Energy metabolism
Chris Elliott
cje2@york.ac.uk
Learning Objectives:

To understand how oxygen consumption is used
as an index of metabolic rate

To be familiar with the variation in energy
expenditure between species (related to life
strategies)

To appreciate the extent to which energy
expenditure is elevated by animal movement in
the laboratory and field
Reading …
 http://biolpc22.york.ac.uk/303/
 Schmidt-Nielsen, K (1997) Animal Physiology CUP
So what is the cost of living?
Living organisms need energy
just to exist
 Basal metabolic rate
 First estimated by Lavoisier
~1790.
Energy as ATP…
Although Lavoisier measured heat production, the
possibilities include measuring:
1 mole glucose
+
36 moles
ATP
heat
6 moles O2
At equilibrium: rate of ATP hydrolysis = rate of ATP synthesis
(but ATP hydrolysis levels difficult to measure!)
Metabolic rate
 Metabolic rate: rate at which an organism uses
energy
 determine from





heat produced
work done
food ingested
oxygen consumed
CO2 production




?
And CO2
 Respiratory quotient
 RQ = CO2 produced / O2 consumed


1 for carbohydrate
0.7 for fat
Cost of doing nothing??
O2 consumption = energy consumption
1ml O2 = 5 cals = 20 Joules
For a 70 kg human:
• 65 kg ATP/day
• 350l O2/day
• 70 W (one light bulb)
• 7 million joules of energy/day
Physiological food value
How long live on a Kitkat?
 20 g so 2105/100*20 = 421 kJ
 421*1000/70 /60 ≈ 100 min
Warning!
 food is often labelled in cal when
they mean kcal
12% for a Mars bar!
In terms of energy cost (O2 required/kg/h), what is
the most expensive tissue in the human body?
[ and why ? ]
Oxygen consumption by pigeon tissue slices maintained
in saline
Tissue
Kidney
Liver
Brain
Heart
Muscle
ml O2 consumed kg-1h-1
41
34
23
13
10
(metabolic rate of tissue slices = 70% of basal metabolic rate
of intact animal)
Spending: 20% ion pumping; 20% protein synthesis and turnover
Summary so far
 oxygen consumption as proxy for energy
consumption
metabolic rate
Getting bigger… BMR = K * mass0.75
mass
and bigger…
 0.75 law is called Kleiber’s law
 Homeotherms need more energy
 Getting bigger


requires more metabolism
become more efficient

for a 10% increase in size, metabolic rate increase is 5.6%
 Evolutionary drive to get bigger
why 0.75 ?
 If directly proportional


big animals overheat
small animals need several cm of fur
 if index were 2/3

would agree with
surface / volume
ratio
Pumping matters…
 West suggested 0.75 was due to


fractal branching
pulsatile blood flow
Summary so far
 oxygen consumption as proxy for energy
consumption
 basal metabolic rate increases with size
Now onto: effect of activity on metabolic rate
Another day at the office……
Cost of running
0
10
speed of running, km/hr
total cost
net cost
0.01
1000
body mass, kg, log scale
Classic study by Dick Taylor
on ponies, three gaits: walking,
Trotting, galloping on a treadmill:
Same gait: different speeds
Black boxes: preferred speed at each gait = minimal metabolic cost
In general:
Animals use uniform amounts of energy/distance covered
at all speeds.
i.e. walk or run from here to Heslington Hall, uses the same
amount of energy, but running takes you there faster
Metabolic costs of running (kJ kg-1 km-1):
Larger animals more efficient than small; think evolution!
Number of legs does not matter
Flying v swimming v running
Swimming flying graph
maximum MR when running is 10 x BMR
European Eel (Anguilla anguilla)
Migrates 6000miles from W. Europe to Sargasso sea to spawn
Swimming eels consume 23mg fat/kg/h
2kg eel (20% fat) swims at 43.2 km/day for 139 days
consumes 153g fat = 38% of stored fat
…….leaving 413g fat for egg production
Sprinting…
 Some species can ‘sprint’: running faster than O2 can
be supplied to the muscles
 How possible?


Red Fibres: ATP production by aerobic respiration, fuelled
by O2 delivery via blood during exertion of moderate force
for extended periods
White Fibres: ATP production independent of O2 (little
blood supply) during exertion of strong force for short
periods – build up lactate during exercise.
 All muscles have mixture, but some have high ratios
of white to red.
Red and white
muscle fibres
Limits to power output
 <1sec 4500 W muscle output
 <2 min 1500 W anaerobic energy store

kettle
 <2 hours 350 W oxygen transport
 All day 150W need to eat/sleep

2 light bulbs
Mars bars again…
 How far can you run on a Mars bar ?
Summary so far
 oxygen consumption as proxy for energy
consumption
 basal metabolic rate increases with size
 cost of movement


independent of speed
up to 60x BMR transiently, 5x BMR for several hours
Now onto: Life in the field
Back to the original
question:
What is the cost of living
and how do you measure
it?
How do you study
metabolic rate in the field?
Double Labeled Water technique
 Key Technique:
 Aim: to measure FMR (field metabolic rate) over
period of time
 Only works on air-dwelling animals
 Inject animals with 2H218O (harmless heavy
isotopes)
 Later time point: blood sample and
quantify 2H and 18O
DLW - 2
loss of water : carries away 2H and 18O
loss of CO2 : just carries 18O
How does 18O get from water to carbon dioxide ?
carbonic anhydase (very high turnover)
CO2 + H2O  H+ + H2CO3¯ [proton + bicarbonate]
difference between loss of 18O and 2H measures CO2
used in expiration
So how much extra energy does an
active bird use?
Using double labeled water technique:
FMR
--------BMR
Blue-tits breeding in evergreen oak woodlands
in south of France (new leaves in May) and Corsica (new
leaves in June)
FMR ~3x BMR
Index of 0.80
Nagy, K. A. J Exp Biol 2005;208:1621-1625
Summary to end
 oxygen consumption as proxy for energy
consumption
 basal metabolic rate increases with size
 cost of movement


independent of speed
up to 60x BMR transiently, 5x BMR for several hours
 field studies FMR 3-6 x BMR