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Biology_A-level_Cie

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  1. 1-1-the-microscope-in-cell-studies
    5 主题
  2. 1-2-cells-as-the-basic-units-of-living-organisms
    5 主题
  3. 2-1-testing-for-biological-molecules
    3 主题
  4. 2-2-carbohydrates-and-lipids
    8 主题
  5. 2-3-proteins
    6 主题
  6. 2-4-water
    2 主题
  7. 3-1-mode-of-action-of-enzymes
    5 主题
  8. 3-2-factors-that-affect-enzyme-action
    8 主题
  9. 4-1-fluid-mosaic-membranes
    4 主题
  10. 4-2-movement-into-and-out-of-cells
    12 主题
  11. 5-1-replication-and-division-of-nuclei-and-cells
    6 主题
  12. 5-2-chromosome-behaviour-in-mitosis
    2 主题
  13. 6-1-structure-of-nucleic-acids-and-replication-of-dna
    4 主题
  14. 6-2-protein-synthesis
    5 主题
  15. 7-1-structure-of-transport-tissues
    4 主题
  16. 7-2-transport-mechanisms
    7 主题
  17. 8-1-the-circulatory-system
    7 主题
  18. 8-2-transport-of-oxygen-and-carbon-dioxide
    5 主题
  19. 8-3-the-heart
    4 主题
  20. 9-1-the-gas-exchange-system
    6 主题
  21. 10-1-infectious-diseases
    3 主题
  22. 10-2-antibiotics
    3 主题
  23. 11-1-the-immune-system
    4 主题
  24. 11-2-antibodies-and-vaccination
    6 主题
  25. 12-1-energy
    5 主题
  26. 12-2-respiration
    11 主题
  27. 13-1-photosynthesis-as-an-energy-transfer-process
    8 主题
  28. 13-2-investigation-of-limiting-factors
    2 主题
  29. 14-1-homeostasis-in-mammals
    8 主题
  30. 14-2-homeostasis-in-plants
    3 主题
  31. 15-1-control-and-coordination-in-mammals
    12 主题
  32. 15-2-control-and-coordination-in-plants
    3 主题
  33. 16-1-passage-of-information-from-parents-to-offspring
    5 主题
  34. 16-2-the-roles-of-genes-in-determining-the-phenotype
    7 主题
  35. 16-3-gene-control
    3 主题
  36. 17-1-variation
    4 主题
  37. 17-2-natural-and-artificial-selection
    7 主题
  38. 17-3-evolution
    2 主题
  39. 18-1-classification
    5 主题
  40. 18-2-biodiversity
    7 主题
  41. 18-3-conservation
    6 主题
  42. 19-1-principles-of-genetic-technology
    11 主题
  43. 19-2-genetic-technology-applied-to-medicine
    4 主题
  44. 19-3-genetically-modified-organisms-in-agriculture
    2 主题
  45. 1-1-the-microscope-in-cell-studies
  46. 1-2-cells-as-the-basic-units-of-living-organisms
  47. 2-1-testing-for-biological-molecules
  48. 2-2-carbohydrates-and-lipids
  49. 2-3-proteins
  50. 2-4-water
  51. 3-1-mode-of-action-of-enzymes
  52. 3-2-factors-that-affect-enzyme-action
  53. 4-1-fluid-mosaic-membranes
  54. 4-2-movement-into-and-out-of-cells
  55. 5-1-replication-and-division-of-nuclei-and-cells
  56. 5-2-chromosome-behaviour-in-mitosis
  57. 6-1-structure-of-nucleic-acids-and-replication-of-dna
  58. 6-2-protein-synthesis
  59. 7-1-structure-of-transport-tissues
  60. 7-2-transport-mechanisms
  61. 8-1-the-circulatory-system
  62. 8-2-transport-of-oxygen-and-carbon-dioxide
  63. 8-3-the-heart
  64. 9-1-the-gas-exchange-system
  65. 10-1-infectious-diseases
  66. 10-2-antibiotics
  67. 11-1-the-immune-system
  68. 11-2-antibodies-and-vaccination
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Forming the glycosidic bond

  • To make monosaccharides more suitable for transport and storage, and to have less influence on a cell’s osmolarity, they are bonded together to form disaccharides and polysaccharides

  • Disaccharides and polysaccharides are formed when two hydroxyl (-OH) groups (on different saccharides) interact to form a strong covalent bond called the glycosidic bond (the oxygen link that holds the two molecules together)

  • Every glycosidic bond results in one water molecule being removed, thus glycosidic bonds are formed by condensation

Diagram showing the condensation reaction of two alpha-glucose molecules forming a disaccharide (maltose) and water, highlighting the glycosidic bond.
The formation of a glycosidic bond by condensation between two monosaccharides (glucose) to form a disaccharide (maltose)

  • Each glycosidic bond is catalysed by enzymes specific to which OH groups are interacting

  • As there are many different monosaccharides, this results in different types of glycosidic bonds forming (e.g maltose has a α-1,4 glycosidic bond and sucrose has a α-1,2 glycosidic bond)

Chemical reaction diagram showing glucose and fructose forming sucrose and water via condensation, highlighting the glycosidic bond.
The formation of a glycosidic bond by condensation between α-glucose and β-fructose to form a disaccharide (sucrose)
Diagram of amylopectin showing 1,4 and 1,6 glycosidic bonds between glucose units. Branching occurs at the 1,6 bonds.
The formation of glycosidic bonds to create a polysaccharide (amylopectin)

Sugar

Type of Glycosidic bond

Type of Molecule

Maltose

α 1,4

Disaccharide

Sucrose

α 1,2

Disaccharide

Cellulose

β 1,4

Polysaccharide

Amylose

α 1,4

Polysaccharide

Amylopectin

α 1,4 and α 1,6

Polysaccharide

Examiner Tips and Tricks

Make sure you can identify where the glycosidic bond is in a carbohydrate.

Breaking the glycosidic bond

  • The glycosidic bond is broken when water is added in a hydrolysis (meaning ‘hydro’ – with water and ‘lyse’ – to break) reaction

  • Disaccharides and polysaccharides are broken down in hydrolysis reactions

  • Hydrolytic reactions are catalysed by enzymes, these are different to those present in condensation reactions

  • Examples of hydrolytic reactions include:

    • The digestion of food in the stomach and intestines

    • The breakdown of stored carbohydrates in muscle and liver cells for use in cellular respiration

Diagram illustrating hydrolysis of maltose. Glycosidic bond breaks with water addition, forming two alpha-glucose monosaccharides.
Glycosidic bonds are broken by the addition of water in a hydrolysis reaction

  • Sucrose is a non-reducing sugar which gives a negative result in a Benedict’s test

    • When sucrose is heated with hydrochloric acid this provides the water that hydrolyses the glycosidic bond resulting in two monosaccharides that will produce a positive Benedict’s test

Diagram illustrating sucrose hydrolysis. Sucrose reacts with water, breaking the glycosidic bond, forming glucose and fructose molecules.
A molecule of glucose and a molecule of fructose are formed when one molecule of sucrose is hydrolysed; the addition of water to the glycosidic bond breaks it

Examiner Tips and Tricks

Remember that disaccharides hydrolyse to two monosaccharides whereas polysaccharides must undergo many hydrolytic reactions until they form monosaccharides.