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

  1. Carboxylation stage

CO2 fixation yields two 3-PGAs

Initial fixation product is C3 acid

catalyzed by rubisco

  1. Reduction Stage

reduce 3-PGA to triose phosphate (TP)

C3 acid becomes C3 carbohydrate

Main site of NADPH and ATP use

  1. 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:

  1. initiation

elongation

  1. 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