Notes for Carbohydrate

• Carbohydrates


  Carbohydrates are made of C, H and O (‘carbo’ – contains carbon ; ‘hydrate’ – contains H and O)

  • Carbohydrates are composed of recurring monomers called monosaccharides (‘mono’ – single ; 'saccharide’ – sugar)
• Monosaccharides


  They are typically form ring structures as a result of a chemical reaction between functional groups at opposite ends of the molecule

  • A hydroxyl group (-OH) links to a carbonyl group (=O) to form a cyclic structure connected by an oxygen atom

  
• Types of Monosaccharides


  Most monosaccharides have either 5 carbons (pentose sugars) or 6 carbons (hexose sugars)

  • The name describes the number of carbons – not the shape (e.g. fructose is a hexose sugar but forms a pentagon)
• An example of a hexose sugar


  It's glucose – which is primarily used as a source of energy (it is digested via cell respiration to produce ATP)

  • Glucose can exist as one of two isomers (⍺-D glucose or ß-D glucose) depending on the orientation of the 1’-OH group

  
• An example of a pentose sugar



  It is ribose – which is a core component of RNA nucleotides and is also found in co-enzymes (such as ATP)

  DNA nucleotides have a modified form of this pentose sugar in which an oxygen atom is removed (deoxyribose)

  
• Function of Monosaccharides


  The primary role of most monosaccharides is to function as a source of energy for the cell

  • Monosaccharides are oxidised (broken down) to produce large quantities of biological energy (ATP) via cellular respiration
• Glucose


  It's the most common monosaccharide to be used as an energy source due to its various chemical properties:

  • Solubility
  • Stability
  • Transport
  • Potential Energy
  • ATP Yield
• Solubility


  Glucose is a polar molecule (due to –OH groups) and so will dissolve in water (it is hydrophilic)
• Stability


  Glucose is a very stable molecule as cyclic structures are generally more energetically favourable than straight chains
• Transport


  Because glucose is soluble and stable, it is easier to transport within aqueous solutions (like blood or cytosol)
• Potential Energy


  • Glucose has many high energy electrons (between C–C and C–H bonds) which can be released via oxidation
• ATP Yield


  Glucose can by oxidised to produce a large yield of ATP via aerobic cell respiration
• Production of macromolecules by condensation reactions that link monomers to form a polymer


  Monosaccharides may be linked together via condensation reactions (water is formed as a by-product)

  • Two monosaccharide monomers may be joined via a glycosidic linkage to form a disaccharide
  • Many monosaccharide monomers may be joined via glycosidic linkages to form polysaccharides

  
• Types of Polysaccharides


  Polysaccharides are carbohydrate polymers comprised of many monosaccharide monomers

  Three key polysaccharides can be produced from glucose alone – cellulose, starch and glycogen
• • Cellulose


  Cellulose is composed of ß-glucose subunits with glycosidic linkages between the 1’ – 4’ carbon atoms and form microfibrils

  
• Starch


  Starch is composed of ⍺-glucose subunits and may involve one of two distinct bonding arrangements and does not draw water.

  • Amylose is solely comprised of 1’ – 4’ linkages organised into a helical structural arrangement
  • Amylopectin is a branched molecule comprised of both 1’ – 4’ linkages and additional 1’ – 6’ linkages

  
• Glycogen


  Glycogen is similar to amylopectin in structure but is more highly branched (more frequent 1’ – 6’ linkages)

  
• Energy Storage


  Polymers of ⍺-glucose are used in energy storage – glycogen is used in animals and starch is used in plants

  • Glucose monomers can be added or removed (by condensation or hydrolysis reactions) to build or mobilise these energy stores
• Starch & Glycogen 


  In both glycogen and starch, the ⍺-glucose monomers are connected via 1’ – 4’ glycosidic linkages to form helical structures. Glucose & starch are efficient storage molecules but not suitablfore transport within aqueous solutions (like the blood or sap)

  • Starch can exist as linear strands (amylose) or be branched (amylopectin) due to the presence of additional 1’ – 6’ linkages
  • Glycogen is a more highly branched molecule than amylopectin as it possesses more frequent 1’ – 6’ linkages
• Branching


  Branching causes the polysaccharides to adopt a more compact structure, but their large molecular size renders them insoluble in water

  • This means that glycogen and starch are efficient storage molecules but not suitable from transport within aqueous solutions (like the blood or sap)
  • However the carbohydrates can be readily digested to release monomers or dimers for transport to other tissues

  
• Structure of cellulose related to its function as a structural polysaccharide in plants


  Polymers of ß-glucose are used to form cellular structures – cellulose is a component of plant cell walls

  • Cellulose is composed of ß-glucose subunits in an alternating arrangement (every second glucose is inverted)
  • This allows cellulose to form straight linear chains that can be grouped in bundles and cross-linked with hydrogen bonds 
  • These cross-linked bundles function to increase the structural integrity and mechanical stability of the polymer

  
• Glycosylation


  It's the process of attaching carbohydrates to proteins, forming glycoproteins.
• The role of glycoproteins in blood cell recognition


  Glycoproteins on blood cells function as identification tags to allow the immune system to recognize the cells as 'self'. This is why blood transfusions are not compatible between individuals with different blood groups, except for AB blood individuals who can accept any blood type due to possessing both glycoproteins.
• The advantages of having both lipid and carbohydrate as energy stores in the human body.

  Lipid is long-term energy storage. Carbohydrate is short-term energy storage/readily available

  Lipids are insoluble, therefore easier to store. Carbohydrates/sugars are soluble, therefore easy to transport by blood. Lipids store more energy «per gram» and they occupy less space «per energy/kJ»
• The advantages of having both lipid and carbohydrate as energy stores in the human body.

  lipid is long-term energy storage. Carbohydrate is short-term energy storage/readily available. Lipids are insoluble, so easier to store. Carbohydrates/sugars are soluble, so easy to transport by blood. lipids store more energy «per gram» and they occupy less space «per energy/kJ»

  
• Example of Disaccharide


  lactose, glucose and galactose
• Example of monomers


  Maltose, glucose and glucose

  Sucrose, glucose and fructose
• Name the three disaccharides and their monomers

  1. sucrose: glucose + fructose
  2. maltose: glucose + glucose
  3. lactose: glucose + galactose
• Name four polysaccharides


  1. Starch
  2. Glycogen
  3. Cellulose
  4. Chitin