AP Syllabus focus:
‘All living systems require a continuous input of energy to survive and carry out cellular processes.’
Living systems are open systems that must constantly obtain energy to build and maintain cellular structures, power essential work, and respond to internal and external changes. Without ongoing energy input, cells lose organization and fail.
Why organisms require a continuous energy supply
Cells never “stop working.” Even when an organism appears inactive, its cells must continually spend energy to remain alive.
Core reasons energy is always needed
Maintain cellular organisation: assembling, repairing, and replacing macromolecules and membranes as they wear out
Support homeostasis: sustaining stable internal conditions (ion balance, pH, water balance) despite changing environments
Enable growth and reproduction: producing new cellular material and dividing cells
Allow responsiveness: sensing signals and adjusting gene expression and cellular activity
Prevent loss of function: keeping proteins properly folded and cellular compartments properly maintained
Energy demand is continuous, not occasional
Many essential processes are time-sensitive (for example, maintaining ion gradients).
This figure shows the sodium–potassium pump (Na⁺/K⁺ ATPase) using ATP hydrolysis to move ions against their concentration gradients (typically 3 Na⁺ out and 2 K⁺ in per cycle). It visually connects ATP spending to the maintenance of electrochemical gradients that underlie membrane potential and homeostasis. Source
If energy input drops, these processes fail quickly, causing cascading cellular dysfunction.
What “energy” means in a biological context
Cells use energy in practical, usable forms rather than “raw” energy. Biological energy is typically captured, stored briefly, and then transferred to cellular work.
Major forms of biologically useful energy
Chemical potential energy: energy stored in covalent bonds of nutrients and in high-energy phosphate bonds
Electrochemical potential energy: energy stored in ion gradients across membranes
Light energy: captured by photosynthetic organisms and converted into chemical energy
ATP as the immediate energy currency of cells
Cells commonly convert diverse energy inputs into a single, spendable form to efficiently power many reactions and processes: ATP.
ATP (adenosine triphosphate): A nucleotide that stores usable cellular energy in phosphate bonds and transfers energy by phosphate group transfer or by hydrolysis to ADP + Pi.
ATP is continuously produced and consumed; cells maintain only a small, rapidly turned-over pool of ATP. This contributes to why energy input must be continuous: when ATP production cannot meet ATP use, essential work stops.
The ATP/ADP cycle supports constant energy transfer
ATP hydrolysis releases energy that can be used to drive cellular processes
ADP is recycled to ATP using energy derived from the environment (such as food molecules or light)
The cell’s ability to function depends on balancing ATP demand with ATP regeneration
This diagram summarizes the ATP/ADP cycle, showing ATP hydrolysis (ATP → ADP + Pi) as the energy-releasing step that powers cellular work. It also emphasizes that regenerating ATP from ADP requires an external energy input, which is why energy supply must be continuous. Source
What cells spend energy on: three categories of cellular work
ATP (and sometimes stored gradients) power cellular work that is necessary for survival.
Chemical work (building and rearranging molecules)
Biosynthesis of proteins, nucleic acids, carbohydrates, and lipids
Repair and turnover of damaged cellular components
Driving reactions that are not energetically favorable unless supplied with energy
Transport work (moving substances across membranes)
This OpenStax figure depicts primary active transport via the Na⁺/K⁺ pump, emphasizing that ATP hydrolysis drives ion movement against gradients. It illustrates how maintaining membrane ion distributions is an ongoing energy cost that supports membrane potential and downstream cellular processes. Source
Active transport of ions and molecules against concentration gradients
Maintaining membrane potentials and ion concentrations required for cell function
Loading and unloading cellular compartments to keep distinct internal environments
Mechanical work (movement)
Movement of cellular structures (for example, chromosome movement during cell division)
Cell and organism movement produced by motor proteins and cytoskeletal dynamics
Consequences of insufficient energy input
When energy input is reduced, cells prioritise immediate survival needs, and energy-expensive processes are curtailed.
Common cellular effects of low energy availability
Reduced active transport, leading to disrupted ion balance and water movement
Slower biosynthesis, impairing growth, repair, and reproduction
Impaired cell signalling and responsiveness due to limited resources for phosphorylation-based regulation
Loss of membrane and organelle function as maintenance processes fail
Because these failures can occur rapidly, living systems require a continuous input of energy to survive and carry out cellular processes.
FAQ
Cells monitor ratios such as ATP:ADP and ATP:AMP.
A key regulator is AMP-activated protein kinase (AMPK), which becomes more active when AMP rises, promoting energy-conserving changes (e.g. reducing biosynthesis) and increasing energy-producing processes.
ATP provides small, controllable energy transfers that can be rapidly coupled to many different cellular tasks.
Glucose contains far more energy than is usually needed for a single step, so converting energy into ATP allows safer, stepwise energy release and regulation.
Some cells use short-term phosphate buffers such as phosphocreatine to rapidly regenerate ATP from ADP.
This supports brief bursts of high ATP demand before longer-term ATP production catches up.
Energy demand reflects cellular function.
Neurones require continuous ATP for ion pumping to maintain membrane potentials.
Muscle cells may show large, rapid ATP demand changes during contraction.
Secretory cells need ATP for synthesis and vesicle transport.
They may reduce energy demand (slower growth, lower activity) and use highly efficient energy-harvesting strategies.
Examples include microbes using scarce chemical gradients or low-yield redox reactions, often paired with streamlined genomes and reduced maintenance costs.
Practice Questions
A toxin causes a rapid fall in cellular ATP concentration. Explain how this would affect (i) transport across membranes and (ii) the cell’s ability to build macromolecules, and link your explanations to ATP’s role. (6 marks)
Active transport decreases because ATP is required to move substances against concentration gradients (1)
Ion gradients/membrane potential are disrupted due to reduced pump activity (1)
Water balance/osmotic stability may be affected as solute gradients collapse (1)
Biosynthesis slows because many anabolic reactions require energy input from ATP (1)
Reduced ATP limits phosphorylation/activation steps needed to assemble polymers (1)
Clear link that ATP acts as an immediate energy source/transfer molecule (1)
State two reasons why living systems require a continuous input of energy. (2 marks)
Any two valid reasons (1 mark each), e.g. maintaining ion gradients/homeostasis; biosynthesis and repair; movement; growth and cell division; cell signalling and responsiveness.
