Biological Molecules (OCR Gateway GCSE Biology: Combined Science)

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Sugars in Complex Carbohydrates

  • 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, IGCSE & GCSE Biology revision notes

Glycogen, cellulose and starch are all made from glucose molecules

Amino Acids in Proteins

  • 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, IGCSE & GCSE Biology revision notes

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

Fatty Acids & Glycerol in Lipids

  • 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

Structure of a triglyceride, IGCSE & GCSE Biology revision notes

The structure of a triglyceride

Exam Tip

You should be able to explain the importance of sugars, amino acids, fatty acids and glycerol in the synthesis and breakdown of carbohydrates, proteins and lipids. There will be many examples of each of these molecules throughout the course. You will not need to remember the molecular structure.

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Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.