International A Level Biology / Photosynthesis

ATP: the immediate energy supply

Edexcel International A Level Biology (YBI11), chapter 14: Photosynthesis, specification point 5.2. Free lesson.

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ATP is a nucleotide that supplies cells with energy in small amounts when it is hydrolysed. Cells continually remake ATP by phosphorylation, using energy from respiration or light.

What an ATP molecule contains

ATP stands for adenosine triphosphate. Adenine joined to ribose is called adenosine, and three phosphate groups make it triphosphate. ATP is a nucleotide: it has the adenine nucleotide found in RNA with two extra phosphate groups. Adenine is a nitrogenous base, meaning it contains nitrogen. Ribose is a pentose sugar, with five carbon atoms. The last phosphate group is removed when ATP is hydrolysed, leaving ADP with two phosphate groups.

How hydrolysis releases energy

Hydrolysis is a reaction in which a bond is broken using water. ATPase catalyses the hydrolysis of ATP. The reaction releases about 30 kJ per mole of ATP, a relatively small amount of energy. ATP + H₂O → ADP + Pᵢ, with energy released. ADP means adenosine diphosphate. It has two phosphate groups. Pᵢ is inorganic phosphate, the phosphate group that has come off and is now free in solution. ATPase, also called ATP hydrolase, is the enzyme that catalyses the reaction.

How ATP is remade

ATP is continually recycled rather than used once and thrown away. Phosphorylation adds inorganic phosphate to ADP. ATP synthase catalyses this reaction, which needs energy and releases water because it is a condensation reaction. ADP + Pᵢ → ATP, with energy needed. The energy comes from respiration, or from light in a chloroplast. Making ATP using energy from light is called photophosphorylation. ATP hydrolysis supplies energy for processes such as muscle contraction, active transport and building large molecules such as proteins.

Why ATP is an immediate energy supply

Glucose stores much more energy per molecule than ATP, but cells release energy from glucose through many steps of respiration. ATP releases a small, manageable amount in one reaction, using one enzyme, so its energy is available straight away. Small energy releases help limit energy wasted as heat, because each reaction releases an amount closer to what is needed. ATP is not an energy store. Cells hold very little ATP and remake it constantly. Energy is stored in substances such as starch, glycogen and lipids. ATP is a molecule, not energy itself. Its hydrolysis releases energy.

How to describe energy transfer accurately

Energy cannot be produced or destroyed; it is transferred. Respiration releases energy from glucose, and that energy is used to phosphorylate ADP and make ATP. Say that energy is released and ATP is made. For ATP synthase in a chloroplast, name the substrates, reaction and energy source: it catalyses phosphorylation of ADP, joining ADP and inorganic phosphate to make ATP using energy from light.

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