Biological Molecules (OCR Gateway GCSE Biology: Combined Science)

• Carbohydrate molecules contain the elements carbon, hydrogen and oxygen
• In the human body, carbohydrates are broken down by enzymes in the mouth and small intestine
• Carbohydrates can be classed into different groups (monosaccharides, disaccharides and polysaccharides) depending on their complexity
• Monosaccharides are simple sugars that consists of a single monomer unit e.g. glucose (C₆H₁₂O₆) or fructose
  • Glucose molecules contain lots of energy which can be released in respiration by breaking the bonds between the carbon atoms
• A disaccharide is made when two monosaccharides join together
  • Maltose is formed from two glucose molecules
  • Sucrose is formed from one glucose and one fructose molecule
• A polysaccharide is formed when lots of monomer units join together in long chains to form a polymer
  • Starch, glycogen or cellulose are all formed when lots of glucose molecules join together
  • Polysaccharides are insoluble and therefore useful as storage molecules

Glycogen, cellulose and starch are all made from glucose molecules

• Proteins are polymers formed from long chains of monomers called amino acids joined together
  • Amino acids contain carbon, nitrogen, hydrogen and oxygen atoms
• Just 20 different standard amino acids make up all the proteins found in the human body (although many more amino acids occur in nature)
• Amino acids can be arranged in any order, resulting in hundreds of thousands of different proteins
  • Examples of proteins include enzymes, haemoglobin, ligaments and keratin
• In the human body, proteins that we ingest are broken down into the constituent amino acids
  • Enzymes in the stomach and small intestine break down the protein
  • These amino acids can then be used in the synthesis of new proteins

Amino acids join together to form proteins

Protein shape

• Different proteins have different amino acid sequences resulting in the peptide chain folding into different shapes
  • The function of a protein is determined by its shape
• Even a small difference in the amino acid sequence will result in a completely different protein being formed
• In this way, every protein has a unique 3-D shape that enables it to carry out its function

• Lipids (fats and oils) are made up of triglycerides
• Their basic unit is one glycerol molecule chemically bonded to three fatty acid chains
• Lipids contain carbon, hydrogen and oxygen atoms
• Unlike carbohydrates and proteins, they don't form polymers (they don't form a long chain of repeating sub-units)
• Lipids are divided into fats (solids at room temperature) and oils (liquids at room temperature)
• In the human body lipids are broken down by enzymes called lipases in the small intestine

The structure of a triglyceride