Explain why cells need a continuous supply of energy from respiration, referring to THREE different uses of energy in living organisms.

Cells require a continuous supply of energy because the energy released from glucose during respiration is used to power essential life processes that cannot be stored for long periods.

1. Muscle contraction – ATP produced by respiration is hydrolysed to provide the energy for actin–myosin cross‑bridge cycling, allowing muscles to contract so the organism can move, breathe or pump blood.
2. Active transport – ATP is required to power pumps such as the Na⁺/K⁺‑ATPase, which moves ions and other solutes against concentration gradients across cell membranes.
3. Biosynthesis – the synthesis of large molecules (e.g. proteins from amino acids, nucleic acids during DNA replication) needs ATP to activate substrates and drive the formation of peptide or phosphodiester bonds.

Because cells cannot store sufficient energy for extended periods, respiration must continuously oxidise glucose to keep ATP levels high and meet these ongoing demands.