1. CHAPTER 42 CONTROL OF ENERGY BALANCE, METABOLIC RATE, AND BODY
TEMPERATURE
Prepared by
Brenda Leady, University of Toledo
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. All animals require sources of carbon, nitrogen, and other nutrients
Used to assemble macromolecules or generate ATP
Animals must have mechanisms to maintain blood levels of important nutrients despite irregular nutrient flow

3. Metabolism – all the activities and chemical reactions in an animal’s body
Metabolic rate – rate at which animal uses fuel to supply ATP for these reactions
Metabolism and body temperature are related – heat is generated as a by-product of nutrient breakdown and ATP synthesis

4. Nutrient utilization and storage
Nutrient utilization divided into 2 alternating phases
Absorptive phase – occurs when ingested nutrients enter blood stream from GI tract
Some of ingested nutrients used for immediate energy needs, others stored
Postabsorptive stage – occurs when GI tract empty of nutrients so body’s stores used

5. Absorbed carbohydrates
Chief monomers absorbed are glucose, galactose, and fructose
Glucose is 1 of 2 major energy sources during absorptive phase (other is fat)
Much of absorbed glucose used to synthesize ATP
Skeletal muscle major consumer of glucose
Can store as glycogen
Excess glucose stored as glycogen in liver or triglycerides

7. Triglycerides
Too large to diffuse across plasma membrane of intestinal epithelial cells
Digested into monoglycerides and fatty acids
Diffuse into intestinal epithelial cells
Resynthesized into triglycerides
Packaged into chylomicrons for blood transport
Lipoprotein lipase releases fatty acids to diffuse into cells of the body
Some ingested fat used during absorptive phase for energy

8. Amino acids Taken up by all body cells Used to synthesize proteins
Excess amino acids not stored as protein
Excess converted by liver cells into carbohydrate or fat

9. Postabsorptive period
Synthesis of glycogen and fat slows and breakdown begins
Blood glucose concentration maintained
Reactions that provide glucose to blood
Using fat instead of glucose for energy (glucose sparing)

11. Reactions that provide glucose to blood
Glycogenolysis – glycogen hydrolysis in liver
Gluconeogenesis – liver converts noncarbohydrates into glucose
Lipolysis releases fatty acids (alternate energy source to glucose) and glycerol (converted to glucose by liver)
Proteins can also be converted to glucose in prolonged fasting

12. Glucose sparing
Most organs and tissues increase fat utilization for energy
Reserves glucose for use by nervous system
Essential step is lipolysis
Fatty acids used by cells for energy
Liver uses fatty acids to make ketones
Prolonged energy source for many tissues, including brain
Less protein breakdown required

13. Regulation of phases Insulin Made by pancreas
Binds to cell surface receptor, stimulates signaling pathway, facilitates glucose diffusion into cells
Glucose transporters (GLUTs) in intracellular vesicles move to the surface
Also inhibits glycogenolysis and gluconeogenesis in the liver

15. GLUT Proteins Transport Glucose in Animal Cells
All animal cells require transporters to move glucose across plasma membranes
GLUTs make up a family of at least 14 related proteins
Expressed in different tissues
Structures similar across phyla – gene duplication of common ancestral gene
Different GLUTS vary in ability to bind glucose
High affinity – binds even at low glucose concentrations
Muscle and fat have low affinity GLUT4 (also only one requiring insulin)
Brain cells have high affinity GLUT

16. Controlling insulin secretion
Blood glucose rises during absorptive state so insulin secretion rises
Blood glucose levels fall during postabsorptive state so insulin secretion falls
Negative feedback
Input from nerve cells also plays role
Eating meal increases insulin secretion
Stressful situation decreases insulin secretion

18. Maintaining blood glucose levels
Blood glucose must remain within the normal homeostatic range
Too low and even high affinity GLUT receptors in the brain will not take in glucose
Glucagon – from the pancreas stimulates glycogenolysis, gluconeogenesis, and ketone synthesis

20. Energy expenditure
Amount of fuel used during a given period of time to power all of its metabolic requirement
Breakdown of organic molecules liberates energy to be transferred to ATP
Not all energy used for ATP to do work
Some energy appears as heat

21. Metabolic rate
Calorie – amount of heat required to raise the temperature of 1 gram of water 1 degree Celsius
Commonly measured in kilocalories (kcal)
Metabolic rate – total energy expenditure of an animal per unit time

22. Basal metabolic rate (BMR) – at rest, in postabsorptive state, and at a standard temperature
Endotherms – generate their own heat and maintain relatively constant body temperature – BMR measured in thermoneutral zone
Ectotherms – body temperature changes with environment – BMR measured at standard temperature for each species
SMR or standard metabolic rate

23. BMR estimation Direct calorimetry Indirect calorimetry
Energy expenditure and heat production directly related
Measures heat given off by animal
Indirect calorimetry
Measures rate of oxygen use by animal
Underestimates BMR due to anaerobic metabolism

26. Not all tissues use oxygen or produce heat at the same rate
Metabolic rates of different tissues can vary independently of each other

27. Factors impacting metabolic rate
Altered skeletal muscle activity
Food-induced thermogenesis – greatest effect from ingested protein increasing heat production
Mass-specific BMR – amount of energy expended per gram of body mass – exponential curve
Smaller animals lose heat more rapidly so higher BMR
Does not explain the same relationship in ectotherms

29. Smallest endotherms face the special challenge of fueling their very high mass-specific metabolic rates
Torpor – nightly lowering internal body temperature to just a few degrees above that of the environment
Hibernation – extended for months

30. Balance
Body weight remains stable when amount of energy consumed equals the energy expended
Appetite and metabolism change when food intake is more or less than the amount needed to maintain the body’s set point

31. Food intake
Satiety (fullness) – involved in short term control of feeding
Satiety signals – remove sensation of hunger and set time period before hunger returns again
Stretch receptors in the stomach and small intestines send signals to the brain
Stomach and small intestine release hormones to suppress appetite

33. Leptin – produced in adipose tissue in proportion to fat mass
Long-term control of food intake mediated by many different brain molecules, by hormones, and by emotional state, particularly in humans
Leptin – produced in adipose tissue in proportion to fat mass
As more fat is stored in the body, more leptin is secreted into the blood
Acts on brain centers to reduce appetite and increase metabolic rate
Decreased fat mass decreases leptin which decreases BMR and increases appetite

35. Coleman Revealed a Satiety Factor in Mammals
Parabiosis – surgical connection between bloodstreams of 2 animals
1st experiment – connected wild type mouse to ob mouse
Ob mouse ate less and gained less weight than usual
Blood of wild type mouse contained factor signaling sufficient fat stores
2nd experiment – connected wild type mouse to db mouse
Db mouse continued to gain weight while wild type mouse lost weight
Obesity phenotype the same – factor was absent in ob mice and present but unable to act in db mice
Most obese humans like db mice – produce leptin but fail to respond to it

37. Regulation of body temperature
Most animals can only survive a narrow range of temperatures
Chemical reactions depend on temperature
Enzymes have optimal temperature
Proteins can become denatured at high temperatures
Heat alters structures of plasma and intracellular membranes

38. Extreme cold is better tolerated
Ice crystals can rupture cell membranes
Some animals can block ice crystal formation in their cells

39. Maintaining body temperature
Classify animals according to
Source of heat – internal or environmental
Ability to maintain body temperature – fluctuating (heterotherms) or stable (homeotherms)
Endothermic homeotherms – birds and mammals
Ectothermic heterotherms – other vertebrates and most invertebrates
Not all animals can be classified neatly all the time
Hibernating animal (heterothermic) or deep ocean fish (homeothermic)

41. Birds and mammals can quickly adjust the body’s mechanisms for retaining or releasing heat so that body temperature remains stable
A suddenly awakened mammal is instantly capable of intense activity
A reptile must wait until it can warm itself using the environment

42. Disadvantages of endothermy
Must consume larger amounts of food to produce sufficient heat by metabolic processes
Run the risk of overheating during intense activity, even in cold weather

43. Heat exchange
Radiation
Conduction
Convection
Evaporation

44. Radiation
Emission of electromagnetic waves by the surfaces of objects
Rate determined by temperature of radiating surface
Conduction
Body surface loses or gains heat through direct contact with cooler or warmer substances
Water has a higher heat capacity than air
Surface area important – elephant ears or bat wings

46. Convection Evaporation
Transfer of heat by the movement of air or water next to the body
Sitting near fans, bat flapping wings, or elephant waving ears
Evaporation
Water vaporizing from surface conducts heat used to drive process away from surface

47. Heat gain or loss
Core temperature of endotherms remains constant but skin surface varies considerably
Blood flow to the skin can increase or decrease heat loss

48. Countercurrent heat exchange
Conserves heat
Dolphin flipper or bird legs
Heat moves from warm arteries to adjacent veins carrying cooler blood
Reduces heat lost to environment
Fish keep muscles warm without losing heat through gills

49. Behavioral adaptations
Evaporative heat loss
Change rate of evaporation via perspiration
Humidity determines rate of actual evaporation
Panting, licking skin, bathing in water are alternatives to sweat glands
Behavioral adaptations
Changing exposed surface area – curling into ball, huddling, hunching
Changing surroundings – critical for endotherms obtaining or losing heat

51. Increased heat production
Changes in muscle activity major control in endotherms
Shivering thermogenesis – skeletal muscle contractions without locomotion
Nonshivering thermogenesis – primarily in brown fat’s mitcohondria where uncoupling proteins use H+ gradient to generate heat, not ATP synthesis

52. Impact on public health
Obesity
Body Mass Index (BMI) of 30 or greater
Lack of physical activity reduces daily energy expenditure
“Apples” versus “pears” – location of fat has health consequences
Genetic factors – thrifty genes may have been survival mechanism
Underweight
Anorexia nervosa – reduce food intake to the point of starving, biological and psychological causes
Bulimia nervosa – recurrent episodes of fasting and overeating, may include self induced vomiting or laxative use
Treatments rely on counseling, nutritional education and medication