Metabolism and Metabolic Rate

Metabolism Overview

• Glucose metabolism starts with glycolysis, converting glucose to pyruvate.
• Pyruvate then gets oxidized into acetyl-CoA to enter the Krebs (Citric Acid) Cycle.
• The Krebs cycle converts pyruvate into a series of intermediate compounds, which results in 2 ATP, FADH, and NADH production.
• FADH and NADH then enter the electron transport chain for further ATP production.
• Glucose metabolism requires oxygen consumption and produces carbon dioxide, specifically in aerobic respiration.
• Besides glucose, proteins and lipids are metabolized, also consuming oxygen and producing carbon dioxide.
• Oxygen consumption and CO2 production measures metabolic rate or energy consumption rate.

Metabolic Rate Basics

• Animals ingest food and is converted into chemical energy and then it is used for:
  • Biosynthesis: macromolecule construction
  • Maintenance: basic physiological/biochemical processes
  • External work: locomotion
• Some energy is lost in feces or exported in organic forms like milk or mucus; energy use is inefficient, producing metabolic heat as waste.
• Metabolic rate is the rate of energy consumption, measured by heat production or monitoring oxygen consumption and carbon dioxide production.
• Metabolic rate dictates food needs and activity levels; a higher rate means needing more food and enabling greater activity.
• Metabolic rate impacts an animal's "drain on the ecosystem," because a higher rate demands more resource extraction from the environment.

Direct Calorimetry

• Direct calorimetry measures metabolic rate by directly assessing heat production.
• Animals are put in a chamber inside a larger, ice-filled chamber during direct calorimetry.
• The animal's heat melts the ice, and the resulting water goes into a collection container.
• Measuring the collected water tells the amount of energy needed to melt the ice, giving a direct measure of metabolic rate.
• Heat production is determined by knowing that 1 calorie is about 1 degree Celsius to heat 1 gram of water.
• Large Calorie (food calorie) is equal to 1000 real calories.

Direct and Indirect Calorimetry Combination

• Human metabolic rate can be measured using direct calorimetry with a calorimeter, with or without exercise.
• Heat produced during exercise warms water flowing through tubes in the calorimeter.
• Measuring the water temperature increase determines the amount of heat produced, indicating metabolic rate.
• Direct and indirect measurements can be combined by measuring oxygen consumption and carbon dioxide production.
• Air entering the chamber has roughly 21% O2 and 0.03% CO2; the difference between these and the levels in exhaled air indicated O2 consumption and CO2 production.
• Respirometry is the process of measuring oxygen consumption and carbon dioxide production levels.

Closed Respirometry

• Closed respirometry measures metabolic rate indirectly via oxygen consumption in a sealed system.
• Two glass chambers, animal and reference, are connected by a U-shaped manometer with colored water.
• The reference chamber contains an inert object, matching the animal's volume in the animal chamber (respirometer).
• In the animal chamber, the tested animal consumes oxygen and produces CO2, which is absorbed by a CO2 absorbent in both chambers.
• Oxygen used decreases air volume in the animal chamber, causing a negative pressure that moves the colored fluid in the manometer.
• The volume of oxygen used determines how much oxygen to inject back to return the colored water to the original position.

Open Respirometry

• Air flows constantly through the animal chamber, unlike closed respirometry with a sealed chamber.
• Oxygen and Carbon Dioxide electrodes measure inflowing and outflowing air levels; that difference indicates oxygen consumption and carbon dioxide production.

Material Balance

• Material balance compares energy consumed to energy excreted in feces for long-term metabolic rate measurement.
• Animals get a known diet in a known quantity, measuring how much energy it is ingesting, then feces are collected to measure energy lost.
• The difference between energy in via food and energy out via feces tells the amount of energy for metabolic processes.
• A bomb calorimeter is used to measure energy in feces is a device to measure the energy in the feces.
• Feces get placed in an oxygen-filled inner chamber surrounded by a water-filled outer chamber.
• When the ignition wires spark the explosion to combust the feces completely which heats the water in the outer chamber, measuring the water with a thermometer.
• The heat produced measures how much energy was in the feces.

Endothermy and Homeothermy

• Poikilothermy and homeothermy are on a temperatures continuum, with body temperature fluctuating (poikilotherms) or staying consistent (homeotherms).
• Ectothermy and endothermy are on a heat origin continuum; ectothermic animals derive body heat from environment, and endothermic animals produce their own heat metabolically.
• Poikilothermy and ectothermy usually occur together, similar to homeothermy and endothermy.

Thermoneutral Zone

• A graph plots resting metabolic rate against ambient temperature in endothermic/homeothermic animals that can keep a constant body temperature except in extreme temperatures.
• Metabolic rate stays steady inside the thermoneutral zone (TNZ), which is a certain range of ambient temperatures.
• Metabolic rate increases below a lower-critical temperature or above an upper-critical temperature to maintain body temperature constantly.
• Resting (basal) metabolic rate calculations for an endotherm must be done when the tested animal is within its thermoneutral zone.

Specific Dynamic Action

• Specific dynamic action increases metabolic rate from feeding, which depends in magnitude and duration depends on the meal size.
• Fasting is time without specific dynamic action, so there is no increase in metabolic rate.

Resting (Basal and Standard) Metabolic Rate

• Resting metabolic rate can be broken down into two definitions dependent on the animal:
  • For endotherms (producing heat from metabolism):
    • Basal metabolic rate represents metabolic rate while fasting, at rest, and within is thermoneutral zone.
  • For ectotherms (producing body heat through environment):
    • Standard metabolic rate represents the metabolic rate while fasting and at rest.

Metabolic Rate and Body Size

• Energy and food needs don't scale proportionally to body size.
• Smaller animals need more energy per weight vs larger animals.
• For example, a 1900 Kg rhinoceros needs to eat approximately one-third (650/1900 = 0.34) of its body weight per week. On the other hand, a 30g mouse requires 6 times its body weight every week (175/30=5.8).

Wieght-Specific Metabolic Rate

• As animals get bigger, each gram of their tissue requires less energy and therefor weight-specific BMR is a function of the body weight.
• A “shrew-to-elephant” curve or “Klieber line” demonstrates this relationship (discovered by Max Klieber).
• A shrew's weight-specific MR is roughly 7 ml O2 per hour per gram of tissue.
• An elephants is 0.2 ml O2 per hour per gram of tissue.

Metabolic Rate vs Body Size

• As the body weight increases, the whole animal metabolic rate climbs, but the weight, or mass, specific metabolic rate is higher in smaller animals compared to larger animals.
• Smaller animals have higher weight-specific metabolic rate (MR) versus larger animals.
• Smaller animals have faster heart and breathing rates and more mitochondria/tissue.
• Their tissues need more oxygen versus larger animals for body weight, so they breathe faster, have faster heart rates, and more mitochondria to make more ATP.
• Smaller animals commonly have shorter lifespans and faster generational times versus larger animals.

B8. The Mathematics of Body Weight and Metabolic Rate

• Metabolic rate and body weight are expressed: M = aW^b
  • M = metabolic rate (BMR or SMR)
  • W = body weight
  • a = constant, representing MR of 1 gram of an animal type (e.g., mammal, bird)
  • b is roughly 0.65–0.75 for all animals