BIO3082 Lecture Notes - Lecture 4: Rubisco, Carbon Fixation, Carboxylation
Lecture 4 – Temperature
Heat Adaptation – Major Selection Pressure
• Metabolism in plants and animals is optimal between 10˚ to 30˚c
• Some organisms are adapted to subzero temperatures (e.g. polar, alpine) and
others tolerate >50˚c (e.g. deserts, tropics)
o Some microbes at extreme temperatures (e.g. 121˚c)
• Survival depends on suite of anatomical, physiological and behavioural
adaptations
• Global surface temperatures has risen by an average 1˚c and predicted to rise
by further 1-3.7 degrees
Methodologies for Measuring Thermal Performance and Limits
• Most performance/fitness traits show temperature
dependence
• Optimal temperature – reflects niche of organism
• Below – decrease of availability of kinetic energy
• Above optimal: sharp drops in performance – protein
denaturation
• Case study: photosynthetic performance
1. Below 15˚c, photosynthesis is limited by slow
diffusion and enzyme turnover
2. Moderate temperatures: photosynthesis is limited
by binding of CO2 to RuBisCO
3. Optimal temperature: various steps involved in
photosynthesis are more balanced
4. Above 40˚c, protein denaturation and membrane deformation rapidly
occurs
o Ambient CO2 concentrations: limiting to photosynthetic performance
• Optimal temperatures reflect ecological niches
o E.g. cold adapted species have optimal photosynthetic performance at
lower temperatures
Contrast Thermal Photosynthetic Performance of C3 and C4 Plants
• Both photosynthesis ~40˚c
• C4 plants (e.g. Tidestromia oblongifolia) is superior to C3
(e.g. Larrea divaricata) plants at higher temperatures
o C4 have mechanisms available to concentrate
carbon before it reaches RuBisCO apparatus
o Co2 diffuses into cells then RuBisCo fixes it –
continually limiting factor for C3
• RuBisCO poorly discriminates between CO2 and O2
o O2 solubility increases relative to CO2 at higher
temperatures
o Beneficial carboxylase activity is increasingly counteracted by
wasteful oxygenase activity
o C4 plants compensate by concentrating carbon and delivering it to
RuBisCO
▪ Ensures efficient carbon fixation even at heightened
temperature
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