[AP Biology 1.3] Introduction to Biological Macromolecules
In this section of the AP Biology Curriculum – Introduction to Biological Macromolecules – we find out what macromolecules are, what they are made of, and why life on Earth would not be possible without them! This section covers the basics of macromolecules as you learn about monomers and polymers – and how they are formed and destroyed through dehydration reactions and hydration reactions, respectively. Check it out!
The following video summarizes the most important aspects of this topic!
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Resources for this Standard
Living Systems are organized in a hierarchy of structural levels that interact.
Hydrolysis and dehydration synthesis are used to cleave and form covalent bonds between monomers.
The molecular structure of specific nucleotides and amino acids is beyond the scope of the AP Exam.
The molecular structure of specific carbohydrate polymers is beyond the scope of the AP Exam.
1.3 Introduction to Biological Macromolecules Overview
As we continue our journey up through the biological hierarchy, we need to make a quick pit stop between molecules and organelles. Some of the most important things in biology happen right at the level between individual molecules and organelles. Before organelles and the most primitive cells can be functional, large biological macromolecules need to be constructed.
A macromolecule is just what it sounds like – a very large molecule. Macromolecules are most often made of many smaller molecules, bonded together into a much larger structure. Macromolecules bridge the gap between small molecules and much larger cellular components. This is important for several reasons.
First, small molecules by themselves cannot store information. This is an essential function of DNA and RNA, both of which store information for how to build proteins. DNA and RNA are made from smaller molecules called nucleotides and DNA is stored in massive macromolecules known as chromosomes. Each human cell contains 46 chromosomes, made of around 6 billion nucleotides! This allows each cell to carry all the information needed to create your body.
Second, individual molecules are too small to serve as catalysts for important biological reactions or structural elements of cells. Proteins serve this role in the cell, and are constructed from much smaller molecules known as amino acids.
Lastly, there is an important need for energy within cells. This role is mainly filled by both lipids and carbohydrates. Lipid molecules – known as fatty acids – combine into much larger macromolecules that can both store energy and create the lipid bilayer needed to separate cells from the surrounding water. Carbohydrates, such as glucose, are the base unit of energy in cells. However, some large carbohydrate macromolecules also serve structural roles in both plants and animals!
Regardless of whether a macromolecule is a protein, carbohydrate, or lipid, we use the same language to describe its parts. An individual unit of the macromolecule is known as a monomer. The prefix “mono-” means one, whereas “-mer” simply refers to a molecule or chemical structure. Therefore, “monomer” literally means one molecule.
Likewise, when you combine two base molecules, you get a “dimer.” “Di” means two. Similarly, “oligo” means “a few” – usually an oligomer is made of 3 or more monomers. Lastly, a “polymer” simply means “many molecules.” Macromolecules are simply long polymer chains!
Believe it or not, the large majority of macromolecules are created by the exact same reaction – the dehydration reaction. While not all monomers have these exact same functional groups, most of them have two parts that are essential to combining molecules – a hydroxyl group (OH) and hydrogen. During a dehydration reaction, these elements combine into a unit of water. Without these elements, one monomer is left with excess electrons while the other is left with a deficit of electrons. This is the perfect situation to form a new covalent bond. While this reaction is pretty standard for all macromolecules, there are many different enzymes and processes required for this to happen with different monomers.
If a dehydration reaction is what forms polymers, then it should make sense that a hydration reaction is needed to break a polymer. In a hydration reaction, enzymes typically put stress on the covalent bond holding two monomers together. As they break, a water molecule rushes in. The water molecule splits, allowing a hydroxyl group to bind to one monomer and a hydrogen atom to bind to the other monomer. Thus, the monomers are effectively separated!
Carbohydrate monomers are called “monosaccharides.” These include glucose, galactose, and fructose, and store a lot of energy. They can be broken down by cells to create ATP. However, these molecules can also be combined into dimers – known as disaccharides. Sucrose – also known as table sugar – is the most recognizable carbohydrate dimer. However, carbohydrate monomers can also be combined into long polymers. Let’s take a look at two very important polysaccharides – cellulose and chitin.
Cellulose is the material that creates plant cell walls. To create this strong fiber that helps plants stand tall, glucose molecules are linked together in massive chains. These fibers are layered on the outside of the lipid bilayer, creating a rigid wall that allows plants to have a defined structure.
Like cellulose, insects and crustaceans use glucose to create chitin. Chitin is slightly more modified, though it works in a similar way. As they grow, arthropods must shed their exoskeleton as they outgrow it – because this exoskeleton is made almost entirely of chitin it is very rigid. In fact, chitin is so strong that arthropods do not need any sort of internal bones or support structures.
Lipids are built out of monomers of fatty acids, which are simply hydrocarbons with polar head groups. Triglycerides are units of three fatty acid chains, bonded together with a glycerol molecule. Triglycerides are used to store massive amounts of energy in a relatively small space. Saturated fatty acids – with no double bonds – create thick fats that are usually solid at room temperature. Saturated fatty acids are usually created by animals, and are considered the more unhealthy of the two. Unsaturated fatty acids – with double bonds between some carbons – tend to be liquid at room temperature. Plants usually create unsaturated fatty acids, which are considered more healthy.
Phospholipid molecules are created using two fatty acids with a hydrophilic head group. Cells use phospholipids to create membranes. Since water cannot easily pass through the hydrophobic core, a bilayer of phospholipids effectively separates different parts of the cell – allowing different conditions to form in the cytosol and organelles that are much different than the outer environment. Fatty acids can also create molecules like steroids and waxes, which serve a number of purposes in both plants and animals. Steroids act as hormones in your body, while waxes are used as protection and passively repel water because they are very hydrophobic.
Arguably the most important monomers for life are nucleic acids. Nucleic acids are simply a sugar molecule, bound to both a phosphate group and a nitrogenous base. The sugar and phosphate molecules can bind together in a long chain. DNA and RNA are constructed in a way that the order of the nitrogenous bases is conserved – essentially allowing cells to store information. DNA molecules are like a storage polymer for this information and form a double helix as the nitrogenous bases from each strand form hydrogen bonds with each other. Hundreds of millions of nucleic acids create a chromosome. Most organisms have many chromosomes, which store all the information needed to create proteins.
Proteins are built out of monomers known as amino acids. All amino acids have the same basic structure – a carbon bonded to an amino group, a carboxyl group, and a side chain. Amino groups can bond to carboxyl groups in a dehydration reaction, while the side chains are responsible for giving each amino acid-specific properties. There are around 20 common amino acids in nature, differing only by the molecules found in their side chains. These side chains react in unique ways to create the structure of proteins.
First, the amino acids are linked together into a primary structure. Then, this structure self-organizes into secondary structures based on reactions between amino acids in the chain. Tertiary structure is determined by interactions between secondary structures. Finally, many proteins actually consist of subunits – separate protein molecules that come together into a much larger functional structure. Proteins serve a huge number of roles in cells, from catalyzing reactions as enzymes to structural elements within cells.