This lesson will explain the chemical reactions of metabolism that occur in cells and will give an overview of the two metabolic pathways.

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What's Covered

Welcome to today’s lesson on metabolism. In this lesson today you are going to learn about:

  1. The Role of Metabolism
  2. Metabolism Pathways

1. The Role of Metabolism

Metabolism describes the chemical reactions that occur in cells.

Term to Know


Metabolism describes the various chemical reactions that are occurring in cells.

There are various types of chemical reactions that occur in cells all of the time and these chemical reactions are necessary in order for cells to be able to function properly and in order for us to be able to survive. Some of these reactions that happen in cells use ATP, and some produce ATP.

ATP is an energy storage molecule and it stands for adenosine triphosphate. It's a molecule that takes place in these reactions and is either produced or used by these reactions. ATP is a nucleotide and it's made up of a ribose sugar, an adenine-- remember that's one of our nitrogen bases and ATP consists of three phosphate groups.

If you'll think back to previous lessons we had on nucleotides, you'll remember most nucleotides only have one phosphate group. But ATP is different because it has a total of three phosphate groups and the reason that ATP is an energy storage molecule is because energy is stored in this bond between the second and third phosphate.

What happens in a reaction is that enzymes will be able to break that bond and the third phosphate will be transferred to another molecule, therefore providing stored energy. This is how ATP is different from other nucleotides that you've looked at; it has energy stored in this extra phosphate.

When that bond is broken, energy can be released and enzymes, which are usually proteins, will take part in speeding up metabolic reactions in cells. Enzymes can make these reactions happen millions of times faster than they would otherwise and enzymes are also responsible for helping to break that bond between the second and third phosphate in ATP.

Take a look at the diagram below. This is the kind of a cycle that ATP will go through when it's used or produced in chemical reactions.

If you'll remember, ATP stands for adenosine triphosphate.

The prefix tri- means three for the three phosphate groups. When ATP takes part in a reaction and energy is used, one of those phosphates is removed and added on to another molecule. You’re actually getting rid of one of our phosphate groups from ATP.

What you end up with then is something called ADP, the D stands for Di; so it is now called adenine diphosphate, which tells you that it now only has two phosphate groups. One of these phosphates has been eliminated, put on with another molecule as energy storage. Now our ATP is changed into ADP with only two phosphate groups.

Then maybe the ADP will take part in a reaction where energy is released. If energy is released the ADP can pick up another phosphate and now it will become ATP again. It can go through this cycle where it gains or loses phosphates and can either be an ATP if it has three phosphates or an ADP if it has two.

2. Metabolism Pathways

There are two types of metabolic pathways depending on what is happening with the molecules in each of these. As you go through these two pathways, please refer to the image below.

In one type of metabolic pathway, small molecules are actually turned into larger molecules and they're combined and turned into larger molecules. This process is called anabolism.

Term to Know


A type of reaction in cells where small molecules combine to form larger molecules.

In this type of metabolic pathway, it assembles complex carbohydrates, proteins, and other molecules. This is the way in which complex carbohydrates are built or proteins are built. Smaller molecules are being combined together to build a larger molecule.

Because everyone has all of these bonds-- you have several bonds between all of the smaller molecules holding them together to form this larger molecule-- there is a lot of energy stored in these bonds because there are so many bonds.

Example An example would be when monosaccharides are built into polysaccharides, you’re starting with those simple building blocks of simple sugars, monosaccharides, and you’re putting a whole bunch of them together being held by bonds and turning them into something called a polysaccharide.

The other type of metabolic pathway is catabolism.

Term to Know


A type of reaction in cells where large molecules are broken down into smaller molecules.

This is basically just the opposite; large molecules are being broken down into smaller molecules. Rather than assembling, it's disassembling complex carbs, proteins, and other molecules, breaking them down into smaller molecules.

In this case, compounds released are used by cells so these cells are taking these large molecules and breaking them down into something smaller. Then it's able to use those smaller molecules for whatever reason it needs.

Example Sometimes you eat complex carbohydrates, and those are getting broken down into glucose, which your cells can then use for cellular respiration to make ATP. You’re starting with a large molecule, and we're breaking it down into a smaller molecule that cells can then use.


Today you learned about the role of metabolism and as well as the two metabolic pathways and the role that it has in your body.

Keep up the learning and have a great day!


  • Metabolism

    Metabolism describes the various chemical reactions that are occurring in cells.

  • ​Anabolism

    ​A type of reaction in cells where small molecules combine to form larger molecules.

  • Catabolism

    ​A type of reaction in cells where large molecules are broken down into smaller molecules.