Biology 029: Life on other planets

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Biology 029Life on other planets

• Cold tolerance in ectotherms

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Two strategies

• Freeze Avoidance
• Freeze Tolerance

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When its cold, water

• Becomes solid
• Becomes less dense (it expands)
• But it remains liquid if salt is added

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Water doesnt always freeze at 0C

• Small quantities of water can supercool to -20C
• Freezing will be induced in the presence of a
  nucleus
• Imposes structure on the water
• Silver iodide, certain proteins

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Application of a bacterial ice nucleating protein
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How do we know that an insect has frozen?
Exotherm ( latent heat of crystallisation)
Temperature
Supercooling Point (SCP)
Time
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Freeze Avoidance

• Keep body fluids liquid at temperatures below
  their melting point
• Theoretical lower limit c. -50 C
• Probability of freezing is proportional to
• Temperature
• Time
• Volume

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Freeze Avoidance Adaptations

• Colligative Antifreezes
• Sugars and Polyols eg Glycerol, Trehalose
• More stuff lower melting and freezing points
• 1 Mole of solutes -1.86 C

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Freeze Avoidance Adaptations

• Non-colligative antifreezes
• Antifreeze proteins and glycoproteins
• aka Thermal Hysteresis proteins
• Act to prevent the growth of ice crystals in
  solution
• Depress the freezing point much more than would
  be predicted by their concentration.

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Antarctic Springtail

• Gomphiocephalus hodgsoni
• Ross Island, Antarctica

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Freeze Avoidance in Antarctic Fishes

• Trematomus bernachii
• (Notothenioidea)
• Body fluid m.p. c. -1.2 C
• Seawater -1.86 C
• Antifreeze glycoproteins

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Freeze Tolerance

• Survive the formation of ice inside the body

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Freeze tolerance in alpine weta

• Turn a freezing problem into a desiccation
  problem
• Control Ice nucleation
• Osmotic dehydration

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Freeze tolerance adaptations in insects

• Ice Nucleation
• Control it!
• Polyols
• Protect membranes and proteins
• Act as stuffing for osmotically dehydrated cells
• Antifreeze proteins
• Inhibit recrystallisation

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Adaptations to freeze tolerance and freeze
avoidance
Freeze tolerant Freeze avoiding
SCP high low
Ice nucleators yes no
Polyols yes yes
Antifreeze proteins yes yes
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Adaptations to freeze tolerance and freeze
avoidance
Freeze tolerant Freeze avoiding
SCP Slow freezing Die when freeze
Ice nucleators Initiate freezing easily Prevent initiation of freezing
Polyols Protect membranes and proteins act as stuffing Protect membranes and proteins depress SCP
Antifreeze proteins Prevent recrystallisation Reduce chance of freezing
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Vertebrate freeze tolerance!

• Wood Frog (Rana sylvatica)
• Nearly 70 of body water converted into ice
  (extracellular)
• Can survive being frozen for at least 4 weeks!

Also some lizards, salamanders hatchling
turtles