Biology · Homeostasis, Nutrition & Biological Organisation
Energy budgets, body size and physiological trade-offs
Two animals can receive food containing the same energy yet retain different amounts in growth or reserves. To explain why, keep unabsorbed material, excreted compounds, heat, external work and retained chemical energy in one consistent account.
Animal Diversity & Body Plans supplies the varied arrangements used in selected examples. Cells, Membranes & Organisation explains boundaries and transport; Energy, Enzymes & Metabolism explains energy conservation and coupling. The first lesson reintroduces cells, tissues and local exchange before using them. Gene Regulation & Cell Signalling is a useful return path for feedback, and Chemistry’s Solutions, Concentration & Solubility for the distinction between amount and concentration.
Back to the Biology mapBy the end, you can
Account for energy over a stated interval and separate total metabolic expenditure from expenditure per unit mass.
An energy budget needs a boundary and an interval
Food contains molecules that can supply chemical energy and material building blocks. Cells couple transformations of those molecules to ATP production and other processes. ATP then participates in coupled transport, movement and synthesis. Digestion, ATP production and the use of ATP are different stages; breaking one bond is not a complete explanation of energy release.
The constructed animal receives 100 kJ of food chemical energy during one interval. Twenty kJ leave in egested material, and five leave in excreted compounds after absorption. The remaining seventy-five are divided among heat, external mechanical work and newly retained chemical energy. These are energy amounts, not grams of food, oxygen volumes or counts of ATP molecules.
In this account, assimilation means input minus egested food energy: 100−20=80 kJ. Five of those eighty subsequently leave in excreted compounds, leaving 75 kJ available in the remaining account. Explicit definitions matter because ecological and nutritional budgets sometimes use different conventions. Real faecal material can also contain microbes and endogenous secretions; the simplified ledger assigns no such extra contributions.
One input and six ending energy entries
Read the diagram in words
The 100 kJ input ends as 20 egested, 5 excreted, 45 heat, 5 external work, 15 growth and 10 reserves. Assimilated energy is 80; after both material losses, 75 remains in the account. Heat/work total 50 and growth/reserves total 25 are intermediate sums, not extra outputs.
| Quantity | Assigned value | Unit or counting convention |
|---|---|---|
| Food chemical-energy input | 100 | kJ |
| Identifier | Ending entry | Energy (kJ) |
|---|---|---|
| egested | Energy in egested material | 20 |
| excreted | Energy in excreted compounds | 5 |
| heat | Heat released | 45 |
| work | External mechanical work | 5 |
| growth | Chemical energy retained in growth | 15 |
| reserve | Chemical energy retained in reserves | 10 |
| Identifier | Intermediate sum | Energy (kJ) |
|---|---|---|
| assimilated | Input minus egested energy | 80 |
| available | After egested and excreted losses | 75 |
| spent | Heat plus external work | 50 |
| retained | Newly retained chemical energy | 25 |
| Intermediate sum | Identifier | Included ending entry |
|---|---|---|
| Input minus egested energy | excreted | Energy in excreted compounds |
| Input minus egested energy | heat | Heat released |
| Input minus egested energy | work | External mechanical work |
| Input minus egested energy | growth | Chemical energy retained in growth |
| Input minus egested energy | reserve | Chemical energy retained in reserves |
| After egested and excreted losses | heat | Heat released |
| After egested and excreted losses | work | External mechanical work |
| After egested and excreted losses | growth | Chemical energy retained in growth |
| After egested and excreted losses | reserve | Chemical energy retained in reserves |
| Heat plus external work | heat | Heat released |
| Heat plus external work | work | External mechanical work |
| Newly retained chemical energy | growth | Chemical energy retained in growth |
| Newly retained chemical energy | reserve | Chemical energy retained in reserves |
| Meaning | Numerator | Denominator | Fraction | Percentage |
|---|---|---|---|---|
| Retained among input | 25 | 100 | 0.25 | 25 |
| Retained after the two material losses | 25 | 75 | 0.3333333333333333 | 33.33333333333333 |
| Axis | Structure or interpretation | Minimum | Maximum | Unit or counting convention |
|---|---|---|---|---|
| energy | Energy during the assigned interval | 0 | 100 | kJ |
| Axis | Tick value |
|---|---|
| energy | 0 |
| energy | 20 |
| energy | 40 |
| energy | 60 |
| energy | 80 |
| energy | 100 |
| Identifier | Meaning |
|---|---|
| entry | Ending entry |
| energy | Energy |
| aggregate | Intermediate total |
| noDoubleCount | Do not add intermediate totals to their components |
| notMass | Energy amounts, not food masses |
Constructed one-interval energy ledger. No extra endogenous faecal contribution, pre-existing reserve drawdown, thermal-energy storage or unlisted energy input is assigned.
A retained pool and a spent process are different entries
The seventy-five remaining kilojoules become 45 kJ of heat, 5 kJ of external work, 15 kJ retained in new growth and 10 kJ retained in reserves. Heat and work together account for fifty. Listing that fifty again beside its forty-five and five components would double-count one process. Chemical energy retained in growth has not all been released as heat during the selected interval.
Maintenance includes ongoing activities of a living animal, even when it is not moving visibly. Active transport, molecular turnover and other processes require energy. Growth, reproduction, movement and regulation can compete for a limited supply, but their exact allocation depends on conditions. A universal fixed percentage cannot replace an animal's actual budget.
A tradeoff means that improving one outcome constrains another under specified limitations. A smaller retained reserve after greater activity is consistent with an energy constraint, but does not by itself identify the responsible mechanism. Changed food input, absorption, temperature or activity may also alter the balance. No dietary requirement or optimal allocation follows from our invented account.
Compare like rates before comparing sizes
A metabolic rate is energy turnover per unit time. Whole-animal rate and mass-specific rate answer different questions. In a separate constructed rested condition, animal A has mass 1 kg and an assigned rate of 100 kJ per day; B has mass 8 kg and rate 400 kJ per day. B's total is four times greater, while its rate per kilogram is half A's.
Body size affects exchange surfaces, distances, insulation and the proportions of different tissues. The cube comparison in the first lesson isolates geometry, not an entire animal. Actual metabolic scaling also depends on lineage, temperature, activity and measurement conditions. A cube's surface-to-volume ratio cannot by itself establish a universal metabolic exponent.
Basal and standard metabolic rates use defined physiological conditions, and neither means every animal's full daily expenditure. Our comparison is simply an assigned rested state, not measurements certified under those definitions. Larger total expenditure and smaller expenditure per mass can coexist without implying that every large animal is cheaper to sustain in every environment.
Total rate and rate per mass
Read the diagram in words
A has mass 1 kg, total rate 100 kJ/day and mass-specific rate 100 kJ/day/kg. B has mass 8 kg, total rate 400 kJ/day and mass-specific rate 50 kJ/day/kg. B/A ratios are 8 for mass, 4 for total rate and 0.5 for rate per mass.
| Identifier | Animal | Body mass (kg) | Whole-animal rate (kJ per day) | Rate per unit mass (kJ per day per kg) | Series mark |
|---|---|---|---|---|---|
| A | Animal A | 1 | 100 | 100 | Circle |
| B | Animal B | 8 | 400 | 50 | Diamond |
| Identifier | Selected panel | Selected quantity | Unit or counting convention |
|---|---|---|---|
| total | Whole-animal metabolic rate | totalRate | kJ per day |
| specific | Metabolic rate per unit mass | massSpecificRate | kJ per day per kg |
| Axis | Structure or interpretation | Minimum | Maximum | Unit or counting convention |
|---|---|---|---|---|
| total | Whole-animal metabolic rate | 0 | 500 | kJ per day |
| specific | Metabolic rate per unit mass | 0 | 125 | kJ per day per kg |
| Axis | Tick value |
|---|---|
| total | 0 |
| total | 100 |
| total | 200 |
| total | 300 |
| total | 400 |
| total | 500 |
| specific | 0 |
| specific | 25 |
| specific | 50 |
| specific | 75 |
| specific | 100 |
| specific | 125 |
| Quantity | Assigned value | Unit or counting convention |
|---|---|---|
| Body mass | 8 | Dimensionless ratio |
| Total rate | 4 | Dimensionless ratio |
| Rate ÷ mass | 0.5 | Dimensionless ratio |
| Identifier | Meaning |
|---|---|
| animal | Animal |
| mass | Body mass |
| massUnit | kg |
| total | Total rate |
| specific | Rate ÷ mass |
| condition | Separate assigned rested condition |
| noExponent | No universal metabolic exponent assigned |
Two original values under a stated rested condition; not empirical BMR data, a fitted allometric law or a prediction of intermediate masses.
Worked example
Close the budget and change the denominator
At the whole-animal boundary, 20 egested + 5 excreted + 45 heat + 5 external work + 15 growth + 10 reserves = 100 kJ. Retained chemical energy is 25/100=25% of input, or 25/75=one third of the account after the two material losses. Those percentages describe different denominators.
For the separate size example, 100/1=100 kJ per day per kg for A; 400/8=50 for B. B uses four times the total energy, despite half the rate per mass. This is a comparison of stated values, not an extrapolation rule for intermediate sizes.
Check your understanding
Why would adding '50 kJ spent' to 45 kJ heat and 5 kJ work be wrong here?
Show reasoning
The fifty already comprises those two outcomes. Adding the aggregate and its components would count the same energy twice.
Is 25% retained energy inconsistent with one third retained?
Show reasoning
No. Twenty-five percent uses the 100 kJ input; one third uses the 75 kJ remaining after egested and excreted energy leave. The retained numerator is the same 25 kJ.
Which animal has the greater total rate, and which the greater mass-specific rate?
Show reasoning
B has the greater total, 400 versus 100 kJ per day. A has the greater mass-specific rate, 100 versus 50 kJ per day per kg.
What to carry forward
Keep energy entries mutually exclusive, name the time interval and denominator, and treat body-size comparisons as conditional.
References
Resources supporting this lesson’s explanations and examples.
- Biology 2eMary Ann Clark, Matthew Douglas and Jung ChoiOpen textbook · 2018Biology 2e33.1 Animal Form and Function33.2 Animal Primary Tissues33.3 Homeostasis34.1 Digestive Systems34.2 Nutrition and Energy Production34.3 Digestive System Processes
- Mammalian basal metabolic rate is proportional to body mass²⁄³Craig R. White and Roger S. SeymourOriginal comparative analysis · 2003