We are what we eat - the biochemical basis of life is similar for all living organisms

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 sequence → ALWAYS 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)