Biological Polymers

Polymers are molecules made up of long chains of smaller units called monomers. Biological polymers are those that are naturally produced by living organisms. Some common examples of biological polymers include:

  • DNA
  • Starch and cellulose
  • Proteins

DNA

DNA (deoxyribonucleic acid) is essential to all living organisms. It is a large molecule that holds the genetic instructions for organisms to develop and function.

The structure of DNA is a double helix, in which two strands wrap around each other. The chains in DNA are made up of repeating units called nucleotides. Each nucleotide contains three smaller molecules: a sugar molecule, a phosphate molecule, and a base molecule.

The four bases found in DNA are:

  • Adenine (A)
  • Cytosine (C)
  • Guanine (G)
  • Thymine (T)

These bases are arranged in a specific sequence along the DNA strands and they code for genes with essential instructions for an organism’s growth and development. The order of the bases is what determines the genetic information that is passed down from parent to offspring.

A labelled nucleotide with its base, sugar, and phosphate highlighted, is placed next to the different bases (Adenine, cytosine, guanine, and thymine). Below, a DNA structure depicts the bonds.

Starch and Cellulose

Carbohydrates are one of the three macronutrients that are essential for the human body. They are classified into two categories:

  • Simple carbohydrates – These are also known as monosaccharides and include molecules such as glucose and fructose. They are the simplest form of carbohydrates and cannot be broken down into smaller sugars.
An illustration comparing the structures of two sugars. On the left, under a pink label reading "glucose", is the molecular structure of glucose, depicted as a six-membered ring with various hydroxyl groups attached. On the right, under a lavender label reading "fructose", is the molecular structure of fructose, shown as a five-membered ring with hydroxyl groups and a CH2OH group extending from it.

  • Complex carbohydrates – These are also called polysaccharides and include molecules such as starch, glycogen, and cellulose. They are made up of long chains of monosaccharides.
Illustrations show "starch" represented by a bowl of a white substance, "glycogen" represented by a liver, and "cellulose" represented by a leaf.

Polysaccharides are formed by linking monosaccharides together through a process called condensation. During this process, a water molecule is eliminated, and a glycosidic bond forms between the two sugar molecules.

A linear chemical structure showing a series of repeating monomer units. Each unit consists of a hexagonal ring with alternating single and double bonds. At the top of each ring is a CH2OH group, and at the bottom, there's an O linked to the adjacent ring. Hydrogen atoms are attached to the sides of the hexagons. Between the rings, the bond connecting them is labelled as "glycosidic bond".

Proteins

Amino acid monomers link together in a long chain to form a polypeptide. The polypeptide chain folds up or combines with other polypeptides to form a protein. This shape is essential to the protein’s biological function, as it determines how the protein interacts with other molecules.

A graphical representation showing the progression from individual amino acids to proteins. On the left, multicoloured spheres represent individual amino acids. Moving to the centre, these spheres are connected by lines to depict a short chain termed "Peptide - A short chain of amino acids". To the right, a more intricate assembly of these spheres and connections represents "Protein - A longer chain of amino acids with a more complex structure". Two arrows guide the viewer from the individual amino acids, through the peptide, and finally to the protein.

The sequence of amino acids determines the unique structure and function of the protein. There are 20 different amino acids that can link together in different ways to create a vast number of protein structures. This diversity of structures allows proteins to perform a wide variety of biological functions.

Some examples of protein functions include:

  • Enzymes – catalyse chemical reactions in the body and are essential for metabolism
  • Structural proteins – provide support and shape to cells and tissues
  • Transport proteins – move molecules and ions across cell membranes and throughout the body

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