⚡ Lecture 7: Energy

Energy Terminology

Energy

• Capacity of a physical system to perform work

Forms of Energy

• Mechanical energy

• Chemical energy

• Light energy

• Heat energy

Food Energy

Definition of Food Energy

• Chemical energy that animals obtain from certain nutrients in food

Sources of Food Energy

• Carbohydrates

• Proteins

• Fats

Energy Release

• Nutrients are metabolized (broken down and processed by the body)

• During metabolism, energy is released or absorbed

Energy Terminology – Heat of Combustion and Energy Transfer

Heat of Combustion

• Heat of combustion: energy released as heat during the combustion (burning) of a substance, such as food

Catabolic Reaction

• Breaks down complex molecules into simple molecules

• Transfers energy from complex molecules to ATP

• Heat is released

• Complex molecules: starches, proteins, lipids

• Simple molecules: monosaccharides, amino acids, glycerol, fatty acids

Anabolic Reaction

• Transfers energy from ATP to form complex molecules

• Heat is released (energy is used from ATP)

ATP and ADP

• ATP: involved in energy transfer

• ADP: involved in the ATP energy cycle

Functions of Food Energy

Energy Requirements

- Food energy is required to sustain an animal’s metabolism during:

• Maintenance — maintaining normal body functions

• Growth — increasing body size

• Reproduction — gestation (pregnancy) and/or lactation (milk production)

• Physical activity — energy used for movement and activity

Energy Requirements

Factors Affecting Energy Needs

• What an animal’s metabolism needs to support

• How long those needs must be supported

Thermoregulation: Endotherms vs Ectotherms

Endotherms

- Regulate body temperature primarily using heat produced by their metabolism

Ectotherms

- Rely primarily on external environmental heat sources to regulate body temperature

Endotherms

Endotherms

- Body temperature primarily determined by heat derived from metabolic activity

Endotherm Characteristics

Metabolism and Food Requirements

• Generate body heat through metabolic activity

• Higher mitochondrial density per cell than ectotherms

  • More mitochondria → higher metabolic rate → more energy needed → more food required

Endotherm Thermoregulation

Cooling Mechanisms

• Eating less → reduces heat produced during digestion and metabolism

• Reducing physical activity → decreases heat produced by muscles

• Panting and sweating → evaporative cooling as water evaporates and removes heat

• Vasodilation → blood vessels near the skin widen, increasing heat loss to the environment

Thermoregulation

Endotherms

• Body temperature primarily determined by heat produced through metabolic activity

• Higher mitochondrial density per cell than ectotherms → higher metabolic rate → greater food requirements

Ectotherms

Ectotherms

- Body temperature primarily determined by heat derived from the environment

Ectotherms: Environmental Temperature and Metabolism

Body Temperature

• Depends primarily on heat from the environment

• Fluctuates with environmental temperature

Metabolic Activity

• Metabolic heat production is minor or negligible

• Lower metabolic rate than endotherms

Ectotherm Cooling Mechanisms

Reducing Body Temperature

• Reduce physical activity → limits heat produced by movement

• Seek shade, shelter, or water → avoids heat or transfers heat to cooler surroundings

• Vasodilation → widens blood vessels, increasing heat transfer to the environment

• Mucus secretions → moisture evaporates from the body surface, helping remove heat

Ectotherm Temperature Regulation

Warming Mechanisms

• Increase physical activity → generates some metabolic heat

• Seek external heat sources → bask in sunlight or contact warm surfaces

• Countercurrent heat exchange → transfers heat from warm blood in arteries (from core) to cooler blood in veins, reducing heat loss

Countercurrent Heat Exchange

How Heat Is Conserved

• Warm arterial blood flows toward the cold tail, flipper, or foot

• Cool venous blood flows back toward the body

• Heat transfers from the warm artery to the cool vein

• Returning blood warms up before reaching the body → reduces heat loss to the environment

Ectotherms: Advantages and Disadvantages

Pros

• Require less food due to lower metabolic rates

• Higher carrying capacity → more individuals can be supported in the same environment (since they don’t consume as much food since they don’t need it for heat)

Cons

• Less functional in cold temperatures because body temperature and activity depend on environmental heat

Body Temperature vs. Environmental Temperature

Endotherms

• Maintain a relatively stable body temperature despite changes in environmental temperature

• Temperature regulation works up to a limit

Ectotherms

• Body temperature fluctuates with environmental temperature

• Body temperature rises or falls as environmental temperature changes

Stability of Body Temperature

Homeotherms vs. Poikilotherms

• Homeotherms → maintain a relatively constant body temperature

• Poikilotherms → body temperature fluctuates with environmental temperature

Memory Cue

• Homeo = same → stable body temperature

• Poikilo = variable → changing body temperature

Maintaining Body Temperature

Homeotherms

- Maintain a relatively constant body temperature, regardless of environmental temperature, up to a point

Body Temperature Stability

Homeotherms

• Maintain a relatively constant body temperature, regardless of environmental temperature, up to a point

• Colloquially called “warm-blooded”

• Includes mostly mammals and birds

Normal Body Temperatures

• Birds → 40°C

• Placental mammals → 37–38°C

• Marsupials → 34°C

• Monotremes → 30°C

Scientific Terminology

- “Warm-blooded” and “cold-blooded” are not scientifically precise terms

Homeothermy vs. Endothermy

Homeotherms Are Not Always Endothermic

• Homeotherms maintain a relatively constant body temperature

• Endotherms generate body heat primarily through metabolism

• Some fish in tropical or very deep waters can maintain a relatively constant body temperature without relying primarily on metabolic heat production

Body Temperature Variation

Poikilotherms

- Body temperature fluctuates considerably, usually according to environmental temperature

Variation in Body Temperature

Poikilotherms

• Body temperature fluctuates considerably, usually according to environmental temperature

• Colloquially called “cold-blooded”

• Includes mostly fish, reptiles, and amphibians

Exceptions to Mammalian Temperature Regulation

Naked Mole-Rat

• The naked mole-rat is considered a mammalian poikilotherm

• Its body temperature varies with environmental temperature

Thermoneutral Zone (TNZ)

Environmental Temperature Range

• Range of environmental temperatures where endotherms do not need to actively warm or cool themselves

• Body temperature is maintained without additional energy expenditure for thermoregulation

Factors Affecting the Thermoneutral Zone

Animal Characteristics

• Body composition → adiposity (body fat) affects insulation

• Fur/pelage → reduces heat loss

• Life stage → affects thermoregulatory needs

• Body size (surface area-to-volume ratio, SA/V) → affects heat exchange with the environment

  • Smaller animals have a higher metabolic rate and lose heat more quickly relative to their size

Surface Area-to-Volume Ratio

Comparing Different Body Sizes

- Cube with sides of 2 units

• Surface area = \(2^2 \times 6 = 24\) square units

• Volume = \(2^3 = 8\) cubic units

• SA/V = \(24/8 = 3\)

- Cube with sides of 1 unit

• Surface area = \(1^2 \times 6 = 6\) square units

• Volume = \(1^3 = 1\) cubic unit

• SA/V = \(6/1 = 6\)

Key Relationship

• Smaller animals have a higher surface area-to-volume ratio, so they lose heat more quickly relative to their volume

• Larger animals have a lower surface area-to-volume ratio, helping them retain heat more effectively

Surface Area-to-Volume Ratio (SA/V) *FROM CHAT

Effect of Body Size on Heat Loss

• Surface area → area through which heat is exchanged with the environment

• Volume → amount of space inside the body, related to body mass

• Smaller animals have a higher SA/V ratio

  • More surface area relative to their volume → lose heat more quickly

  • Higher metabolic rates help replace lost heat

• Larger animals have a lower SA/V ratio

  • Less surface area relative to their volume → retain heat more effectively

Upper Critical Temperature

Upper Limit of the Thermoneutral Zone

• Environmental temperature at the upper limit of an endotherm’s thermoneutral zone (TNZ)

• Above this temperature, the animal must use additional cooling mechanisms to maintain body temperature

Effects of Exceeding the Upper Critical Temperature

Heat Stress

• Remaining above the upper critical temperature (UCT) for too long can cause heat stress

• Anorexia → reduced appetite

• Excessive sweating/panting → dehydration

• Cellular dysfunction → occurs in severe cases

• Death → possible if heat stress is severe and prolonged

Lower Critical Temperature

Lower Limit of the Thermoneutral Zone

• Environmental temperature at the lower limit of an endotherm’s thermoneutral zone (TNZ)

• Below this temperature, the animal must use additional mechanisms to produce or conserve heat to maintain body temperature

Effects of Falling Below the Lower Critical Temperature

Cold Stress

• Remaining below the lower critical temperature (LCT) for too long can cause cold stress

• Excessive shivering → produces heat through muscle contractions

• Bodily processes may slow

• Cellular dysfunction → occurs in severe cases

• Death → possible if cold stress is severe and prolonged

Basal Metabolic Rate (BMR)

Minimum Daily Energy Requirements

• Minimal amount of energy required by an endothermic animal each day to maintain homeostasis

• Measured in the absence of factors that affect energy expenditure

Total Energy Expenditure

• BMR → 70% of total energy expenditure

• Physical activity → 20%

• Thermogenesis → 10%

Determining Basal Metabolic Rate (BMR)

Conditions for Measuring BMR

• At rest → awake but not physically active

• Post-absorptive state → after a 12-hour fast

• Thermoneutral environment → within the thermoneutral zone (TNZ), without needing to warm or cool the body

• No physical or psychological stress

• Measured in kcal/day

Determining Standard Metabolic Rate (SMR)

Conditions for Measuring SMR

• At rest → awake but not physically active

• Post-absorptive state → after a 12-hour fast

• Specific environmental temperature → measured at a defined temperature

• No physical or psychological stress

• Measured in kcal/day

Metabolic Body Weight

Definition

• Body weight raised to a power, typically BW^X, to account for differences in metabolic rate between animals of different sizes

• Used to scale energy requirements relative to body size

Energy Requirements and Body Size

Metabolic Body Weight (BW^X)

• Energy requirements do not increase directly with total body weight

• A 100 kg animal does not require twice the energy of a 50 kg animal

• Smaller animals generally require more energy per unit of body weight than larger animals

• The graph compares animals from shrews and mice to horses and elephants, showing that energy expenditure per gram decreases as body size increases

Memory Cue

• Smaller animal → more energy per kg

• Larger animal → more total energy, but less energy per kg

Estimating Energy Requirements Across Body Sizes

Kleiber’s Law

• Developed by Max Kleiber (1893–1976), a Swiss agricultural biologist

• Estimates basal metabolic rate (BMR) for mature (non-growing) endothermic animals

• Used because energy requirements do not increase linearly with body weight

Kleiber’s Equation

• Y = 70BW^0.75

• Y = energy expenditure (kcal/24 hours)

• 70 = constant

• BW = body weight (kg), raised to the power of 0.75 to account for metabolic body weight

Kleiber’s Law: Metabolic Body Weight

¾ Power Law

• In most animals, metabolic body weight follows the ¾ power of body weight

• Metabolic body weight = BW^0.75

• This accounts for the fact that energy requirements do not increase proportionally with body weight

Daily Energy Expenditure (DEE)

Total Daily Energy Use

• Total energy used to support essential body functions, digestion, metabolism, and physical activity

• DEE = BMR + metabolic heat production + physical activity

  • BMR → energy for essential body functions at rest

  • Metabolic heat production → energy used to produce heat

  • Physical activity → energy used for movement