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The Evolution of Endothermy: Role for Membranes and Molecular Activity By Paul L. Else, N. Turner and A.J. Hulbert Endothermy in Vertebrates: A Defining Property Characteristics (Compared to Ectotherms) High Rate of Metabolism/Oxygen Consumption

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The evolution of endothermy role for membranes and molecular activity l.jpg

The Evolution of Endothermy:Role for Membranes and Molecular Activity


Paul L. Else, N. Turner and A.J. Hulbert

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Endothermy in Vertebrates:A Defining Property


(Compared to Ectotherms)

  • High Rate of Metabolism/Oxygen Consumption

  • Tissues have high levels of aerobic enzyme activity

  • Large mitochondrial volume and membrane densities (Table 1)

  • Internal organs are larger

  • Organisms with a “high cost of living” which impact every aspect of their life.

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Why is there a higher rate of metabolism/O2 consumption???

There are differences in the overall rates of cellular processes when comparing endotherms to ectotherms.

(ex. Sodium pump / Protein synthesis

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Three Models of Metabolism Show Recurring Themes

  • Compared body temp between ectotherms and endotherms (vertebrates) and mass-matched species.

  • Compared animals of varying body mass within a vertebrate class

  • Metabolic changes that occur during different phases of mammalian development

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The Three Ideas of Membrane Pacemaker Theory of Metabolism

  • Only a few cellular processed consume most of the energy used by most organisms

    • Ex. Na+/K+ pump drives other processed (moves other ions across membranes)

  • What does this say????

    • No new or special processes acct for the differences in metabolism between species

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Pacemaker Theory (continued)

  • Energy consuming processes acct for similar proportions/% of metabolism in animals with very different rates of metabolism

    • What does this say???

    • The amt of energy associated with a process may be different among species, but as a percentage of metabolic rate, it is similar.

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Pacemaker Theory (cont.)

  • Most processes are directly or indirectly linked to membranes.

    • (examples – maintenance of ion gradients, protein synthesis, etc.

  • What does this mean…

    • The variations in animal metabolism is due to MEMBRANES setting the pace of a small number of Energy consuming processes.

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Testing the Theory

  • “The Sodium Pump” A ubiquitous protein in the cells and tissues of all animals

  • Endotherms vs. Ectotherms tissues

  • There are no differences in the number or density of sodium pumps

  • There are differences in the rate of enzyme activity/ molecular activity.

  • (Figure 1)

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Sodium Pump Study More Findings…

  • Changes in the Pump molecular activity is common during development

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Study of the Membrane Environment

  • Ectotherms have more monounsaturated membranes and fatty acids with only one C=C.

  • Endotherms compared to Ectotherms, Smaller compared with Larger –sized mammals and birds, and adult compared with young mammals tend to have higher molecular activities and fatty acids with more C=C double bonds.

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Membrane “Crossover” Experiments

  • Used tissues from rat (with high mol. Activity)

    • Brain and Kidney

  • Used same tissues from toad (low molecular activity.

  • METHOD- detergent to lower activity to similar levels between the two, heat to destroy pump while fatty acid comp remained same.

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Cross Over

  • Enzyme activity was reconstituted with same species membrane

    • Mol activ. Back to normal level

  • Membrane reconstituted with membrane of another species.

    • Mol activ. Shifts to the added membranes orig.activity.

    • Ex. Toad added to rat…reduced Mol. Act.

  • Other CO exper- During Development

    • Similar Findings

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Possible Mechanisms

  • The use for polyunsaturates suggests an improvement in the activity of membrane proteins.

  • Looked at differences in acyl composition of membrane which suggests an altering of the electrical fields within the bilayer

    • It is still unknown how they influence lipid protein interactions.

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Mechanisms (continued)

  • Experiments on Physical Properties of Membrane Lipids and influence on Molecular activity

    • Shows increasing molecular activity with increasing area occupied by lipids.

  • What does this say???

    • MA and area lipids correlate to polyunsaturation and

    • Electrical field strengths are dispersed by the “kinked” carbon chains of the unsaturated lipids

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Summing it up

  • During evolution, as body mass increased, there was a need for animals to reduce their mass-specific met rates.

    • (too little surface area to get rid of their heat)

  • Membrane lipid composition becomes important.

  • If, during evolution, increased polyunsaturated membranes occurred, then an increase in Molecular activity would occur