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HBIO1 > Food molecules
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Large Molecules

  • Monomer (-OH) + monomer (-H) polymer + H2O(l)
  • Condensation: monomers join to form polymers
    • Amino acids join to form a dipeptide (protein)
      • Two amino acids release -H and -OH groups (H2O)
      • Peptide bond forms between the alpha-carbon and nitrogen
    • Monosaccharides join to form disaccharides
      • Glycosidic bond forms between both monomers
  • Hydrolysis: break down of a polymer
    • Reverse of the condensation reaction
    • This is the process of digestion

Carbohydrates

  • Organic molecules which contain C, H and O
  • Bind together in the ratio Cx(H2O)y
  • Monosaccharides → single sugar (monomer)
    • Ribose found in RNA and DNA
    • Deoxyribose part of nucleic acids
    • Glucose                is the main energy source in brain
    • Fructose is found in sweet-tasting fruits
  • Disaccharides → two sugar residues (2 monomers)
    • Sucrose (glucose + fructose) → transport carbohydrates in plants
    • Maltose (glucose + glucose) → formed from digestion of starch
    • Lactose (glucose + galactose) → found in milk
  • Polysaccharides → many sugar residues (polymer)
    • Starch (alpha-glucose) → main storage of carbohydrates in plants
    • Glycogen (alpha-glucose) → main storage of carbohydrates in humans
    • Cellulose (beta-glucose)                → component of plant cell wall, important for digestion

Starch

  • Consists of amylopectin and amylose (both are made of α-glucose)
    • Amylopectin is branched via 1,6-glycosidic bonds
    • Amylose forms a stiff helical structure via 1,4-glycosidic bonds
    • Both are compact molecules → starch can be stored in small space
  • The ends are easily broken down to glucose for respiration
  • Does not affect water potential as it is insoluble
  • Readily hydrolysed by the enzyme amylase found in the gut and saliva
  • Found in corn (maize), wheat, potato, rice

Glycogen

  • Found in skeletal muscle and liver
  • Insoluble, branched polymer, made of α-glucose linked via glycosidic bonds
  • Glycogen is broken down to glucose by glycogenolysis (glycogen phosphorylase)
  • Major site of daily glucose consumption (75%) is the brain via aerobic pathways
  • Most of the remainder is utilized by erythrocytes, skeletal muscle, and heart muscle
  • Glucose is obtained from diets or from amino acids and lactate via gluconeogenesis
  • Storage of glycogen in liver are considered to be main buffer of blood glucose levels

Cellulose

  • Polysaccharide consisting of long beta-glucose chains
  • Linked together by hydrogen bonds to form microfibrils
  • Humans have no enzymes to break down beta-glucose

Lipids

  • Easily dissolved in organic solvents but not in water
  • Triglycerides (fats and oils)
    • Also called triacylglycerides (TAG)
    • Consists of 3 fatty acids linked by ester bonds to glycerol
      • Require 3 condensation reactions (but are not polymers!)
      • Glycerol contains 3 -OH groups
      • One fatty acid contains a -COOH group
    • Excess energy available from food is stored as TAG
    • Can be broken down to yield energy when needed
    • Contain twice as many energy stored per unit of weight as carbohydrates
  • Saturated fatty acids
    • -COOH group without double bonds in the carbohydrate chain
    • May cause blockage of arteries which can lead to strokes and heart attacks
    • High melting point / solid at room temperature (fats) / typical animal fats
  • Unsaturated fatty acids
    • -COOH group with double bonds in the carbohydrate chain
    • Low melting point / liquid at room temperature (oils)
    • Found in plants
  • Phospholipids
    • Found in cell membrane
    • Formed by replacing one fatty acids in a triglyceride with a phosphate group
    • Phosphate is polar / hydrophilic / does mix with H2O
    • Fatty acid tails remain non-polar / hydrophobic / insoluble, does not mix with H2O

Proteins

  • Proteins are made up by different combinations of 20 amino acids
    • Common structure
      • -COOH group
      • -NH2 group
    • Amino acids differ in their R-group
  • Tertiary structure
    • Complex globular 3D shape
    • Folding and twisting of polypeptides (H-bond, ionic bonds, disulphide bridges)
    • Polypeptides contain many peptide bonds
  • Same amino acid sequenceALWAYS same shape
  • Bonds found in proteins
    • Hydrogen bonds
      • Between R-groups
      • Easily broken, but present in larger numbers
      • The more bonds, the stronger the structure
    • Ionic bonds
      • Between -COOH and -NH2 groups
    • Disulphide bridges
      • Between two sulphur-containing cysteine side chains
      • Strong bonds found in skin and hair
  • Denaturation
    • Destruction of tertiary structure, can be done by heat
    • Protein structure is lost and cannot reform → dysfunctional
  • Background Reading: Structure of Proteins http://www.chemguide.co.uk/organicprops/aminoacids/proteinstruct.html

Digestion

  • Large molecules (starch, proteins, TAG) are too big and insoluble to be absorbed
    • Polymers have to be broken down into monomers
    • With help of hydrolytic enzymes - reaction requires H2O
    • Note: TAGs are not polymers but also need to be broken down
  • Different enzymes break down different food
    • Work best at body temperature (37°)
    • Work in different conditions at different pH (stomach is acidic, intestine is alkaline)
  • Hydrolysis
    • Proteins → amino acids
      • Essential amino acids: cannot be synthesised and must be present in diet
      • Non-essential amino acids: synthesised from essential amino acids by transamination in the liver
    • TAG → glycerol and fatty acids
    • Polysaccharides → monosaccharides

Chromatography

  • Separates a mixture to identify its components
  • Stationary phase: paper (cellulose)
  • Mobile phase: liquid solvent
  • Method
    • Pencil is used to draw a horizontal line on paper to mark the origin
    • Small drop of solution is placed on the pencil line to form a spot
    • Paper is fastened with a drawing pin to a bung
    • This is placed in a boiling tube with some amount of solvent inside
    • Spot needs to be suspended above the level of solvent!
  • Analysis
    • Rf = (distance moved by substance) / (distance moved by solvent)