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Work and Heat (Dr. Haynie, November 16, 2005) Work and heat Food Intake Metabolic reactions in the body Heat Work Weight Weight Gain Loss Excretion & Secretion Example: Rubber band contracts when it is heated and can be made to lift a weight, hence doing work. Entropy 1. Measure of increasing disorder of the universe 2. Measure of increasing inability of energy to do work 3. How do we measure it? Heat Source Work Heat How do we measure entropy? Cannot generally measure it directly. E.g. use a calorimeter. What is the entropy involved in holding an amino acid sequence together? Use a calorimeter, with protein molecules, heat the solution, and measure the amount of heat absorbed by the protein molecules as they go from folded to unfolded proteins. Then measure the temperature at which that occurs. E.g. if it’s 60C (330K), S q T . Free Energy What is it? Gibbs free energy Momement by moment, temperature and pressure (and no. of particles) are constant, so have Gibbs free energy instead of Helmholtz free energy (constant volume). GFE is a thermodynamic potential. Combination of the First and Second Laws of thermodynamics. Familiar with the concept of potential from E&M and Gravity. Can predict the spontaneous movement of an electron, knowing the potential. G H TS H q p is the heat transferred when the temperature is constant. If you transfer heat when the pressure is constant, then GFE is the energy change of the system. If G 0 non-spontaneous reaction if G 0 spontaneous reaction We will need GFE to describe how enzymes work. H is 1st law. TS is 2nd law. For thermodynamics to apply rigorously, everything must be in equiliburium (unless you talk about non-equilibrium thermodynamics). For reaction rate: Q10 rule: for every 10C increase in reaction temperature, the enzyme will react twice as fast. Transition state free energy. i.e. if you heat up molecules, they will collide more with one another, the collision rate depends on thermal energy, increasing the rate of reaction. Cannot hold absolutely since the protein will denature if it is too hot. Transition state free energy: Reaction rate theory Eyring Theory Energy G 0 Reaction will occur spontaneously Reactants Products Reactants must go over the energy barrier for the reaction to occur, and likelihood of a reaction having enough energy increases with temperature. Basic concepts of energy balance and metabolism Metabolism = “change,” chemical-energy transformations Catabolism = breakdown of compounds, heat releasing Anabolism = forming of compounds, heat absorbing Metabolic rate: Energy output = external work + energy storage + heat Metabolic rate = energy output per unit time Muscle contraction Isotonic (free) contraction Biceps shortens freely, Weight is lifted Physical work is done Isometric contraction Biceps generates force, muscle does not shorten Weight is not lifted Physical work is not done Muscle Contraction Head pivots and moves filament (power stroke). Depends on the interaction between actin and myosin and the hydrolysis of ATP. Heat generated when you run comes from hydrolysis of ATP. Energy Balance Balance between caloric intake and energy output is based on First law Negative balance utilize endogenous store glycogen body fat, body protein catabolized weight loss Positive balance You get fat if you eat too much, but don’t exercise. Regulation of appetite Complex Partly based on caloric needs - ~2000 kcal/day for average adult, depending on activity Can store chemical energy in the form of: Glycogen Fats Protein Someone who becomes severely malnourished has little muscle – body is using the muscle as an energy store. Energy Transfer Energy of catabolism not used directly Formation of ester bonds Phosphoric acid residues (high-energy phosphate compounds) Certain organic compounds Examples: ATP (phosphodiester), creatine phosphate, coenzyme A (thioester) Energy Production Net energy production of “energy-rich” phosphate compounds Amount depends on metabolic pathway Anaerobic (not much) Aerobic (a lot) Energy is consumed in the process Electron transfer reactions are involved Glycolysis: Glycogen is broken down into ATPs What affects metabolic rate? Muscular exertion -> increase in oxygen consumption Recently ingested foods -> obligatory energy expenditure during assimilation (specific dynamic action) Proteins – deamination of aa’s in liver (?) Fats may play a role in the stimulation of metabolism (?) Carbohydrates – energy required to fomr glycogen (?) Environmental temperature shivering acceleration of metabolic rate at higher temperature (fever) Because enzymes work faster at higher temperature Height, weight, surface area Sex Age Emotional state Some people eat (or don’t eat) when they get nervous or are under stress Body temperature Pregnancy/menstruation Thyroid hormones Neurotransmitter levels Concepts Covered during Day 2 Body Heat Thermoregulation cellular level system level body level Thermoception Non-shivering thermogenesis Body Temperature Balance between production and loss Cold weather clothes have low thermal conductivity Heat production in the body Muscular activity, exercise, contraction Assimilation of food, specific dynamic action All vital processes contributing to basal metabolic arte Endocrine mechanisms: epinephrine, norepinephrine, thyroid hormones Nervous system: sympathetic discharge Brown fat Heat Loss Radiation Conduction (tissue conductance) Vaporization of water in the respiratory passages/skin Excretion Normal body temperature changes with Time of day Age Part of body Circadian fluctuation of core temperature Sex (ovulation) Exercise Temperature of subject G 0 Reaction will occur spontaneously Caloremic Temperature Regulatory Mechanisms Mechanisms activated by cold Shiver Hunger (body tells you to eat when you are cold) Increased voluntary activity (get up and move – push ups, etc.) Increased secretion of neurotransmitters (more neurons firing, need more ATP to regenerate the neural potential, heat released will help keep the body warm). Cutaneous vasoconstriction (blood vessels shring in diameter) Curling Up (reduce surface area) Horripilation Regulatory Mechanisms activated by heat Cutaneous vasodilation Sweating Increased respiration Anorexia Apathy and inertia Rate of Sweating 37 C Internal Temperature (C) Thermoregulation Maintenance of an optimum temperature range by an organism Poikilotherms (variable temperature ones) Pick up or lose heat by way of the environment (lizard) move from one place to another as necessary Homiotherms (similar temperature ones) Have additional means by whichy they can heat or cool their bodies. More independent from their environment Thermoreception Birds & Mammels Birds: Homiothermic Maintain temperature within a range of < 1 C Study suggests existence of thermosensors in a lower part of the brain (hypothalamus) and skin. Direct electrophysiological evidence of thermoreceptors has been found in tongues of chickens and skins of pigeons Individual fibers of nerves serve each receptor. Chicken Tongue At a constant temperature of 20C, have a high level of static activity in cold receptors in a chicken tongue At a constant temperature of 44C, individual cold fibers show a steady rate of 2-4 Hz. Temperature drop of 9 C gives an initial response of 30 Hz, declining gradually to 8 Hz Rewarming tongue gives cessation of electrical activity for several seconds In other words, firing rate increases with decreasing temperature, hence these are cold receptors. Warm receptors have not been found. Megapodes Bury their eggs Depend on thermal sensitivity of their face or mouthparts (measuring temperature of the eggs) to guide control of temperature of eggs during hatching Eggs are incubated in mounds, heat is generated by fermentation of rotting vegetation and irradiation of sun Male covers and uncovers eggs, keeping temperature constant at 34C for up to 63 days. Mammals E;ectrophysiologica studies have been done on sensitive nerve fibers in the noses of cats and monkeys. Nose of the cat Numerous cold and warm receptors Highly specific in responding to thermal stimuli Not excited by mechanical deformation of the skin Each receptor is connected to a single nerve fiber One can use finely-tipped thermal stimulator to locate sites of warm and cold receptors One can use electron microscopy to study underlying cellular structure at these spots. Cold sensitive spots A thin, myelinated (insulated) nerve fiber penetrates the dermis and divides into several unmyelinated branches about 70 microns beneath the skin surface Tips of branches are cold-sensitive nerve endings, in close contact with basal cells of epidermis. Nerve endings are embedded in small concavities on lower … Skin of the monkey Warm receptors Innervated by unmyelinated nerve fibers Diameter ~1 micron Impulse conduction velocity ~ 0.5 – 1.5 m/s Cold receptors Served by unmyelinated fibers or thin myelinated fibers Diameter ~2-4 microns Conduction velocity ~20 m/s Cold fibers Continuously active at a rate of 8 Hz at constant temperature. Range of static activity 5-42C Cold receptors will be activated again above 45 C On sudden cooling, rate can be up to 240 Hz Myelinated cold fibers are blocked below 10 C. Warm fibers Average static max. rate at 41 C Range of static activity 5-42 C Average max freq at 27 C Transient overshooting can occur at several times the static maximum frequency Degree of overshooting is maximum at 27C when cold stimulus is applied. Cold fibers and warm fibers alter behavior at similar temperature Information Processing Numerous nerve cells in the thalamus respond only to cooling of the tongue Show static discharge in range of 15-44C Maximium frequency at 21-31 C Considerable transient overshooting on rapid cooling of the tongue A few brain neurons are excited by mechanical stimulation and cooling of the tongue. Summery: Have presented aspects of metabolism and thermoregulation and demonstrated how various physiological systems are related to the 1st and 2nd laws of thermodynamics. Of course, one could spend an entire quarter on just the neurology alone.
