Edexcel IGCSE Biology: Double Science 复习笔记 2.3.1 Carbohydrates, Fats & Proteins

• Most of the molecules in living organisms fall into three categories: carbohydrates, proteins and lipids
• These all contain carbon and so are described as organic molecules

Carbohydrates

• Carbohydrates contain the elements carbon, hydrogen and oxygen
• A monosaccharide is a simple sugar 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 monosaccharides join together
  • 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

Fats

• Most fats (lipids) in the body are made up of triglycerides
• Their basic unit is one glycerol molecule chemically bonded to three fatty acid chains
• The fatty acids vary in size and structure
• Lipids are divided into fats (solids at room temperature) and oils (liquids at room temperature)

The structure of a triglyceride

Proteins

• Proteins are formed from long chains of amino acids
• There are 20 different amino acids
• When amino acids are joined together a protein is formed
• 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

   

Amino acids join together to form proteins

Protein shape

• Different proteins have different amino acid sequences resulting in them being different shapes
• Even a small difference in the amino acid sequence will result in a completely different protein being formed
• The different sequences of amino acids cause the polypeptide chains to fold in different ways and this gives rise to the different shapes of proteins
• In this way, every protein has a unique 3-D shape that enables it to carry out its function
• The shape of a protein determines its function
• For example:
  • Enzymes have a specifically shaped active site - this is where a specific substrate molecule fits in order for a reaction to take place
  • If the shape of the active site does not match the shape of the molecule that fits into it, the reaction will not take place
  • Antibodies are proteins produced by certain types of white blood cells that attach to antigens on the surface of pathogens
  • The shape of the antibody must match the shape of the antigen so that it can attach to it and signal it for destruction

   

Every enzyme has a different shaped active site-specific to one substrate