Lecture 10.1: Metabolic Scaling Flashcards
What is isometric scaling?
relationship among geometrically similar objects
- variables change in direct proportion with size
What is allometric scaling?
relationship is often curvilinear
- variables do not change in direct proportion with size
- Y = aM^b
equation: Y = aM^b
log equation: log Y = b log M + log a
- body mass (M): mass of the organism
- intercept (a): proportionality constant – aids in comparing between datasets and asking questions related to levels
- slope (b): mass exponent (or scaling exponent/coefficient) – provides information on how a variable of interest changes with body mass
What is isometry?
slope (b) = 1
What is hyperallometry?
slope (b) > 1
What is hypoallometry?
slope (b) < 1
Should allometric scaling of metabolic rate be viewed as a fundamental law in biology?
NO
What is metabolic rate?
amount of energy expended per unit time
- sum of all energy transformation processes
- rate of ATP turnover
What are the 3 types of metabolic rate?
- standard or basal metabolic rate
- maximal metabolic rate
- hypometabolic rates (metabolically suppressed)
What are the 3 measurements of metabolic rate?
- O2 consumption rate (VO2 or MO2)
- CO2 production rate (VCO2 or MCO2)
- heat dissipation
What contributes to an organism’s whole animal basal metabolic rate (BMR)?
- not all organs contribute equally to whole body basal metabolic rate (BMR) – how much an organ makes up part of your body does not correlate with how much it contributes to BMR
- subcellular processes contribute differently to whole animal metabolic rate – protein turnover and ion regulation contribute the greatest to dictating whole animal metabolic rate
How is metabolic rate scaled?
allometric relationship (can determine from the slope, which is not 1)
- as body size increases, whole body metabolic rate increases – increases in heat production in various organisms
- same allometric relationship between body size and whole body metabolic rate exists in unicellular organisms < ectotherms < homeotherms (but slopes differ)
Basal metabolic rate (BMR) scales with body mass with what scaling exponent (b)?
vast majority of studies indicate somewhere around 0.67 and/or 0.75
Do 1/3 laws or 1/4 laws govern scaling of metabolic rate?
–
What is the 1/3 law?
b = 0.67 or ⅔
- based on 3 dimensions – ie. surface area to volume ratio
- if size is growing by 1, metabolic rate is growing ⅔ – growth is proportional to the mass exponent
What is the 1/4 law?
b = 0.75 or ¾
- includes additional component(s) beyond the 3D size/volume relationship
What are the 2 broad categories of theories explaining the relationship between metabolic rate and body mass, and what are the 4 theories?
categories:
- due to physical properties of being 3D organism
- intrinsic properties of the cell/organism
theories:
- surface area/volume
- system composition
- resource demand
- resource transport
Theories Explaining the Relationship Between Metabolic Rate and Body Mass
- Surface Area/Volume
- oldest theory – “Rubner’s Law”
- basal metabolic rate (BMR) is proportional to mass^2/3
- based on simple geometric and physical principles
- rate of metabolic heat production is matched to the rate of heat dissipation across the body surface
Theories Explaining the Relationship Between Metabolic Rate and Body Mass
- Surface Area/Volume
Do smaller or larger animals lose more heat?
smaller animals lose more heat per volume due to their large SA:V ratio
- very small organisms (ie. amoeba) have larger SA:V ratio compared to larger animals – absorb gasses and nutrients across their integument, which limits their maximal size
- larger animals are more energy-efficient than smaller ones on a per-unit-mass basis – this is why there is an allometric relationship
Theories Explaining the Relationship Between Metabolic Rate and Body Mass
- System Composition
- proportion of metabolically active and inert tissues differ with body size – LARGER animals have GREATER proportion of metabolically inert tissue (ie. bone)
- skeletal mass scales with body mass with b = ~1, which does NOT explain allometric relationship between metabolic rate and body mass
- this theory is largely ignored
Theories Explaining the Relationship Between Metabolic Rate and Body Mass
- Resource Demand
intrinsic properties of the cell define the metabolic rate scaling relationship
- different properties → different resource demands → different scaling relationship as body size increases
- 1 g of cells from a small animal has a higher intrinsic metabolic rate than 1 g of cells from a large animal
- metabolic rate: scaling coefficient = 0.75
- mass-specific metabolic rate: scaling coefficient = -0.25
- larger animals have smaller mass-specific metabolic rate
Theories Explaining the Relationship Between Metabolic Rate and Body Mass
Conclusion
- big theories that explain metabolic rate scaling relationships (increase in metabolic rate with increase in body size) is resource demand and intrinsic properties of the cell
- degree of membrane unsaturation decreases with increasing body mass in mammals and birds
How does membrane unsaturation change with body mass in mammals and birds?
degree of membrane unsaturation DECREASES with INCREASING body mass in mammals and birds
- smaller animals have greater % of unsaturation
- larger animals have greater % saturation level in membrane
- suggests that the membrane can function as a metabolic pacemaker
What are control coefficients (Q)?
quantifies the contribution of each step in a pathway to regulating overall flux through that pathway
- to determine Q for one site in a metabolic pathways, double the quantity of the enzyme at the site and determine the effect on overall pathway flux (rate)
- Q of an entire pathway must add up to 1
