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What is ATP?
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?
The principles of metabolic pathways
- Types of Nutrition
How do different organisms get their food?
- What is ATP?
An introduction to ATP
- The Structural Formula of ATP
What does ATP look like?
- How does ATP store energy?
How is energy stored in ATP for use by the cell?
- Why use ATP?
Why does the cell use ATP instead of Glucose?
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.
When did you first hear about ATP?
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
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
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
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
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
© 2014 Rhys Baker