The burden of food

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The burden of food
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Why animals need energy

• Animals are organized or ordered systems.
• Energy is brought into the system to maintain
  that order
• Energy definition The capacity to do mechanical
  work or more broadly the capacity to increase
  order.
• Animals are open ordered systems that rely on the
  input of energy to maintain that order.

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Figure 6.1 The second law of thermodynamics in
action
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Fundamental of animal energetics 4 energy forms

• Chemical energy energy liberated by the
  rearrangement of atoms
• Electrical energy produced by the separation of
  positive and negative charge.
• Mechanical energy produced by organized motion
  of the molecules.
• Molecular kinetic energy (heat) energy of random
  atomic-molecular motion.

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Physiological work increases order

• Chemical energy can be used to do all forms of
  physiological work (totipotent).
• Electrical and mechanical energy can be used for
  some physiological work.
• Animals cannot use heat to do any form of
  physiological work in themselves.
• Work produced by heat requires a temperature
  difference.

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Transformation of high-grade energy

• Efficiency of energy transmission is less than 1.
• 70 of energy released from glucose is
  incorporated into ATP bonds. 30 is lost in heat
• 35-30 of the energy released from ATP is used in
  muscular contraction.

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The flow of energy in animals

• Flow of energy starts as ingested chemical
  energy.
• Egested chemical bond energy is fecal chemical
  energy.
• Ingested energy that is assimilated into the
  cells is absorbed chemical energy and this energy
  is available to perform physiological work.
• Ingested energy-egested energyabsorbed chemical
  energy

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The 3 major physiological functions

• Biosynthesis
• Produce chemicals with chemical-energy content,
  i.e. ATP.
• Produce new cells and tissues.
• Produce organic compounds that are exported from
  the body during an animals life.

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The 3 major physiological functions

• Maintenance functions maintain the integrity of
  the body and this energy is entirely degraded to
  heat.
• Circulation, respiration, nervous coordination,
  gut motility, tissue repair.
• Mechanical work in the body is internal work

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The 3 major physiological functions

• Generation of external work is the application of
  forces to objects or forces outside the body.
• Energy is transmitted to the environment and is
  sometimes converted into potential energy.

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Figure 6.2 The uses of energy by an animal
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Metabolic rate

• The metabolic rate is the rate of energy
  consumption or the rate at which an animal
  converts chemical energy into heat and external
  work.
• Metabolic rate affects how much food needs to be
  ingested.
• Metabolic rate can be measured to quantitatively
  determine total activity of all physiological
  mechanisms.
• Metabolic rate measures the drain produced by an
  animal on its ecosystem
• The rate of degradation of chemical energy or
  organic compounds.

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Measurement of metabolic rate

• Metabolic rates can be measured using a direct
  calorimeter.
• Traditional unit of measurement is calorie, which
  is the unit needed to increase 1g of water 1oC, a
  kilocalorie is kcal or C.
• Measurement of energy is joule (J). A watt is 1
  J/s.
• 1 cal4.186J
• Indirect measurements of metabolic rate use the
  rate of O2 consumption.

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Fig. 6.4 Lavoisiers direct calorimeter
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Indirect calorimeter
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Injestion of food and metabolic rate

• The increase in metabolic rate caused by food
  ingestion is called specific dynamic action (SDA)
  or the heat increment of feeding.
• Magnitude of the increase is the total excess
  metabolic heat produced by the meal.
• The SDA is proportional to the food eaten and it
  rises after food absorption, particularly
  proteins.
• Diet can induce chronic changes in metabolic
  ratediet induced thermogenesis (DIT).

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Figure 6.5 Specific dynamic action (Part 1)

• Specific dynamic action is the calorigenic effect
  of ingested food.

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Figure 6.5 Specific dynamic action (Part 2)

• SDA is proportional to the amount of food eaten

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Basal metabolic rate (BMR) and standard metabolic
rate (SMR)

• BMR applies to homeotherms and is the animals
  metabolic rate while it is in its
• Thermoneutral zonethe temperature zone within
  which metabolic rate is minimal
• Fasting
• Resting
• SMR applies to poikilotherms and is the animals
  metabolic rate while it is
• Fasting
• Resting
• Routine metabolic rate is used by fish
  physiologist because of the constant motion of
  fish.

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Figure 6.6 The effect of body size on weekly
food requirements

• The energy needs of an organism are NOT
  proportional to their body size.

Ratio5.83
Ratio0.34
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Figure 6.7 BMR as a function of body weight in
various species of placental mammals
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Figure 6.8 Weight-specific BMR as a function of
body weight in various species
The relations can be expressed as M (total BMR or
SMR)aWb where W is body weight and a and b are
constants derived from the fitted curve. The
value of b is typically 0.7. This equation is an
allometric equation.
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Figure 6.9 Weight-specific metabolic rate as a
function of body weight in four groups of
vertebrates
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Figure 6.10 Metabolic rate and body weight are
related linearly on loglog coordinates (Part 1)
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Figure 6.10 Metabolic rate and body weight are
related linearly on loglog coordinates (Part 2)
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Physiological and ecological implications of
metabolic rate-body weight

• Cellular properties differ allometrically,
  particularly in reference to density of
  mitochondria.
• Resting heart rate and breathing rates differs
  allometrically.
• The structure of ecosystems is affected by
  allometric relations.
• Ex a square kilometer can support 95kg of
  warthogs, or 460kg of zebras, or 1250kgs of
  elephants.
• Environmental pollutants are concentrated in
  animals with a higher weight specific metabolic
  rate.
• Catabolism and excretion are higher in small
  species.

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Fig. 6.1 Proportionality of heart to body size
horse
cat
mouse
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Figure 6.12 Herbivores of different body sizes
coexisting on an African grassland
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Theories to explain the allometric relation
between metabolic rate and body weight

• Rubners surface law Surface areas increase only
  as the 2/3 power of their volume.
• The ratio of surface area to volume declines as
  organisms increase in size

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Rubners Logic

• Rubners logic rested on data from homeotherms
• Homeotherms tend to lose body heat to the
  environment
• Heat loss is proportional to the animals
  body-surface.
• Small homeotherms with a large surface area to
  volume lose heat more rapidly.
• An increase in metabolic processes is required
  for small homeotherms to maintain body
  temperature.

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• Problems with Rubners surface law
• Data contradicted the lawthe law predicts an
  exponent of 0.67 and the exponent is
  significantly higher 0.7
• Allometric relationships are also found among
  poikilotherms and the law only accounts for
  homeotherms.
• The explanation for the allometric relationship
  between metabolism and body weight should include
  all organisms

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Figure 6.13 Theories to explain the allometric
relation between metabolic rate and body weight
(1)

• Allometric relations are the result of key
  internal transport systems, which bring in the
  raw materials of metabolism. Rates of transport
  are constrained by geometry.

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Figure 5.12 Theories to explain the allometric
relation between metabolic rate and body weight
(2)

• Multiple causes theory states that the rate of
  any major processes depends on the interaction
  between multiple underlying processes.

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Energetic efficiency

• Lipids are twice as high in energy density as
  proteins and carbohydrates.
• The efficiency of energy absorption absorbed
  energy/ingested energy
• Absorbed energy is available for metabolism
• Not all energy is absorbed.
• Growing animals accumulate chemical-bond energy
  in their body.

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Figure 6.14 Net growth efficiency during each
year of life in Pacific sardines (Sardinops sagax)
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• Energy is the common currency of life.
• Most processes can be expressed in units of
  energy.
• The impact of organisms on the ecosystem can be
  determined by examining energy costs of
  organisms.
• Energy of mental effort?
• Resting brain consumes 20 of the energy
  absorbed.
• Francis Benedict determined the working brain did
  not consume more energy.
• is this still a good study?