B1 biological molecules

• atoms combine by covalent bonding, ionic bonding, and hydrogen bonding
• in covalent bonding, atoms share a pair of electrons in their outer shells, forming a more stable compound called a molecule
• in ionic bonding, ions with opposite charges attract each other, the electrostatic attraction is an ionic bond, this is weaker than covalent bonding but stronger than hydrogen bonding
• in hydrogen bonding, in polarised molecules, negative regions of one are attracted to positive regions of another, and vice versa, the electrostatic attraction is a hydrogen bond, these are individually weak but can be strong when there are many of them
• monomers are the small sub-units, polymers are made from many monomers
• monomers can be linked to form long chains called polymers by a process called polymerisation
• condensation reactions are when two molecules are joined together, forming a chemical bond, releasing a water molecule
• hydrolysis reactions are when a chemical bond is broken between two molecules, using a water molecule
• all the chemical processes that take place in living organisms are called the metabolism
• carbohydrates are made up of carbon, hydrogen and oxygen atoms, forming long chains
• the sub-units of carbohydrates are monosaccharides
• a condensation reaction between two monosaccharides forms a glycosidic bond
• common monosaccharides are glucose, fructose and galactose
• glucose has two isomers, alpha glucose and beta glucose, these have the same molecular formula but a different arrangement of atoms
• all monosaccharides and some disaccharides are reducing sugars
• reducing sugars can be tested for using a Benedict's test:

  • add 2cm^3 of the sample to a test tube
  • add an equal volume of Benedict's solution
  • heat in a water bath for 5 minutes
  • positive = colour change from blue to brick-red
• disaccharides are formed by the condensation of two monosaccharides, and can be broken down by hydrolysis into two monosaccharides
• common disaccharides are maltose, sucrose and lactose
• maltose = glucose + glucose
• sucrose = glucose + fructose
• lactose = glucose + galactose
• disaccharides, apart from maltose, are non-reducing sugars
• the test for non-reducing sugars is:

  • carry out normal Benedict's test, produces a negative result
  • add 2cm^3 of sample to 2cm^3 of hydrochloric acid in a test tube
  • heat in water bath for 5 minutes
  • add sodium hydrogencarbonate to neutralise the solution
  • carry out normal Benedict's test
  • positive = colour change from blue to brick-red
• polysaccharides are polymers formed from many monosaccharides with glycosidic bonds between them
• polysaccharides are insoluble as they are so large, so they can be used for storage or structural support
• starch is a polysaccharide found in plants, made up of chains of alpha glucose monomers joined by glycosidic bonds, formed by condensation reactions
• the main role of starch is energy storage, it has adaptations for this:

  • insoluble - does not affect water potential so does not affect osmosis
  • large - does not diffuse out of cells
  • compact - lots can be stored in a small space
  • alpha-glucose - easily transported and readily used in respiration
  • branched - many ends can be acted on by enzymes simultaneously so release is rapid
• the test for starch is:

  • place 2cm^3 of sample in a test tube
  • add 2 drops of iodine solution
  • positive = colour change from orange-brown to black
• glycogen is a polysaccharide found in animals, made up of chains of alpha-glucose monomers joined by glycosidic bonds, formed by condensation reactions
• the main role of glycogen is energy storage, it has adaptations for this:

  • insoluble - does not affect water potential so does not affect osmosis
  • large - does not diffuse out of cells
  • compact - lots can be stored in a small space
  • highly branched - many ends can be acted on by enzymes simultaneously, so release is rapid
• cellulose is a polysaccharide found in plants, made up of unbranched chains of beta-glucose, joined by glycosidic and hydrogen bonds
• the main role of cellulose is structural support, it has adaptations for this:

  • beta-glucose - forms long straight unbranched chains
  • chains - run parallel to each other, are cross-linked by hydrogen bonds which add strength
  • microfibrils, chains are grouped into these, which are grouped into fibrils, which add strength
• lipids are made up of carbon, hydrogen and oxygen atoms
• the main types of lipid are triglyceride and phospholipid
• lipids have many roles:

  • cell membrane - phospholipids contribute to flexibility of membranes and transfer of lipid-soluble substances across
  • energy source - provide energy and release water when oxidised
  • waterproofing - insoluble in water, eg waxy cuticle
  • insulation - slow conductors of heat so retain body heat, slow conductors of electricity so make up myelin sheaths
  • protection - fat is stored around delicate organs
• triglycerides have three fatty acids and one glycerol
• triglycerides have the following properties:

  • good energy source as they have a high ratio of C-H bonds to C atoms
  • good storage molecules as they have a low mass to energy ratio
  • can be stored without affecting osmosis as they are insoluble in water so do not affect water potential
  • release water when oxidised as they have a high ratio of H atoms to O atoms
• phospholipids have two fatty acids, one glycerol and a phosphate group
• phospholipids have hydrophobic tails and hydrophilic heads, because fatty acids are hydrophobic and phosphate groups are hydrophilic
• phospholipids have the following properties:

  • polar, able to form a bilayer in a membrane, as they have hydrophobic tails and hydrophilic heads
  • cell recognition as they can form glycolipids with carbohydrates