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Quite simply, molecules are many atoms chemically bonded together. They are the most basic structural formation of an element and make up everything, from the air we breathe to ground we walk on. Some important molecules make up cells which are the most basic forms of life; every living thing is made up of cells.
Cells mainly consist of water molecules but there are a number of other important biological molecules vital to allow life to exist. Macromolecules are among the most important as they can offer structural support to the cell, hold energy like a battery, act as a catalyst for biochemical reactions and store genetic information as well as having many other important functions. Macromolecules are formed of monomers – simple molecules that can bond together with others to form larger more complicated molecules.
When two monomers react, they create a separate water molecule as monomers always have hydrogen and oxygen atoms in their molecular structure. A covalent bond is formed in the absence of the hydrogen and oxygen atoms, connecting the two monomers together. This is called a condensation reaction and it can continue happening until a large chain of molecules forms, eventually making a macromolecule. The reverse effect, called a hydrolysis reaction, can also occur where water will displace a covalent bond in a macromolecule reverting it back to a monomer though a catalyst is usually needed for this reaction to take place. The following image displays these reactions very well.
These larger molecules are called polymers and macromolecules are biopolymers (naturally produced polymers). These consist of four types; the carbohydrates, proteins, lipids and nucleic acids with each type being made up of its own specific monomers.
Carbohydrates are molecules made from the elements carbon, hydrogen and oxygen. They are often called sugars and they supply a large percentage of energy to animal and human cells. The monomers of carbohydrate macromolecules are called monosaccharides (simple sugars) with only one unit of sugar. All monosaccharides have the chemical structure (CH2O)n with n equalling 3, 4, 5, 6 or 7 depending on the number of carbon atoms the monosaccharide has – for example as glyceraldehyde has three carbon atoms the formula will be C3H6O3 and as fructose has six carbon atoms the formula will be C6H12O6. Most monosaccharides form ring shaped molecular structures when dissolved in water (see below image, depicting glucoses three forms). One of the most common monosaccharides is glucose – a vital ingredient for almost all life on earth. Plants make this monomer by using photosynthesis in the following way (CH2O represents glucose).
H2O + CO2 + Sunlight and Chlorophyll = (CH2O) + O2
When these simple monosaccharide monomers such as glucose join together to form carbohydrate polymers they become disaccharides (with two sugar units), oligosaccharides (between 3 and 10 units) and polysaccharides (10 or more units also known as complex carbohydrates). The term for this joining together is called glycosidic linkages where the molecules will covalently bond by a condensation reaction (mentioned in the introductions) with each other. The disaccharide sucrose is a polymer of glucose combined with fructose, and lactose is a polymer of glucose bonded with galactose – both of these are also known as table sugar and the sweetness in milk. Cellulose, a polysaccharide, is made from many thousand conjoined glucose molecules and is the main part of plant cell walls. Another polysaccharide, starch, is again made from a large amount of glucose molecules but it is found in many food stuffs such as many vegetables, seeds, grains and fruits. Starch is very useful in that it can be stored in reserves and broken down quickly to release the energy when it’s most needed – for example a deciduous tree will need starch in the winter when it can’t carry out photosynthesis.
As polysaccharides are just repeating units of monosaccharides, their structural formations look just like them, except in larger numbers and all connected. Some have relatively simple structures like starch which is just a long helix structure, whereas others such as glycogen have much more complicated branching structures (see left image).
Proteins have perhaps the largest amount and most important functions of the four macromolecules from catalysing chemical reactions (enzymes) and keeping check on many processes (hormones) to providing structural integrity (structural proteins like collagen and elastin) and helping to defend the cell (immunoglobulin) as well as many more. Proteins are made from the amino acid monomers, of which there is vast amount, but only 20 are actually able to form proteins. The general structure of amino acids is the same in that there are four groups surrounding a carbon atom – a hydrogen atom, a carboxyl group (a combination of a carbon atom and two oxygen atoms), an amino group (nitrogen and hydrogen atoms) and a variable group (the group that will define the amino acid).
Much like the carbohydrates monosaccharides, the proteins amino acids form covalent bonds between each other (or peptide bonding) to create peptides, which contain two amino acids, such as oxytocin (a hormone released during labour to help stimulate contractions as milk production). If there are any more than two amino acids in a peptide it becomes a polypeptide, like insulin (a hormone that regulates blood sugar levels), and if there’s any more than fifty it is classed as an actual protein such as haemoglobin (an oxygen delivery macromolecule that form parts of red blood cells). The peptide linkages are created by a condensation reaction between the hydrogen of the amino group and the oxygen of the carboxyl group. Once the amino acids have linked, they form long chains and when enough of them have bonded to it, they’ll fold to make a secondary chain, creating a either a helix or B pleated sheet. As you can see from the secondary structure part of the picture, the lone hydrogen and oxygen atoms on each monomer have formed hydrogen bonds between each other, thus affecting the shape of the chain. These folded chain structures can become even more complex because of non covalent interactions which include hydrophobic interactions (where nonpolar atoms are repelled from water and forced together), ionic bonds (electrical bonds where oppositely charged atoms will be attracted and like charged atoms will be repelled, much like a magnet) and aforementioned hydrogen bonds (weak electrical attractions that occur between a positively charged hydrogen atom in one molecule and a negatively charged atom in another). Disulfide bridges can also affect the structure of the protein but will only occur in between proteins with sulphur in their monomer variable groups, such as cysteine. Many of these interactions will cause the proteins chains to fold back and forth upon itself into a more compact shape, thus changing it into its tertiary. Some proteins will consist of more than one polypeptide chain, making a quaternary structure, also sustained by the various non covalent interactions.
Proteins can also unfold and revert back to a less advanced structure stage so they may not be able to perform their primary biological function. This process is called denaturation and can happen when;
the protein is heated as its atoms will vibrate more violently making the non covalent interactions a lot weaker
the protein is mixed with a strong acid or base as this will weaken its ionic bonds
the protein comes into contact with other chemicals for example certain metals are highly positively charged therefore they will bond with the negative parts of the protein and disrupt the ionic bonds.
Lipids are the fat and oil molecules
Nucleic acids are unique in that their specialised function is for storing information about the cell such as DNA and RNA.
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