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What is ATP?

Updated on April 5, 2014
Our 'metabolism' is often linked to how we process our food. This is a common misconception.
Our 'metabolism' is often linked to how we process our food. This is a common misconception. | Source

Metabolism and ATP

ATP is the 'energy currency' of the cell. It is a phosphorylated nucleotide. Removing the terminal phosphate group from ATP (to form ADP), liberates energy. This energy is used to power every anabolic reaction in an organism.

Metabolism and nutrition form a key part of the A-level curriculum in the UK. This topic is also found on AP Biology courses. It is vital you understand metabolism - and it's key molecule, ATP - before moving on to learn about cellular respiration or photosynthesis.

When teaching this topic, my students start to realise how much chemistry is involved in biology. You don't need to know all of the math I will set out, but it will help round out your understanding of why ATP is the 'energy currency' of the cell, and why all organisms on the planet go through the trouble of making it.

What is a Metabolic Pathway?

A living organism is made of cells. There may be just one, or there may be many cells. To stay alive, each cell must perform many chemical reactions at the same time. All the chemical reactions occurring in an organism are called metabolism. This is broken down into two parts:

  • Anabolic Reactions: These build up small molecules into larger ones (think anabolic steroids) and need energy to happen.
  • Catabolic Reactions: These break down large molecules into smaller ones (think digestion) and release energy.

Anabolism or catabolism are rarely single reactions. Usually they occur as a number of chemical reactions that are linked together. A metabolic pathway is where the product of one reaction acts as the substrate for the next reaction.

Metabolic Pathways

A Metabolic Pathway.  Most reactions are enzyme-controlled. In this pathway, the enzymes a, b, c and d each control a different step.
A Metabolic Pathway. Most reactions are enzyme-controlled. In this pathway, the enzymes a, b, c and d each control a different step. | Source

When did you first hear about ATP?

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Types of Nutrition

Metabolism does not describe how much weight we gain after eating. But without the energy gained from our food, our cells would not be able to power the chemical reactions that keep us alive.

Not all organisms have to eat food to gain their energy. Energy can be obtained by living things in two ways:

  • Autotrophs - make their own food out of raw materials. E.g. Plants photosynthesise
  • Heterotrophs - eat other organisms for food.

Types of Nutrition

Autotrophs
Heterotrophs
Photoautotrophs - plants and bacteria who fix carbon using energy from light
Saprotrophs - bacteria and fungi that break down dead matter for energy
Chemoautotrophs - (mainly) bacteria who fix carbon using energy from a chemical compound
Parasites - feed on food already digested by their host organism
 
Holozoic - plants, animals and fungi that feed on solid organic matter from living or dead organisms
A selection of different types of nutrition. Remember, autotrophs produce their own food out of raw materials; heterotrophs must eat other living things.

Review of ATP - Khan Academy

What is ATP?

ATP (adenosine triphosphate) is a phosphorylated Nucleotide. It is used as an energy store for cells, and is often called the 'energy currency' of the cell. A cell does not store large amounts of ATP; it uses it to transfer small packets of energy from one reaction to another.

In the image below, you can see that ATP looks very similar to the RNA nucleotide that contains Adenine; in fact it is the same, just with two extra phosphates put on the end.

ATP is produced from adenosine diphosphate (ADP) and an inorganic phosphate. This requires energy, which becomes trapped in the resulting ATP molecule. An enzyme called ATP synthase catalyses the reaction.

The circled phosphate group is easily hydrolysed (split using water and the enzyme, ATPase). When this happens, a small amount of energy is released. The cell can use this liberated energy for many different things.

Adenosine Triphosphate Structural Formula

The structural formula of ATP. ATP is a phosphorylated nucleotide - two extra phosphates have been added to the Adenine nucleotide seen in DNA and RNA.
The structural formula of ATP. ATP is a phosphorylated nucleotide - two extra phosphates have been added to the Adenine nucleotide seen in DNA and RNA. | Source

Key Point

In any chemical reaction where energy is released, it is always caused by electrons going from a higher to a lower energy state.

How does ATP Store Energy?

ATP is an energy store. The energy to power different reactions is stored in the bond between the terminal (last) phosphate groups [this is circled in the above diagram].

Bonds are electrons shared between atoms. Because oxygen is more electronegative than phosphorus, oxygen 'hogs' outer electrons from phosphorus. In other words, these electrons are 'stretched' uncomfortably away from the phosphorus nucleus. Imagine a stretcing a rubber band between your hands.

When the bond between the terminal phophate groups is broken (hydrolysed) the 'stretched' electrons can spring back to a 'more comfortable' lower energy state. Imagine letting go of the elastic band with one hand

As the electron moves to a lower energy state, energy is released. Imagine using the elastic band as a slingshot. This can be used to do useful work.

Why does a cell use ATP, not Glucose, for Energy?

Cells use carbohydrates (like Glucose) to make ATP. This ATP - the energy currency of the cell - provides energy for many processes, such as:

  • Muscle contraction;
  • Active transport;
  • Anabolism (synthesising macromolecules);
  • ATP synthesis.

But why not use glucose directly? Why bother making lots of small molecules of ATP? Isn't this inefficient?

The answer lies in how much energy is released at once.

Compared to hydrolysing ATP, oxidising Glucose releases a lot of energy. Oxidising one molecule of glucose during cellular respiration makes 38 molecules of ATP. Releasing all the energy stored in glucose at once would be catastrophic for the cell:

The Screaming Jelly Baby

In the above video, molten Potassium Chlorate, a strong oxidising agent, was used to add oxygen to the sugar in the jelly baby. It's a little bit like burning the jelly baby.

The Potassium Chlorate made the sugar react with oxgygen very quickly, releasing all of the energy stored in the sugar at once.

If our cells used glucose as a direct energy source, the amount of energy released would kill the cell. This is why we use ATP

ATP releases small, useful packets of energy that can be safely used by the cell. If a cell needs more energy, it uses more ATP. The equations below show how much energy is released by ATP hydrolysis compared to Glucose combustion.

Hydrolysis of ATP - Bond Enthalpies

 
Enthalpy of Formation
ATP
-2982kJ/mol
Water
-287kJ/mol
ADP
-2000kJ/mol
Phosphate
-1299kJ/mol
The enthalpy of formation for bonds in the hydrolysis of ATP reaction
Looking at the enthalpy change between reactants and products shows that hydrolysing ATP releases around 30kJ/mol of energy.
Looking at the enthalpy change between reactants and products shows that hydrolysing ATP releases around 30kJ/mol of energy. | Source

The equivalent reaction for glucose releases over 2800kJ/mol. This is 93 times more energy than released by hydrolysing ATP!

If this energy were released at once, the cell would burn out.

Metabolism and ATP Quiz


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Key Metabolism Definitions

  • Metabolism - all the chemical reactions occurring in an organism.
  • Anabolism - building up large, complex molecules from smaller, simpler ones. Requires energy.
  • Autotrophs - produce their own food out of raw materials.
  • ATP - Adenosine Triphosphate: the 'energy currency' of the cell used for all anabolic reactions.
  • Bond - electrons shared between atoms.
  • Catabolism - breaking down large, complex molecules into smaller, simpler ones. Releases energy.
  • Heterotrophs - feed on other organisms that have made their own food
  • Hydrolyse - a chemical reaction where a large molecule is broken down into a smaller molecule using water.

Ball and Stick Model of ATP

This ball and stick model of ATP shows how the molecule would look in 3-D Space
This ball and stick model of ATP shows how the molecule would look in 3-D Space | Source

© 2014 Rhys Baker

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    • The Examiner-1 profile image

      The Examiner-1 3 years ago

      It was very informative and interesting. I always like to try learning something new, it intrigues me. I could not believe that I only got one correct on the quiz though. Still, I rated it high and pinned it. I clicked on thumbs up (more than once) but it would not highlight.

      Kevin

    • Jevannel profile image

      Jevannel 3 years ago from Davao City

      The article is very informative. But, I could suggest that ATP and ADP should be spell-out on the first part of your post. However, overall, it is a very good article.

    • TFScientist profile image
      Author

      Rhys Baker 3 years ago from Peterborough, UK

      Thank-you, Jevannel. I'm glad you found the hub interesting. Your summary is very apt and succinct-you have certainly grasped the basics of ATP.

      Is there anything about the hub that could be improved?

    • Jevannel profile image

      Jevannel 3 years ago from Davao City

      I can only summarize ATP as the source of energy for metabolic processes of all living organism :-) Interesting article!