Biological molecules


1.1 Introduction to biological molecules

Biological molecules are molecules found in living organisms.

Many biological molecules, including carbohydrates, proteins, and nucleic acids, are polymers. This means they are long molecules made up of lots of smaller building blocks.

1.1 Bonding in biological molecules

All molecules are composed of atoms that have joined together. There are three main types of bonding between biological molecules:

  • Ionic bonding: atoms with opposite charges are strongly attracted to each other.
  • Covalent bonding: atoms share a pair of electrons in their outer shells.
  • Hydrogen bonding: electrostatic bonds form between oppositely charged regions.

1.1 Introduction to biological molecules

Many biological molecules are polymers, like polysaccharides, polypeptides, and polynucleotides. This means they are made up of lots of monomer building blocks.

Polysaccharides, like starch, are made of lots of monosaccharides.

Proteins or polypeptides are made of lots of peptides which have amino acids as their basic subunits.

Polynucleotides, like DNA, are made of lots of mononucleotides.

Monomers are joined together by condensation reactions and are broken down by hydrolysis reactions.

1.1 Moles

A mole is the molecular mass of a substance expressed as grams, which is the same as 6.022 × 1023 atoms or molecules of a substance. This is Avogadro number: it is the number of atoms in 12 g of carbon-12.

A molar solution is a solution that contains one mole of solute for each litre of solution.

1.2 Carbohydrates: monosaccharides – introduction

Carbohydrates are molecules created when carbon is combined with water.

Carbohydrates are very important in metabolism and they are different sizes, ranging from very small to very large.

Monosaccharides are small carbohydrates and the monomer building blocks, as they can join together to form polymers, also called polysaccharides.

1.2 Structure of monosaccharides



Monosaccharides are small carbohydrates made up of a backbone of carbon with hydrogen and oxygen molecules attached to it. 

Monosaccharides have the general formula , where n can be 3-7. 

Glucose is a common monosaccharide with the formula C₆H₁₂O₆, but the atoms can be arranged in more than one way, creating different isomers of sugar.

1.2 Benedict’s test for reducing sugars

All monosaccharides and some disaccharides are reducing sugars. This means that they are able to donate electrons to another molecules.

Benedict’s reagent is a solution of copper(II) sulfate. When heated with a reducing sugar, the copper(II) sulfate accepts electrons from the sugar forming an insoluble red precipitate of copper(I) oxide.

1.3 Carbohydrates: disaccharides and polysaccharides – introduction

Monosaccharides are small carbohydrates, but they can join together in pairs to form disaccharides, or in long chains to form polysaccharides.

Disaccharides and polysaccharides have different properties to monosaccharides and have different roles in cells.

1.3 Carbohydrates: disaccharides and polysaccharides

Monosaccharides can join as pairs to form disaccharides. For example, glucose joined to fructose forms sucrose.

When the two monosaccharides are joined a molecule of water is produced. This is a condensation reaction. The bond formed is a glycosidic bond.

Under suitable conditions, the glycosidic bond can be broken


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