrolyzes froming two 3-PGA molecules
two conditions must be met for chloroplast to take up CO2
- 3-PGA continually removed
- Adequate supply of acceptor molecule must be maintained
- > require ATP and NADPH
- 3-PGA removed by reduction
convering 3-PGA to G-3-P
- ATP phosphorylates 3-PGA = 1,3, biphosphate glycerate
2. NADPH reduces 1,3-bisphosphate glycerate to G3P (reduced form)
of g3p pool, what are 2 routes
- regeneration. contribute to regen of RuBP
2. utilization: 1/6 of G3p exported for sugar synthesis
how to get from G3P to RuBP?
interconversions to generate Ribulose-5-phosphates.
ATP phosphorylates to RuBP
what is net effect of regeneration reactions?
recycle 5/6 G2P to three RuBP
what is net effect of regeneration reactions?
recycle 5/6 G2P to three RuBP
how many turns of the cycle to get an additional G3P?
three.
6 CO2 = one hexose sugar.
12CO2 = 1 molecule of sucrose
what is photorespration
photosynthesis associated event that generates CO2.
Rubisco react with O2 (oxygenase activity) as well as CO2 (carboxylase
compete.
rubisco + O2: photorespiration
1 3-pga and 1 2c phosphoglycolate.
- drain C from and inhibit enzymes in C3 cycles
phosphoglycolate is metabolized by C2 oxidative photosynthetic carbon cycle, which releases CO2 and forms 3-PGA (glycine created, releases CO2. 2 glycine combine to make serine with NAD+. serine phosphorylated to reform phosphoglycerate.
C2 oxidative photosynthetic carbon cycle
- recover carbon from phosphoglycolate . 3 organelles
1. chloroplasts
2. peroxisomes
3. mitochondria
how do chloroplast help to recover phosphoglycolate
rubisco oxygenase activity = 2-phosphoglycolate. dephosphorylate to glycolate
what does peroxisome do to recover carbon from phosphoglycolate?
- glycolate oxidized to glyoxylate.
glyoxylate transminated to form glycine
how mitochondira helps recover carbon from phosphoglycolate
- 2 glycine = 1 serine. release CO2 and NH4+.
- serine to peroxisome.
- glycerate to chloroplast
- energetic requirements are higher than for C3 (CO2 cycle) = more metabolic action
three factors that affect photorespiration?
- kinetic properties of Rubisco (carb to oxy = 3:1)
- ratio of atmospheric CO2:O2 (lower CO2 increases oxygen)
- temperatture (increase favours oxygen, increase affinity for O2, lower solubility of CO2, increase stomatal closure
benefits of C2 (photorespiration) cycle
scavenges phosphoglycolate, returning to carbon pool (recovers 3/4 carbon, recovers nitrogen)
contribute to aa glycine + serine metabolism.
- may minimize photo-oxidative damage
other carbon-concentrating mechanisms that land plants could use
C4 and CAM
- adaptations to minimize photorespiratory losses under conditions where CO2 is limited +/or temps are high
c4 : separated how
spatially.
what are 5 stages of carbon cycle in C4
- mesophyll: PEPcase catalyzes HCO3- + 2CPEP = 4C
- 4C acid flows across diffusion barrier to bundle sheath cell
- stromal space: decarboxylating enzyme release CO2 from 4c yelding 3C. = build up of CO2, increase affinity where Rubisco is found
- 3c back to mesophyll cell
- pyruvate-phosphate dikinase catalyzes regeneratio of PEP
what is kranz anatomy?
adaptation that separates carboxylation reaction from CB cycle.
two concentric layers of diff cells around veins.
1. vascular bundle surrounded by ring of tightly-packed bundle sheath cells (sites of decarboxylation of 4c and Co2 assimilation
2. outer ring of mesophyll cells peripheral to bundle sheath. carbon fixation vie PEPcase
how are CAM plants separated?
temporally
stomatal cycle of CA
stomata open at night for gas exchange (store carbone)
stomata closed during day to limit evaporative water loss (use carbon stores up)
where do CAM plants grow?
arid environments + display anatomical features that minimize water loss
CAM: uptake of atmospheric O2 when?
at night, when stomata are open.
- mt respiration increases.
PEPCase concentrates CO2
4C oxaloacetate reduced to malate; accumulated in vacuole + stoed
limitations to CAM?
PEP same as in C4. large pool to fix + store.
- only so much starch can be stored. lower rate of carbon fixation, slower growth.
how stored carbon is used during day in CAM
malic acid flows back to cytosol
NAD-malic enzyme transforms malate to CO2.
CO2 refixed into carbon skeletons - increases CO2 near rubisco.
- stomata remain closed
CAM photosynthesis
compared to C3 and C4
conservation of water bc stomata are closed during light period. lower transpiration rate than C3,C4
- CAM reassimilate CO2 more readily than C3,C4. store trios phosphates as starch limits sucrose synthesis
-CAM can be facultative (utilized as pathway for potosynthesis only when needed)
similarities between C4 and CAM
both utilie PEP carboxylase to form 4C acid
- both concentrate CO2 around Rubisco to increase its efficiency
differences btween C4 and CAM
CAM - 4C carboxylation and CB cycle at different times but in same cell TEMPORAL separation
C4: 4C carboxylation + CBcycle in diff cells = SPATIALLY separated
sun - light absorbed? generates what?
- 50% of sunlight absorbed.
- generates heat-load that must be dissipated in order to avoid damage to leaf
3 types of heat loss
radiative heat loss: emission of long-wave radiation
sensible heat loss: air circulation around leaf enables convective cooling (transfer heat from leaf to air)
latent heat loss: water evaporation from leaf a (transpiration) removes large amounts of heat, thus cools it
optimal/max temp for photosynthesis
max photosynthesis within narrow temp range.
- photosynthetic thermal optimum
- varies between species and between C3/C4
three phases of temperature -response curves.
- low temp: enzmatic events stimulated
- temp range for optimal photosynthesis
- high temp range where deleterious events occur.
=c4 has higher optimum than c3.
C4 optimal temp for photosynthesis
higher than c3.
-> higher temps, where CO2 solubility decrease, O2 increases.
increased rubisco activity
increase rubisco affinity for O2 relative to CO2
c4 more sensitive to lower temp (slower for PEP to be regenerated
increased photorespiration in C3 plants = reduce photosynthetic eficiency
yield of C3 vs C4 plants at changing temps
C4 is steady yield across temps.
C3 has higher yield than C4 below 30C (bc C4 inefficient ATP costs), whereas above 30C C4 have higher yield. (C4 independent of temp)
temp on quantum yield and photorespiration
reduced QY and increase PR.
- > high temp (equator) C4 not affected. C3 affected by photorespiration rates.
- > low temp (further from equator) C4 not affected by photorespiration, basically stays the same. C3 benefits. CO2 preferred.
ecological differences btw C3 ,C4
c3 thrive in cooler northern environments
c4 thrive in warmer, drier southern environments
How has atmospheric CO2 changed since industrial revolution?
increased. due to human use of fossil fuels and deforestation.
increase CO2 in atmosphere = what effect on plants?
increased photosynthesis and productivity in C3 plants.
not in C4.
intercellular CO2 and net CO2 assimilation of C3 and C4
C4 saturates qquickly. doesnt respond to increase in CO2
high compensation point (moderate rates of photorespiration when co2=0) in C3. saturated by NADPH and ATP to regenerate RuBP
c4 - very low or no photorespiration.
which plant more likely to benefit from increasing atmospheric CO2?
C3 > c4.
because greater co2 would decrease photorespiration rates
availability of CO2 - biggest factor?
concentration gradient - stomatal resistance is greatest effect on CO2 diffusion.
- regulate stomatal aperture is only effective way plants control gas exchange
- boundary layer helps increase stomatal length, decreases cO2 diffusion.
how C4 reduces water loss?
intercellular CO2 is low. stomatal closure restricts CO2 diffusion.
low CO2 compensation points (no photorespiratio) and photosynthetic rate saturates at low Ci.
-> C4 sustain high photosynthetic rates when stomata are relatively closed therefore restirct water loss.
C4 vs C3 in photosynthetic efficiency.
C4 more efficient in hot dry enviro than C3.
- more resistant to drought than C3
C4 vs C3 in nitrogen efficiency
CO2-concentration mechanism in C4 plants maintain Rubisco close to saturated.
C4 less nitrogen rich rubisco to maintain CO2 assimilation. = C4 need less nitrogen to grow.
Photosynthates AKA
- types? what do they do/
photoassimilates.
- generated by CBcycle.
triose phosphates (1 every 3 cycles) are used by plants to synthesize starch + sucrose
carbon allocation= transitory starch in chloroplast; sucrose in cytosol,
2 ways carbon can be allocated
starch - storage
sucrose: transported long distances (local or exported)
sucrose biosynthesis
photosynthate produced in leaves transported as sucrose to meristems and developing organs when demand is high.
- in cytosol
- triose phosphate exported to cytosol, converted to hexose => sucrose
what is cytosolic [sucrose] dependent on?
- carbon import of triose phosphates (diurnal) and maltose (nocturnal) from chloroplasts to cytosol
- carbon export: delivery of sucrose from leaf cytosol to other tissues = low demand - favour storage of starch.
- enviro factors: fruiting - increase fructose or starch to store rather than sucrose
starch biosynthesis
main storage carb.
- synthesized + stored as insoluble granules in stroma of chloroplasts.
- diurnal synthesis + noctural breakdown of transitory starch in chloroplast
starch biosynthesis
main storage carb.
- synthesized + stored as insoluble granules in stroma of chloroplasts. [glucose-1-phosphate -> ADP-glucose. starch synthase adds glucosyl groups.
- diurnal synthesis + noctural breakdown of transitory starch in chloroplast
what is purpose of diurnal/nocturnal switching in starch synthesis
energy reserve that provides an adequate supply of carbohydrates at night
diurnal overflow that stores photosynthate when photosynthetic CO2 assimilation proceeds faster than synthesis of sucrose.
two principle forms of starch
amylose (a(1->4)linkages)
amylopectin (a(1->6)linkages) = branched
starch is dominant where?
seeds, stems, roots, tubers,
Modifications to starch biosynthesis - change quality
supersweet (sh2) corn have 4-10x more sugar than normal.
increase sugar to starch ratio by perventing formation of ADP-glucose
modification to starch biosynthesis for crop quality
potato, differ in starch content of tubers. baking has more starch than boiling potatoes
sugar beet tap-root: stores sucrose. no starch
Daytime carbon allocation
- what are triose phosphates used for?
- synthesis of chloroplast ADP-glucose. (glucosyl donor for starch synthesis)
- translocation to cytosol for synthesis of sucrose via hexose phosphate pool
= external + internal stimuli control carbon allocation
nightime carbon allocation
starch degrades mainly at night.
cleave glcosidic linkages to release maltose + glucose. then transported to cytosol to convert to hexose phosphates for sucrose synthesis
define allocation
regulated distribution of photosynthats into metabolic processes with source cells
define partitioning
differential distribution of photosynthates to multiple sinks within the plant
allocation in source leaves includes
- synthesis of storage compounds
- metabolic utilization to meet energy needs + to provide carbon skeletons
- synthesis of transport compounds to be exported from the leaf
Source leaves regulate allocation
control distribution of triose phosphates between CB cycle (regen of intermediates)
- starch synthesis
- sucrose synthesis
- respiration
key enzymes in regulating distn of triose phosphates?
ADP-glucose pyrophosphorylase
- starch synthase
- fructose 1,6-bbisphosphatase
- sucrose phosphate synthaase
most photosynthates transported from?
from mature healthy leaves. through phloem from sources to sinks
define source
tissue or organ that is capable of exporting photosynthates
what are net C exporters?
source that produces more photosynthate than in uses.
define sink
tissue or organ that must import photosynthates to support metabolism.
what are net C importers?
sink that consumes more photosynthates than it produces
amount of photosynthate exported from leaf depends on?
age + photosynthetic rates
import cassation + export initiation are separate events
import allowed once veins are mature.
in mature leaf, import stops + export starts. photosynthate loads into minor veins, while major veins are for export
partitioning is determined by?
competition between sink tissues.
- greater sink strength = will get larger fraction of photosynthate pool
what is sink strength?
- function of what?
ability of sink organ to mobilize assimilates toward itself.
- > sink size: biomass of sink tissue
- > sink activity: rate of uptake of photosynthates per unit weight of sink tissue
- proximity of sink to source:
- development
- vascular connections
sink activity determined by?
rate of photosynthate acquisition form translocation system
- rate of photosynthate utilization
- rate of C storage
how proximity affects sink strenght?
upper leaves provide photosynthates to growing shoot tip + young immature leaves.
lower leaves supply root system
how development affects sink strength?
may change as plant develops.
shoot + root apices are major sinks during growth. but seeds + fruits become dominant during reproductive development
- seasonal paritioning patterns
how vascular connections affect sink strength
source leaves preferentially supply sinks with which they have direct vascular connections.
- shoot: leaf connected to other leaves via vascular sstem.
- plasticity of translocation pathway dpends on interconnections between vascular bundles
phleom - define
transports photosynthates from sources to sinks.
- usually phloem fibers, phloem parenchyma, sieve elements + companion cells
conducting cells of phloem?
sieve elements.
- highly differentiated in angiosperms
- relatively unspecialized in sieve ells of gymnosperms.
- mature sieve elements living cells specialized for transport
- attached end to end to form tube/phloem network
what are sieve areas
pores that interconnect protopast of adjacent sieve elements
what are sieve plates
groups of larger sieve area pores usually at ends of sieve elements.
end-to-end connection of adjacent sieve element protoplasts.
found in angiosperm
sieve elements - living cells - how developed?
living (in contrast to dead xylem). highly modified.
lose many organelles, including nuclei. retain PM, mt, plastids, SmoothER
CW non-lignified (xylem are), may have secondarly CW thickening
sieve element companion cell - function?
specialized parenchyma cell derived from mother cell of sieve element.
- provide metabolic support (protein, ATP)
aid in loading.
= need both to survive.
diff varieties of companion cells = diff smplastic connections
what is phloem sap?
water + solutes that flows through phloem
mostly sucrose. up to 12% dry matter.
solute potential between -1.2 and -1.8 MPa.
= v important to keep phloem intact
sugars transported by phloem?
non-reducing sugars.
less chemically reactive.
ketone/aldeyde group reduced to alcohol or combined with another sugar to eliminate groups
most commonly translocated sugar
sucrose.
other mobile carbs?
bbound to sucrose or other galactose molecules.
sugar alcohols include mannitol + sorbitol
what are p-proteins?
structural proteins in phloem sap that seal damaged sieve tube elements of most angiosperms
- absent in gymnosperms
- in periphery of sieve tube elements
- block loss of sap (energy and carbon rich)
- short-term solution
sieve tube pressure + damage?
under high pressure. when damaged release in pressure. cytoplasm surges towards damaged end and p-proteins clog sieve plate pores, sealing the sieve tube
callose + sieve element damage
long-term mechanism.
- glucose polymer, synthesized by PM-localized callose synthase. deposited outside PM.
callose is deposited around sieve pores in response to wounding and infection by pathogens. decrease pore diameter until blockage.
-reversible
average rate of phloem transport?
~1m/h
pressure-flow model of phloem transport
osmotically generated pressure gradient btw source + sink drives phloem transport.
- bulk flow is passive by positive hydrostatic pressure.
- pressure gradient established by phloem loading at source + phloem unloading at sink
where does phloem loading occur?
source.
- move photosynthates from source cells into sieve elements
increasing sugar concentration in sieve elements does what?
generates low solute potential = water potential declines + high water turgor pressure because water follows.
phloem unloading increases solute potential
phloem unloading
at sink. photosynthates from sieve elements into sink cells.
- decrease in solute concentration in sieve elements generates higher solute potentials. water potential of sieve elements increases and flows to xylem
post-unloading
water diffuses from sieve elements back to xylem.
down water potential gradient
- decreased turgor pressure at site of phloem unloading
phloem sap movement due to?
bulk flow in response to pressure differential created by phloem loading/unloading.
- water+ solutes move together thru phloem.
- phloem sap occurs passively without direct input of energy
bulk flow in phloem
driven by pressure gradients, whereas transport of water across membranes driven by water potential gradients.
- transport within sieve tubes occurs in continuous symplastic space, cell-to-cell via sieve plates
direction of movement in single sieve element
no bidirectional movement. only move in one direction at any moment in time.
- bidrectional transport can occur through diff bundles, or diff sieve tubes within same bundle.
phloem loading
photosynthates move from source cells through symplast via plasmodesmata by diffusion to reach phloem.
- phloem loading occurs when sugars are transported into sieve elements and companion cells
phloem loading via apoplast or symplast
-symplast allows for transport directly into SE-CC (sieve element-companion cell complex)
- apoplastic pathway involves export of sucrose into cell wall apoplast and movement via apoplastic pathway into SE-CC
- many species use both
apoplastic phloem loading requires?
metabolic energy
- sucrose efflux occurs from phloem parenchyma to apoplast near SE-CC.
= against concentration gradient= requires active transport.
-respiratory inhibitors decrease ATP concentrations and inhibit loading of apoplastic sucrose
sucrose-H+ symporters in apoplastic phloem loading
sucrose across PM,
PM H+-ATPase establishes PMF to drive cotransport.
- apoplastic loading is ATP dependent.
sucrose-H+ symporter found along veins in leaves
how are raffinose + stachyose transported?
in symplastic phloem loading
what is polymer-trapping model
sucrose in sheath cell diffuses to intermediary cell.
- polymer synthesized from sucrose in intermediary cells
- larger polymers only diffuse into sieve elements. no sucrose in intermediary drives sucrose to continue diffuse down to intermediary cells.
passive symplastic loading
rely on high sugar concentrations to drive movement rom mesophyll cells to sieve elements via diffusion
sink regulation of phloem loading
sink demand regulates phloem loading when photosynthate flow is not limited by source.
- alteration of sink demand immediately e effects photosynthate loading.
- apoplastic phloem loading capacity determined by sucrose - H+ symptorter abundance. - decrease trxn = less sucrose loading.
import of photosynthates into sink cells involves:
- phloem unloading: removing sugar from sap, increase water potential, water leaves
- short-distance transport
- storage or metabolism of sugars in sink cells
symplastic phloem unloading + short-distance transport
- always happeneing bc sucrose being degraded/utilized.
most common.
young, developing leaves + root tips - invovles sucrose diffusion followed by utilization of sucrose in sink cells
apoplastic unloading + short-distance transport
discontinuous symplastic pathway.
- apoplastic at times where developmental stages of sink accumulate high concentrations of sugars.
using apoplastic vs symplastic unloading?
apoplastic - when trying to accumulate sugar ( fruit, seeeds, storagE)
symplastic - when sink is consuming sugar quickly
nitrogen assimilation?
plants incorporate inorganic nutrients into carbon contituents necessary for growth + development.
- requires large energy inputs.
- need biologiccal nitrogen fixation to get bioavailable N.
main reservoir of N is?
atmosphere N2.
hard to convert to NH4+ or NO3-. need bbiological nitrogen fixation
plant forms of N?
NH4+ or NO3-
- most abundant mineral element in plants (other than CHO)
why is N important limiting nutrient?
deficiency is v common.
sympotms: inhibits plant growth; chlorosis of older leaves
N cycle
atmospheric N fixation = natural, lightning + photochemical reactions. rains onto ground.
biological N fixation
- most. bacteria + cyanobacteria convert N2 and NH3 to NH4+
organic nitrogen -> NH4+ by ammonification
NH4+ oxidied to NO3- by nitrification
denitrification: No3- to N2.
plant transport for NH4+ and NO3- uptake
rapidly take up nitrogen in these forms..
No3- transporters, 2 forms. high affinity and low affinity
how can NH4+ and NO3- be used?
primary assimilation - incorporated into organic compounds.
discuss nitrate assimilation
reduction to nitrite by nitrate reducatase in cytosol.
- requires NADH - costly. donates 2e-.
regulation of nitrate reductase
NR is active in light. light upregulates txn.
increased light, nitrate + carbs = NR synthesis + activation.
- inactive in dark. NR phosphorylated, inactivated eventually degraded.
- quick regulation of NR by changing phosphorylation/dephosphorylation status.
nitrite - bad news why?
reactive/toxic.
- transported to chloroplasts + plastids.
reduced to ammonium by nitrite reductase.
-reduced ferredoxin from photosynthetic light reaction used as electron donor.
NR contains?
iron-sulfur cluster + specialied heme. components work together to bind nitrite and transfer electrons.
increase light or increase [NO3-} induce NR expression.
energetics of N assiilation
energy-requiring.
coupled to photosynthetic e- transport, which generates powerful reducing agents.
No3- reduction to NH4+ consumes 35% of total energy expenditure in roots + shoots
why bother with NO3- if energy consuming. ? why not just take in NH4+
NH4+ uptake requires less energy, and produced by bacteroids during biological N fixation
- nitrifying bacteria oxidize NH4+ to NO2- then to NO3-
- NH4+ is toxic to plants + animals
what does ammonium do in plants?
dissipates transmembrane H+ gradients - diminishes PMF
- de-protonates at high/neutral oH
- uncharged NH3 is membrane permeable
- NH3 accumulates in low pH compartments + is protonated again. - diminishes OH- and H+.
how plants overocme NH4+ toxicity?
assimilating NH4+ near site of uptake from soil or bacteroids.
- or store nitrate and transport to xylem when needed (only then converted to NH4+
Primary assimilation of NH4+
to avoid NH4+ toxicity. NH4+ => aa via GS- GOGAT pathway.
two enzymes in GS-GOGAT
- GS: glutamine synthetase. NH4+ and glutamate = glutamine.
2. glutamate synthase (GOGAT) : tarnsfer 2 amide groups from glutamine to 2-oxyoglutarate = 2 glutamate
GS isoforms
cytosolic GS
- more prominent in roots.
- major role in primary assimilation of NH4+ from soil
- vascular tissue
- chloroplast/plastic GS: primary assimilation. present in green tissues. reassimilates NH3 lost during photorespiration
GS-GOGAT PATH + energy
required energy (ATP) and electrons (NADH or Fd)
- > nadh: non-photosynthetic tissue. active in assimilation of NH4+
- > Fd : in chloroplast of leaves.reassimilating NH3 lost during photorespiration
ammonium asimilation
glutamate dehydrogenase (GDH)
- reversible reaction, cannot assimilate NH4+as substitute for GS-GOGAT path.
- reallocation of N during senscence
Nitrogen recycled in plant
during senescence, protein broken down + aa converted to N-rich aa for long-distance transport.
asparagine is key compound for N transport + storage. same as glutamine + glutamate.
aa acids go to actively growing tissue. synthesize storage proteins
storage protein broken down for aa gain in Spring
biological N fixation accounts for how much NH3 fixed? how?
90%
- free-living prokaryotes.
N-fixation genes spread among bacteria by HGT
-symbiotic associations with specific taxa of plants
= rhizobia: symbiosis with legumes.
N-fixing symbiotic associations exchange?
N for carbon (carbs and other nutrients)
- fixation is energetically expensive
reduction of N2 to NH3
catalyzed by nitrogenase enzyme complex.
-> requires 16ATP and 8 e- to reduce one N2 molecule to 2NH3 and 1H2.
two components of nitrogenase enzyme complex
ferredoxin is e- donor to Fe protein which hydrolyes ATP, reducing MoFe protei
MoFe reduces N2 to NH3.
-legumes particularly sensitive to iron deficiency
Nitrogenase enzymatic function and O2.
O2 strong electron acceptor, can damage e- exchange sites of both nitrogenase subunits
-> O2 irreversibly inactivates nitrogenase
Nitrogenase enzymatic function - fixed under what kind of conditions?
anaerobic conditions.
- free-living bacteria fix N2 in anaerobic soils.
- nodules are specialized organs of plant host that contain symbiotic N-fixing bacteria
= nodules are low-O2 enviro for n-fixing. , provide microbe-rich, highly competitive rhizosphere
how nodule protect rhizobial nitrogenase from O2 (4)
- layer of lignin-rich sclerenchyma cells is barrier to O2 diffusion to nodule parenchyma
- centrally located in rhizobia differentiate into bacteroids that are enclosed in plasma membrane-derived vesicles (symbiosomes) within root cells
- high respiratory rates of rhizobia use O2
- Nodules synthesize leghemoglobins (high affinity O2 binding) most abundant proteins in nodules. localized to cytoplasm of nodule cells hosting symbiosomes -> facilitates diffusion of O2 to the respiring symbiotic bacteroids
nodule development steps
- attraction and colonization
- root hair curling + invasion of rhizobia
- nodule initiation + development in cortex
- release and differentiation of rhizobia
attraction and colonization of nodule development
- develop root nodules due to chemical signaling btw rhizobia + root cells.
- free-living rhizobia migrate to roots
= attracted by flavanoids - signalling molecule bind to rhizobial nodulation gene (nodD) inside rhizobia. - nod factors are glucosamine polymers with fatty acyl chains attached.
what is NodD protein?
transcription factor that induces expression of other rhizobial nod genes that synthesie Nod factors that act as specific singalling molecule for symbiosis
what are Nod factors
lipochitin olirgosaccharides (glucosamine polymers w fatty acyl chains) signal molecules that regulate host gene expression during nodule formation
how are nod factors recognized by host?
PM bound Nod factor receptors
- host specificity = root hair curling
consequence of nod factors stimulating root growth?
increased cell division
increased root hair production
development of shorter + thicker roots
induce root hair curling - envelopes rhizobia bacteria
nod factors + cell wall (root hari curling)
local cell wall degradation - rhizobia access to root hair plasma membrane.
- infection thread: continuous w original PM and extends t to adjacent PM. rhizobia invades root cortical cells. - not into cytoplasm tho.
- infection thread fuses with PM releasing rhizobia into apoplast. rhizobia create new branched infection threads in subepidermal cells to form continuous channel
nodule initiation + development in cortex
- infection elongates through root tissue towrads nodule primordim.
- as primordium grows, more connections allow for exchange of C and N
release and differentiation of rhizobia
infection thread buds off into vesicles containing rhizobia (symbiosomes) release into cytosol of host
rhizobia grow larger + differentiate into N2-fixing
nodule development sumary
- root emits chemical signal, stimulate root hair elongation, rhizobium aggregates + invaginates into PM
- infection containing bacteria penetrates root cortex. start N-fixing
- growth continues in cortex + pericycle. use = nodule.
- nodule develops vascular tissue, supplies nutrients + carries N compound for distn around plant
- mature nodule grows. lignin rich sclerenchyma forms, reducing absorption of O2 and maintain anaerobic enviro
rhizobial nitrogenase complex does what?
N2 reduced to NH3. NH4+ exported from nodules rapidly assimilated by GS-GOGAT path
