IAL Edexcel Biology - Carbohydrates

1.2(i) Know the difference between monosaccharides, disaccharides and polysaccharides, including glycogen and starch (amylose and amylopectin)

1.2(ii) Be able to relate the structures of monosaccharides, disaccharides and polysaccharides to their roles in providing and storing energy

1.4 Know how monosaccharides (glucose, fructose and galactose) join together to form disaccharides (maltose, sucrose and lactose) and polysaccharides (glycogen, amylose and amylopectin) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions

• Carbohydrates are made up of carbon, hydrogen and oxygen
• The chemical formula is (CH₂O)_n
• The smallest unit is called a monosaccharide
• When two or more monosaccharide join together, they can form a disaccharide (di- means two) or polysaccharide (poly- means many)

• Single saccharides
• Soluble with osmotic effect (i.e. can cause change in water potential)
• Examples:
  • Glucose – main substrate for respiration
  • Gelactose
  • Fructose – “fruit sugar”

• Two saccharides joined together
• Soluble
• Examples:
  • Sucrose (glucose + fructose) – main transport sugar in plants
  • Lactose (glucose + gelactose) – ‘milk’ sugar
  • Maltose (glucose + glucose)

• Many saccharides joined together
• Generally insoluble

👇Two important polysaccharides to remember for IAL Edexcel Biology 👇

Starch

• Energy store in plants
• Made up of:
  • Amylose
    • α -glucose molecules joined by 1,4-glycosidic bonds only
    • The unbranched chain forms a spiral and makes it more compact for storage
  • Amylopectin
    • α-Glucose molecules joined by 1,4- and 1,6- glycosidic bonds
    • The branched chain has more terminal glucose units that can be used to release energy more rapidly  

Glycogen

• Energy store in animals
• Branched chains of glucose molecules
• Similar structure with amylopectin but with more 1,6-glycosidic bonds, allowing it to be broken down very rapidly

• In a condensation reaction, glycosidic bonds form between saccharides and a water molecule is released
• In a hydrolysis reaction, glycosidic bonds are broken and a water molecule is required for the reaction 

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That's a wrap!

Written by Vivian | GCE Biology Specialist @ TUTTEE

References:

1. Edexcel AS Biology Revision Guide. Edexcel.

2. https://www.nutritionaldoublethink.com/carbohydrates.html

3. https://futurefoodchemist.weebly.com/amylose-or-amylopectin.html

4. https://zhuanlan.zhihu.com/p/91924400