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Macromolecules are large, complex molecules that are found in all living things. They are usually the products of smaller molecules and can be divided into four main classes: carbohydrates, lipids, proteins and nucleic acid.

Macromolecules also known as polymers are made up of many monomers joined together by a process called polymerization. Polymers can either be formed or broken up by dehydration or hydrolysis. Dehydration is the process of connecting monomers in which two molecules are covalently bonded together with the removal of water molecule. This process is repeated many times until a polymer is formed.

The process of breaking up polymers is hydrolysis and is essentially the opposite of dehydration. The bond between the monomers making up the polymer is broken with the addition of a water molecule and the polymer is split into its monomers.

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The Four Major Classes of Macromolecules

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The Four Major Classes of Macromolecules

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Even though there is immense diversity between molecules we are still able to group molecules together by the molecular structure and functions of each molecule. The major classes of Macromolecules are the following

1) Carbohydrates

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Carbohydrates are one of the four basic macromolecules of life. It is made up of just three major elements which are carbon, hydrogen and oxygen. Carbohydrates are important as it is a major source of energy for our bodies. Since carbohydrates are a long chain polymer it is made up of unique monomers for carbohydrates and can be classified into three subtypes: monosaccharides, disaccharides and polysaccharides.

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Monosaccharides also known as simple sugars are the monomers of carbohydrates and combine in a dehydration reaction to form complex carbohydrates. Since they are monomers they are unable to be split into smaller molecules

Structure of Monosaccharides

- has a general molecular formula of CnH2nOn , for example glucose which has the molecular formula of C₆H₁₂O6 .
- Length of carbon skeleton ranges from three carbons to seven carbons.
- Contains the carbonyl and hydroxyl functional group that is distinct to sugars
- Can be either an aldehyde sugar or a ketone sugar due to the location of the carbonyl group
- Can be trioses, pentoses and hexoses depending on the number of carbons (three carbons, five carbons and six carbons)

Functions of Monosaccharides

Monosaccharides function as an important source of energy for living things. Glucose for example is broken down in a series of reactions which releases energy, this energy in return is used to make adenosine triphosphate (ATP).

It is also used in the synthesis of other small organic molecules such as amino acid and fatty acids due to the raw material in their carbon skeleton.

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Structure of Disaccharides
A disaccharide is formed when two monosaccharides undergo a dehydration reaction, during the reaction a hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide releasing a water molecule and forming a covalent bond. This covalent bond is known as a glycosidic linkage/bond. The glycosidic bonds can be of two types an alpha or the beta type.

Functions of a Disaccharide

In everyday life disaccharides are used as sweeteners, an example would be sucrose which is used as table sugar. Sucrose is also used in plants as a form of energy storage and transporting nutrients in the phloem. One example is sugar cane which is high in sucrose. Disaccharides are also used to transport monosaccharides like glucose, fructose and galactose between cells as they are less likely to break down during transport

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Polysaccharides are long chain monosaccharides linked by glycosidic bonds. The chains may be branched or unbranched and contain different types of monosaccharides.

Functions of Polysaccharides

- used as storage material
- used as building materials for structures to protect cells or whole organisms

Uses as a storage material

Plants and animals alike store excess sugars for later use in the form of storage polysaccharides. Plants store these excess sugars as starch granules within cellular structures known as plastids. The sugar can later be withdrawn by hydrolyzing the starch into glucose monomers. In animals  excess glucose is stored in the form of glycogen, a form of polysaccharide which is more extensively branched. The glycogen is usually stored in the liver and muscle cells of animals.

Uses as a structural polysaccharide

The polysaccharide cellulose is used as a strong wall that encloses plant cells. It is generally used to provide mechanical support and shape to plant cells in addition to preventing it from bursting due to excess water intake. Cellulose like starch is a polymer of glucose but are different in terms of structure as cellulose are made by linkages of beta-glucose monomers

In animals the polysaccharide used to provide support is known as chitin which is used to form the exoskeletons of arthropods (insects, spiders and crustaceans). The chitin exoskeleton functions to protect the soft internals of arthropods and is also similar to glucose with its beta linkages.

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Lipids like carbohydrates are another class of large biological molecule. Lipids are unique compared to other classes of macromolecules due to its inability to dissolve in water. The hydrophobic property of lipids are due to its molecular structure and its abundance in hydrocarbon regions. Lipids can be divided into three categories according to its importance biologically which are fats, phospholipids and steroids.

Fats

Fats even though not a polymer is still categorized under lipids due to its inability to dissolve in water. Fats are large molecules assembled from smaller molecules by dehydration reaction. Fats are made up of two smaller molecules which are glycerol and fatty acids. Glycerol is an alcohol as its 3 carbons bear a hydroxyl group, while a fatty acid is a long carbon chain usually 16-18 carbons long. It is called a fatty acid due to the carbon at the end of the skeleton being a part of the carboxyl group. The presence of the non-polar C-H bonds in the hydrocarbon chain of fatty acids are the reason of the hydrophobic properties of fatty acids.

A triacylglycerol is a fat made by joining three fatty acid molecules to glycerol by an ester linkage. An ester linkage is formed by a dehydration reaction between a carboxyl group and a hydroxyl group.

Functions of Fats

- A source of energy
- Vitamin Absorption
-Insulation and Temperature Regulation

Fats are able to function as a backup energy source when our main source of energy, carbohydrates are not available.

Certain vitamins such as vitamins A,D,E and K are fat soluble vitamins and rely on fat for absorption and storage.

Fat cells stored in adipose tissue helps to insulate our body and sustain a normal core body temperature. Besides that fats surrounding vital organs help keep them protected from sudden movements or outside impacts.

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The terms saturated and unsaturated are used to describe the structure of the hydrocarbon chains of the fatty acids. The term saturated is used when there are no double bonds between carbon atoms composing a chain, and there are maximum number of hydrogen atoms bonded to the carbon skeleton. The fatty acid is then called a saturated fatty acid. Saturated fatty acids are typically from animal fats and are solid at room temperature. Saturated fats are packed together tightly due to its lack of double bonds.

An unsaturated fatty acid is a fatty acid containing one or more double bonds on its carbon chain. This causes it to have one less hydrogen atom on each double-bonded carbon compared to an saturated fatty acid. Most of the double bond in fatty acids are a cis double bond. Unsaturated fats are usually liquids at room temperature as the double bonds prevent the molecules from packing together closely enough to solidify. It is usually found in plant and fish fats and are referred to as oil. One difference that it has is its ability to be converted to a saturated fat by adding hydrogen.

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Phospholipids are also a class of lipids that are a major component of all cell membranes. A phospholipid is similar to a fat molecule but has only two fatty acids attached to the glycerol rather than three.

The ends of phospholipids show different behaviors towards water. The hydrocarbon tails of phospholipids are hydrophobic while the phosphate groups and its attachment form a hydrophilic head that has an affinity towards water due to its polar characteristic. When added to water the hydrophobic and hydrophilic properties of phospholipids can be observed by how it assembles into a double-layered sheet that shields the hydrophobic fatty acids from water.

Functions of Phospholipids

The phospholipids help to form a boundary between the cells and its external environment. Being a membrane component, phospholipids are selectively permeable, and controls the movement of molecules in and out of the cells

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Nucleic acid are polymers made of monomers called nucleotides. Nucleic acids are important as it is necessary for the continuity of life and it carries the genetic blueprint of a cell and the instructions for the functioning of the cell. Nucleic acid exists as polymers called polynucleotides.

Structure of Nucleotides

Nucleotides that make up the polynucleotide is composed of three parts, a five carbon sugar, a nitrogen containing base and one to three phosphate groups. Nucleic acids can be divided into two which are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The monomer used to build polynucleotides have three phosphate groups, but are lost during polymerization.

As stated before each single nucleotide has a five carbon sugar, a nitrogenous base and a phosphate group. The nitrogenous consists of two families which are pyrimidines and purines. A pyrimidine has one six-membered ring of carbon and nitrogen atoms. Pyrimidines typically consists of cytosine (C), thymine (T) and uracil (U).

The other type of nitrogenous base which is purines are larger, with a six-membered ring fused to a five member ring. Purines are adenine (A) and guanine (G). Adenine, guanine and cytosine are found in both DNA and RNA, thymine only in DNA and uracil only in RNA.

In DNA the sugar attached to the nitrogenous base is deoxyribose while in RNA it is ribose.

To complete a nucleic acid phosphate is required either one to three groups are attached to carbon number 5 in the chain.

Structures of DNA and RNA molecule

DNA
- Has two polynucleotides that wind around an imaginary axis, forming a double helix
- Contains two sugar-phosphate backbones running in opposite directions, called an antiparallel
- Sugar phosphate backbones are on the outside of the helix while
- Held together by hydrogen bonds between paired bases
- The bases pair up with the ones they are compatible with such as adenine with thymine and cytosine with guanine

RNA
- Exists as single strands
- Base pairing can occur but instead of having the base thymine (T), uracil is used to replace it and pairs up with adenine (A)
- More variable in shape and versatile

Functions of DNA (Deoxyribonucleic acid)

- Stores hereditary information efficiently
- Contains instructions to construct other components of cells

Functions of RNA (Ribonucleic acid)

- involved in protein synthesis where RNA molecules direct the assembly of proteins on ribosomes. This process uses transfer RNA molecules to deliver amino acids to the ribosome, where ribosomal RNA then links amino acids together to form proteins.

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Proteins are the most structurally sophisticated molecules consistent with their diverse function. Proteins are constructed from the same set of 20 amino acids, linked in unbranched polymers. A polymer of protein is called a polynucleotide due to bonds between amino acids being called peptide bonds. Proteins are molecules made up of one or more polypeptides, giving them distinct shapes due to it being coiled and folded into specific three-dimensional structures.

Monomers of Proteins (Amino acids)

An amino acid is an organic molecule with an amino group and a carboxyl group.

Structure of an amino acid

- contains four different groups bonded to the carbon atom called the alpha carbon
- physical and chemical properties of amino acids are determined by the side chain

Polypeptides

Polypeptides are the result of amino acids (monomers of proteins) undergoing a dehydration reaction by the formation of a peptide bond with the removal of a water molecule.

Protein Structure and Function

The specific functions of protein arise due to its intricate three-dimensional structure. A functional protein is not due to a single polypeptide chain but one or more polypeptides precisely twisted, folded and coiled into a molecule of unique shape

Enzymes

-proteins acts as catalysts to speed up chemical reactions

-enzymes can be reused repeatedly after reactions as they don't get used up and only catalyse the reaction

Defensive proteins

- are antibodies

- produced to fight against diseases
- identifies and binds to very specific foreign molecules. By binding to foreign proteins they can help neutralize them and tag them, facilitating their engulfment and removal by defensive cells

Transport Proteins

-transport substances that are unable to pass through the plasma membrane

- involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane
- The proteins may assist in the movement of substances by facilitated diffusion or active transport

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Proteins can be divided into four levels of structure that being primary, secondary, tertiary and quaternary structure.

Primary Structure of Proteins

- Is the sequence of amino acids
- The amino acids are linked by peptide bonds or covalent bonds

Secondary Structure of Proteins

- formed by segments of polypeptide chains repeatedly coiled or folded in patterns that contribute to the proteins shape
- the coils and folds are referred to as the secondary structure of proteins
- consists of alpha-helix and beta-pleated sheets
- can be divided into two main types of secondary structure which are the alpha-helix and the beta pleated sheets

Alpha helix

- a coil held together by hydrogen bonding between every fourth amino acid
- hydrogen bonds between the fourth amino acid holds the helix together and makes it stable

Beta-pleated sheets

- two or more segments of a polypeptide chain lying side by side
- connected by hydrogen bonds between two parallel segments of polypeptide backbone

Tertiary Structure of Proteins

- three-dimensional in shape
- results from the combination of secondary structures
- undergoes hydrophobic interactions, where amino acids with hydrophobic side chains ends up in clusters at the core of a protein away from water
- hydrogen bonds between polar side chains and ionic bonds between positively and negatively charged side chains help stabilize the structure
- contains disulfide bridges which reinforces the shape of proteins

Quaternary Structure of Proteins

- consists of two or more polypeptide chains aggregated into one functional macromolecule
- may consist of more than one of the same type of protein subunit