outline the need for energy in living organisms, as illustrated by active transport, movement and anabolic reactions, such as those occurring in DNA replication and protein synthesis

Energy in Living Organisms

All cellular processes require a continuous supply of usable energy. In biological systems this energy is most commonly transferred by the molecule adenosine‑triphosphate (ATP). The hydrolysis of ATP releases free energy that can be coupled to otherwise non‑spontaneous reactions, allowing cells to maintain order, grow and respond to their environment.

Why Energy Is Needed

• Active transport: movement of solutes against their concentration gradient.
• Mechanical movement: muscle contraction, flagellar rotation, cytoplasmic streaming.
• Anabolic reactions: synthesis of macromolecules such as DNA and proteins.

Active Transport

Transport proteins use the energy from ATP hydrolysis to move ions or molecules from a region of low concentration to a region of high concentration. This creates and maintains electrochemical gradients essential for nerve impulse transmission, nutrient uptake and pH regulation.

Cellular Movement

Movement at the cellular level is driven by the polymerisation and depolymerisation of cytoskeletal filaments, which is powered by ATP. Examples include:

1. Muscle fibre contraction (actin‑myosin interaction).
2. Flagellar rotation in prokaryotes.
3. Cytoplasmic streaming in plant cells.

Anabolic Reactions

Anabolism involves the formation of complex molecules from simpler precursors. Two key anabolic processes are DNA replication and protein synthesis, both of which consume large amounts of ATP.

DNA Replication

During each round of replication the following ATP‑dependent steps occur:

• Helicase unwinds the double helix (\$2 \, \text{ATP} \rightarrow 2 \, \text{ADP} + 2 \, \text{P}_i\$ per base pair).
• DNA polymerases add nucleotides, using the energy released from the cleavage of the nucleoside‑triphosphate.
• DNA ligase seals nicks in the backbone, consuming \$1 \, \text{ATP}\$ per phosphodiester bond formed.

Protein Synthesis

Translation requires energy at several stages:

• Charging of tRNA (aminoacyl‑tRNA synthetase) uses \$2 \, \text{ATP}\$ equivalents per amino acid.
• Initiation, elongation and termination steps each involve GTP hydrolysis (functionally equivalent to ATP).
• Ribosomal translocation consumes \$1 \, \text{GTP}\$ per codon moved.

Energy Balance Equation

The overall free‑energy change for a cellular process can be expressed as:

\$\Delta G = \Delta H - T\Delta S\$

When \$\Delta G\$ is positive, the reaction is non‑spontaneous and must be coupled to ATP hydrolysis (which has \$\Delta G^\circ' \approx -30.5 \, \text{kJ mol}^{-1}\$) to proceed.

Summary Table