3.2.1Inquiry Process
Adenosine Triphosphate (ATP) — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 108 of 160
Section 1 of 4Structure of ATP
ATP is a small, soluble molecule made of adenine, ribose and three phosphate groups.
Adenine
- A nitrogenous base that forms one part of the molecule and is also found in the nucleotides of DNA and RNA.
Ribose
- A five-carbon sugar that joins the adenine to the phosphate groups and forms the backbone of the molecule.
Three phosphates
- A chain of three phosphate groups is attached to the ribose;
- the bonds between them hold energy that can be released.
Nucleotide
- ATP is a modified nucleotide, which links its structure to the building blocks of nucleic acids.
Section 2 of 4Releasing and Restoring Energy
1
Hydrolysis of ATP with water, catalysed by an enzyme, removes the end phosphate group and forms ADP and inorganic phosphate: ATP + H₂O → ADP + Pᵢ.
2
This reaction releases energy that the cell can use, and the energy is transferred in small amounts that match the needs of cell reactions.
3
The phosphate removed can be passed to another molecule, which makes it more reactive and easier to change in the next reaction.
4
Energy from respiration is later used to rejoin ADP and phosphate, which is phosphorylation and forms ATP again: ADP + Pᵢ → ATP.
5
A cell therefore recycles its ATP continually instead of storing large amounts.
Section 3 of 4Why ATP Is Useful
Why ATP Is Useful
Immediate source
- ATP provides energy quickly because only one bond needs to be hydrolysed, in a single step.
Soluble and mobile
- ATP dissolves in water and moves easily to where energy is needed, such as the cell surface or the myosin in muscle.
Small packets
- Releasing a modest amount of energy in each reaction avoids wasting energy as heat.
Not a long-term store
- ATP is unstable, so long-term energy is stored as fat or glycogen, and then converted to ATP when it is required.
| Process | Use of ATP |
|---|---|
| Active transport | Drives pumps that move ions against concentration gradients |
| Muscle contraction | Allows myosin heads to move along actin |
| Anabolic reactions | Supplies energy for building proteins and DNA |
| Nerve impulse | Powers the sodium-potassium pump |
Section 4 of 4Exam Checklist & Tips
3.2.1 – Key Points for the Exam
- ATP Adenosine triphosphate, the immediate energy currency of the cell.
- ADP Adenosine diphosphate, formed when ATP loses one phosphate group.
- Hydrolysis Breaking a bond by adding water.
- Phosphorylation Addition of a phosphate group to a molecule.
Common Mistakes
- Saying ATP is stored in large amounts; cells hold small amounts and make it continuously.
- Writing that energy is contained inside the phosphate itself rather than released when the bond is broken and new bonds form.
- Mixing up ATP and ADP, so that hydrolysis is described as forming ATP.
- Describing ATP as a long-term energy store when fats and glycogen are used for that.
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