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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 High Rate of Metabolism/Oxygen (Compared to Ectotherms) 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. 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 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 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 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. 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. And thus the Membrane Pacemaker Theory of Metabolism… was born!!!!! 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) Sodium Pump Study More Findings… Changes in the Pump molecular activity is common during development 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. 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. 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 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. 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 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