IB Biology HLCellular RespirationPaper 1 & 2~12 min read
Adenosine Triphosphate (ATP)
Your cells cannot spend glucose directly, in the same way you cannot pay for a coffee with a gold bar. Energy has to be broken into small, spendable units first. ATP is that unit.
📚 What you need to know
Energy released by respiration is transferred to ATP in a series of small steps. Heat is lost at each step, and endotherms use it to keep warm.
ATP is a phosphorylated nucleotide: a ribose sugar, an adenine base and three phosphate groups.
It is a short-term store of chemical energy. It is small and soluble, so it moves easily around the cell by facilitated diffusion.
ATP is the universal energy currency: universal because all organisms use it, currency because it is spent on many different reactions and reused endlessly.
Hydrolysis of ATP gives ADP + Pi and releases energy. One enzyme, ATPase, does it quickly wherever energy is needed.
Synthesis of ATP from ADP and Pi is a condensation reaction that requires energy and releases water.
ATP is not stored in quantity. Cells make it as they need it.
ATP powers anabolic reactions, active transport, movement of the whole cell, and movement of components such as chromosomes inside it.
What ATP is made of
ATP is a nucleotide with extra phosphates bolted on — the same basic build as the nucleotides in DNA and RNA.
Count the phosphates and the names make sense: triphosphate has three, diphosphate has two, monophosphate has one.
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Building the name
Adenine + ribose = adenosine. Adenosine + three phosphates = adenosine triphosphate. The name is a description of the diagram.
Why ATP works so well as a currency
ATP is described as the universal energy currency. Both halves of that phrase mean something:
Universal — every organism on Earth uses it, from bacteria to blue whales.
Currency — like money, the same unit is spent on completely different things, and it is reused countless times.
Feature of ATP
Why that is useful
Releases a small but sufficient amount of energy
Enough to drive a reaction without wasting the rest as heat, and it gives the cell fine control over what happens
Stable at cellular pH
It does not fall apart on its own — it only breaks down when ATPase is present, so energy is not wasted
Hydrolysis is quick and easy
Needs just one enzyme, so a cell can respond immediately to a sudden demand for energy
Can be recycled
The reaction is reversible: ADP and Pi are rebuilt into ATP and used again
Soluble and small
Moves easily to wherever in the cell the energy is needed
Forms phosphorylated intermediates
Adding a phosphate to another molecule makes it more reactive and lowers the activation energy of its reaction
That last row is easy to skip over and it matters later. When ATP phosphorylates glucose at the start of glycolysis, it is not “giving glucose energy” — it is making glucose unstable enough to react. Keep that in mind when you reach the glycolysis page.
The ATP–ADP cycle
ATP is not a battery you charge once. It is spent and rebuilt continuously, thousands of times a second across a cell.
The same molecules go round this loop all day. What runs out is not ATP but the glucose needed to keep rebuilding it.
Hydrolysis: spending it
Hydrolysis of ATP
ATP + water → ADP + Pi + energy (catalysed by ATPase)
Removing the third phosphate releases free energy the cell can use for work such as DNA synthesis. A second phosphate can be removed to give AMP, releasing a similar amount again.
Synthesis: earning it
Synthesis of ATP
ADP + Pi + energy → ATP + water (a condensation reaction)
Water is released, which is what makes it a condensation reaction, and energy has to be supplied — which is exactly what respiration is for.
Why cells cannot stockpile ATP. ATP is very reactive, so it is not stored, and it rarely crosses the cell surface membrane. On average a human turns over more than 50 kg of ATP a day but holds only about 200 g at any moment. The molecule is recycled, not hoarded.
Do not use “energy” and “ATP” as if they were the same word. Energy is the capacity to do work; ATP is a molecule that carries it to where the work happens. Writing “the cell releases ATP for movement” when you mean “hydrolyses ATP to release energy for movement” costs marks.
Worked examples
WE 1
Explain why ATP suits its role
Explain why ATP is a suitable molecule for transferring energy within cells. (4 marks)
Point 1: the right size of payment
Hydrolysis releases a small but sufficient amount of energy, so little is wasted and the cell keeps control of which processes run.
Point 2: quick to use
Hydrolysis needs only one enzyme, ATPase, so energy can be released immediately wherever it is required.
Point 3: stable until needed
ATP is relatively stable at cellular pH, so it does not break down and waste energy on its own.
Point 4: mobile and reusable
It is small and soluble so it moves easily around the cell, and the reaction is reversible so ADP and Pi are rebuilt into ATP.
Small payment, fast release, stable, soluble, recyclablegive a property AND its benefit each time — a bare list of properties rarely scores full marks
WE 2
Calculate how often ATP is recycled
An adult uses about 50 kg of ATP in a day but the body contains only about 200 g of ATP at any one time. Calculate how many times, on average, each ATP molecule is recycled per day. (2 marks)
Step 1: same units
50 kg = 50 000 g of ATP used in a day.
Step 2: divide50 000 ÷ 200 = 250Step 3: what it means
Each ATP molecule is broken down and rebuilt roughly 250 times a day, which shows ATP is a carrier rather than a store.
about 250 times per dayconvert to the same unit before dividing — that is where most marks are lost
WE 3
Compare hydrolysis and synthesis
Compare the hydrolysis and the synthesis of ATP. (3 marks)
Hydrolysis
ATP is broken down to ADP + Pi using water, and energy is released for the cell to use.
Synthesis
ADP and Pi are joined in a condensation reaction that releases water; this requires energy, supplied by respiration.
What they share
Both involve the same molecules and the same bond, and both are catalysed by enzymes — the reaction is reversible.
Same reaction run in opposite directions: one spends, one earnsmention water in both directions — used in hydrolysis, released in condensation
💡 Exam tips
Learn the three components in order: ribose, adenine, three phosphates.
Name the enzyme — ATPase — when describing hydrolysis.
You are not expected to recall exact energy values in kJ, only that the amount released is sufficient for cellular tasks.
Say ATP is a short-term energy store; glycogen, starch and triglycerides are the long-term ones.
Use the phrase universal energy currency and explain both words if asked.
Remember ATP synthesis is a condensation reaction, so water is a product.
⚠ Common mistakes
Using “energy” and “ATP” as the same thing. ATP is a molecule that carries energy.
Saying ATP is stored in cells. It is made on demand; only about 200 g exists in the body at once.
Writing that ATP contains adenosine and a ribose sugar. Adenosine already includes the ribose.
Calling ATP a protein. It is a phosphorylated nucleotide.
Saying respiration produces energy. Energy is transferred, not created.
Forgetting the water in the hydrolysis and synthesis equations.
Up next: Cell Respiration. You know what the cell is trying to make. The next page looks at the process that makes it, and at what changes when oxygen runs out.
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