Topic 1 - A: Biological molecules

• What is the theory of evolution?

  The theory that all organisms on Earth are descended from one or a few common ancestors and that they have changed and diversified over time
• Do all organisms share biochemistry?

  Yes - contain the same groups of carbon-based compounds that interact in similar ways - e.g. use the same nucleic acids (DNA and RNA) as genetic material and the same amino acids to build proteins.
• What do these similarities suggest?

  That animals and plants have a common ancestor - provides indirect evidence for evolution
• Monomers and polymers: most carbohydrates, proteins and nucleic acids are polymers.
• What is a polymer?

  a large complex molecule composed of long chains of monomers joined together
• What is a monomer?

  a small, basic molecular unit that can form a polymer e.g. monosaccharides, amino acids and nucleotides
• How do polymers form?

  Formed from their monomers by condensation reactions. A condensation reaction forms a chemical bond between monomers, releasing a molecule of water
• How do polymers break down?

  Biological polymers can be broken down into monomers by hydrolysis reactions. A hydrolysis reaction breaks the chemical bond between monomers using a water molecule.
• What are the monomers carbohydrates are made from?

  Monosaccharides e.g. glucose, fructose and galactose
• Describe glucose?

  a hexose sugar

  a monosaccharide with 6 carbon atoms in each molecule

  2 types - alpha and beta - isomers
• what is an isomer?

  molecules with the same molecular formula as each other, but with the atoms connected in a different way
• How are disaccharides formed?

  When two monosaccharides join together by condensation reactions forming a glycosidic bond between the monosaccharides and releasing a molecule of water
• What is maltose formed from?

  2 alpha glucose molecules
• What is sucrose formed from?

  Sucrose is formed from glucose and fructose.
• What is lactose formed from?

  Galactose and glucose.
• Describe the test for reducing sugars?

  Reducing sugars include all monosaccharides and some disaccharides e.g. maltose and lactose

  1. add Benedict's reagent to a sample and heat in a water bath brought to the boil (or about 70 degrees)
  2. A positive test will form a coloured precipitate with solid particles suspended in the solution
  3. green - yellow - brick red
• Why does the colour vary in a positive Benedict's test?

  The higher the concentration of reducing sugar the further the colour change goes - can be used to compare the amount of sugar in different solutions.
• What is a more accurate way to compare the amount of sugar in different solution?

  filter the solution and weigh the precipitate, or remove the precipitate and use a colorimeter to measure the absorbance of the remaining Benedict's reagent
• Describe the test for a non-reducing sugar?

  If the result of the reducing sugars test is negative - could still be a non-reducing sugar present e.g. sucrose

  1. first need to break them down into monosaccharides by hydrolysing the glycosidic bond - get a new sample and add dilute hydrochloric acid and carefully heat in a water bath brought to the boil (or 70 degrees)
  2. neutralise by adding sodium hydrogencarbonate - Benedict's doesn't work in acidic conditions
  3. Benedict's test - same results
• What is a polysaccharide?

  formed when more than two monosaccharides are joined together by glycosidic bonds (condensation reactions). Broken down into constituent monosaccharides by hydrolysis reactions
• What is starch?

  Cells get energy from glucose - plants store excess glucose as starch (when a plant needs more glucose for energy, it breaks down starch (hysdrolysis) to release the glucose). Starch is a mixture of 2 polysaccharides of alpha glucose - amylose and amylopectin
• Describe the structure of amylose?

  long, unbranched chain of alpha glucose

  angles of the glycosidic bonds give it a coiled structure, almost like a cylinder

  makes it compact - good for storage as you can fit more in a small space
• Describe the structure of amylopectin?

  long, branched chain of alpha glucose

  side branches all enzymes that break down the molecule to easily get to the glycosidic bonds

  means glucose can be released quickly
• Is starch soluble?

  No - it is insoluble in water and doesn't affect water potential so it does not cause water to enter cells by osmosis, which would make them swell. This makes it good for storage
• What is glycogen?

  Animals store excess glucose as glycogen - anothr polysaccharide of alpha glucose
• Describe the structure of glycogen?

  Very similar to that of amylopectin except it has more side branches. Lots of branches means that the stored glucose can be released quickly, which is important for energy release in animals. Also very compact molecule - good for storage
• What is cellulose?

  Made of long, unbranched chains of beta glucose. When beta glucose molecules bond they form straight cellulose chains. Cellulose chains linked together by hydrogen bonds to form strong fibres called microfibrils. Strong fibres mean cellulose provides structural support for cells (e.g. in plant cell walls)
• Describe the iodine test for starch?

  Add iodine dissolved in potassium iodide solution to the test sample. If starch is present, sample changes from orange-brown to dark blue-black colour
• What are lipids made from?

  Not polymers formed from long chains of monomers. Lipids are made from a variety of different components, but they all contain hydrocarbons. The components they are made from relates to the lipid's function
• What are the two types of lipids?

  triglycerides and phospholipids
• What are triglycerides?

  Have one molecule of glycerol with three fatty acids attached to it.
• Describe fatty acids?

  Fatty acid molecules have long 'tails' made of hydrocarbons that are hydrophobic - repel water. These tails make lipids insoluble in water
• All fatty acids consist of the same basic structure but the hydrocarbon tail varies. There are 2 kinds of fatty acid - saturated and unsaturated. The difference is in their hydrocarbon tails (R groups)
• Saturated fatty acids?

  don't have any double bonds between their carbon atoms - the FA is saturated with hydrogen
• Unsaturated fatty acids?

  Do have double bonds between carbon atoms - causes the chain to kink
• Hoe are triglycerides formed?

  By condensation reactions. An ester bond forms between the two molecules, releasing a molecule of water - process happens 3x to form a triglyceride
• What are phospholipids?

  The lipids found in cell membranes. Similar to triglycerides except one of the fatty acid molecules is replaced by a phosphate group which is hydrophilic and attracts water.
• Properties of triglycerides?

  mainly used as energy storage molecules

  good for storage because the long hydrocarbon tails of the fatty acids contain lots of chemical energy

  due to their tails - lipids contain about 2x as much energy per gram as carbohydrates

  insoluble in water - don't affect water potential and cause water to enter cell be osmosis which would make them swell.

  triglycerides bundle together as insoluble droplets in cells because the fatty acid tails are hydrophobic (water-repelling) - the tails face inwards shielding themselves from water with their glycerol heads
• Properties of phospholipids?

  make up the bilayer of cell membranes - control what enters and leaves the cell

  Phospholipid heads are hydrophilic and their tails are hydrophobic so they form a double layer with their heads facing out towards the water on either side. Centre of the bilayer is hydrophobic, so water-soluble substances can't easily pass through - membrane acts as a barrier
• Describe the emulsion test for lipids?

  Shake the test substance with ethanol for about 1 min

  Pour the solution into water

  any lipid will show up as a milky emulsion - the more lipid there is the more noticeable the milky colour