Carbohydrates Study Notes (LO1-LO3)

LO1: Synthesis of biological polymers

  • Dehydration synthesis (condensation): a reaction in which a monomer and another monomer or polymer join to form a polymer and a water molecule is released.
    • General idea: monomers covalently bond to build polymers; water is a byproduct.
    • Representative equation: ext{Monomer} + ext{Monomer}
      ightarrow ext{Polymer} + H_2O
  • Hydrolysis: a reaction in which a polymer is broken into 2 monomers or a monomer and a polymer, and a water molecule is consumed.
    • Water is used to separate monomer units.
    • Representative equation: ext{Polymer} + H_2O
      ightarrow ext{Monomer} + ext{Monomer}
  • Macromolecules are built from subunits called monomers; monomers are linked via covalent bonds into polymers.
  • Example of dehydration formation: two glucose units linked to form the disaccharide maltose; a glycosidic bond forms and a molecule of water is released.
    • Representation: ext{Glucose} + ext{Glucose}
      ightarrow ext{Maltose} + H_2O
  • Monosaccharides are named by the number of carbons they contain; they can be linear or circular in solution.
  • Monosaccharides are held together with glycosidic bonds.
  • Alpha glycosidic bonds form coiled polysaccharides; beta glycosidic bonds form strong polysaccharide sheets.
  • Energy storage: starch (plants) and glycogen (animals) store energy in the C–C and C–H bonds of their sugar units.
  • Structural carbohydrate: cellulose is dietary fiber; its function is structural, and it is found in cell walls.
  • Carbohydrates attach to proteins within the plasma membranes of cells and within the extracellular matrix (ECM); these oligosaccharides function in signaling and cell identity.

LO2: Structure of Carbohydrates

  • Learning focus: structure of carbohydrates; differentiate monosaccharides, disaccharides, polysaccharides; classify monosaccharides by carbon count, carbonyl location, and alpha/beta ring forms.
  • General carbohydrate formula: (CH2O)n ; ratio C:H:O = 1:2:1.
  • Three major subtypes: monosaccharides, disaccharides, polysaccharides.
  • Monosaccharides
    • Typically have 3-7 carbons.
    • End with the suffix \(-\text{ose}\").
    • Contain a carbonyl group: C=O .
    • Classification by carbonyl position: external carbonyl = aldose; internal carbonyl = ketose.
    • Carbon-count groups: \text{Trioses} \ (3\text{C}),\quad \text{Pentoses} \ (5\text{C}),\quad \text{Hexoses} \ (6\text{C}).
  • Monosaccharides exist as linear chains or ring-shaped molecules.
    • In aqueous solution, ring forms are common; five- and six-carbon sugars cycle between linear and ring forms.
    • Ring closure locks the molecule into an α or β configuration depending on the orientation of the substituent at the anomeric carbon.
    • Examples: fructose and ribose can form five-membered rings; glucose commonly forms a six-membered ring.
  • Disaccharides
    • Formed by dehydration reaction when two monosaccharides join via a glycosidic bond.
    • Example: glucose + fructose → sucrose; water is released.
    • General form: two monosaccharides linked by a glycosidic bond; represented as a disaccharide.
  • Polysaccharides
    • Long chains of monosaccharides joined by glycosidic linkages.
    • May be branched or unbranched.
    • May consist of multiple types of monosaccharides.
    • Molecular weight can exceed 10{,}000\ \text{daltons}.
    • Major functional distinctions:
    • Alpha glycosidic linkages: used for energy storage.
    • Beta glycosidic linkages: provide structural support.
    • Oligosaccharides: small, branched sugars that can be part of glycoproteins or proteoglycans.
  • 3 major functions of polysaccharides (and oligosaccharides):
    • Energy storage (via alpha linkages)
    • Structural support (via beta linkages)
    • Cell identity and signaling (via oligosaccharides attached to proteins in the plasma membrane and ECM)
  • Polysaccharides with alpha glycosidic linkages (energy storage):
    • Starch in plants is composed of amylose and amylopectin; glycogen in animals.
    • These polymers tend to form helical coils.
    • Function: store energy in the C–C and C–H bonds of the sugar units.
  • Polysaccharides with beta glycosidic linkages (structure):
    • Cellulose in plant cell walls.
    • Chitin in animal exoskeletons and fungal cell walls.
    • These form strong sheet-like structures providing support.
  • Cell identity (glycobiology):
    • Oligosaccharides attached to proteins embedded in the plasma membrane and within the ECM have unique structures.
    • These structures contribute to cell identity (e.g., blood typing, ABO system).

LO3: Functions of Carbohydrates

  • Carbohydrates perform three core roles in cells and in the extracellular matrix:
    • Energy provision and storage (via monosaccharides and polysaccharides with alpha linkages).
    • Structural support and protection (via beta-linked polysaccharides like cellulose and chitin).
    • Cell identity and signaling (via oligosaccharides attached to proteins in membranes and ECM; examples include ABO blood group determinants).
  • Olgiosaccharides and glycoproteins:
    • Small, branched sugar chains attached to proteins contribute to signaling and recognition at the cell surface.
    • They help determine how cells recognize each other and respond to their environment.

Key terms & quick references

  • Dehydration synthesis (condensation): monomer + monomer → polymer + H2O
  • Hydrolysis: polymer + H2O → monomer + monomer
  • Glycosidic bond: covalent bond linking sugar monomers; can be alpha (α) or beta (β)
  • Monosaccharides: 3–7 carbons; end with -ose; include aldoses and ketoses; ring and linear forms
  • Disaccharides: two monosaccharides linked by a glycosidic bond (e.g., glucose + fructose → sucrose)
  • Polysaccharides: long polymers of monosaccharides; may be branched or unbranched
  • General carbohydrate formula: (CH2O)n
  • Energy storage polysaccharides (alpha): starch (amylose and amylopectin), glycogen
  • Structural polysaccharides (beta): cellulose, chitin
  • Cell identity: oligosaccharides on proteins/ECM; ABO blood typing as an example

Connections to foundational ideas

  • Dehydration synthesis and hydrolysis are fundamental reaction types in biology, underlying polymer assembly and breakdown across macromolecules (carbohydrates, proteins, nucleic acids, lipids).
  • The concept of monomers forming polymers via covalent bonds helps explain how complex carbohydrates achieve diverse structures (linear vs branched, alpha vs beta linkages) and functions (energy storage vs structure).
  • The idea that macromolecules have roles in signaling and identity foreshadows broader topics in cell communication, immune recognition, and tissue compatibility.

Practical and ethical considerations

  • Dietary fiber (cellulose) is not digestible by humans but is important for digestive health; emphasizes how structural carbohydrates contribute to nutrition.
  • Carbohydrate structures on cells influence blood typing and transplantation compatibility; understanding glycosylation patterns is critical in medicine and transfusion science.

Examples to remember

  • Monosaccharide examples: Glucose (C6H12O6), Fructose (C6H12O6), Ribose (C5H10O5)
  • Disaccharide example: Sucrose = glucose + fructose
  • Polysaccharide examples:
    • Alpha-linked energy storage: Amylose (starch), Amylopectin (starch), Glycogen
    • Beta-linked structural: Cellulose, Chitin

Equations to recall

  • General carbohydrate formula: (CH2O)n
  • Dehydration synthesis (disaccharide formation): ext{Monomer} + ext{Monomer}
    ightarrow ext{Disaccharide} + H_2O
  • Hydrolysis (polymer breakdown): ext{Polymer} + H_2O
    ightarrow ext{Monomer} + ext{Monomer}
  • Glucose + Fructose → Sucrose (disaccharide): ext{Glucose} + ext{Fructose}
    ightarrow ext{Sucrose} + H_2O
  • Energy storage polymers form coils via alpha linkages; structure polymers form sheets via beta linkages. In symbols: ext{Alpha-linkage}
    ightarrow ext{coils / helices} "; ext{Beta-linkage}
    ightarrow ext{structural sheets}