build up in NADH means a drop in NAD+, inhibiting glycolysis
lactate dehydrogenase (LDH) converts pyruvate, NADH, and H+ to lactate and NAD+
reversible process
other pathways form less toxic end products and more ATP than lactate
- ex. succinate and proprionate in invertebrates

Lipids:

hydrophobic
carbon backbone in linear or ring structure
used for energy metabolism, cell structure, and signaling

Fatty acids:

chain of carbons with a carboxyl end
saturated: no double bonds
unsaturated: one or more double bonds

Beta-oxidation:

fatty acids are dense, more reduced forms of carbon (require more oxidation) to unlock energy
takes place in the mitochondria, aerobic metabolism
results in the formation of Acetyl CoA
Acetyl CoA is then oxidized

Ketones:

tissues that cannot metabolize fatty acids metabolize ketones
- ex. brain and shark muscle
ketogenesis:
- fatty acids converted to Acetyl CoA
- Acetyl CoA converted to ketones
- ketone bodies move through circulation
ketolysis:
- ketones are broken down into Acetyl CoA
- Acetyl CoA then participates in oxidative phosphorylation

Mitochondrial Metabolism

glucose → metabolites
metabolites → Acetyl CoA
Acetyl CoA enters the krebs cycle and is oxidized to form reducing agents
reducing agents are oxidized to release energy
O2 is the final electron acceptor

Parasites lack mitochondrial genome and loss their ability to rely on O2.

Tricarboxylic Acid (TCA) Cycle:

generates reducing equivalents
amphibolic pathway: some intermediates are used for synthesis while others are used for breakdown

Electron Transport System (ETS):

within the inner mitochondrial membrane
composed of four multisubunit proteins (complexes I, II, III, IV)
composed of two electron carriers (ubiquinone and cytochrome c)
oxidation: 4e- + 4H+ + O2 → 2H2O
generates a proton gradient, heat, water, and ROS

ATP Synthesis:

ADP + P → ATP
proton motive force
F1F0ATPase uses energy in proton force to produce ATP
no physical link
functionally coupled

Phosphocreatine:

alternative high-energy compound
creatine + ATP ←→ ADP + phosphocreatine
phosphocreatine can move throughout the cell like ATP
enflux of high energy

Spectroscopy:

measures ATP turnover
pros:
- measures cellular energy currency
- accurate over short time scales
cons:
- difficult
- subject must be restrained
- equipment not portable

Hess’ Law

total amount of energy released for breakdown of given amount of fuel always the same
regardless of intermediate chemical steps

Direct Calorimetry:

measurement of heat during chemical/physiological processes
pros:
- quite accurate
- accounts for aerobic and anaerobic energy production
cons:
- restrained
- heavy and complicated
- makes assumptions about anabolic vs catabolic actvitity

Indirect Calorimetry: Respirometry

inferring metabolic heat production
pros:
- friendly and easy
- can be portable
- very accurate with assumptions
- easy on active animals
cons:
- aerobic metabolism only
- must sample gases
- animal must be ‘tied’ at least

Respirometry Quotient:

rate of CO2 production/O2 consumption
RQ for carbohydrates > proteins > lipids
differ depending on which fuel is being oxidized
a greater RQ occurs during anaerobic exercise
fats produce less CO2 per O2 consumed
14.9-18.7% more O2 required when oxidizing fats compared to carbs

Hummingbirds hovering VO2 will be 15-19% greater when fasted (burning fat) compared to when fed (burning carbs). When fasted, they are burning fats which require more oxygen to burn.

Chamber Respirometry:

animal enclosed in chamber
pros:
- easiest approach
- more accurate
- easier to control
cons:
- contained animal
- risk of asphyxiation
- can be messy (all functions in the chamber)
closed system:
- make sure all gases are accounted for
- more accurate with longer time scales
- switching between rest and activity complicated the study

Mask Respirometry:

only covers mouth/nose/head
small volume, faster flow rates, animal can behaviour nearly naturally
poorer signal to nose ratio, composition of gases harder to control, assumes gas concentrations

The ATP turnover rate and the rate of fuel delivery to or supply in the cell do not necessarily match instantly. ATP is used instantly, while fuel delivery and processing take time.

O2 consumption/CO2 production measured at the respiratory interface with the environment does not reflect instantaneous O2 consumption/CO2 production in tissues.

during transition between exercise and rest, there can be an offset in tissue level O2 consumption or metabolic rate, and VO2
actual rate of oxygen consumption and CO₂ production in tissues is not reflected instantly in the air you breathe

Inverse relationship between max power and sustainability.

at maximum power, energy is used instantly
ATP Pool → Phosphocreatine → Anaerobic Glycolysis → Aerobic Glycolysis → Aerobic Lipolysis

O2 consumption remains high even after returning to low activity levels as glycogen stores are replenished.

Lactate produced is used to…
- Cori Cycle: Gluconeogenesis in the liver to return glucose to liver and rebuild glycogen
- Lactate Shuttle: Used as fuel → pyruvate → Acetyl CoA
Creatine produced is used to…
- rebuild phosphocreatine at the expense of ATP
- since ATP is largely being generated using aerobic metabolism, this can be done

Diving:

O2 consumption may not be possible
reduce exchange of O2/CO2
hypoxic (low oxygen) and hypercapnia (high carbon dioxide)
perform compensatory hyperventilation → store a lot of O2 in tissues and “blow off” CO2

Basal Metabolic Rate (BMR):

metabolic rate of homeothermic animal at rest
at a temperature within thermoneutral zone

Standard Metabolic Rate (SMR):

BMR for poikilothermic animal at a defined environmental temperature

Resting Metabolic Rate (RMR):

metabolic rate when animal is at rest under defined conditions
slightly higher than BMR

Maximum Aerobic Metabolic Rate (VO2 max):

maximum sustainable VO2
during intense exercise or when a homeotherm is exposed to very cold temperatures

Supermaximal Metabolic Rate:

burst only

Field Metabolic Rate:

the actual realized metabolic rate when the animal is behaving naturally in the wild

Daily Energy Expenditure:

total energetic cost of a day of life
not a rate, just an energy amount

Allometric scaling describes metabolic rate, where one variable scales to the power of the other.

whole-animal metabolic rate increases with increasing size (larger animals have more cells)
mass-specific metabolic rate decreases with increasing size (smaller animals have a larger surface area to mass ratio which means they burn more energy per unit of body mass)
the exponent for one is the inverse of the other
Scaling Exponent (b) tells you how a trait changes with body size. It’s a power that shows whether the trait increases faster, slower, or proportionally with size. About 0.67 for mammals and 0.75 interspecifically.
Scaling Coefficient (a) is a constant that sets the starting point for the scaling. It determines how large or small the value of the trait is to begin with.

Surface area to volume scaling affect gas exchange and nutrient absorption too.

VO2 max scales at an exponent greater than BMR but it is limited by O2 absorption and distribution.

Note