Physiology 8: Extreme temp Flashcards
What happens when regulatory strategies are not enough…
Too cold:
Proteins denature → disrupt metabolism
Ice formation = tissue death (hypothermia)
Too hot:
Proteins denature → disrupt metabolism
Dehydration and death (hyperthermia)
What 2 strategies are there to minimise damage at extreme temps?
Tolerance
Avoidance
Define tolerance in relation to extreme temp
capacity to endure environment conditions without adverse reaction
Define avoidance in relation to extreme temp
prevent damage from occurring
What is the zone of tolerance?
The central range at which an animal is comfortable:
Bounded by zones of physiological stress, within which they can survive for a limited period of time
What affects tolerance range?
- Ability to tolerate varies between taxa, life stages, pop.s, individuals
- Ability to tolerate varies within an individual’s thermal exposure (builds on idea of
acclimatisation)
What are heat shock proteins (HsPs)
Found in all major compartments of all cells (animals, prokaryotes, plants) → highly conserved
Limit consequences of damage from heat stress
Molecular chaperones
= better name would be stress proteins (as they operate in response to all kinds to stressors, not just heat!)
What are the primary functions of heat shock proteins?
- To promote the proper folding/ refolding of a protein
- To prevent potentially damaging interactions with proteins
- Aid in the disassembly of protein aggregates
Describe the expression of heat shock proteins
Some are constitutively expressed (present at all times)
Others are increased during/after stress
Or others are exclusively induced by stress
Describe how HsPs work
- Under stress conditions proteins start to denature = malformation
- Detected by other molecules
- In organisms where HsPs are not constitutively expressed, these molecules induce the expression, transcription & translation of HsPs
- HsPs bind to malformed proteins and refold into correct formation
Why don’t heat shock proteins dennature?
Better H bonds
More stable secondary structure
= hard to denature, but in extreme conditions even HsPs would denature
What are the secondary function of HsPs?
Immune function:
Usually found intracellularly, so if found extracellularly it suggests cell membranes have been damaged
Helps to present antigens from diseased cells to T cells, which destroy diseased cells
What is the cost of having too many HsPS?
Drosophila larvae were genetically engineered to have more copies of HsP genes
Compared to the wild type they had:
Greater Mortality
Slower development
Describe what happens inside organisms at sub-zero temps
- Ice formation
- Internal fluid freezes so solute conc. of internal fluid increases = water leaves cells by osmosis and cells shrink
= Cell membrane structure degrades because vol shrinks too much
Name and define 2 strategies organisms use to deal with sub-zero temps.
Freeze avoidance → avoid freezing by keeping the bodily fluids liquid (Northern hemi)
Freeze tolerance → can tolerate the formation of internal ice crystals and the effects of dehydration (Southern hemi arctic)
Describe freeze avoidance
- Selection of a dry hibernation site where no ice nucleation (crystallisation) from an external source can occur
- Physical barrier e.g wax-coated cuticle that provides protection against external ice across the cuticle
- Depress the temps at which bodily fluids will freeze (supercooling)
What is supercooling?
Where water is cooled below freezing without ice forming
Describe how organisms use supercooling to stop ice formation
- Water requires a particle e.g dust inorder to crystallise (nucleate)
- Ice-nucleating agents (dust, food particles, bacteria) in the gut or intracellular compartments are removed / inactivated
- If no source of nucleation is introduced, water can cool to -42oCwithout freezing
- Physical process
Describe cryoprotectant synthesis
- Alteration of an organism’s biochemistry
- Increased solute conc. so decreased freezing point
- Most common is glycerol
Describe how glycerol is commonly used in cryoprotectant synthesis
- Attracts water molecules
- Some water is held inside cells (increases osmolarity inside cells)
- Reduces amount of ice formed outside cells
- Reduced cellular dehydration
Describe how organisms are freeze tolerant
They avoid a sudden, total freeze:
1. Limit supercooling, initiate freezing of body fluids at relatively high temps
2. Produce ice structuring proteins (antifreeze proteins)
3. Produce ice nucleating proteins
What are antifreeze proteins?
actually help ice to form in a controlled way by
binding to small ice crystals to inhibit growth/recrystallisation of ice
Describe ice nucleating proteins.
Used by freeze tolerant organisms:
- Regulated production of ice nucleating proteins allows control of the formation of ice crystals within their bodies
- Allows organisms to moderate the rate of ice growth, adjust more slowly to the mechanical and osmotic pressures imposed by ice formation
Give an example of a freeze tolerant organism
Arctic wooly bear moth:
- Withstand -70°C during annual period of diapause (hibernation)
- Accumulate cryoprotectants (glycerol and betaine) in late Arctic summer
- Form hibernation to eliminate nucleators
Give an example of a freeze avoidant organism
Goldenrod gall moth larvae
Supercooling points dropped from -14°C to -38°C during autumn
Water content decreased Glycerol content increased
Describe how the Wood frog combines freeze tolerance and avoidance
Supercools to -3°C
Survived weeks at -8°C at frozen state with 48% of total body water as ice - tolerates freezing of extracellular water
No anticipatory accumulation of cryoprotectants during autumn - triggered by initiation of ice formation in the body
