ATP and the Need for Energy

A2 · 14 min

Every living cell is constantly doing work: pumping ions, moving structures, and building large molecules from small ones. The energy for all of this comes from one molecule, adenosine triphosphate (ATP). This note explains why organisms need energy, what makes ATP the ideal "energy currency", and the two ways cells make it. It is the foundation for the whole of Topic 12 and for the light-dependent stage of photosynthesis, and Paper 4 regularly opens a respiration question with a few marks on exactly these ideas.

Why living organisms need energy

Energy is the capacity to do work. In biology, "work" means any process that would not happen on its own: moving substances against a gradient, moving a structure, or building a complex molecule that is less stable than its building blocks. Cells cannot use heat to do this work, so they need energy in a chemical form they can release in controlled amounts exactly where it is needed.

The syllabus asks you to illustrate the need for energy with three types of work.

Active transport

Active transport moves ions or molecules across a membrane against their concentration gradient, using carrier proteins and energy from ATP. The best example is the sodium–potassium pump in the cell surface membrane of almost every animal cell: for each ATP hydrolysed it moves 3 $\ce{Na+}$ out of the cell and 2 $\ce{K+}$ in. This maintains the resting potential of neurones, drives the uptake of glucose by co-transport in the proximal convoluted tubule and the small intestine, and helps control cell volume. Other examples are the loading of sucrose into phloem companion cells (via a proton pump) and the uptake of mineral ions by root hair cells.

Movement

Movement at every scale needs ATP:

Muscle contraction: myosin heads use ATP to detach from actin and re-cock before the next power stroke (see Muscle Contraction).
Cilia and flagella: the beating of cilia in the airways and the movement of sperm tails are driven by ATP hydrolysis by motor proteins.
Inside cells: the movement of chromosomes on the spindle during mitosis and meiosis, and the transport of vesicles along microtubules, both use ATP.

Anabolic reactions

Anabolic reactions build large molecules from smaller ones. They are condensation reactions that need an input of energy. The syllabus highlights two:

DNA replication: free nucleotides are activated (they arrive as nucleoside triphosphates), and DNA polymerase uses the energy released when two of the phosphates are removed to form each phosphodiester bond. ATP is also used by helicase to unwind the double helix.
Protein synthesis: ATP is used to attach each amino acid to its specific tRNA (amino acid activation), and energy is also needed for the ribosome to move along the mRNA and form peptide bonds. RNA polymerase also uses activated nucleotides during transcription.

Other anabolic examples include the synthesis of glycogen, starch and cellulose from glucose, and the synthesis of triglycerides.

Tip

Other processes that use ATP and that you can quote in a "suggest" question: the transmission of nerve impulses (restoring ion gradients), exocytosis and endocytosis, maintaining body temperature in mammals and birds (heat released by respiration), and the Calvin cycle in plants.

The structure of ATP

ATP is a phosphorylated nucleotide. It has three parts:

the nitrogenous base adenine
the pentose sugar ribose
a chain of three phosphate groups

Adenine plus ribose is called adenosine; adding one, two or three phosphates gives AMP, ADP and ATP. ATP is therefore closely related to the nucleotides in RNA, which is one reason it is found in every cell.

\text{adenine} - \text{ribose} - \text{P} - \text{P} - \text{P}

When ATP is hydrolysed, the bond to the terminal phosphate is broken by the addition of water. The reaction is catalysed by enzymes called ATPases:

Key result

\ce{ATP + H2O -> ADP + P_i}\qquad \Delta G \approx -30.5\ \text{kJ mol}^{-1}

ATP is made again by the reverse reaction, a condensation catalysed by ATP synthase:

\ce{ADP + P_i -> ATP + H2O}

This needs an input of $30.5\ \text{kJ mol}^{-1}$ , supplied by respiration (or by light energy in photosynthesis).

Watch out

Do not say that energy is "stored in the phosphate bonds" or that "breaking the bond releases energy". Breaking any bond needs energy. Energy is released by the whole hydrolysis reaction, because the products (ADP and inorganic phosphate) are more stable than ATP and water. The safe wording is "hydrolysis of ATP releases energy" or "energy is released when ATP is hydrolysed to ADP and phosphate".

Why ATP is the universal energy currency

A currency is something that can be exchanged for many different goods. ATP is called the universal energy currency because every cell in every organism uses it, and it can be "spent" on any energy-requiring process. Glucose is a good energy store, but a poor currency: oxidising one mole releases about $2870\ \text{kJ}$ , far too much to use in one reaction, so it would mostly be wasted as heat.

Features of ATP that make it suitable as the universal energy currency

Feature	Why it matters
Hydrolysis releases a small, manageable quantity of energy (about $30.5\ \text{kJ mol}^{-1}$ )	Enough to drive a typical cellular reaction, so little is wasted as heat; several ATP can be used if more is needed
Energy is released in a single step (one hydrolysis reaction)	Release is rapid and can be triggered exactly when needed
ATP is soluble in water	It diffuses easily through the cytoplasm to where energy is needed
ATP is stable in the absence of an enzyme	It does not hydrolyse spontaneously, so energy is released only where an ATPase is present
ATP is easily regenerated from ADP and phosphate	It can be recycled thousands of times; the cell needs only a small pool
ATP cannot pass through the cell surface membrane (it is charged)	It stays inside the cell where it was made
Phosphate from ATP can be transferred to other molecules (phosphorylation)	The phosphorylated molecule becomes more reactive, lowering the activation energy of the next reaction (for example glucose in glycolysis)
ATP is found in all organisms	The same molecule links energy-releasing and energy-requiring reactions everywhere

The idea of ATP as a link between reactions is important. Catabolic (energy-releasing) reactions such as respiration are coupled to the synthesis of ATP; anabolic and other energy-requiring processes are coupled to its hydrolysis. ATP is not a long-term store: a cell holds only a few seconds' supply and must make it continuously.

How ATP is synthesised

The syllabus requires you to state that ATP is made in two ways.

Definition

Substrate-linked phosphorylation (substrate-level phosphorylation) is the synthesis of ATP by the direct transfer of a phosphate group from a phosphorylated substrate molecule to ADP, catalysed by an enzyme.

Chemiosmosis is the synthesis of ATP using the energy of a proton ( $\ce{H+}$ ) gradient across a membrane: protons diffuse back down their gradient through ATP synthase, which uses the energy to phosphorylate ADP.

	Substrate-linked phosphorylation	Chemiosmosis
Where	Cytoplasm (glycolysis); mitochondrial matrix (Krebs cycle)	Inner mitochondrial membrane (oxidative phosphorylation); thylakoid membrane (photophosphorylation)
Source of phosphate	A phosphorylated intermediate, e.g. a phosphorylated 3C compound in glycolysis	Inorganic phosphate ( $\ce{P_i}$ )
Membrane needed?	No	Yes, a membrane that is impermeable to protons except through ATP synthase
Proton gradient needed?	No	Yes
Oxygen needed?	No (works in anaerobic conditions)	Yes in mitochondria (oxygen is the final electron acceptor); no in chloroplasts
Share of ATP from aerobic respiration	Small (4 per glucose)	Large (most of the ATP)

Chemiosmosis was proposed by Peter Mitchell in 1961. Its central idea is that an electron transport chain uses energy released from electrons to pump protons across a membrane, building a gradient of both concentration and charge. The flow of protons back through ATP synthase is what turns ADP into ATP. The details for mitochondria are in Oxidative Phosphorylation and for chloroplasts in The Light-Dependent Stage.

Exam tip

"State" and "outline" questions on ATP are short. For "describe the features of ATP that make it suitable as the energy currency" you need a feature and its benefit for each mark: "small amount of energy released per molecule, so energy is not wasted" scores; "releases small energy" alone often does not. Examiners repeatedly reject "ATP stores energy in its bonds" and "ATP is energy". ATP is a molecule that transfers energy.

Worked examples

Uses of ATP in a cell

Outline three different ways in which a cell in the proximal convoluted tubule of the kidney uses ATP. [3]

Solution

Any three, each tied to a specific process:

Active transport of sodium ions out of the cell into the tissue fluid by the sodium–potassium pump in the basal membrane, keeping the sodium concentration inside the cell low.
Protein synthesis, for example making the carrier and channel proteins of the membranes: ATP is used to attach amino acids to tRNA.
Endocytosis of small proteins that have entered the filtrate (vesicle formation needs ATP).
DNA replication before cell division, to replace damaged cells.

A general answer such as "for respiration" or "for energy" earns nothing. Each mark needs a named process.

Explaining the features of ATP

Glucose is described as an energy store and ATP as an energy currency. Explain why glucose is not used directly as the immediate source of energy for cellular reactions. [3]

Solution

Complete oxidation of glucose releases a large amount of energy (about $2870\ \text{kJ mol}^{-1}$ ), much more than any single reaction needs, so most would be wasted as heat. [1]
Energy is released from glucose in many steps (glycolysis, link reaction, Krebs cycle, oxidative phosphorylation), so release is slow; hydrolysis of ATP is a single, rapid step. [1]
ATP releases a small, manageable amount (about $30.5\ \text{kJ mol}^{-1}$ ) and is readily regenerated, so the energy from one glucose molecule can be packaged into many ATP and released only where an ATPase is present. [1]

Turnover of ATP

A resting adult hydrolyses about $40\ \text{kg}$ of ATP per day, but the body contains only about $50\ \text{g}$ of ATP at any moment. The relative molecular mass of ATP is $507$ .

(a) Calculate how many times, on average, each ATP molecule is regenerated per day.

(b) Using $\Delta G = -30.5\ \text{kJ mol}^{-1}$ , calculate the energy released per day by this hydrolysis. Give your answer to 3 significant figures.

Solution

(a)

\frac{40\,000\ \text{g}}{50\ \text{g}} = 800 \text{ times per day}

(b)

\text{moles of ATP} = \frac{40\,000}{507} = 78.9\ \text{mol}

\text{energy} = 78.9 \times 30.5 = 2410\ \text{kJ (3 s.f.)}

(c) ATP is regenerated from ADP and phosphate so quickly that a large store is unnecessary. ATP is also relatively unstable compared with glycogen or fat, and a large mass of a soluble, charged molecule would affect the water potential of cells. Energy is therefore stored long-term as glycogen and triglycerides and transferred to ATP as needed.

Comparing the two ways of making ATP

A drug makes the inner mitochondrial membrane permeable to protons. Explain why cells treated with this drug still produce some ATP, but much less than untreated cells. [4]

Solution

The drug allows protons to diffuse back into the matrix without passing through ATP synthase, so the proton gradient across the inner membrane is lost. [1]
Without the proton gradient, chemiosmosis cannot occur, so no ATP is made by oxidative phosphorylation. [1]
ATP is still made by substrate-linked phosphorylation in glycolysis (in the cytoplasm) and the Krebs cycle (in the matrix), which do not need a proton gradient. [1]
Substrate-linked phosphorylation produces only 4 ATP per glucose (2 in glycolysis, 2 in the Krebs cycle), whereas most ATP from aerobic respiration is made by chemiosmosis, so the yield falls greatly. [1]

(The energy that would have made ATP is released as heat instead. Brown fat uses exactly this mechanism, with a natural uncoupling protein, to generate heat.)

Efficiency of aerobic respiration

Aerobic respiration of one mole of glucose releases $2870\ \text{kJ}$ . Under cellular conditions about $32$ ATP are made per glucose. Using $30.5\ \text{kJ mol}^{-1}$ for each ATP, calculate the efficiency of energy transfer to ATP, and state what happens to the rest of the energy.

Solution

\text{energy transferred to ATP} = 32 \times 30.5 = 976\ \text{kJ}

\text{efficiency} = \frac{976}{2870} \times 100 = 34\%

The remaining $66\%$ is released as heat. In mammals and birds this heat helps to maintain a high, constant body temperature.

Practical: ATP and muscle contraction

Practical skills

A classic demonstration of ATP as the immediate energy source uses glycerinated muscle fibres (muscle soaked in glycerol, which removes soluble substances including the cell's own ATP and glucose but leaves the actin and myosin intact).

Place thin strands of fibre on a microscope slide and measure their length with an eyepiece graticule or a ruler under a hand lens.
Add a few drops of ATP solution (with magnesium and potassium ions) to one set, glucose solution to a second set, and distilled water to a third (control).
Measure the length again after a few minutes and calculate the percentage change in length.

Only fibres given ATP contract. Glucose has no effect because the respiratory enzymes and mitochondria have been destroyed, so glucose cannot be used to make ATP. The independent variable is the solution added; the dependent variable is the percentage change in length; volume and concentration of solution, temperature, and the initial length and thickness of fibres should be standardised. Repeat with several fibres and calculate a mean.

Summary

Cells need energy for active transport (e.g. the $\ce{Na+}$ / $\ce{K+}$ pump), movement (muscle, cilia, chromosomes, vesicles) and anabolic reactions (DNA replication, protein synthesis, polysaccharide synthesis).
ATP is a phosphorylated nucleotide: adenine + ribose + three phosphates.
$\ce{ATP + H2O -> ADP + P_i}$ releases about $30.5\ \text{kJ mol}^{-1}$ ; the reaction is catalysed by ATPase. Never say energy is "stored in bonds".
ATP suits its role because it releases a small, manageable amount of energy in one step, is soluble, stable without an enzyme, quickly regenerated, stays inside the cell, can phosphorylate other molecules, and is used in all organisms.
ATP is made by substrate-linked phosphorylation (direct phosphate transfer, glycolysis and Krebs cycle) and by chemiosmosis (proton gradient and ATP synthase in mitochondria and chloroplasts).
ATP is an energy currency, not a store; glycogen, starch and lipids are stores.

Practice

Question

State the three components of a molecule of ATP. [2]
Explain why ATP is described as the universal energy currency. [2]
Outline how ATP is used in protein synthesis. [2]
Distinguish between substrate-linked phosphorylation and chemiosmosis. [3]
State two places in a eukaryotic cell where ATP is made by chemiosmosis and two where it is made by substrate-linked phosphorylation. [2]
A student wrote: "ATP has high-energy bonds which release energy when they break." Explain what is wrong with this statement and rewrite it correctly. [2]
Cyanide stops the electron transport chain. Explain why a person poisoned with cyanide quickly loses the ability to contract muscles, even though glycolysis continues. [3]
A muscle cell contains $5 \times 10^{-3}\ \text{mol dm}^{-3}$ ATP. During intense exercise ATP is hydrolysed at $1.5 \times 10^{-3}\ \text{mol dm}^{-3}\ \text{s}^{-1}$ . Calculate how long the ATP would last if it were not regenerated, and comment on what this shows about ATP. [3]
Discuss the features of ATP that make it suitable as the energy currency of cells, and explain why the hydrolysis of ATP, rather than the oxidation of glucose, is coupled directly to energy-requiring reactions. [6]

Answers

Adenine (a nitrogenous base), ribose (a pentose sugar) and three phosphate groups.
It is used by all organisms / all cells to transfer energy from energy-releasing reactions (such as respiration) to any energy-requiring process (active transport, movement, anabolism). It is "exchanged" for work in the way money is exchanged for goods.
ATP is hydrolysed to provide energy to attach amino acids to their specific tRNA molecules (amino acid activation); energy is also used for peptide bond formation and movement of the ribosome along mRNA. Transcription uses activated nucleotides.
Substrate-linked phosphorylation: a phosphate group is transferred directly from a phosphorylated substrate to ADP by an enzyme; it needs no membrane or proton gradient. Chemiosmosis: energy from electrons passing along an electron transport chain is used to pump protons across a membrane, and protons diffusing back through ATP synthase provide energy to combine ADP with inorganic phosphate. It occurs only on membranes.
Chemiosmosis: inner mitochondrial membrane (cristae); thylakoid membranes of chloroplasts. Substrate-linked: cytoplasm (glycolysis); mitochondrial matrix (Krebs cycle).
Breaking bonds requires energy, so bonds do not "release" energy when broken. Correct version: "The hydrolysis of ATP to ADP and inorganic phosphate releases energy, because the products are more stable than the reactants" (about $30.5\ \text{kJ mol}^{-1}$ ).
Without a functioning electron transport chain, no ATP is made by oxidative phosphorylation, which normally provides most of the cell's ATP. Glycolysis makes only 2 ATP (net) per glucose, far too little to meet the demand of contracting muscle. Muscle contraction needs ATP for myosin heads to detach from actin and re-cock, and for pumping $\ce{Ca^{2+}}$ back into the sarcoplasmic reticulum, so contraction fails.
$\dfrac{5 \times 10^{-3}}{1.5 \times 10^{-3}} = 3.3\ \text{s}$ . ATP is not an energy store; the cell holds only a few seconds' supply, so ATP must be regenerated continuously from ADP and phosphate by respiration (and, in muscle, by creatine phosphate).
Mark points (any six):
- Hydrolysis of ATP releases a small quantity of energy ( $30.5\ \text{kJ mol}^{-1}$ ), which closely matches the needs of individual reactions, so little energy is wasted as heat.
- Release is in one step (single hydrolysis reaction), so it is rapid.
- ATP is soluble and moves easily within the cell.
- ATP is stable unless an ATPase is present, so energy release is controlled.
- ATP is rapidly regenerated by phosphorylation of ADP, so a small pool is enough.
- ATP can phosphorylate other molecules, making them more reactive.
- ATP cannot leave the cell, and it is used by all cells.
- Oxidation of glucose releases a very large amount of energy ( $2870\ \text{kJ mol}^{-1}$ ) in many enzyme-controlled steps, so it is too slow and too large to be coupled to individual reactions; instead its energy is transferred to about 30 ATP molecules.

Why living organisms need energy

Active transport

Movement

Anabolic reactions

The structure of ATP

Why ATP is the universal energy currency

How ATP is synthesised

Worked examples

Practical: ATP and muscle contraction

Practice

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