ecological adaptations of photosynthesis

Question 1

C3 photosynthesis

Answer 1

• most plants
• first stable molecule produced in carbon cycle is G3P, 3 carbon
• product of RuBisCO

Question 2

problem with C3 photosynthesis

Answer 2

• when environment is hot/dry plant closes stomata to reduce water loss
• gas exchange is interrupted
• O2 builds up and CO2 is depleted in plant cells

Question 3

photorespiration

Answer 3

• RuBisCO is a carboxylase/oxygenase enzyme
• can bind with O2 and add it to the calvin cycle at low levels of CO2 (dry, high temperatures, RuBisCO has high affinity with O2 at high temperatures)
• produces 1 3C and 1 2C product
• can’t produce sugars, so net loss of carbon, also uses ATP
• 2C compound leaves chloroplast, peroxisomes and mitochondria rearrange and split compound into CO2

Question 4

strategies to prevent high rates of photorespiration

Answer 4

• CAM photosynthesis, accumulates CO2 at night so stomata are not needed on hot dry days
• C4 photosynthesis, concentrates CO2 into bundle sheath so calvin cycle can operate efficiently, outcompetes RuBisCO reaction with O2
• both use PEP carboxylase (enzyme found in cytoplasm) as well as RuBisCO, has a higher affinity for CO2

Question 5

plants that use CAM photosynthesis

Answer 5

• ‘Crassulacean’ Acid Metabolism
• discovered in succulent Crassulaceae family e.g. Kalanchoe
• not unique to family, convergent evolution
• dry arid environment
• includes cacti, pineapple

Question 6

CAM pathway

Answer 6

• stomata close in day
• stomata open at night, CO2 into the cycle
• PEP carboxylase catalyses addition of bicarbonate to phosphenolpyruvate (PEP, 3C) to form oxaloacetate (4C)
• OAA forms malate which forms malic acid that is stored in the vacuole
• in the day, malic acid can be split into CO2 and pyruvate
• CO2 is used in the calvin cycle
• pyruvate is converted into PEP

Question 7

plants that use C4 photosynthesis

Answer 7

• 15,000spp. many grasses, no trees
• hot and high light environments like Savannah, Steppe etc
• 20-30% terrestrial CO2 fixation
• 30% agricultural grain, maize, sorghum, millet etc
• all contain bundle sheath cells between mesophyll and vasculature

Question 8

C4 pathway

Answer 8

• stomata are partially open on hot sunny days, allowing some CO2 to enter
• PEP carboxylase fixes CO2 in mesophyll to PEP, forms oxaloacetate (4C)
• forms malate (4C), which releases Co2 in the bundle sheath cell for the carbon cycle
• pyruvate reenters the cycle, into PEP
• spatial separation of process, light-dependent in mesophyll, calvin cycle in bundle sheath cells
• CO2 concentrated in bundle sheath, calvin cycle can function efficiently

Question 9

rise of C4 grasslands

Answer 9

• 5-8mya
• rise in 13C (heavy isotope, extra neutron, 1% natural carbon) in herbivore tooth enamel (proboscid and equid fossil history)
• RuBisCO favours 12C
• PEP carboxylase favour 13C
• convergent evolution,C4 photosynthesis evolved independently at least 62 times across the world

Question 10

why did RuBisCO evolve inefficiently?

Answer 10

• high CO2 and low O2 when plants colonised land
• no need for high CO2 specifity

Question 11

history of atmospheric CO2

Answer 11

• carboniferous forests depleted atmospheric CO2 400-300mya (fossil fuels)
• Palaeozoic forests declines, biggest ever mass extinction 250mya
• Mesozoic Co2 still higher than today
• increasing CO2 starvation during teriary
• grasslands only appear in mid Tertiary, c4 grasslands better adapted to low CO2 levels

Question 12

genetic engineering of C4 rice

Answer 12

• aspiration to establish C4 pathway in C3 crops like rice
• increases photosynthetic efficiency and therefore crop yields
• increased water efficiency and temperature tolerance, useful in light of climate change
• complicated, enzymes, transporters and anatomy to engineer