Carbs

• Carbohydrates


  Molecules containing carbon, hydrogen and oxygen
• Glucose


  • Carbon 1 can be above or below the plane of the ring
  • Molecules polymerise into branched or linear chains
• What are carbohydrates for?

  • Store of energy (e.g. starch)
  • Part of other biological molecules like nucleic acids and glycoproteins
  • Three main types: monosaccharides, disaccharides, polysaccharides
• Monosaccharides


  The simplest carbohydrates, cannot be hydrolysed into smaller carbohydrates
• Disaccharides


  Made when two monosaccharides join together, can be hydrolysed into monosaccharides
• Formation of disaccharides


  1. Two monosaccharides join via a condensation reaction, removing a water molecule

  2. Glycosidic bond forms between the two monosaccharides
• Hydrolysis of disaccharides


  Addition of water breaks the glycosidic bond, separating the two monosaccharides
• Examples of disaccharides


  • Sucrose (glucose + fructose)
  • Maltose (glucose + glucose)
  • Lactose (glucose + galactose)
• Disaccharides can be reducing sugars (maltose, lactose) or non-reducing (sucrose)
• Hydrolysis of polysaccharides like starch produces monosaccharides
• Polysaccharides


  • Polymers of monaccharides
  • Made of hundreds or thousands of monosaccharide monomen bonded together
• Types of polysaccharides


  • Homopolysaccharides (made solely of one kind of monosaccharide)
  • Heteropolysaccharides (made of more than one monomer)
• Examples


  • Starch (homopolysaccharide)
  • Hyaluronic acid (heteropolysaccharide)
• Glucose


  Source of energy as it is a reactant in respiration
• Glucose + Oxygen


  Carbon dioxide + Water
• By joining lots of glucose molecules together into polysaccharides, you can create a store of energy</b>
• Plants store energy as starch in chloroplasts and in membrane-bound starch graina, and humans store energy as glycogen in cells of the mud and ver
• Why polysaccharides are good energy stores

  • Compact, so they do not occupy a large amount of space
  • Occur in dense granules within the cell
  • Glucose molecules held in chains, so they can be easily 'snipped off' from the rest of the chain by hydrolysis when required for respiration
  • Hydrolysis reactions catalysed by enzymes
• Types of polysaccharides


  • Amylose (unbranched)
  • Amylopectin (branched)
  • Glycogen (branched)
• Amylose


  Unbranched chain of a-glucose molecules
• Amylopectin


  Branched chain of a-glucose molecules, with 1-4 glycosidic bonds between carbons 1 and 4, and 1-6 glycosidic bonds forming branches
• Glycogen


  Highly branched molecule, similar to amylopectin, found in animal cells
• Amylase hydrolyses 1-4 glycosidic linkages, and glucosidase hydrolyses 1-6 glycosidic linkages
• A 1-4 glycosidic linkage is one between carbon 1 of one glucose and carbon 4 of the other
• Amylose forms a spiral shape, with hydrogen bonds holding the spiral in place
• Hydroxyl groups on carbon 2 of glucose are found on the inside of the amylose spiral, making it less soluble and allowing hydrogen bonds to form
• Amylopectin also coils into a spiral shape held together with hydrogen bonds, but with branches emerging from the spiral
• Glycogen is more branched than starch, so it has less tendency to coil. This makes it more compact and easier to move monomer units as needed
• Cellulose is a homopolysaccharide made from long chains of up to 15,000 b-glucose molecules bonded together through condensation reactions to form glycosidic bonds
• The second glucose molecule in cellulose is rotated forwards by 180 degrees compared to the first, as if it is doing a 'handshake'
• The chains of b-glucose joined by condensation reactions are straight and form a tough cable structure
• Hydroxyl groups on carbon 2 of glucose


  Found on the inside of the amylose
• 5 Carbohydrates 3: Polysaccharides as structural units
• By the end of this topic, you should be able to demonstrate and apply your knowledge and understanding of the structure and properties of starch (amylose and amylopectin), glycogen and cellulose, and their function in living organisms
• Cellulose


  • Found in plant cell walls, it is a tough cable and the most common polysaccharide in the world
  • A homopolysaccharide made from long chains of up to 15,000 glucose molecules bonded together through condensation reactions to form glycosidic bonds
• Cellulose structure


  • Glucose molecules are rotated 180 degrees compared to the previous one, forming straight chains
  • Hydrogen bonding between the rotated glucose molecules in each chain gives the chain additional strength and prevents twisting
• Cellulose chains are bundled together into microfibrils, which are then bundled into macrofibrils to form the plant cell wall
• Cellulose has very high tensile strength due to the strength of the glycosidic bonds and hydrogen bonding between chains
• Cellulose is difficult to digest because the glycosidic bonds between glucose molecules are resistant to hydrolysis
• Functions of plant cell walls

  • Provide strength to support the whole plant
  • Allow water and mineral ions to pass in and out of the cell
  • Protect the delicate cell membrane