12.1.1—Energy needs in living organisms

Living cells need a continuous energy supply because maintaining life requires work. Energy is transferred to particular processes; it is not a material that a cell simply stores and spends unchanged.

• Transport: Active transport moves substances against a concentration gradient, and bulk transport such as exocytosis also requires coordinated cellular work.
• Synthesis and growth: Anabolic reactions join smaller units to build larger molecules such as DNA or proteins; cell growth therefore requires energy input.
• Movement and contraction: Motor proteins, muscle contraction and the spindle-driven movement of chromosomes all require energy to change position or shape.
• Division and control: Cell division reorganises and separates cell contents, while homeostatic control continually adjusts cellular conditions; these coordinated changes also require energy.
• Immediate coupling: ATP acts as a short-term, immediately usable carrier that transfers energy from energy-releasing reactions to a nearby energy-requiring task. It is the coupling intermediary, not the original respiratory substrate and not a long-term store.

Energy-releasing reactions → ATP as a transferable immediate carrier → a specific cellular task. The same principle can support different work types, but each task still requires its own molecular machinery and control.

Do not say that ATP is energy itself or that respiration directly “makes energy”. This card explains why work needs energy and the boundary of ATP’s role; ATP structure, hydrolysis and synthesis are developed in the next objectives, while respiratory-substrate values are later in Topic 12.1.