plant adaptations (29)

why is rubisco considered an inefficient enzyme

rubisco has a slow catalytic rate and can bind both co2 and o2

• its oxygenase activity leads to photorespriation, which wastes energy and carbon

under what conditions does rubisco act as a carboxylase vs. oxygenase

carboxylase - low o2, high co2 inside the leaf → fixes co2 into 3PG

oxygenase - high o2, low co2 → binds o2, producing 2 phosphoglycerate and triggering photorespiration

what is photorespiration and how does it affect photosynthesis

photorespiration is the process where rubisco fixes o2 insteadof co2, producing phosphoglycolate

• reduces carbon fixation efficiency, consumes atp and releases co2

what is the pathway of phtorespiration in c3 plants with cellular locations

• chloroplast stroma - rubisco oxygenase activity → phosphoglycolate → glycolate

• peroxisome - glycolate → glycine

• mitcohondria - glycine → serine +co2

• peroxisome - serine→ glycerate

  • chloroplast stroma - glycerate → 3PG → calvin cycle

how much does photorespiration decrease photosyntehsis efficiency

• carbon fixation drops to 75% efficieny; atp is consumed to recycle intermediatesw

hat is the gas composition of earth’s atmosphere relevant to photosyntehsis

• 21% oxygen, ~ 0.04 % carbon dioxide ( 400 ppm)

what is the funciton of stomata

stomata regulate gas exchange : co2 uptake for photosyntehsis and water vapor release ( transpiration)

how is stomatal opening and closing regulated

controlled by ion transport in guard cell

• open : K+ influx → water enters → guard cells swell

  • close : k+ efflux → water exits → guard cells shrink

how are plants adapted to dry and hot environments

waxy cuticle reduces water loss, stomata opens at night(CAM), C4 pathway concentrates co2, smaller leaves reduce transpiration

C3

• co2 fixed directly by rubisco; prone to photorespriation

• roses, wheat,rice,soybean, barley

• mesophyll cells have fubisco and fix co2 to RuBP to form 3PG

• Bundle shealth cells have few chloroplast and little rubisco; they do not fix co2

c4

• spatial separation ; PEP carboxylase fixes co2 in mesophyll, Rebuscio acts in bundle-sheath cells

• corn, sugar cane, tropical grasses

• bundle sheath cells have modified chloroplasts that concenetrate co2 around rubisco

• closes proximity permits co2 pumping from mesophyll cells to bundle sheath cells

• fixes co2 at very low co2 levels

• rubisco works as a carboxylase; increase co2 concentration in the bundle shealth cells; increase the efficieny calvin cycle and photosyntehsis

CAM

temporal separation; co2 fixed at night ( PEP carboxylase), calcin cycle runs during the day

how much of earth;s light is used in photosyntehsis

only about 5% of incoming sunlight is capture for photosyntehsis

What factors reduce the amount of light available for photosynthesis

• reflection,scattering,absorption by nonphotosyntehtic pigments, shading by other leaves, and atmospheric filtering