Energy Transfer and Metabolic Pathways

This lesson brings together the concepts of photosynthesis and respiration into a unified understanding of energy transfer in living systems. It covers the role of ATP, the coupling of metabolic reactions, and the integration of anabolic and catabolic pathways as required by the Edexcel A-Level Biology (9BI0) specification.

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ATP: The Universal Energy Currency

Adenosine triphosphate (ATP) is the immediate energy source for virtually all cellular processes. It is often described as the "energy currency" of the cell.

Structure of ATP

ATP consists of:

• Adenine — a nitrogenous base
• Ribose — a 5-carbon sugar
• Three phosphate groups — linked by high-energy phosphoanhydride bonds

ATP Hydrolysis

When the terminal phosphate bond is broken by the enzyme ATPase (or ATP hydrolase), energy is released:

ATP + H₂O → ADP + Pᵢ + energy (~30.5 kJ mol⁻¹)

This is an exergonic (energy-releasing) reaction.

ATP Synthesis

ATP is synthesised from ADP and inorganic phosphate (Pᵢ) in a condensation reaction:

ADP + Pᵢ + energy → ATP + H₂O

This is an endergonic (energy-requiring) reaction.