Photosynthesis
Leaves
Parts:
Monocot: blade + sheath
Tissues:
Dicot: blade + petiole
epidermis
epidermal cells
guard cells/stomata
Palisade Parenchyma
Spongy mesophyll
Vascular tissue
Chloroplast
site of photosynthesis
Photosynthetic Electron Transport (PET)
C3 photosynthetic carbon cycle (CO2-->sugar)
Dual membrane system
inner and outer envelopes that regulate metabolite flux in and out of the chloroplast
Thylakoids contain PET and are impermeable to H+
Stroma: space between envelope and thylakoids that contains C3 photosynthetic enzymes
Stromules: tubules that connect stroma of chloroplasts and permit exchange of small metabolites
Semiautonomy
endosymbiotic theory
derived from primitive cyanobacterium
dual membrane system
maternal inheritance pattern
Prokaryotic Features
circular DNA chromosome that may be linear but branched
divides by fission
replicates independently of nucleus, with the number within a cell type remaining constant and with some coordination with the nucleus
70S ribosomes for protein synthesis, inhibited by CAP
Genome
contains gene expression machinery (DNA, RNA, Ribosomes 70S)
Contains limited number of genes
tRNAs
ribosome components (rRNAs, ribosomal proteins)
some PET components
most proteins important from cytosol, whose genes are encoded in the nucleus
Ploidy Levels
ctDNA + mtDNA = 1/3 Nuclear DNA
Chloroplast genome increases polyploidy with maturity
immature leaf: mtDNA > ctDNA
Propagation
mature leaf: ctDNA > mtDNA
Divide by Binary Fission
ctDNA replication and fission indpendent of nuclear DNA replication and cell division
Require nuclear encoded proteins to divide
requires light
Development in seedlings
initiated from proplastids
inherited maternally via egg cell
light-dependent process
morphogenesis
chlorophyll biosynthesis
Proplastid phenotype:
few (if any) internal membranes
no chlorophyll
incomplete complement of photosynthetic enzymes
found in meristematic cells
Conversion to chloroplasts:
requires light
rapid enzyme formation from chloroplast and nucleus genes
Formation of light-absorbing pigments (chlorophylls, carotenoids)
Rapid membrane proliferation (stromal lamellae, grana stacks)
Conversion to etioplasts:
occurs in seedlings maintained in dark
prolamellar bodies formed (semicrystalline tubular arrays)
accumulation of protochlorophyllide (pale yellow-green pigment, precursor of chlorophyll)
Etioplast conversion to Chloroplast
occurs within minutes in light
prolamellar bodies become thyllakoid and stroma lamellae
protochlorophyllide becomes chlorophyll
Biosynthesis
FAMILIARIZE BUT DON'T MEMORIZE
Chlorophyll Breakdown process--FAMILIARIZE
Photosynthesis
light reactions
absorption of sunlight by pigments
light converted into chemical energy (ATP, NADPH)
primary source of O2 in atmosphere
carbon reactions
CO2 fixed into organic compounds
requires energy from light reactions (ATP, NADPH)
primary source of biomass in biosphere
primary energy input into biosphere
light and carbon reactions coordinated via gene expression, energy transfer, and regulation of enzyme activities
Properties of light
MEMORIZE OVERALL RXN
Solar Energy
dual nature: a particle and a wave
photon (particle nature)
each photon carries a discrete amount of energy that acts as a function of its wave properties (frequency and wavelength)
1 Einstein = 1 mole photons
Electromagnetic wave properties
E = hv
defined by wavelength and amplitude
speed of light constant: 3 x 10^8 m s-1; related to wavelength and frequency
Colors make up white light or visible spectrum
Red region has longest wavelengths and least energy
Blue region has shortest wavelengths and most energy
Photosynthetically active radiation: abbr = PAR, wavelengths = 400-700 nm
MEMORIZE LIGHT PROPERTY EQNS
Photosynthetically Active Radiation
light is derived from the sun
wide range of wavelengths
not all wavelengths reach earth's surface
plant pigments
unique, overlapping absorption spectra
small fraction of solar output absorbed
Chl a has only 2 major absorption peaks: 400 nm and 700 nm
harvest specific light wavelengths reaching earth's surface
solar energy to chemical energy, with carbohydrates being the storage form of chemical energy
most light is not absorbed, as it is out of the pigment range
reduction in absorption efficiency via reflection and transmission, heat dissipation, and metabolism
PAR
400-700 nm wavelengths
85%-90% absorbed by plant
green: 500-600 nm not absorbed as well, therefore green plants
far-red/infrared wavelengths (>700) not absorbed well
Blue and red light absorbed by chloroplasts in upper layers of leaf, where they are effieciently absorbed and used for photosynthesis
Green light penetrates to deeper layers of leaf, as it is less efficiently absorbed by chloroplasts and is able to drive photosynthesis
Leaf anatomy and Light Absorption
leaf anatomy is designed for maximum light absorption
epidermis
transparent cells without pigments
convex shape focus light like lens
increases internal light intensity several fold
important in shade species
Palisade mesophyll (paranchyma)
columns of closely packed cells
sieve effect
only chloroplasts absorb light
chloroplasts not uniformly distributed
gaps in light absorption
more light absorbed by equal concentration of chlorophyll in solution than by chloroplasts
light channeling
light transmitted via intracellular spaces, cell walls, and central vacuole to cells below
no chloroplasts in these structures
Spongy mesophyll cells
irregular shaped cells around large air spaces
reflects light within leaf, resulting in light scattering and a path of light 4x leaf thickness
results in more uniform light absorption across entire leaf
Leaf anatomy may be modified to reduce light absorption
high light environments such as deserts
potentially dangerous light levels
modifications to reflect light such as hairs, salt glands, and epicuticular wax
decrease light absorption by 40%
Chloroplast movement regulates light absorption
low light
chloroplasts align along cell surface parallel to plane of leaf lamina
perpendicular to incident light
High light
chloroplasts align along sides of cell
chloroplasts shade each other
reduces light absorption about 15%
Move along cytoskeleton
usually respond to blue light
phototropins (PHOT) are blue light receptors
PHOT activates protein kinase and induces a rapid change in cytoskeleton
some species respond to red light using phytochromes
Leaf Movement and Light Absorption
Chloropast movement regulates light absorption
in low light, chloroplasts align along cell surface parallel to plane of leaf lamina so they are perpendicular to incident light
in high light, chloroplasts align along sides of the cell and shade one another to reduce light absorption about 15%
chloroplasts move along cytoskeleton, usually responding to blue light
MEMORIZE: PHOT INDUCING CHANGES IN BLUE LIGHT
leaf movement regulates light absorption, with maximum absorption occurring when lamina are perpendicular to incident light
solar tracking is the adjustment of lamina to be perpendicular to incident light, with lamina being within 15 degrees of perpendicular and moving up to 90 degrees per hour
heliotropism
diaheliotropic
paraheliotropic
maintains maximal light harvesting throughout day, maximizes photosynthesis when water loss is lower
nyctinasty
MEMORIZE MECHANISM
Water stress affects leaf movements
Light saturation and Growth Conditions
competition for light occurs when little light gets through the canopy
light saturation occurs lower in shade vs sun plants
light saturation depends on growth conditions; leaves adapted to lower light levels saturate more quickly
the whole plant canopy is rarely saturated
Light Absorption
MEMORIZE LIGHT RESPONSE CURVE
the more light there is, the faster photosynthesis occurs in the exponential phase
CO2 fixation limits photosynthesis in the saturation phase
point at which the rate of photosynthesis and respiration are equal
Excited Energy Absorption
light is absorbed by isolated pigments
this excites chlorophyll a
red and blue wavelengths absorbed
blue: shorter wavelength, high energy, high excited state
red: long wavelength, low energy, low excited state
MEMORIZE MECHANISM
fates of excitation energy include heat, fluorescence, resonance energy transfer, and photochemistry
FAMILIARIZE WITH PATHWAY
Quantum Yield
Photosynthetic Pigments
Chl a and Chl b are present in higher plants, and all photosynthetic organisms have carotenoids
Chlorophylls
Carotenoids
FAMILIARIZE WITH TRAITS
Phycobiliproteins
FAMILIARIZE WITH TRAITS
FAMILIARIZE WITH TRAITS AND VARIATIONS
Absorption spectrum is the light absorbed by chlorophyll
Each pigment has a unique absorption spectrum that overlaps with that of other pigments
action spectrum is the light that drives photosynthetic O2 production
Light Harvesting and Energy Transfer to Reaction Centers
Light harvesting complex (LHC) is a protein complex bound to antenna pigments that transfers excitation energy by resonance to RC
MEMORIZE PATHWAY
Reaction center (RC) is a special chl a pair coordinated with photosystem proteins that recieves excitation energy
Light Harvesting complex II (LHCII)
Light harvesting complex I (LHCI)
Reaction Center Redox Properties
good reducing agents have a strongly negative redox potential and donate elections to another molecule~
Good oxidizing agents have positive redox potential and accept electrons from another molecule
Energy is captured when excited chl transfers e-
MEMORIZE MECHANISM
Photosynthetic Electron Transport (PET)
Cytochrome b6f complex
connects PSII with PSI
accepts 2 e- from PQ, directing one to PSI and one to the Q-cycle
generates H+ gradient for ATP synthesis
multisubunit complex with several prosthetic groups
redox centers in cyt b6f complex include b-type cytochrome, c-type cytochromes, and Reiske Fe-S protein
FAMILIARIZE WITH Q-CYCLE
Plastocyanin (PC)`
small water soluble molecule in thylakoid lumen that carries one e- from cyt b6f complex to PSI after PSI loses e- following absorption of a photon
analogous to cyt c in MET
Two Photosystems in Plants
Photosystem I (PSI, P700)
lost e- replaced with e- from plastocyanin
MEMORIZE PATHWAY ILLUSTRATION, STEPS, AND PARTS P73-98, MAKE UNIQUE WEB
a proton-pigment complex including chl a
very strong reducing agent; very unstable
light absorption by PSI initiates e- transfer
transfers e-to ferredoxin (Fd)
Fd transfers e- to multiple molecules
most e- used to produce NADPH for C3 carbon reactions
some e- used to reduce thioredoxin to regulate various enzyme activities in chloroplast
some e- used to convert NO2 to NH4 for incorporation into organic molecules
can operate in cyclic mode
MEMORIZE PATHWAY
Found in bundle sheath of C4 plants
heterocysts of cyanobacteria
Photophosphorylation
ATP synthesis in chloroplast via chemiosmosis
catalyzed by ATP synthase
uses H+ gradient formed by PET
process determined by Jagendorf experiment: generation of electrochemical gradient in the dark without PET activity
Proton Motive Force
MEMORIZE EQNS
MEMORIZE PROCESS
antennae pigments funnel energy to RC like tuning forks, and red shift toward RC traps energy flow in 1 direction, promoting movement from carotenoids to chl b to chl a and inhibiting movement in the opposite direction
FAMILIARIZE P62 DIAGRAM
energy of excited RC used for phytochemistry
chlorophyll a/b antenna proteins are members of a large family of structurally related proteins
associated primarily with PSII
trimeric transmembrane complex, with each monomer containing three alpha helices spanning the thylakoid membrane and 14 chl a and chl b molecules plus four carotenoid molecules
associated primarily with PSI and structurally similar to LHCII
MEMORIZE QUANTUM YIELD EQN
Emerson Red drop effect: quantum yeield constant across spectrum until far-red light due to overlapping absorption spectra of carotenoids, chl a, and chl b
FAMILIARIZE W P66 FLASH ENERGY V O2
MEMORIZE ENERGETICS OF PHOTOSYNTHESIS P67
PET Measurements
spectroscopy measures absorbance of sample at selected wavelength
spectrofluorometry measures fluorescence of sample by excitation wavelength and emitter wavelength and is a more sensitive method than spectrophotometry
O2 Electrode
MEMORIZE MEASUREMENT TYPES P68-70
Hill Reaction
oxygenic photosynthesis
series of redox reactions
isolated chloroplasts oxidize and reduce many compounds inc. electron donors and electron acceptors and this is used to dissect PET
use O2 electrode to monitor oxygen evolution
use spectroscopy to monitor redox state of electron donors/acceptors
Emerson enhancement effect: red and far-light are synergistic on photosynthesis due to presence of two photosystems; green light has no effect or is inhibitory on cytochrome oxidation (PET)
transforms far-red light into charge separation
absorbs maximally at 700 nm (far-red)
was experimentally isolated first, hence its name, but occurs second in pathway
genes that make up complex designated psaX
Photosystem II (PSII)
absorbs maximally at 680 nm (red light)
was isolated experimentally second but occurs first in pathway
Genes that make up complex designated psbX
Z Scheme is seen when PET is arranged by redox potential
MEMORIZE PATHWAY P73
PET generates negative change in proton motive force across thylakoid membrane
H+ translocated from stroma to thylakoid lumen
water-splitting reaction in thylakoid lumen
Dissipation of electrochemical gradient occurs through ATP synthase
Change in pH is main component of change in proton motive force in chloroplasts
Chloroplast ATP Synthase
CFo-CF1 ATP Synthase is a multimeric enzyme complex
CFo: hydrophobic core
CF1 subunit: hydrophilic catalytic subunits
peripheral membrane protein complex on stromal side of thylakoid with several protein subunits and regulatory units
forms channel that H+ pass through
Chemiosmosis
MEMORIZE MECHANISM P102
MEMORIZE MECHANISM P101
Herbicide inhibition of PET
some herbicides kill plants by blocking PET
DCMU binds to QA site of PSII and prevents e- transfer to PQ
Paraquat binds to reducing side of PSI and prevents e- transfer to Fd
Excess Light leads to Photoinhibition
photoinhibition can occur in form of reduction in photosynthesis due to excess light energy
dynamic photoinhibition occurs even under optimum growth conditions and is used to dissipate excess energy as a short term response
Chronic photoinhibition occurs under stress conditions as photosystem damage as a long-term response
Photosystem Protection and Repair
photooxidation occurs through highly reducing reactions leading to instability and generate reactive oxygen species
carotenoids vent excess energy, prevent singlet oxygen, and radiate heat
regulation of energy flow between PSI and PSII
xanthophylls dissipate energy
PSII easily damaged via photoinhibition, with D1 being the main target and needing to be resynthesized
in PSI, Fd forms superoxides
FAMILIARIZE WITH P105 CHART
moderate excess of light energy
quantum efficiency decreased
maximum photosynthesis unchanged
overload of protective mechanisms
occurs at mid-day
quantum efficiency decreased
maximum photosynthesis decreased
overload of protective mechanism and photochemical reactions
MEMORIZE PATHWAY AND CONVERSIONS P110-111
Photosynthesis and the Environment
Temperature
C3 plants more sensitive to temperature than C4 plants
Leaves dissipate excess heat
long-wave radiation
sensible heat loss via conduction/convection of heat to cool air
evaporative heat loss (latent) via evaporative cooling and transpiration
Bowen Ratio
Bowen Ratio = sensible heat loss / evaporative heat loss
MEMORIZE
in well-watered plants, increased transpiration = decreased Bowen ratio
on a still day, decreased sensible heat loss means decreased bowen ratio
approaches infinity in desert plants
may be negative--cotton leaves cooled below air temp by evaporation, resulting in sensible heat flow into leaf
C3 Photosynthesis
MEMORIZE REGULATION AND SEPCIFIC ENZYMES, NOT MECHANISM
Rubisco
a multimeric enzyme complex
holoenzyme assembled in chloroplast, folded by chaperonins
40% soluble protein in leaves
rbcS encoded in nucleus
rbcL encoded in chloroplast
Variations: L2 in non-sulfer bacteria and dinoflagellates; some brown, red, and green algae encode both rbcL and rbcS
Rubisco is a dual-function enzyme: used in both C3 photosynthetic cycle and photorespiratory cycle
Rubisco CO2 Fixation
rubisco catalyzes CO2 fixation with product being 3-PGA in C3 photosynthetic pathway
FAMILIARIZE WITH BASIC CYCLE
CO2 concentration in solution relative to O2 is important
increased temp leads to decreased CO2/O2
rubsico active sites about 4 mM in stroma
carboxylation favored, but O2 competes and reduces net photosynthesis
CO2 concentrating mechanism deals with O2 problem
MEMORIZE RUBISCO PATHWAY/DETAILS
Stages
- Carboxylation stage
CO2 fixation yields two 3-PGAs
Initial fixation product is C3 acid
catalyzed by rubisco
- Reduction Stage
reduce 3-PGA to triose phosphate (TP)
C3 acid becomes C3 carbohydrate
Main site of NADPH and ATP use
- Regeneration Stage
regenerate RuBP from TP
synthesis of C5 sugar-P
utilizes ATP
TP is major branch point
Reductive phase of CO2 fixation
TP acts as branch point in photosynthesis
sucrose in cytoplasm
starch in chloroplast
reduction of 3-PGA to TP
NADP-GAPDH is an isozyme similar to glycolysis except for its cofactor
regenerates RuBP
Regulation of C3 Pathway via Specific Enzymes
Ionic Regulation
Regulation of rubisco
Rubsico Activase
removes bound substrate before carbamylation
carbamylation
removes carboxyarabinitol-1-P (natural inhibitor that binds in dark and is removed in light)
activation step requiring CO2 binding
uses non-catalytic CO2
MEMORIZE ALL
MEMORIZE DIAGRAM P145
Ionic movements
light induces H+ pumps (stromal pH goes from 7 to 8)
pH alters Mg2+ distribution
Enzymes that require Mg2+ for catalysis more active at alkaline pH, including rubisco and others
Ionic regulation
final activation state requires Mg2+ complex
optimum activity at alkaline pH
Thioredoxin-Mediated Regulation
thioredoxin coordinates PET activity with other pathways or processes
mechanism of action:
PSI reduces Fd via PET
Fd-thioredoxin reductase
transfers electrons and reduces disulfide bridge to dithiols
Thioredoxin reduces target enzyme by altering enzyme activyt or sensitivity to allosteric effectors
dithiols on enzyme oxidized by O2
Light-regulated C3 Enzymes
MEMORIZE, MEMORIZE, MEMORIZE FOUR ENZYMES REGULATED P150
glyceraldehyde-3-phosphate dehydrogenase
Phospho-ribulokinase
Fructose-1,6-Bisphosphatase
Sedoheptulose-1,7-Bisphosphatase
Thioredoxin-Inactivated Enzymes
Supercomplex Formation
complex formation regulated by ratio of oxidized and reduced thioredoxin
the more light there is, the more reduced thioredoxin is present
increased oxidized thioredoxin results in oxidization of CP12 and creation of complexes that bind the enzymes PRK and G3PDH to inactivate them
increased reduced thioredoxin dissociates CP12 complex to allow PRK and G3PDH to become active
inhibits enzyme G6PDH, thereby inhibiting the oxidative pentose phosphate pathway
oxidative pentose phosphate pathway changes NADP+ to NADPH in chloroplast
thermal inhibition of photosynthesis impacts protein stability, membrane stability, and water relations
MEMORIZE P154 CHARACTERISTICS OF LIMITATIONS TO PHOTOSYNTHESIS RATE
control rubisco activity and RuBP regeneration
CO2 fixation versus temperature
high CO2: rubisco saturated; limited by light reactions
ambient CO2: rubisco limits
at low temperature, photosynthesis Pi limited
sucrose and starch synthesis strongly inhibited by low temperature
carotenoid system dissipates excess light energy via conversion among epoxidation states
Epoxidation occurs when NADPH transfers e- to xanthophylls
de-epoxidation occurs when xanthophylls transfer e- to H2O via ascorbate
isoprene emissions related to heat dissipation
gives forest odor and blue haze
transgenic plants without isoprene emission more heat sensitive
Photosynthetic Carbon Metabolism
Photosynthetic CO2 fixation
C3 Photosynthesis
Calvin-Benson-Bassham cycle
CO2 concentrating mechanisms
Includes:
C4 photosynthesis in angioperms in hot environments
C4 Photosynthesis
C4 Leaf Anatomy
plasmodesmata connect mesophyll and bundle sheath cnells
mesophyll cells only 2-3 cells from bundle sheath cell
Overall Pathway:
C4 acid are first detectable intermediates (malate, aspartate)
PEPCase carries out CO2 fixation
Three flavors of C4 metabolism:
NADP-ME pathway
PEP Carboxykinase pathway
MEMORIZE REGULATION AND REDUCING AGENT FOR EACH
no taxonomic relationship between types of C4 photosynthesis; has arisen via convergent evolution 62 times
Energetics: one additional ATP required per CO2 fixed
reduces PR loss:
PEPCase has high affinity for HCO3-
O2 does not compete with HCO3-
Permits reduced stomatal aperature, reducing H2O loss and permitting growth at higher temperatures
MEMORIZE OVERALL PATHWAY 165
regulated by PEP carboxylase, which is activated by protein phosphorylation
regulated by pyruvate-Pi dikinase, which is controlled by adenylate energy charge
regulated by malate dehydrogenase, which is activated by thioredoxin system (light)
NAD-ME pathway
regulated by pyruvate-Pi dikinase, which is controlled by adenylate energy charge
regulated by PEP carboxylase, which is activated by protein phosphorylation
regulated by pyruvate-Pi dikinase, which is controlled by adenylate energy charge and whose regulation is mitigated by PEP transport
C4 plants are more efficient than C3 plants
respiration increases with temperature, rather than decreasing like in C3 plants
C4 mechanism traps and recaptures respired CO2, so quantum yield is stable with increasing temperature
Reductive pentose phosphate (RPP) cycle
Photosynthetic carbon reduction (PCR) cycle
primary source of earth's biomass
200B tons of CO2 fixed per year
40% fixed by phytoplankton
located in stroma of chloroplast and coupled to light reactions involving ATP, NADPH, and Fd
Assisted by C4 and CAM pathways
Atmospheric CO2
is rising
C3 plants grow better at elevated CO2 due to lower photorespiration
CO2 enrichment used in greenhouses, with nutrients and light optimized
CO2 Diffusion into leaves
photosynthesis requires diffusion of CO2 into leaf to rubisco, with cuticle preventing CO2 diffusion and stomata regulating CO2 diffusion
water and CO2 move in opposite directions in same pathway
Photosynthetic CO2 measurements
infrared gas analyzer
net photosynthesis
net CO2 fixation
net CO2 assimilation
CO2 response curve
FAMILIARIZE P121
CO2 compensation point where net photosynthesis = 0, with photosynthesis = respiration
Photorespiration (PR or PCO)
oxygenation of rubisco leads to loss of CO2
involves operation of three organelles, each using O2
increases with temperature
chloroplast
peroxisome
mitochondria
recovers 75% of carbon lost by oxygenation
regulated by PEP carboxylase, which is activated by protein phosphorylation
some plants exhibit little to no PR
works as add on to C3 photosynthesis
found in maize, sugar cane, sorghum, etc
CO2 pumps in aquatic plants
Bundle Sheath Cells
CAM photosynthesis in angiosperms in deserts
algae adapted to low CO2 do not photorespire
inorganic C actively transported into cell, driven by ATP derived from light reactions
effectively increases CO2 around Rubis
CO2 Fixation and Stable Isotopes
most stable carbon isotope: 12C, followed by 13C
12CO2 diffuses faster than 13CO2
Rubisco discriminates against 13CO2
PEPCase discriminates against 13CO2 to a small degree
improves water-use efficiency
CO2 fixation occurs at night and end of day to reduce water loss while fixing CO2 and allow stomates to close during high temperature
CAM idling recycles respired CO2 for months at a time with little H2O loss and slow growth
Pathway:
In the dark. open stomata permit CO2 entry and H2O loss; CO2 uptake and fixation; leaf acidification
In light, closed stomata prevent H2O loss and CO2 uptake; decarboxytlation of stored malate and refixation of internal CO2; deacidification of leaf
C4 mechanism both spatially and temporally separate from C3 photosynthesis
malate stored in vacuole at night
utilize C3 pathway during day
FAMILIARIZE WITH COMPARISON P192-193
carbon partitioning acts as the metabolic fate of photosynthetic products, with TP as a decision point
sucrose is synthesized directly from photosynthesis during light or synthesized from starch at night
starch is accumulated during the day and converted to sucrose during the night
use of triose-phosphates and triose-phosphate translocators
cytosolic F-1,6-P2 phosphatase (F-2,6-P2); cytosolic PP-F6P kinase (F-2,6-P2); F-1,6-P2; F-2,6-P2
FAMILIARIZE WITH P198-201
sucrose-phosphate synthase
allosterically regulated--activated by G6P and inhibited by Pi
protein phosphorylation--activated in light and inactivated in dark
chloroplastic F-1,6-P phosphatase interacts with thioredoxin system
ADP-Glucose pyrophosphorylase activated by thioredoxin
Trehalose-6-phosphate links leaf carbon metabolism in cytoplasm to starch synthesis, with positive correlation; trehalose synthesis controlled by levels of UDP-glucose and glucose-6-phosphate
starch is main storage carbohydrate, second only to cellulose in abundance; long linear chains of glucose units; packs into regular semi-crystalline arrays leading to insoluble granules
two main types
amylose
amylopectin
low molecular weight and low branching; relatively extended shape; synthesized by granule-bound starch synthase
high molecular weight, moderate branching; compact shape; synthesized by soluble starch synthase
Starch synthesis occurs in three steps:
- initiation
elongation
- termination
Starch degradation requires phosphorylation of starch granules
concerted action of glucan water dikinase and phosphoglucan water dikinase is essential to convert crystalline glucans to soluble maltodextrins at night
starch debranching enzymes such as pullulanases and isoamylases are essential for complete breakdown of starch granules
starch degradation products exported from chloroplast to cytoplasm
maltose exported to cytoplasm by MEX1 protein bc maltose metabolizing enzymes not active in chloroplast
glucose transported to cytoplasm by hexose transporter
MEMORIZE CO2 CURVE P121