life at the extremes

• Cells are adaptable and able to cope with a wide variety of environments
• Environments cells can cope with

  • Temperature extremes
  • Low oxygen environments
  • Low nutrient environments
  • Toxic environments
• Cells


  • Compartmentalised
  • Specialised
  • Dynamic
  • Social
  • Robust
• Robustness


  The ability of a cell to maintain performance and function in the face of internal and external perturbations
• Robustness has long been recognized as a key property of living systems
• Dynamic cells


  Are able to adapt to changes in their environment
• Redundancy in cells


  Cells are therefore fault tolerant
• Caspases


  • Caspase-1
  • Caspase-9
  • Caspase-3
• Surviving high temperature


  Slight increases in temperature can be beneficial in the short term
• Fever during infection can help the immune response
• Benefits of fever during infection

  • Increased movement of white blood cells
  • Increased proliferation of white blood cells (T cells)
  • Enhanced rate of phagocytosis
• At higher temperatures cells encounter problems and become stressed
• Most enzymes are optimised to work at 37°C
• The cell membrane becomes much more fluid at higher temperatures
• Proteins don’t fold properly at higher temperatures
• How membranes respond to heat

  1. Change lipid composition of the membrane

  2. More saturated fatty acids

  3. More cholesterol

  4. Unsaturated = more fluid

  5. Saturated = more viscous

  6. Cholesterol = more stable
• Heat shock proteins


  A group of proteins produced when a cell is exposed to elevated, sub-lethal temperatures
• Heat shock proteins protect the cell if it is subsequently exposed to lethal temperature rises
• Heat shock proteins increase in expression in response to other stresses such as deprivation of oxygen or nutrients
• Main function of heat shock proteins

  Help proteins fold properly so that they can then function properly
• Geogemma barossii is a prokaryote from the Archaea domain that was found growing in a sulphur and iron-rich hydrothermal vent at 121°C and can survive at 130°C
• Adaptations of Geogemma barossii

  • High GC levels in third codon position
  • Supercoiling DNA
  • Temperature stable proteins
  • Changes to membrane composition (including ether bonds in phospholipids rather than ester bonds)
• Slow enzyme reactions occur at low temperatures
• Low temperatures lead to rigid and viscous membranes
• Problems caused by cold

  • Slow enzyme reactions
  • Rigid & viscous membranes
  • Ice crystals
• Glycoproteins


  Proteins with sugars attached
• Glycosylation


  The process of attaching the sugar to proteins
• Many glycoproteins are present in the plasma membrane with the carbohydrate region presented outside the cell
• The attached sugars function to stabilise the protein structure in the extracellular environment
• Glycolipids are also present in the plasma membrane and have similar protective functions
• Glycoproteins are attached to proteins in the rough ER
• A glycosidic bond links a carbohydrate to the side chain of the amino acids asparagine (N-linked) or to the side chain of serine or threonine (O-linked)
• Antifreeze glycoproteins are used by Antarctic fish to stop them freezing
• Antifreeze glycoproteins can stop ice crystal formation down to -2 ⁰C
• These biological antifreezes have important medical applications
• Low temperature storage is required and ice crystal formation is damaging
• Medical applications of antifreeze glycoproteins

  • Improved protection of blood platelets
  • Protection of human organs at low temperatures
• The wood frog, Rana sylvatica, can survive multiple freeze-thaw cycles through the use of antifreeze glycoproteins and high glucose levels
• Many bacterial species thrive at low temperatures
• Psychrophiles


  Have proteins and metabolism optimised for cold temperatures