ATP – Biological Molecules Ep 5

Today we are looking at adenosine triphosphate, often just abbreviated to ATP because as with most scientific terms, it’s a bit of a mouthful. This molecule is very important in providing a quick source of energy for processes in cells. Let’s have a closer look at its structure and function.

Structure of ATP

ATP is actually very similar to a nucleotide; in fact you could call it a phosphorylated nucleotide, or a nucleotide derivative. It has a pentose sugar (ribose), a base (adenine), and three phosphate groups.

An ATP molecule

When one of the phosphate groups is removed, energy is released and we are left with adenosine diphosphate (ADP, with two phosphate groups) and an inorganic phosphate molecule which is written as Pi. Conversely, adding a phosphate group requires energy.

Function and use of ATP

The enzyme that synthesises ATP from ADP and inorganic phosphate is called ATP synthase. You will come across this enzyme quite a lot if you study biology. For example, it appears in both respiration and photosynthesis. It adds inorganic phosphate to ADP in a condensation reaction. The opposite reaction (removing inorganic phosphate from ATP) is a hydrolysis reaction catalysed by ATP hydrolase. ATP hydrolysis releases energy from the high energy bonds between phosphate groups. So don’t forget that water is involved – water is produced in a condensation reaction and used in a hydrolysis reaction.

ATP synthesis and hydrolysis

ATP is made during respiration using energy released from glucose. It is used in a huge number of reactions to provide energy. Sometimes the hydrolysis of ATP is coupled to another reaction so the energy can be used directly. One key use for ATP is to provide energy for active transport. Active transport moves a molecule against a concentration gradient across a membrane. For example, absorption of glucose in the small intestine requires active transport. We’ll be coming across ATP a lot more as we cover biology topics.


  • ATP consists of ribose, adenine, and three phosphate groups.
  • It is synthesised from ADP and Pi by ATP synthase in a condensation reaction.
  • It is broken down by ATP hydrolase in a hydrolysis reaction. This releases energy from a high energy phosphate bond.

p.s don’t worry too much about what inorganic phosphate means – just think of it as a single phosphate group!

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