PYTR Class

Learning Objectives

1. Identify the structure and function of monosaccharides in biological systems.
2. Explain the role of polysaccharides in energy storage.
3. Relate the structure of cellulose to its function as a structural polysaccharide in plants.
4. Understand the function of glycoproteins in cell–cell recognition.

Part 1: Structures Roles of Monosaccharides

• Monosaccharides contain three to seven carbon atoms.
  • Pentoses have five carbon atoms
  • Hexoses have six carbon atoms
• Both pentoses and hexoses usually form ring structures.
• The ring consists of one oxygen atom and four or five carbon atoms.

There are two type of glucose: alpha and beta. When becomes polymer, they form different structures

Part 2: Polysaccharides

1. Polysaccharides as Energy Storage:

• Starch (plants) and glycogen (animals) serve as energy storage molecules.
• Both are composed of α-glucose molecules, which can be used in aerobic and anaerobic respiration.

• 2 types of Starch:
  • Amylose:
    • Unbranched chain of α-glucose linked by α-1→4 glycosidic bonds.
    • Forms a helical structure due to bond angles.
  • Amylopectin:
    • Similar to amylose but with some α-1→6 glycosidic bonds, creating branches.
    • Branched structure allows faster glucose removal when needed.
• Glycogen Structure:
  • Similar to amylopectin, with α-glucose linked by α-1→4 bonds and branches via α-1→6 bonds.
  • More branched than amylopectin (1 in 10 glucose molecules vs. 1 in 20).
  • Can contain tens of thousands of glucose subunits.

Advantages of Starch & Glycogen:

• Lower solubility than glucose, preventing osmotic swelling.
• Compact branched structure, allowing efficient storage.

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2. Cellulose

• Cellulose Structure & Composition:
  • Composed of β-glucose instead of α-glucose, giving it different properties from starch and glycogen.
  • Condensation reactions link C1 of one β-glucose to C4 of another, forming β-1→4 glycosidic bonds.
  • Forms unbranched chains that can exceed 10,000 β-glucose molecules and 10 µm in length.
• Molecular Orientation & Bonding:
  • β-glucose structure requires alternating orientation of glucose units, resulting in straight chains instead of helices.
  • Hydrogen bonds between chains form bundles called microfibrils.
  • Regularly spaced hydroxyl groups enable extensive hydrogen bonding.
• Function & Strength:
  • Microfibrils are the structural basis of plant cell walls.
  • High tensile strength due to:
    • Strong covalent bonds within cellulose molecules.
    • Numerous cross-links between molecules.
    • Large number of molecules in parallel arrangement.
  • Prevents plant cells from bursting, even under high internal pressure from osmosis.

Part 3: Glycoproteins

Role of Glycoproteins in Cell–Cell Recognition

• Structure:
  • = protein + carbohydrates (usually oligosaccharides)
  • Found in plasma membranes of animal cells, with carbohydrates facing outward.
• Function:
  • Enable cell recognition by displaying distinct glycoproteins.
  • Recognised by receptors on other cells, facilitating tissue organisation.
  • Help identify and destroy foreign or infected cells.
  • Example: ABO antigens in red blood cells function as glycoproteins for cell recognition.

ABO Glycoproteins & Blood Transfusion

• Red Blood Cell Glycoproteins:
  • Membranes contain glycoproteins with three possible oligosaccharides: O, A, and B.
  • Each person has one or two types but never all three.
• Blood Transfusion & Immune Response:
  • A-type blood is rejected if transfused into a person without A glycoproteins.
  • B-type blood is rejected if transfused into a person without B glycoproteins.
  • O-type blood causes no rejection, as it has the same structure as A and B but with one monosaccharide less.