Metabolism and Energetics

Energy Metabolism

sum of the processes by which animals acquire energy, channel energy into useful functions, and dissipate energy from their bodies in ways that sustain life

• catabolic processes: breaking down

• anabolic processes: constructing molecules

Second Law of Thermodynamics

in an isolated system, internal changes are always towards greater disorder (entropy)

order can only be maintained or increased if a system is not isolated (will require energy to be added from outside)

Order in an Animal System

animals require energy from the outside because energy is necessary to create and maintain their essential internal organization

• protein channels, action potential gradients in neurons, sodium potassium ATPase

Many Animals Must Feed Regularly

jamaican fruit-eating bats - glycogen stores are exhausted very quickly

• study done on their livers to see how long the stores last

• bats were held for different periods → stores were depleted after 24h

common vampire bats die if they go more than 2 nights without a meal → food sharing and regurgitation

Some animals go long periods without feeding

some animals are adapted for long periods without feeding (weeks - months)

common in ectotherms and heterotherms - some insects don’t eat at all in the adult stage; many examples exist in homeotherms

NOTE: long term fasting ≠ starvation

• burmese python might feed once a year, where the meal could be up to 70% of their body mass - digest meal over 10-14 days (ectotherm with low metabolic rate so doesn’t need a constant energy source to maintain body T)

• photosynthetic sea slugs eat photosynthetic algae as larvae and incorporate their chloroplast genes into their own cells → don’t need to eat for up to 10 months provided access to sunlight

Forms of Energy

1. chemical

2. electrical

3. mechanical

4. heat (molecular kinetic)

Physiological Work

a process carried out by an animal that increases order

• synthesizing molecules

• ocntracting muscles

• etc.

What kind of work can be done?

chemical energy can do all forms of physiological work “totipotent”

electrical and mechanical energy can only perform some physiological work (e.g. move ions but not synthesize protein)

heat energy cannot perform any physiological work BUT is still important

Efficiency

ability to do work = high grade energy (chemical, electrical, mechanical)

energy that can’t do work = low grade energy (heat)

• glucose → ATP loses 30-40% as heat; ATP → muscle contraction loses 70-75% as heat = 15021% efficient total

Constraints on Efficency

• max foraging rate

• max digestion rate

• max absorption rate

even if the animal was sitting amongst its main food source, there is an upper limit of how much it can forage, how much it can digest, and absorb

energy is constant being lost as heat = basal metabolic rate

Pathway of Energy Through the Body

energy enters an animal’s body as chemical energy

absorbed chemical energy is used to perform 3 major types of physiological work in the body

energy leaves the body as heat, chemical energy, or external work

3 Major Functions

1. biosynthesis:

   1. energy reminds in chemical form

   2. accumulate chemical energy in body

   3. exported organic matter (gametes, secretions, mucous, skin, hair, exoskeleton)

2. maintenance: circulation, respiration, nervous system, gut motility, tissue repair

3. external work: chemical energy largely degraded to heat inside the body, some work externalized (mostly degraded)

Examples of Biosynthesis

1. caterpillar to butterfly

2. arctic ground squirrel getting ready to hibernate by building up fat in the summer (biosynthesis of fat)

3. cicada shedding exoskeleton

4. christmas island crab going to edge of water during high tide to deposit eggs into the ocean, but if they get washed away they drown

What is internal work?

energy fully degraded to heat

Heat

produced by each major function

conversion of chemical energy to this is irreversible

all animals produce heat, some more than others (endotherm/ectotherm)

Is the conversion of chemical energy to heat reversible?

no; once it is degraded to heat it remains that way

Energy Consumption

energy converted to heat and external work (external work is generally internal work that degrades to heat)

Metabolic Rate

rate at which an animal consumes energy

heat is always the dominant component of metabolic rate

metabolic rate = rate of heat production

Units of Metabolism

is measured as energy

• Watts (joules/second); mW in small animals with low metabolic rate

• kcal/hr

1 calorie = 4.186 J

• mL O2/hr

• mL Co2/min/g

• V CO2, V H2O (rate of consumption)

• Mo2

Significance of Metabolic Rate

one of the most important determinants of how much food an animal needs

• human MR is usually ~23 cal/s

• nutritional calories are actually kcal (1 nutritional calorie = 1000 calories)

represents the intensity of living (metabolic pace of life)

• every E-using process makes heat → MR is the total rate of heat production

• a quantitative measure of the total activity of all physiological mechanisms

ecological relevance of transforming energy in an ecosystem

Ways to Assess Metabolic Rate

• respirometry (earliest public records)

• allometry

• heart rate

• isotopes

• accelerometry

• direct calorimetry

• body mass loss

• infrared thermography

History of Respirometry

discovered that mouse and candle both consume oxygen and release carbon dioxide (makes heat, takes oxygen, gets rid of CO2)

both release heat as part of the process

therefore O2/CO2 = heat

Direct Calorimetry

measure heat production directly

simple concept but technically challenging

commonly used in early MR measurements but not used much now

Indirect Calorimetry

material balance: (E in) - (E out) = E consumed

done in captivity, not in the wild

• get animal, feed them, hold them until they defecate, calculate difference between excretion and initial food to see how much was used

NOTE: assumes that the animal is neither increasing nor decreasing biomass

measure over long time intervals represents the average metabolic rate

Energy Budget

(P + R)/I = assimilation

• p = production

• r = heat lost from respiration

• i = intake

Calorimetry

measuring energy/heat

calor (heat) + metrion (measure) = calorimetry

Lavoisier Calorimetry

to measure an animal’s heat production, he surrounded the animal with an ice-filled jacket

enclosed the entire apparatus in an outer ice-filled jacket that intercepted environmental heat

ice melted by animal heat yielded liquid water, which dripped out of the apparatus for collection and measurement (faster melting and dripping indicated higher MR)

Indirect Calorimetry Based on Respiratory Gas Exchange

2820 kJ per 6 mol O2

• holds true even with multiple intermediate products (doesn’t matter what the animal does with the glucose)

• measuring either O2 or CO2 can indirectly measure MR

Respirometry

find a big or cooperative animal and put a mask on them to measure how much oxygen is being consumed and CO2 being made

measured by a precision O2 meter

Types of Metabolic Rates

• Basal metabolic rate

• Standard metabolic rate

• Maximum metabolic rate

• Resting metabolic rate

• Flight metabolic rate

• Field metabolic rate

• Torpid metabolic rate