This web page contains notes to accompany lectures in Vertebrate Physiology, Biology 410, taught by Dr. Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, 29502, USA.

Metabolism is the sum total of all the chemical reactions in an organism.
It can be divided into anabolism (energy input for building compounds) and catabolism (energy release and break down).
Metabolic rate is an important characteristic of an animal, telling us a lot about it's role in an ecosystem.

Metabolic rate is the energy used by an animal per unit of time. If no work is being done it can be measured as heat production (Hess's Law) and conversion of energy to units of heat is common (1 calorie = the quantity of energy required to raise the temperature of 1g of water from 14.5°C to 15.5°C).
A more convenient measure of metabolism is oxygen consumption.

Using oxygen consumption as a measure of metabolism assumes aerobic pathways are being used, which is generally correct in the long term but can be misleading in the short term.
The disadvantage of measuring oxygen or heat is that they must be done in the lab.

FMR can be measured using labeled water (using isotopes of hydrogen such as tritium) to measure water flux. Water is produced during aerobic resperation and the rate of dilution of injected labeled water is proportional to metabolic rate.

Basal metabolic rate BMR is measured in endotherms (birds and mammals) at rest with minimum stress. It is necessary for the animals to be fasting to avoid measuring digestion.

A similar measurement in ectotherms is the standard metabolic rate, SMR, which is measured at rest and fasting at a given temperature.
Field metabolic rate, FMR, is the average rate of energy used during normal activity.
Metabolic scope is the amount an animal can increase its sustainable metabolic rate compared to its rate at rest.

The respiratory quotient RQ is theproportion of CO₂ produced compared to the O₂ consumed. This varies depending upon the fuel being broken down.
The RQ for carbohydrates is 1.00
The RQ for proteins is 0.80
The RQ for lipids is 0.71

Energy is consumed constantly but energy intake is not constant and so energy must be stored.
Animals store energy as either carbohydrates or lipids.
Lipids are stored as triglycerides. This is the most efficient form of energy storage. 1 g of triglyceride yields 9.3 kcal more than twice the yield of 1 g of carbohydrate yielding 4.2 kcal.

Adding to this problem is that animal starch, glycogen (stored in the liver and muscles) is hydrated and 1g of glycogen contains about 4.5 g of water. Tryglycerides in adipose tissue are not hydrated.
What does this mean to dieters?
Initial weight loss is often mainly water.

Body size effects metabolic rate
A widely applicable relationship exists between body mass and metabolic rate. There is an inverse relationship between body mass of an animal and its metabolic rate.
MR = aM^b or log MR = loga + b(logM)

b is close to 0.75 in all species

This relationship is true within species also but is harder to measure.
Study of a guinea pig indicates b is closer to O.67
How is this difference explained?
Within a species body proportions are the same regardless of size (and suface area varies to a power of 0.67 of body mass).

Initially physiologist thought that it was due to surface area to mass relationships effecting heat loss.
To date no one can adequately explain the relationship between mass and metabolic rate being what it is.

Temperature and metabolic rate
Temperature effects all chemical reactions and chemical reactions in organism, especially those mediated by enzymes, are no different.
Individual reactions and ultimately whole body metabolic rate are temperature dependent.
Endothermy evolved in birds and mammals to keep body temperature within narrow fixed parameters.

Enzymes in endotherms are generally adapted to function efficiently at normal body temperatures.
Temperature effects on metabolism in ectotherms is generally expressed as a Q₁₀.
Q₁₀ = (MR₂/MR₁)^10/T2-T1
This standardizes changes in MR over a 10°C interval.
For ectotherms this is usually between 2 and 3.

Body temperature is a reult of internal heat production + heat absorption - heat radiation.
Animals use different mechanisms to control heat exchange with the environment
Behavioral - e.g. basking
Autonomic - e.g. sweating, fluffing feathers
Acclimatization - increased adipose tissue

Body temperature is a reult of internal heat production + heat absorption - heat radiation.
Animals use different mechanisms to control heat exchange with the environment
Behavioral - e.g. basking
Autonomic - e.g. sweating, fluffing feathers
Acclimatization - increased adipose tissue

Ectothermy was onced viewed as less advanced than endothermy but it is now recognized as an effective and successful adaptation.
It alters our views of energy budgets and behavior.

At a temperature of about 20°C comparing a mouse of similar mass to a lizard, the lizard will have a metabolic rate of about 1/10 that of the mouse.
FMRs of endotherms can be up to 17 times higher than ectotherms of similar size.

Cost benefits of endothermy/ectothermy
1. Ectotherms generally have a lower metabolic rate than ectotherms thus requiring less energy. More of their energy budget can go into growth and reproduction.
2. Ectotherms low capacity for aerobic respiration reduces duration of energy bursts. (Could dinosaurs have been ectotheric?)
3. Ectotherms that control body temperature do not always encounter desired environmental conditions to do so.
4. Endotherms need to have high intake of energy to sustain high metabolic rate.
5. High rate of respiration in endotherms leads to high rates of water evaporation.
6. Endothermy has allowed occupation of colder climates - ectotherms more suited to tropics. No ectotherms in polar regions.

Temperature regulation
Temperature limits exist for organisms. Very few organisms can withstand body temperatures of below 0°C because body fluids freeze. Some fish and turtles are known to survive below 0°C by increasinf levels of an "antifreeze" in their cells and other body fluids.
Above about 50°C the structural integrity of proteins is compromised and function is altered.

Body temperatures are altered by microclimate selection (Fence lizards Sceloporus undulatus). Heat transference can be altered by physiological modification of skin color (Anolis carolinensis). Cardiac output can be altered as can the cpillary recruitment in the skin (Galapagos iguana, Amblyrhynchus sp.)

Endotherms have a thermal neutral zone where BMR supplies sufficient to allow thermoregulation. Within this range heat loss can be adjusted by altering conductance at the body surface.
Above the upper critical temperature metabolic rate increases to facilitate heat loss. Activity such as sweating and panting, blood flow to skin.

Below the lower critical level LCT metaboism increases to produce more heat.
Shivering uses muscle contraction to produce heat. No work is being done but extra heat is produced as a "waste" product.
Countercurrent heat exchange is used in blood vessels in extremities and to heat or cool inspired air.

Non shivering thermogenesis involves breakdown of lipids or carbohydrates without producing ATP.
Some mammals have a special adaptation called brown adipose tissue (BAT) or brown fat. BAT has increased blood supply and many mitochondria in the cells making the color brown.
Brown fat is used to produce heat and little ATP. It is not mobilized to the blood stream to supply energy for other cells.

Dormancy
Many animals go through periods of dormancy or metabolic depression (or torpor) which conserve energy.
Most mammals have daily periods of reduced metabolic rate and temperature. usually during sleep.
Daily torpor periods are common in small birds such as hummingbirds. Body temperature can drop from 40°C to 13°C (in rufous hummingbird with ambientt temperature below 13°C).

Winter periods of dormancy are called hibernation. Common in bats rodents and insectivores. Body temperaure is maintained as low as 20°C. Thermoregulation is still taking place but at a lower level.
Some reptiles hibernate - there SMR drops lower than can be accounted for by lowering temperatures. Seen in alligators and turtles.

Estivation is a warm temperature dormancy, thought to be an energy conservation method. Takes place in lungfish, frogs and turtles.

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This page was created by Peter King. Please contact the author at pking@fmarion.edu with comments.
http://people.fmarion.edu/pking/vertphys/metabolism.html
Last edit January 10, 2011.
Copyright Peter King