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Introduction to macromolecules

Types of large biological molecules. Monomers, polymers, dehydration synthesis, and hydrolysis.


Think back to what you ate for lunch. Did any of your lunch items have a “Nutrition Facts” label on the back of them? If so, and if you had a look at the food's protein, carbohydrate, or fat content, you may already be familiar with several types of large biological molecules we’ll discuss here. If you’re wondering what something as weird-sounding as a “large biological molecule” is doing in your food, the answer is that it’s providing you with the building blocks you need to maintain your body – because your body is also made of large biological molecules!
Just as you can be thought of as an assortment of atoms or a walking, talking bag of water, you can also be viewed as a collection of four major types of large biological molecules: carbohydrates (such as sugars), lipids (such as fats), proteins, and nucleic acids (such as DNA and RNA). That’s not to say that these are the only molecules in your body, but rather, that your most important large molecules can be divided into these groups. Together, the four groups of large biological molecules make up the majority of the dry weight of a cell. (Water, a small molecule, makes up the majority of the wet weight).
Large biological molecules perform a wide range of jobs in an organism. Some carbohydrates store fuel for future energy needs, and some lipids are key structural components of cell membranes. Nucleic acids store and transfer hereditary information, much of which provides instructions for making proteins. Proteins themselves have perhaps the broadest range of functions: some provide structural support, but many are like little machines that carry out specific jobs in a cell, such as catalyzing metabolic reactions or receiving and transmitting signals.
We’ll look in greater detail at carbohydrates, lipids, nucleic acids, and proteins a few articles down the road. Here, we’ll look a bit more at the key chemical reactions that build up and break down these molecules.

Monomers and polymers

Most large biological molecules are polymers, long chains made up of repeating molecular subunits, or building blocks, called monomers. If you think of a monomer as being like a bead, then you can think of a polymer as being like a necklace, a series of beads strung together.
Carbohydrates, nucleic acids, and proteins are often found as long polymers in nature. Because of their polymeric nature and their large (sometimes huge!) size, they are classified as macromolecules, big (macro-) molecules made through the joining of smaller subunits. Lipids are not usually polymers and are smaller than the other three, so they are not considered macromolecules by some sources1,2. However, many other sources use the term “macromolecule” more loosely, as a general name for the four types of large biological molecules3,4. This is just a naming difference, so don’t get too hung up on it. Just remember that lipids are one of the four main types of large biological molecules, but that they don’t generally form polymers.

Dehydration synthesis

How do you build polymers from monomers? Large biological molecules often assemble via dehydration synthesis reactions, in which one monomer forms a covalent bond to another monomer (or growing chain of monomers), releasing a water molecule in the process. You can remember what happens by the name of the reaction: dehydration, for the loss of the water molecule, and synthesis, for the formation of a new bond.
In the dehydration synthesis reaction above, two molecules of the sugar glucose (monomers) combine to form a single molecule of the sugar maltose. One of the glucose molecules loses an H, the other loses an OH group, and a water molecule is released as a new covalent bond forms between the two glucose molecules. As additional monomers join by the same process, the chain can get longer and longer and form a polymer.
Even though polymers are made out of repeating monomer units, there is lots of room for variety in their shape and composition. Carbohydrates, nucleic acids, and proteins can all contain multiple different types of monomers, and their composition and sequence is important to their function. For instance, there are four types of nucleotide monomers in your DNA, as well as twenty types of amino acid monomers commonly found in the proteins of your body. Even a single type of monomer may form different polymers with different properties. For example, starch, glycogen, and cellulose are all carbohydrates made up of glucose monomers, but they have different bonding and branching patterns.


How do polymers turn back into monomers (for instance, when the body needs to recycle one molecule to build a different one)? Polymers are broken down into monomers via hydrolysis reactions, in which a bond is broken, or lysed, by addition of a water molecule.
During a hydrolysis reaction, a molecule composed of multiple subunits is split in two: one of the new molecules gains a hydrogen atom, while the other gains a hydroxyl (-OH) group, both of which are donated by water. This is the reverse of a dehydration synthesis reaction, and it releases a monomer that can be used in building a new polymer. For example, in the hydrolysis reaction below, a water molecule splits maltose to release two glucose monomers. This reaction is the reverse of the dehydration synthesis reaction shown above.
Dehydration synthesis reactions build molecules up and generally require energy, while hydrolysis reactions break molecules down and generally release energy. Carbohydrates, proteins, and nucleic acids are built up and broken down via these types of reactions, although the monomers involved are different in each case. (In a cell, nucleic acids actually aren't polymerized via dehydration synthesis; we’ll examine how they're assembled in the article on nucleic acids. And, dehydration synthesis reactions are also involved in the assembly of certain types of lipids, even though the lipids are not polymers3).
In the body, enzymes catalyze, or speed up, both the dehydration synthesis and hydrolysis reactions. Enzymes involved in breaking bonds are often given names that end with -ase: for instance, the maltase enzyme breaks down maltose, lipases break down lipids, and peptidases break down proteins (also known as polypeptides, as we’ll see in the article on proteins). As food travels through your digestive system – in fact, from the moment it hits your saliva – it is being worked over by enzymes like these. The enzymes break down large biological molecules, releasing the smaller building blocks that can be readily absorbed and used by the body.

Want to join the conversation?

  • aqualine tree style avatar for user kaitlynlebrun
    Can you have a polymer made of different macro molecules? Like a carbohydrate bonded to a protein?
    (30 votes)
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  • piceratops seed style avatar for user karthik2001
    what are macromolecules?
    (2 votes)
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    • aqualine seedling style avatar for user SpinosaurusRex
      There are four classes of macromolecules that constitute all living matter: carbohydrates, lipids, proteins, and nucleic acids. While they have different structures and functions, they are all composed of long complex chains of molecules (polymers) made up of simpler, smaller subunits (monomers). They are joined together in a process known as dehydration synthesis, in which a covalent bond is formed between two monomers by releasing a water molecule.
      (28 votes)
  • blobby green style avatar for user kab4598
    when is it my turn to be happy
    (14 votes)
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  • blobby green style avatar for user pulemiyen
    Why are lipids polynomials
    (10 votes)
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  • piceratops ultimate style avatar for user Sher Gill
    How are Hydrocarbons related to Macromolecules? Are they considered to be macromolecules or are they closely related?
    (5 votes)
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    • leaf red style avatar for user Sammy
      A hydrocarbon is a type of macromolecule. Basically a macromolecule is a "big molecule". Hydrocarbons are just a chain of carbon atoms with hydrogens bonded to all the carbons (hence, hydro-carbon). So, a big chain of carbons and hydrogens is a type of macromolecule. There are other types of macromolecules, but since most are organic (consisting of mostly carbon), hydrocarbons are used as an example.
      (12 votes)
  • starky tree style avatar for user Sadeel Zafar
    Are glycogen molecules stored in every cell of the body or only in liver cells or other cells specialized for storing it? If it is only stored in certain parts of the body then is it broken down into glucose before it is transported to other cells for use?
    (6 votes)
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    • piceratops seed style avatar for user RogerP
      Glycogen is also stored in skeletal muscle cells for use by those cells. Muscle cells can break down glycogen but lack glucose-6-phosphatase, the enzyme that is needed if glucose is to enter the blood stream to be used by other cells.

      Glycogen is also stored by other tissues in amounts smaller than in muscle.
      (7 votes)
  • blobby green style avatar for user wixwebsitelover
    The article states that "Dehydration synthesis reactions... generally require energy, while hydrolysis reactions... generally release energy." However, in chemistry we were drilled that breaking bonds requires energy and creating bonds releases energy; how would these guidelines mesh?
    (4 votes)
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    • leaf red style avatar for user Richard
      The breaking of bonds does require an input of energy, while the formation of bonds results in a release of energy. This is true for all reactions. But then we also have to concern ourselves with the magnitude of these energy inputs and outputs. It’s possible for a reaction to release more energy in bond formation than was put into it for bond breaking, and vis versa.

      We formalize this phenomenon in chemistry with the idea of enthalpy, or specifically the change in enthalpy. Reactions which release more energy than they take in are referred to as exothermic, and reactions which release less energy then they take in are referred to as endothermic.

      Hope that helps.
      (9 votes)
  • marcimus pink style avatar for user Eiman Tahir
    So lipids are macromolecules but not polymers? Is that even possible?
    (2 votes)
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    • orange juice squid orange style avatar for user Amma
      Yes, macromolecules are large molecules that are formed by the joining of smaller units. Lipids fit that description, but they aren't polymers because they are made up of smaller units of different kinds (like glycerol and fatty acids) rather than monomers that repeat themselves.
      (11 votes)
  • marcimus pink style avatar for user Williams, Kamili
    What are the four macro-molecules in living things?
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  • spunky sam blue style avatar for user SV52
    are there any other macromolecules apart from carbs ,fats,protiens and nucleotides
    (3 votes)
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    • female robot grace style avatar for user tyersome
      To some extent that depends on how you choose to define macromolecule, but those are (representatives of) the four groups that are usually considered to be biological macromolecules.

      The second group is actually lipids, which includes compounds like steroids that are not fats.

      The last group is nucleic acids — nucleotides are the monomers that make up nucleic acids.

      Other than these there are many biological macromolecules that contain parts from two (or more) of these types. Some examples are:
      • glycoproteins — proteins with attached carbohydrate chains
      • lipoproteins — proteins with attached lipids
      • lipopolysaccharides — carbohydrates with attached lipids

      There are also many (usually) synthetic (i.e. artificial or "man made") macromolecules such as plastics, and allotropes§ of carbon (e.g. graphene and carbon nanotubes).

      §Note: see — https://en.wikipedia.org/wiki/Allotropes_of_carbon
      (5 votes)