Polymers: Combinations of Monomers: Tutorial

Synthetic Polymers and the Environment

As every coin has two sides, so do polymers. Synthetic polymers pose great hazards to the environment. As synthetic polymers take an extremely long time to biologically degrade, they keep accumulating in nature and remain there for many years, causing environmental damage. That leads to the question of recycling. Low molecular weight polymers such as polyethylene bags are very difficult to recycle. Additionally, when being destroyed or burned, polythene releases large amounts of toxic fumes. That causes more damage to our environment. Polymers are made and modified using additives. These polymers emit harmful gases when burned, which again leads to pollution. However, research is under way to overcome the environmental hazards caused by synthetic polymers. Watch a video to find out more about the hazards caused by synthetic polymers. Note that only synthetic polymers exhibit these disadvantages.

Working with Polymers

As more and more research is being done, we are finding newer ways to effectively work with polymers. Biologists are constantly coming up with various solutions to our ever-growing needs. As synthetic polymers such as polythene are not good for the environment, biologists have found other solutions. For example, banana fibers are now used to make carry bags and other utility items, such as mats and home furnishings. Interest in the field of polymers is also relevant to the field of molecular biology. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, including the interactions between DNA, RNA, and protein biosynthesis as well as learning how these interactions are regulated. These areas of research require deep understanding of how polymers function.

Dehydration Synthesis and Hydrolysis

During the process of polymerization, monomers are joined together by a process called dehydration synthesis. As this name suggests, a covalent bond is synthesized by the loss of a water molecule (dehydration). For example, when two glucose molecules come together to form maltose, a water molecule is eliminated, as shown in the image. Now, if the maltose formed in the earlier reaction is broken down using a water molecule, it will split to give two glucose units. Because water is used to break the covalent bond present in maltose, this reaction is called hydrolysis. It is a complete reversal of the dehydration synthesis that we saw earlier.

Hydrolysis in Carbohydrates

Earlier, we saw dehydration synthesis bind a glucose monomer to a starch polymer. The reverse process, called hydrolysis, is the basis for digestion in animals. Animals ingest organic molecules, like carbohydrates, as food. To break down this food, the digestive system uses water to break the bonds between the monomers, often with the help of enzymes. In image 1, a long chain of glucose monomers exists in the form of a carbohydrate, like starch. A water molecule is available. Then, in image 2, notice that the bond is broken. The OH- ion in the water breaks free to connect to the lone glucose monomer, breaking the covalent bond in the starch. The resulting H+ ion connects to the remaining polymer chain of glucose monomers. This same process breaks down other organic polymers like proteins.

Polymerization

If polymers are chains of monomers, what holds the monomer units of a polymer together? To understand this concept, let's first look at the two types of polymers—linear and branched.

Polymers and Life

In the beginning of this lesson, you read about the different items made up of polymers that David uses every day. Many of the items he used were examples of synthetic polymers that don't occur in nature. In the course of this lesson, you have also seen examples of some natural polymers. These polymers are found in all living organisms in the forms of building materials and storage substances. They also play an important role in biochemical reactions. The basic organic compounds found in all living organisms are carbohydrates, lipids, nucleic acids, and proteins. And each of these organic compounds, except lipids, are polymers. Some examples of natural polymers found in plants include cellulose and lignin, which give structure to plants. Another polymer is starch in plants, which stores glucose and therefore, energy. As you know, starch and cellulose are made up of the same monomer unit of glucose. However, the covalent bonds in starch are weaker compared to those found in cellulose.

Summary

In this lesson, you have learned about the following key concepts: Monomers and Polymers: Just as several atoms come together to form an element and elements form molecules, polymers too are formed by the joining of several smaller units or molecules. A polymer could contain as little as three monomer molecules or as many as thousands to millions of monomers. Polymers are considered to be macromolecules, or combinations of smaller molecules. Types of Polymers: Polymers can be of two types—linear and branched. Linear polymers start at one point and end at another point. Branched polymers have more than one starting point and many ending points. Covalent Bonds and Polymerization: A covalent bond is a bond that helps monomers link to one another. It is a chemical bond involving the sharing of pairs of electrons between atoms. The process where polymers are formed through the creation of covalent bonds between monomers is called polymerization. Dehydration Synthesis and Hydrolysis: During the process of polymerization, water molecules are produced through dehydration synthesis. On the other hand, when the covalent bonds present in a polymer break down in hydrolysis, water molecules are used by the reaction and the polymers are split to form monomers. Exergonic and Endergonic Reactions: Reactions that involve the release of energy are known as exergonic reactions. Such reactions involve the breaking of covalent bonds. On the other hand, those reactions that use energy are known as endergonic reactions, as is seen in the formation of covalent bonds in polymers. Natural and Artificial Polymers: Natural polymers are found in animals, humans, and plants as building materials and storage substances, all playing a role in biochemical reactions. The basic organic compounds found in all living organisms, including animals and humans, are carbohydrates, lipids, nucleic acids, and proteins. Each of these organic compounds are macromolecules, and all of them, except lipids, are polymers. Because synthetic polymers are difficult to degrade, they keep accumulating in nature and remain there for many years.

Monomers: The Building Blocks of Polymers

Just as several atoms come together to form an element and elements form molecules, polymers are also formed by the joining of several smaller units, or molecules. These molecules, which link together to form polymers, are called monomers. Monomers are thus the molecule-level building blocks of polymers. Glucose, amino acids, and nucleotides are examples of monomers. These components are the building blocks of other molecules. For example, glucose units come together to form large cellulose and starch molecules. Similarly, amino acids in various combinations form protein molecules. Nucleotides serve as the monomers for nucleic acids, such as DNA and RNA. Monomers, the building blocks of polymers, can be thought of as a chain like the different types of cars on a train. Hundreds of thousands of monomers, even millions of them, can connect or join together to form polymers. Because polymers are larger structures made up of numerous monomer molecules, polymers are often called macromolecules. While polymers can contain thousands or millions of monomers, a polymer can contain as few as three monomer molecules. For example, raffinose, a carbohydrate commonly found in beans and vegetables, is composed of one glucose, one galactose, and one fructose unit. While starch, which is another carbohydrate, is made up of numerous glucose units. Irrespective of the number of monomers, both of these molecules are polymers.

Polymers: The Macromolecules of Life

Let's start by reviewing what atoms, elements, and molecules are. The atom is the basic unit of all matter. It has a nucleus that contains protons and neutrons. Electrons revolve around the nucleus. Protons are always positive, and electrons are always negative, but neutrons are neither positive nor negative. Similar atoms combine to form elements. Elements occur naturally or can be created in laboratories. Molecules are combinations of elements. A common example of a molecule is water, which is made up of two hydrogen atoms and one oxygen atom. In living beings, small molecules in cells come together to form larger molecules. Carbohydrates, proteins, and even our DNA are examples of these larger molecules. A single protein molecule can contain hundreds of thousands of atoms.

Linear and Branched Polymers

Linear polymers are polymers in which the monomer units hold on to each other in a long chain called the backbone. This chain may not necessarily be straight. The bonds between atoms of monomers on the backbone can also twist to form, say, a spiral structure, which is essentially linear. Therefore, linear polymers start at one point and end at another point. Branched polymers, as the name indicates, have branches like trees. They have more than one starting point and many ending points.

Covalent Bonds and Polymerization

Monomers come together in covalent bonds through a process given the name polymerization because it produces polymers. A covalent bond is a chemical bond involving the sharing of pairs of electrons between atoms. For example, in a molecule of starch, there is sharing of electrons between the atoms of one glucose monomer with another glucose monomer. Other monomers like amino acids and nucleotides form covalent bonds to produce complex proteins and nucleic acids, respectively. These covalent bonds, however, are not of equal strength in different molecules. For example, though polymers like starch and cellulose are made up of the same glucose monomers, the strength of covalent bonds in these polymers differs. The covalent bonds in starch are relatively weak, whereas those in cellulose are much stronger. Thus, little energy is needed to break down starch, whereas the breaking down of cellulose requires a lot of energy.

Exergonic and Endergonic Reactions

Much of the chemistry that makes life happen involves the making and breaking apart of polymers. This process involves energy that can be seen in two types of reactions: exergonic and endergonic. The breaking of covalent bonds releases energy. Processes that release energy are called exergonic reactions. For example, respiration releases a great amount of energy by breaking the bonds between the complex monomers in food molecules and converting them into simpler forms. This energy is then used by an organism for things like locomotion or growth. In respiration, glucose is broken down to release carbon dioxide and water along with energy. Here's a representation of that process: C6H12O6 + O2 → CO2 + H2O + energy In this video, you'll see an example of energy being released and water molecules being formed in an exergonic process. Processes in which energy is used up or absorbed are known as endergonic reactions. A good example of an endergonic reaction is photosynthesis. During photosynthesis, plants use solar energy to produce food, mainly in the form of glucose. Here's a representation of that process: CO2 + H2O + energy → C6H12O6 + O2 Note that unlike exergonic reactions, endergonic reactions are characterized by the absorption of energy.

Polymers in Animals and Humans

Polymers are also found abundantly in animals and humans. Protein is a natural polymer formed of monomers of amino acid molecules. Protein is the main component of our skin, organs, muscles, hair, and fingernails. Human hair and all animal feathers, furs, and even hooves are made of proteins. Carbohydrates are another type of polymer found in all living systems. Carbohydrates play important roles such as acting as energy storage molecules. They also provide structural support by forming hard shells that protect animals. Another polymer, DNA, is responsible for determining who we are—our inheritance. It also directs the creation of proteins at the cellular level.

Dehydration Synthesis Example

To see dehydration synthesis in detail, note this series of images. In image 1, there is a chain of glucose monomers in the form of a growing carbohydrate polymer. At one end of the chain is an OH- ion present in the natural structure of a glucose monomer. Near the polymer chain is an unlinked glucose monomer with an H+ ion as part of its structure. In image 2, note how the OH- ion and the H+ ion break free of the glucose structures and bond together to form water. That allows the starch chain to form a covalent bond with the unlinked glucose, synthesizing a larger carbohydrate polymer.

Polymerization in Proteins

To see how proteins are formed from combinations of amino acids, look at image 1, which shows two generic amino acids. All 20 amino acids have a similar structure that includes an amino group of H3N+, a carboxyl group of CO2H, a solitary hydrogen atom, and an alpha carbon. Each amino acid has a different element, or group of elements, called the R group that makes it unique. In image 2, the carboxyl group from one amino acid bonds with the amino group from another amino acid to form a peptide bond. The two amino acids come together to form a protein through the process of dehydration synthesis. The carboxyl group provides an OH- ion and the amino group provides an H+ ion to produce a water molecule as a byproduct.

Synthetic Polymers

You can see from this lesson that there is a lot of chemistry to biology. Most of this chemistry is called organic chemistry or biochemistry. As biologists and chemists have researched the chemistry of life, they have discovered ways to artificially make organic molecules that have very practical uses. They are artificial polymers, and they make up much of the world we have created around us, as David discovered in the introduction. We encounter synthetic polymers every day, many of which we describe as plastics. From the handle of a toothbrush to the chairs we sit on—all of these substances are made of polymers. Plastic bottles, shower curtains, and even CDs are made of polymers. They are also used in the manufacture of big items, such as airplanes, ships, and cars. A striking feature of synthetic polymers is that they have a hydrocarbon backbone, which can be chemically altered to create newer polymers. Polyvinyl chloride (PVC), nylon, Bakelite, vulcanized rubber, polyethylene, and polystyrene are all examples of synthetically created polymers.