Applications of Thermodynamics
3. Thermodynamics in Biology
Learning outcomes
- I can explain energy transfer in living organisms.
- I can describe metabolism using thermodynamic principles.
- I can explain homeostasis as an energy balance.
- I can identify biological examples of entropy.
- I can relate thermodynamics to ecosystems.
Thermodynamics in Biology
Every living organism depends on continuous energy transfer and energy transformation. Cells require energy to build molecules, transport substances, move, reproduce, repair damage, maintain temperature, and perform thousands of chemical reactions.
Thermodynamics provides the physical framework for understanding these processes.
Living organisms obey the same fundamental thermodynamic laws as every other physical system:
First Law: energy is conserved.
Second Law: energy transformations increase the total entropy of the organism plus its surroundings.
Life does not violate thermodynamics. Organisms maintain highly organized internal structures by continuously taking in energy and matter and releasing energy and waste to their surroundings.
Living Organisms Are Open Systems
Most organisms are open systems.
They exchange both:
matter + energy
with their surroundings.
For example, a human may take in:
- food
- oxygen
- water
and release:
- carbon dioxide
- water
- waste materials
- thermal energy
Plants exchange:
- light energy
- carbon dioxide
- oxygen
- water
- minerals
- thermal energy
This continuous exchange allows organisms to maintain internal organization.
The First Law and Living Organisms
The First Law of Thermodynamics states that energy cannot be created or destroyed.
Therefore organisms cannot:
create energy
Instead, they transform energy from one form into another.
For example:
chemical energy in food → chemical energy in ATP → cellular work + thermal energy
The total energy is conserved.
Energy Transformations in the Body
Suppose a person eats food.
The food contains stored:
chemical potential energy
During cellular respiration, some of this energy becomes available for cellular processes.
It may eventually become:
- mechanical work
- electrical activity
- chemical potential energy
- thermal energy
For example:
food → ATP → muscle contraction → movement + heat
Metabolism
Metabolism is the complete set of chemical reactions occurring within an organism.
It includes reactions that:
- release energy
- store energy
- build molecules
- break down molecules
- maintain cellular conditions
Metabolism can be divided broadly into:
catabolism
and:
anabolism
Catabolism
Catabolic reactions break larger molecules into smaller molecules.
These reactions often release energy that can be captured for cellular use.
For example:
glucose → smaller products + energy
Cellular respiration is an important catabolic process.
Catabolic pathways help transfer chemical energy into forms cells can use.
Anabolism
Anabolic reactions build larger molecules from smaller molecules.
Examples include:
- protein synthesis
- DNA synthesis
- glycogen formation
- lipid synthesis
These processes generally require:
energy input
Therefore cells must couple energy-requiring reactions with energy-releasing processes.
ATP: The Cellular Energy Carrier
A major molecule involved in cellular energy transfer is:
ATP — adenosine triphosphate
Cells use ATP to couple energy-releasing reactions to energy-requiring processes.
A simplified reaction is:
ATP + H₂O → ADP + Pᵢ
This hydrolysis is thermodynamically favorable under typical cellular conditions and can be coupled to processes that require free-energy input.
What ATP Does
ATP can provide energy for:
- active transport
- muscle contraction
- protein synthesis
- cell division
- movement of cellular structures
- maintenance of ion gradients
- biosynthesis
ATP is therefore often described as an:
energy-transfer molecule
It is better understood as a molecule that helps transfer usable chemical free energy between cellular processes rather than as a permanent energy store.
Cellular Respiration
Cells obtain usable energy from nutrients through:
cellular respiration
A simplified aerobic respiration equation is:
glucose + oxygen → carbon dioxide + water + energy
In chemical form:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Some of the released energy is captured through ATP-producing processes.
Some is transferred as:
thermal energy
Cellular Respiration Is Not 100% Efficient
Cells do not convert all the chemical energy from glucose into ATP.
Some energy becomes:
thermal energy
This is not an accidental violation of energy conservation.
It is a natural consequence of real, irreversible energy transformations.
The released thermal energy can also be biologically useful, particularly in maintaining body temperature in endothermic animals.
Mitochondria
In eukaryotic cells, much aerobic ATP production occurs in the:
mitochondria
Mitochondria use controlled sequences of reactions rather than releasing all of glucose's energy in one step.
Important processes include:
- glycolysis, which occurs in the cytoplasm
- pyruvate oxidation
- citric acid cycle
- electron transport
- chemiosmosis
Stepwise energy transfer allows cells to capture useful free energy efficiently.
Energy Gradients
Biological systems frequently use gradients.
A gradient exists when a quantity differs between two regions.
Examples include differences in:
- ion concentration
- electrical potential
- chemical potential
- pressure
- temperature
Cells can store usable free energy in such gradients.
Proton Gradients
Mitochondria create a proton gradient across the inner mitochondrial membrane.
This gradient stores:
electrochemical potential energy
Protons then move through:
ATP synthase
which couples this movement to ATP production.
This is a remarkable biological example of energy conversion.
Energy Conversion in Cellular Respiration
A simplified pathway is:
chemical energy in glucose
↓
electron carriers
↓
proton gradient
↓
ATP
↓
cellular processes
At every stage, some energy becomes more dispersed, including as thermal energy.
Photosynthesis
Photosynthetic organisms capture energy from sunlight.
A simplified equation is:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
The process converts:
radiant energy → chemical potential energy
Plants can then use the chemical energy stored in organic molecules for cellular respiration and growth.
Photosynthesis and Thermodynamics
Photosynthesis produces highly organized energy-rich molecules from simpler substances.
This requires:
energy input from sunlight
Therefore the local increase in biological organization does not violate the Second Law.
The plant is not an isolated system.
Energy enters from the Sun and energy is eventually dispersed to the surroundings.
Gibbs Free Energy
Biological reactions are often analyzed using Gibbs free energy.
A useful relationship is:
ΔG = ΔH − TΔS
where:
- ΔG = change in Gibbs free energy
- ΔH = enthalpy change
- T = absolute temperature
- ΔS = entropy change
At constant temperature and pressure:
ΔG < 0 → thermodynamically favorable process
ΔG > 0 → requires coupling or energy input
ΔG = 0 → equilibrium
Spontaneous Does Not Mean Fast
A reaction with:
ΔG < 0
is thermodynamically favorable.
But this does not mean it occurs quickly.
Many biological reactions would occur extremely slowly without:
enzymes
Thermodynamics predicts energetic favorability.
Kinetics helps determine:
reaction rate
These are different concepts.
Enzymes and Thermodynamics
Enzymes increase reaction rates by lowering:
activation energy
However, enzymes do not change the overall:
ΔG
of the reaction.
They also do not change the equilibrium position.
They simply allow the system to approach equilibrium more quickly.
Coupled Reactions
Cells often connect an unfavorable reaction with a favorable reaction.
For example:
energy-requiring process + ATP hydrolysis
If the total coupled process has:
ΔG_total < 0
the combined process can proceed favorably.
This is known as:
energy coupling
Example: Active Transport
Cells often need to move substances:
against a concentration gradient
This is not spontaneous by itself.
Therefore it requires an energy source.
For example:
ATP hydrolysis → active transport
The sodium-potassium pump is a well-known example.
It uses ATP to maintain different ion concentrations across the cell membrane.
Ion Gradients and Nerve Cells
Neurons maintain ion gradients involving ions such as:
- Na⁺
- K⁺
- Ca²⁺
These gradients store electrochemical free energy.
During electrical signaling, controlled ion movement changes the membrane potential.
The gradients must then be maintained or restored using cellular energy.
Thus nervous-system activity ultimately depends on:
metabolism
Muscle Contraction
Muscle cells convert chemical free energy into:
mechanical work
ATP participates in interactions between:
actin and myosin
During contraction:
chemical energy → mechanical work + thermal energy
This is another example of the First and Second Laws operating in biology.
Biological Efficiency
Muscles cannot convert all available chemical energy into mechanical work.
Some becomes:
thermal energy
Therefore:
useful mechanical output < chemical energy input
This resembles engineered machines.
However, biological systems are complex chemical systems rather than simple heat engines.
Homeostasis
Homeostasis is the maintenance of relatively stable internal conditions despite changes inside or outside an organism.
Examples include regulation of:
- body temperature
- blood glucose
- water balance
- ion concentration
- pH
- oxygen and carbon dioxide levels
Maintaining these conditions requires:
energy
Homeostasis as an Energy Balance
Consider human body temperature.
The body continuously:
- produces thermal energy through metabolism
- exchanges energy with the environment
For approximately stable body temperature:
rate of energy gain ≈ rate of energy loss
If energy gain exceeds loss:
body temperature tends to rise
If energy loss exceeds gain:
body temperature tends to fall
Human Thermal Energy Balance
The human body exchanges energy through:
- radiation
- conduction
- convection
- evaporation
The relative importance of each mechanism depends on environmental conditions.
Radiation
The body emits infrared radiation.
It also absorbs radiation from its surroundings.
Net radiative energy transfer depends on factors including:
- skin and clothing temperature
- surrounding temperature
- surface properties
In cooler surroundings, the body often loses net energy through radiation.
Conduction
When the body touches another material, energy can transfer through:
conduction
For example, sitting on a cold metal surface can transfer energy:
body → metal
because the metal is colder.
Convection
Air or water moving past the body can carry thermal energy away.
This is:
convection
Wind can increase convective heat loss.
This contributes to the effect commonly described as:
wind chill
Evaporation
When sweat evaporates:
liquid water → water vapor
energy is required.
This energy comes partly from the skin and nearby tissues.
Therefore evaporation causes:
cooling
Sweating is an important homeostatic mechanism.
Sweating
When body temperature rises:
- sweat production increases
- sweat reaches the skin
- water evaporates
- energy is transferred away
- body temperature tends to decrease
This is a:
negative feedback mechanism
Shivering
When body temperature becomes too low, muscles may contract rapidly and repeatedly.
This is:
shivering
Increased muscle activity raises metabolic energy conversion.
More energy is released as:
thermal energy
helping warm the body.
Vasodilation
When the body needs to lose more thermal energy, blood vessels near the skin can widen.
This is:
vasodilation
More warm blood reaches the skin.
This can increase energy transfer to the surroundings.
Vasoconstriction
In cold conditions, blood vessels near the skin may narrow.
This is:
vasoconstriction
Less warm blood reaches the surface.
This reduces thermal-energy transfer from the body to the environment.
Endotherms
Endotherms, such as mammals and birds, generate substantial thermal energy metabolically and regulate internal temperature.
This requires continuous energy consumption.
Therefore maintaining a stable high body temperature has a significant:
metabolic cost
Ectotherms
Ectotherms rely more strongly on external environmental energy sources to regulate body temperature.
Examples include many:
- reptiles
- amphibians
- fish
- invertebrates
They may regulate temperature behaviorally by:
- basking
- moving into shade
- changing depth
- changing activity time
Thermodynamics influences their behavior and habitat use.
Entropy in Biology
The Second Law of Thermodynamics states that the total entropy of an isolated system cannot decrease.
Living organisms appear highly organized.
Cells construct:
- proteins
- membranes
- DNA
- tissues
- organs
Does this violate the Second Law?
No.
Living organisms are open systems.
Local Order and Total Entropy
An organism can decrease entropy locally by creating ordered structures.
However, doing so requires energy transformations that increase entropy elsewhere.
Therefore:
local entropy decrease is possible
while:
ΔS_total > 0
for the larger system.
This distinction is fundamental to understanding thermodynamics and life.
Building a Protein
Cells assemble amino acids into highly ordered proteins.
This creates greater local organization.
However, protein synthesis requires:
- ATP
- GTP
- enzymes
- cellular machinery
Energy is transformed and thermal energy is released.
The total entropy change of:
cell + surroundings
remains consistent with the Second Law.
Growth and Development
A seed can grow into an enormous tree.
This represents a major increase in biological complexity and organization.
But the tree:
- absorbs solar energy
- takes in matter
- performs metabolism
- releases thermal energy
- releases waste products
Therefore the tree is not becoming organized without an energy cost.
Organisms and Energy Dispersal
Consider an animal eating food.
The food contains relatively concentrated chemical energy.
The animal uses some of this energy for:
- growth
- movement
- repair
- reproduction
Eventually much of the energy becomes:
dispersed thermal energy
This contributes to increasing entropy in the surroundings.
Ecosystems and Thermodynamics
An ecosystem contains organisms interacting with each other and with their physical environment.
Energy flows through ecosystems.
A simplified pathway is:
Sun → producers → consumers → decomposers → surroundings
Unlike matter, energy is not continuously recycled through an ecosystem in the same way.
Energy:
flows through
the ecosystem.
Producers
Plants, algae, and some microorganisms are:
primary producers
They capture energy, often from sunlight.
Through photosynthesis:
radiant energy → chemical energy
This chemical energy becomes available to other organisms through feeding relationships.
Consumers
Consumers obtain chemical energy by eating:
- producers
- other consumers
The chemical energy is used for:
- respiration
- movement
- growth
- reproduction
- maintenance
At every stage, some energy is dispersed as:
thermal energy
Decomposers
Decomposers break down dead organisms and biological wastes.
Examples include many:
- bacteria
- fungi
They obtain chemical energy from organic matter and release nutrients back into ecosystems.
Their metabolism also transfers energy to the surroundings as heat.
Energy Flow Through Trophic Levels
Only part of the energy available at one trophic level becomes biomass available to the next.
Energy may be:
- used in respiration
- transferred as heat
- used for movement
- lost in waste
- contained in uneaten material
Therefore:
energy available generally decreases at higher trophic levels
Energy Pyramids
An energy pyramid represents the amount of energy available at different trophic levels.
The base usually contains:
producers
Above them are:
- primary consumers
- secondary consumers
- higher-level consumers
The pyramid becomes narrower because less usable energy is available at each successive level.
The 10% Rule
A rough ecological guideline sometimes states that approximately:
10%
of energy at one trophic level becomes biomass available to the next trophic level.
However, this is not a universal thermodynamic law.
Actual transfer efficiencies vary substantially among:
- organisms
- ecosystems
- trophic levels
The important thermodynamic principle is:
energy transfer is never perfectly efficient
Worked Example: Trophic Energy
Suppose producers contain:
20,000 kJ
of energy available to the next trophic level.
If 10% is transferred:
Primary consumers:
20,000 × 0.10
= 2000 kJ
If 10% transfers again:
Secondary consumers:
2000 × 0.10
= 200 kJ
Again:
Tertiary consumers:
200 × 0.10
= 20 kJ
This helps explain why ecosystems generally support fewer organisms at high trophic levels.
Why Food Chains Are Limited
If very little usable energy reaches higher trophic levels, extremely long food chains become difficult to support.
At each level:
some chemical energy → thermal energy
The energy is not destroyed.
Instead, it becomes increasingly dispersed and less available to support another trophic level.
Energy Flows, Matter Cycles
This distinction is extremely important.
Matter
Elements such as:
- carbon
- nitrogen
- phosphorus
- water
can cycle through ecosystems.
Energy
Energy generally:
enters → is transformed → becomes dispersed → leaves
For most ecosystems, the major external energy source is the:
Sun
Carbon Cycle and Energy
Carbon atoms can cycle between:
- atmosphere
- plants
- animals
- soil
- oceans
But the energy associated with biological processes does not simply cycle indefinitely.
For example:
photosynthesis stores energy
while:
respiration releases usable chemical free energy and ultimately disperses much of it as heat
Matter cycles.
Energy flows.
Respiration and Ecosystems
Cellular respiration occurs throughout food webs.
Producers respire.
Consumers respire.
Decomposers respire.
Therefore all major biological groups contribute to the conversion:
chemical energy → cellular work + thermal energy
This continual energy dispersal is consistent with the Second Law.
Photosynthesis and Respiration
These processes are closely connected.
Photosynthesis
CO₂ + H₂O + light energy → energy-rich organic molecules + O₂
Respiration
energy-rich organic molecules + O₂ → CO₂ + H₂O + released energy
They participate in cycling matter while helping move energy through the biosphere.
Thermodynamics and Population Size
The amount of energy available can limit:
- population size
- biomass
- food-chain length
- ecosystem productivity
A habitat receiving more usable energy can potentially support greater biological productivity, although many other factors also matter.
Primary Productivity
Primary productivity measures the rate at which producers store energy in organic matter.
Gross primary productivity
Total rate at which producers capture energy.
Net primary productivity
Energy remaining after producers use some energy for their own respiration.
A simplified relationship is:
NPP = GPP − R
where:
- NPP = net primary productivity
- GPP = gross primary productivity
- R = energy used in respiration
Worked Example: Productivity
Suppose plants capture:
12,000 kJ/m² per year
and use:
5000 kJ/m² per year
through respiration.
Then:
NPP = GPP − R
NPP = 12,000 − 5000
NPP = 7000 kJ/m² per year
This represents energy stored in new biomass and potentially available to consumers and decomposers.
Biological Energy Efficiency
Biologists can define efficiencies for different processes.
For example:
ecological efficiency = energy transferred to next trophic level / energy available at previous level
If:
500 kJ
becomes:
75 kJ
at the next level:
efficiency = 75/500
efficiency = 0.15
efficiency = 15%
Homeostasis Beyond Temperature
Homeostasis involves many forms of energy-dependent regulation.
Examples include maintaining:
- ion gradients
- blood glucose
- water balance
- pH
- oxygen levels
These stable conditions are not maintained automatically.
Cells must continuously:
perform work
Osmoregulation
Organisms regulate water and dissolved substances through:
osmoregulation
Moving ions against concentration gradients requires active transport.
Active transport requires:
energy
Therefore maintaining water and salt balance has a metabolic cost.
Blood Glucose Regulation
Animals regulate blood glucose using coordinated hormonal mechanisms.
Glucose may be:
- absorbed
- stored
- released
- oxidized
These processes involve transformations of chemical energy.
Homeostasis therefore involves both:
matter balance + energy balance
Maintaining pH
Cells must maintain appropriate pH for enzymes and biochemical reactions.
This can involve:
- buffers
- ion transport
- respiratory regulation
- kidney function
Some regulatory processes require active transport and therefore:
ATP
Organisms Away from Equilibrium
A dead organism eventually approaches greater equilibrium with its surroundings.
A living organism actively maintains conditions far from equilibrium.
For example, cells maintain:
- concentration gradients
- membrane potentials
- chemical differences
- organized structures
Maintaining these differences requires:
continuous energy input
Equilibrium and Life
Complete thermodynamic equilibrium would eliminate many gradients required for biological activity.
If all relevant gradients disappeared:
- no useful ion gradients
- no chemical potential differences
- no organized energy flow
Cells could no longer perform the processes required for life.
Life therefore depends on maintaining:
non-equilibrium states
through continuous energy transfer.
Human Metabolism and Energy
The chemical energy obtained from food supports:
- basal metabolism
- physical activity
- thermoregulation
- growth
- repair
Even a resting person continuously uses energy for:
- breathing
- circulation
- nerve activity
- active transport
- protein turnover
- temperature regulation
Basal Metabolic Rate
Basal metabolic rate (BMR) refers to the energy expenditure required to maintain basic body functions under standardized resting conditions.
It supports processes such as:
- heartbeat
- breathing
- brain activity
- ion pumping
- cellular maintenance
Even at rest:
metabolism continues
Exercise
During exercise, muscles require more ATP.
Therefore:
rate of cellular respiration increases
More chemical energy is transformed into:
- mechanical work
- thermal energy
Body temperature tends to increase.
Homeostatic responses such as sweating and increased blood flow to the skin help remove the additional thermal energy.
Worked Example: Human Energy
Suppose muscles receive:
1000 J
of chemical energy and produce:
250 J
of useful mechanical work.
Mechanical efficiency:
η = 250/1000
η = 0.25
η = 25%
The remaining:
1000 − 250
= 750 J
is transferred through other pathways, much of it ultimately as thermal energy.
The energy has not disappeared.
Thermoregulation During Exercise
During vigorous exercise:
metabolic rate increases
↓
thermal energy production increases
↓
body temperature tends to rise
↓
sweating and skin blood flow increase
↓
energy loss increases
This is a biological feedback system involving thermodynamics.
Animals in Cold Environments
Animals living in cold environments may reduce thermal-energy loss using:
- thick fur
- feathers
- fat
- reduced exposed surface area
- countercurrent heat exchange
These are adaptations involving:
heat-transfer control
Countercurrent Heat Exchange
Some animals use countercurrent heat exchange.
Warm blood flowing toward an extremity passes near colder blood returning toward the body.
Energy transfers:
warm outgoing blood → cooler returning blood
This conserves thermal energy in the body's core.
This is similar in principle to counterflow heat exchangers used in engineering.
Surface Area and Heat Transfer
Small animals generally have a larger:
surface-area-to-volume ratio
than large animals.
Therefore they can lose thermal energy relatively quickly.
This helps explain why many small endotherms require high metabolic rates relative to body mass.
Body shape can therefore influence:
thermoregulation
Plants and Thermodynamics
Plants also experience energy balances.
A leaf:
- absorbs solar radiation
- emits infrared radiation
- exchanges energy with air
- loses energy through evaporation
Transpiration can cool leaves because evaporation requires energy.
Therefore plant temperature is influenced by:
radiation + convection + evaporation
Transpiration Cooling
Water evaporating from leaf surfaces requires energy.
Therefore:
liquid water → water vapor
can remove thermal energy from the leaf.
This resembles evaporative cooling through human sweating.
Decomposition and Entropy
When an organism dies, decomposers break complex organic material into simpler substances.
Chemical energy is used by decomposers and increasingly dispersed through metabolism.
This contributes to:
- nutrient recycling
- energy dispersal
- increasing total entropy
Again:
matter cycles while energy flows
Life and the Second Law
The existence of complex living organisms is sometimes incorrectly presented as a challenge to the Second Law.
There is no conflict.
The Second Law concerns:
total entropy
Living organisms maintain local organization by consuming energy and increasing entropy in their surroundings.
For the larger system:
ΔS_total ≥ 0
Life operates because Earth receives a continuous supply of energy, predominantly from the Sun.
Earth, Life, and Solar Energy
A simplified biosphere energy pathway is:
Sun
↓
photosynthetic organisms
↓
chemical energy
↓
food webs
↓
metabolism
↓
thermal energy
↓
environment and eventually space
This flow helps maintain the highly organized structures found in Earth's biosphere.
Thermodynamics Across Biological Scales
Thermodynamic principles apply from molecules to entire ecosystems.
Molecular scale
ATP hydrolysis and biochemical reactions.
Cellular scale
Respiration, active transport, ion gradients.
Organism scale
Movement, metabolism, thermoregulation, homeostasis.
Ecosystem scale
Food webs, productivity, energy pyramids.
Biosphere scale
Solar energy enters Earth and is eventually radiated back to space.
The same fundamental laws apply at every scale.
Common Mistakes
Mistake: Saying organisms create energy.
Organisms transform energy; they do not create it.
Mistake: Saying ATP is energy itself.
ATP is a molecule whose reactions can transfer usable chemical free energy.
Mistake: Saying all food energy becomes mechanical work.
A substantial fraction is ultimately transferred as thermal energy.
Mistake: Saying enzymes make unfavorable reactions favorable by changing ΔG.
Enzymes lower activation energy but do not change the overall ΔG.
Mistake: Saying organisms violate the Second Law because they become more organized.
Organisms are open systems and increase entropy in their surroundings.
Mistake: Saying entropy must increase everywhere.
Entropy can decrease locally as long as total entropy increases appropriately.
Mistake: Saying energy is recycled through ecosystems.
Matter cycles extensively; energy flows through ecosystems and becomes increasingly dispersed.
Mistake: Treating the 10% rule as an exact law.
It is a rough ecological guideline; actual transfer efficiency varies.
Mistake: Saying homeostasis requires no energy because conditions remain constant.
Maintaining stable conditions often requires continuous metabolic work.
Mistake: Saying ectotherms have no thermoregulation.
Many ectotherms regulate temperature behaviorally.
Key Terms
Metabolism — complete set of chemical reactions occurring within an organism.
Catabolism — metabolic pathways that break molecules down and often release usable free energy.
Anabolism — metabolic pathways that build complex molecules and require energy input.
ATP — molecule that couples energy-releasing and energy-requiring cellular processes.
Cellular respiration — set of processes that transfers chemical energy from nutrients into usable cellular forms, including ATP.
Photosynthesis — process that captures light energy and stores some of it as chemical energy.
Gibbs free energy — thermodynamic quantity used to predict energetic favorability at constant temperature and pressure.
Energy coupling — use of an energetically favorable process to drive an unfavorable process.
Homeostasis — maintenance of relatively stable internal conditions.
Thermoregulation — regulation of organism temperature.
Endotherm — organism relying substantially on metabolic energy to maintain body temperature.
Ectotherm — organism relying strongly on external thermal conditions for body-temperature regulation.
Entropy — thermodynamic quantity associated with energy dispersal and the number of accessible microscopic arrangements.
Open system — system exchanging matter and energy with its surroundings.
Trophic level — feeding position within a food chain or food web.
Primary producer — organism that converts an external energy source into chemical energy stored in organic matter.
Primary productivity — rate at which producers store energy in organic matter.
Gross primary productivity (GPP) — total rate of energy capture by producers.
Net primary productivity (NPP) — energy stored by producers after their own respiratory energy use.
Countercurrent heat exchange — heat-transfer arrangement in which fluids moving in opposite directions exchange thermal energy efficiently.
Non-equilibrium state — state maintained away from thermodynamic equilibrium through continuing energy and matter transfers.
Key Takeaways
- Living organisms obey the First and Second Laws of Thermodynamics.
- Organisms are generally open systems exchanging both matter and energy.
- Energy cannot be created or destroyed by biological processes.
- Metabolism transforms energy between different forms.
- Catabolism breaks molecules down and can release usable free energy.
- Anabolism builds molecules and requires energy input.
- ATP helps couple energy-releasing processes to energy-requiring cellular processes.
- Cellular respiration transfers chemical energy from nutrients into ATP and other energy pathways.
- Some energy from metabolism ultimately becomes thermal energy.
- Photosynthesis converts radiant energy into chemical potential energy.
- Biological reactions can be analyzed using Gibbs free energy.
- Enzymes affect reaction rates but do not change the overall thermodynamic favorability of reactions.
- Cells use energy coupling to drive energetically unfavorable processes.
- Ion gradients store electrochemical potential energy.
- Active transport requires energy.
- Homeostasis often requires continuous energy input.
- Human body temperature depends on balancing metabolic heat production with energy transfer to the environment.
- Radiation, conduction, convection, and evaporation all contribute to human thermal balance.
- Sweating cools the body through evaporation.
- Shivering increases metabolic heat production.
- Living organisms can become locally more organized without violating the Second Law.
- Local entropy can decrease while total entropy increases.
- Maintaining biological organization requires continuous energy transformations.
- Ecosystems depend on continuous energy flow.
- Most ecosystems receive their major external energy input from sunlight.
- Producers convert radiant energy into chemical energy.
- Energy is transferred between trophic levels through feeding.
- Usable energy generally decreases at higher trophic levels.
- Energy pyramids represent this decrease in available energy.
- The 10% rule is an approximation, not a universal law.
- Matter cycles through ecosystems, while energy flows through them.
- Primary productivity describes the rate at which producers store energy.
- NPP = GPP − R.
- Organisms maintain important gradients and therefore remain far from thermodynamic equilibrium.
- Thermodynamics links molecular biology, physiology, ecology, and the entire biosphere.
Check Your Understanding
- Why are living organisms described as open systems?
- Explain how the First Law applies to cellular respiration.
- Distinguish between catabolism and anabolism.
- What role does ATP play in cellular energy transfer?
- Describe the main energy transformations involved in cellular respiration.
- Why does cellular respiration release thermal energy?
- Explain how photosynthesis is consistent with the Second Law of Thermodynamics.
- What does ΔG < 0 indicate about a process under appropriate constant-temperature and constant-pressure conditions?
- Why do enzymes not change the overall ΔG of a reaction?
- Explain how ATP can drive an energetically unfavorable cellular process.
- What is homeostasis, and why does maintaining it require energy?
- Describe two mechanisms the human body uses to control thermal-energy transfer.
- Explain why sweating cools the body.
- How can an organism become more organized without violating the Second Law?
- Explain the difference between energy flow and matter cycling in an ecosystem.
- Why does less usable energy generally become available at higher trophic levels?
- Producers have a GPP of 15,000 kJ/m² per year and use 6000 kJ/m² per year in respiration. Calculate NPP.
- Explain how countercurrent heat exchange helps an animal conserve thermal energy.
- Why must living cells maintain conditions away from thermodynamic equilibrium?
- Trace energy from sunlight through a plant, an herbivore, and a predator, explaining how the First and Second Laws apply at each stage.