AP Syllabus focus:
‘Organisms use energy to organize biological systems, grow, reproduce, and maintain homeostasis.’
Energy is required to build and maintain biological order in a universe tending toward disorder. In ecosystems, organisms obtain energy and allocate it among cellular work, growth, reproduction, and stabilising internal conditions.
Core idea: energy allocation drives organismal function
Organisms transform energy from the environment into biological work. Because energy supply is finite, using more energy for one function reduces energy available for others, shaping life-history traits and survival.
What “using energy” means in cells
Most cellular work is powered by ATP produced primarily by cellular respiration (or photosynthesis in autotrophs), then spent to drive reactions that would not proceed spontaneously.
This diagram summarizes how ATP hydrolysis can be coupled to distinct kinds of cellular work, including transport work (e.g., pumping solutes across membranes) and mechanical work (e.g., movement driven by motor proteins). The key idea is energy coupling: ATP provides a transferable “packet” of free energy that can be linked to otherwise nonspontaneous processes. This helps unify many examples of metabolism into a single conceptual model of ATP-driven work. Source
Metabolism: The sum of all chemical reactions in an organism, including energy-releasing pathways (catabolism) and energy-requiring pathways (anabolism).
Metabolic energy is channelled into three broad categories of work: chemical work (biosynthesis), transport work (moving substances), and mechanical work (movement).
This figure illustrates primary active transport by the sodium–potassium pump, which uses ATP hydrolysis to move ions against their concentration gradients. By exporting Na⁺ and importing K⁺, the pump maintains electrochemical gradients that are essential for membrane potential and many downstream transport processes. It’s a concrete example of how ATP expenditure underwrites cellular organization and homeostasis. Source
Energy for organization: maintaining order and building structure
Living systems maintain highly organized structures (cells, tissues, organs) by continually investing energy to counteract entropy.
Maintaining cellular organization
Key ATP-dependent processes that preserve organization include:
Active transport (e.g., ion pumps) to sustain concentration gradients crucial for membrane potential and nutrient uptake
Macromolecule synthesis (proteins, nucleic acids, membranes) to replace degraded components
Vesicular trafficking (endocytosis/exocytosis) to sort and move materials within cells
Repair and quality control (protein refolding and degradation pathways) to prevent accumulation of faulty molecules
Homeostasis: Maintenance of relatively stable internal conditions despite external change, achieved through regulatory mechanisms and energy expenditure.
Homeostasis depends on organized feedback networks, but the physical maintenance of gradients, compartmentalisation, and biomolecular integrity is itself energetically costly.
Energy for growth: increasing biomass and complexity
Growth requires net synthesis of new cellular material, which demands both raw materials and energy.
Biosynthesis and growth efficiency
Growth involves:
Converting absorbed molecules into biomass (anabolism), requiring ATP and reducing power
Expanding cell number (mitosis) and/or cell size, requiring membrane and protein production
Storing energy reserves (e.g., glycogen, lipids) that can later support survival or reproduction
Only a fraction of consumed energy becomes new biomass; much is lost as heat during energy conversions or spent on maintenance.
This multi-panel figure compares pyramids of numbers, biomass, and energy across ecosystems, emphasizing how energy transfer constrains higher trophic levels. The energy pyramid panel highlights that available energy decreases sharply from producers to consumers because much input is lost to respiration and heat at each step. This reinforces why organismal growth (biomass production) represents only a fraction of total energy intake. Source
Energy for reproduction: producing offspring
Reproduction is a major energy sink because it requires:
Gamete production and/or asexual propagule formation
Developmental programs (gene expression, cell division, differentiation)
In many species, parental investment (feeding, protection), which increases offspring survival but raises adult energy demands
Energy allocated to reproduction can reduce energy available for growth and long-term maintenance, contributing to trade-offs such as earlier reproduction versus longer lifespan.
Energy for homeostasis: maintaining internal stability
Homeostasis requires continuous energy input to keep internal variables within tolerable ranges.
Major homeostatic energy costs
Common energy-intensive regulatory targets include:
Thermoregulation (heat production, shivering, brown fat activity; or behavioural strategies that still require energy to move and forage)
Osmoregulation (ion pumps in kidney tubules or gills; synthesis of compatible solutes)
Blood glucose regulation (hormone synthesis/secretion and metabolic pathway switching)
Detoxification and immune function (enzymatic processing and cellular defence responses)
When energy intake drops, organisms often downregulate costly processes (growth, reproduction, some immune functions) to prioritise immediate survival.
Energy budgets: tracking where energy goes
Ecologists summarise allocation using an energy budget, linking intake to growth and maintenance.
= energy consumed (J or kJ)
= net production: growth + reproduction (J or kJ)
= respiration (metabolic work + heat) (J or kJ)
= energy lost in faeces (J or kJ)
= energy lost in excretory products (e.g., nitrogenous wastes) (J or kJ)
A larger proportion of going to supports more growth and reproductive output, whereas higher reflects greater maintenance demands (often under stress or extreme environments).
FAQ
Priority is set by regulatory networks that sense ATP/AMP and nutrient status (e.g., kinase signalling), rapidly suppressing biosynthesis while maintaining membrane potential and essential transport.
Ions constantly leak across membranes down gradients. ATP-powered pumps must continuously counteract leakage to preserve excitability, pH balance, and secondary transport capacity.
Conversion efficiency depends on digestion/absorption, biochemical pathway efficiency, and how much energy must be spent on maintenance (repair, transport, regulation) versus building new tissue.
Stress responses elevate hormone signalling and metabolic rate, increase repair and detoxification demands, and can heighten immune activity, all of which raise ATP use and respiratory losses.
Reproduction is energetically expensive and typically not immediately necessary for short-term survival. Redirecting energy away from $P$ helps maintain core functions that keep the organism alive.
Practice Questions
State two ways organisms use energy to maintain homeostasis. (2 marks)
Any valid homeostatic use of energy (1)
Second distinct valid homeostatic use of energy (1) Examples: active transport to maintain ion gradients; thermoregulation; osmoregulation; synthesis/secretion of hormones for regulation.
An ecologist models an animal’s energy budget using . Explain what each term represents and describe how limited energy intake can affect allocation to growth, reproduction, and homeostasis. (5 marks)
Correct meaning of (1)
Correct meaning of as growth + reproduction (1)
Correct meaning of as respiration/metabolic work and heat loss (1)
Correct meaning of or as energy lost in faeces or excretory products (1)
Explanation that reduced forces trade-offs, typically maintaining essential homeostasis/maintenance while reducing (growth/reproduction) (1)
