Photosynthesis II Flashcards
___% plants use C3?
~95
(all woody trees, temperate crop species)
___ first product of fixation by ___ (catalyzed) - calvin cycle
3-PGA
RubisCO
C3: what kind of cell do light-dependent/independent reactions occur in?
mesophyll cells
no chloroplasts in bundle sheath cells, parenchyma used for conduction
sugar transport C3 leaf
transport sugars (mesophyll-made) -> veins -> down to roots for storage
sugar transport: Phloem (C3)
movement of sugar and other organic molecules can either be symplastic (and transmembrane) or apoplastic
sugar transport: proton pumps
- energy required to move sucrose from apoplasm to symplasm of sieve and companion cells
- a H+ gradient, initially started by proton pump, is used to co-transport one sucrose with every H+
sugar transport: source to sink
sugar source is plant part that makes sugars (leaves, green stems)
sugar sink is the part that mainly consumes or stores sugars (roots, stems, fruits)
sink in summer can become source in winter (not always transported by phloem; maple syrup is xylem sap)
sugar transport: pressure-flow hypothesis steps (4)
- at source, sugar enters by active transport into sieve tubes. water potential of sieve tube decreases and water enters by osmosis
- turgor pressure generated moves water and sugar down sieve tube toward sink
- sugar actively unloaded at sink, water exits sieve tubes, lowering water pressure in sieve tubes
- water diffuses back into xylem
active transport (symplastic) required, so sieve tubes and companion cells being “alive”
why is efficiency of fixing carbon in C3 low?
Rubisco (oxygenase)
photorespiration: RubisCO functions as an oxygenase, adds oxygen from O2
- 2-PG produced can be eventually broken down by mitochondria to release CO2 (NO ATP MADE)
what is “gas exchange catch 22”? (temp increase, what happens?)
warm temp, higher transpiration
- stomata close
- conserves water but CO2 locked out and O2 trapped in
- photorespiration rates increase
C4 fixation occurs in ___ regions; mostly ___ plants wth some cold-tolerant ___ and tropical _____
warm/arid
grass
shrubs
angiosperms
(~3% of flowering plants)
most C4 plants have a distinctive leaf anatomy called ____ ______
Kranz Anatomy
what is kranz anatomy?
spatial separation of C4 pathways (mesophyll) and calvin cycle (bundle sheath)
RubisCO in bundle sheath, mesophyll has no RubisCO
four-carbon (C4) pathway: spatial separation steps (5):
first product of Co2-fixation is 4-carbon oxaloacetate (OAA)
- OAA formed when CO2 fixed to phosphoenolpyruvate (PEP) by PEP carboxylase (PEPC) in chloroplast stroma of mesophyll
- OAA converted to malate (or aspartate) and move to bundle sheath through symplast (plasmodesmata)
- once inside bundle sheath, malate decarboxylated to yield CO2 and pyruvate in chloroplast stroma
- released CO2 enters calvin cycle
- pyruvate recycles back to mesophyll where it is phosphorylated to regenerate PEP
C4 advantages
- allow bundle sheaths to maintain high CO2 conc, favouring binding of RubisCO to CO2 not O2
- PEP carboxylase has high affinity for hydrate form of CO2 (bicarbonate) and not affected by O2
- CO2 released by photorespiration can be re-fixed by C4 pathway
- predominate in hot, dry climates
C4 vs C3
- optimal temp for c4 higher
- same photosynthetic rate in c4 with small stomata openings, less water loss
- c4 have 3-6X less RubisCO
- overall use nitrogen + H2O more efficiently
C4 Pathway: single-cell C4 system
- kranz anatomy not required
- several known Chenopodiaceae plant species achieve spatial separation in a single cell type:
– peripheral chloroplast compartments (PCC; C4 fixation) and central chloroplast compartments (CCC; calvin cycle, c3 fixation)
C4 Pathway: Temporal Separation Steps + detail
threatened by excessive water loss
- some plants take up CO2 and perform C4 pathway at night, then carry out Cavin cycle during daytime (temporal separation)
- allowing malic acid to accumulate in vacuole
- during day, these organic acids are used to supply co2 to calvin cycle
- cam plants can keep stomata closed while using stored CO2 pools for photosynthesis
CO2 fixed at NIGHT with open stomata
- 1. starch from chloroplast broken down as far as PEP
- 2. HCO3- reacts with PEP to make oxaloacetate, then reduced to malate
- 3. malate stored as malic acid in vacuole
DAYTIME with closed stomata:
- 4. malic acid decarboxylated to make CO2 and pyruvate
- 5. CO2 enters calvin cycle, refixed by RubisCO
- 6. stomata closure prevents water & CO2 loss (when malate is decarboxylated)
what does CAM stand for?
crassulacean acid metabolism
Crassulacean Acid Metabolism (CAM)
- temporal separation of CO2 fixation and calvin cycle first discovered in crassulaceae (hence name)
- in vascular plants, cam more widespread than spatial C4 fixation (~5%)
- CAM evolved independently in many succulents
- also some gymnosperms
- ability to fix CO2 in dark through PEP carboxylase in cytosol = defining feature
Carbon Fixation: evolutionary innovations
- improving light harvest by adding another photosystem and varying pigments
- improving energy generation by maintaining potential for cyclic electron flow
- improving co2 fixation:
- spatial separation (C4)
- temporal separation (CAM); storing intermediates for later use
