Photosynthesis II

Question 1

___% plants use C3?

Answer 1

~95
(all woody trees, temperate crop species)

Question 2

___ first product of fixation by ___ (catalyzed) - calvin cycle

Answer 2

3-PGA
RubisCO

Question 3

C3: what kind of cell do light-dependent/independent reactions occur in?

Answer 3

mesophyll cells

no chloroplasts in bundle sheath cells, parenchyma used for conduction

Question 4

sugar transport C3 leaf

Answer 4

transport sugars (mesophyll-made) -> veins -> down to roots for storage

Question 5

sugar transport: Phloem (C3)

Answer 5

movement of sugar and other organic molecules can either be symplastic (and transmembrane) or apoplastic

Question 6

sugar transport: proton pumps

Answer 6

• 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+

Question 7

sugar transport: source to sink

Answer 7

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)

Question 8

sugar transport: pressure-flow hypothesis steps (4)

Answer 8

1. at source, sugar enters by active transport into sieve tubes. water potential of sieve tube decreases and water enters by osmosis
2. turgor pressure generated moves water and sugar down sieve tube toward sink
3. sugar actively unloaded at sink, water exits sieve tubes, lowering water pressure in sieve tubes
4. water diffuses back into xylem

active transport (symplastic) required, so sieve tubes and companion cells being “alive”

Question 9

why is efficiency of fixing carbon in C3 low?

Answer 9

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)

Question 10

what is “gas exchange catch 22”? (temp increase, what happens?)

Answer 10

warm temp, higher transpiration
- stomata close
- conserves water but CO2 locked out and O2 trapped in
- photorespiration rates increase

Question 11

C4 fixation occurs in ___ regions; mostly ___ plants wth some cold-tolerant ___ and tropical _____

Answer 11

warm/arid
grass
shrubs
angiosperms
(~3% of flowering plants)

Question 12

most C4 plants have a distinctive leaf anatomy called ____ ______

Answer 12

Kranz Anatomy

Question 13

what is kranz anatomy?

Answer 13

spatial separation of C4 pathways (mesophyll) and calvin cycle (bundle sheath)

RubisCO in bundle sheath, mesophyll has no RubisCO

Question 14

four-carbon (C4) pathway: spatial separation steps (5):

Answer 14

first product of Co2-fixation is 4-carbon oxaloacetate (OAA)

1. OAA formed when CO2 fixed to phosphoenolpyruvate (PEP) by PEP carboxylase (PEPC) in chloroplast stroma of mesophyll
2. OAA converted to malate (or aspartate) and move to bundle sheath through symplast (plasmodesmata)
3. once inside bundle sheath, malate decarboxylated to yield CO2 and pyruvate in chloroplast stroma
4. released CO2 enters calvin cycle
5. pyruvate recycles back to mesophyll where it is phosphorylated to regenerate PEP

Question 15

C4 advantages

Answer 15

• 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

Question 16

C4 vs C3

Answer 16

• 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

Question 17

C4 Pathway: single-cell C4 system

Answer 17

• 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)

Question 18

C4 Pathway: Temporal Separation Steps + detail

Answer 18

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)

Question 19

what does CAM stand for?

Answer 19

crassulacean acid metabolism

Question 20

Crassulacean Acid Metabolism (CAM)

Answer 20

• 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

Question 21

Carbon Fixation: evolutionary innovations

Answer 21

1. improving light harvest by adding another photosystem and varying pigments
2. improving energy generation by maintaining potential for cyclic electron flow
3. improving co2 fixation:
   - spatial separation (C4)
   - temporal separation (CAM); storing intermediates for later use