Biology Topic 1

• Polymers are

  large, complex molecules composed of long chains of monomers joined

  together.
• Monomers are small, basic molecular units that can form a polymer
• Examples of monomers include monosaccharides, amino acids and nucleotides.
• Polymers are formed from their monomers by condensation

  reactions. A condensation reaction forms a chemical bond between

  monomers, releasing a molecule of water
• A hydrolysis reaction breaks the chemical bond between monomers using a

  water molecule.
• The monomers that

  carbohydrates are made from are monosaccharides, e.g. glucose, fructose

  and galactose.
• Glucose is a hexose sugar — a monosaccharide with six carbon

  atoms in each molecule.
• A disaccharide is formed when two monosaccharides join together.
• Monosaccharides are joined together by condensation reactions - a glycosidic bond forms between the two monosaccharides as a molecule

  of water is released.
• This is an example of a condensation reaction

  
• Alpha glucose : H atom is above OH

  Beta glucose : OH atom is above H

  
• Sucrose is a disaccharide formed from a condensation reaction between a

  glucose molecule and a fructose molecule
• Lactose is a disaccharide formed from a glucose molecule and a galactose molecule.
• All sugars can be classified as reducing sugars or non-reducing sugars.
• Benedict's test for sugars:

  1. Add benedict's reagent (blue) to a sample
  2. Heat the sample in a water bath until it's brought to a boil
  3. If positive, it will form a green -> yellow -> orange -> brick-red percipiatate, solid particles will suspend in the solution
• Benedicts test for sugars (extra)

  The higher the concentration of reducing sugar, the further the colour change

  goes (green -> yellow -> orange -> brick red)

  To compare the amount of reducing sugars in different solutions....

  1. Filter the solution and weigh the percipitate OR use a colorimeter to measure the absorbance of the remaining benedict's reagent
• Non-reducing sugars

  When the reducing sugars test is negative, a non-reducing sugar can still be present so...

  1. Get a new sample of the solution
  2. Add diluted HCl acid and carefully heat it so its been brought to a boil
  3. Neutralise it by adding NaHCO3
  4. Carry out the benedict's test you would for a reducing sugar

  A) Do this step to test for NON-REDUCING sugars

  
• Polysaccharides are carbohydrates. Polysaccharide molecules are made from

  large numbers of their monomers (monosaccharides).
• A polysaccharide is formed when more than two monosaccharides are joined

  together by condensation reactions.
• The three polysaccharides are starch, gylycogen and cellulose
• Cells get energy from glucose. Plants store excess glucose as starch (when a plant needs more glucose for energy, it breaks down starch to release the glucose).
• Starch is a mixture of two polysaccharides of alpha-glucose — amylose and amylopectin:
• Amylose is a long, unbranched chain of a-glucose. The angles of the glycosidic bonds give it a coiled structure making it compact making it good for storage as more into a smaller space
• Amylopectin is a long, branched

  chain of a-glucose. Its side branches

  allow the enzymes that break down

  the molecule to get at the glycosidic

  bonds easily. This means that the

  glucose can be released quickly.
• Starch is insoluble in water and doesn’t affect water potential so it doesn’t cause water to enter cells by osmosis, which would make them

  swell. This makes it good for storage.
• Hydrogen bonds between a-glucose molecules help to hold amylose in its helical structure.
• Animals store excess glucose as glycogen, a polysaccharide of alpha-glucose. It has more side branches than amylopectin meaning that stored glucose can be released quickly. Also a very compact molecule so good for storage.
• Cellulose is made of long, unbranched chains of beta-glucose. When beta-glucose molecules bond, they form straight cellulose chains. The cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils. The strong fibres mean cellulose provides structural support for cells.
• Starch is a large molecule, so it can’t leave the cell — this is another reason why it’s a good storage molecule.
• Iodine test for starch

  Add iodine to the sample and if positive, the sample will change from a browny-orange to a blue-black colour
• Lipids all contain hydrocarbons and there are two types of lipids - triglycerides and phospholipids
• Triglycerides have one molecule of glycerol with three fatty acids attached to it. Fatty acid molecules

  have long ‘tails’ made of hydrocarbons. The tails are ‘hydrophobic’ (they repel water molecules). These tails make lipids insoluble in water.

  
• Phospholipids also have a glycerol backbone but instead of having three fatty acids attached, they have two fatty acids and a phosphate group. This makes them slightly soluble in water because the phosphate head is ‘hydrophilic’. They still have two fatty acid tails which are ‘hydrophobic’, making them mostly insoluble in water.
• There are two types of fatty acids - saturated and unsaturated

  Saturated have no double bonds between C atoms

  Unsaturated have double bonds between C atoms

  
• The hydrocarbon tail varies - unsaturated and saturated

  
• Triglycerides are formed by condensation reactions. An ester bond forms between a fatty acids and a glycerol molecule, releasing a molecule of water. This process happens two more timed to form a triglyceride.

  
• Phospholipids are found in the cell membrane and are made up of a glycerol backbone, a phosphate group and two fatty acids. The phosphate

  group is hydrophilic (attracts water). The fatty acid tails are hydrophobic (repel water).

  
• Triglycerides are mainly used as energy storage molecules. They’re good for this because the long hydrocarbon tails of the fatty acids contain lots of chemical energy — a load of energy is released when they’re broken down. Because of these tails, lipids contain about twice as much energy per gram as carbohydrates.
• Triglycerides are insoluble in water so they don't affect the water potential of the cell and cause water to enter the cell by osmosis (which can cause them to swell). The 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.

  
• The bilayer of cell membranes are made up of phospholipids; the cell membrance controls what enters and exits 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. The centre of the bilayer is hydrophobic, so water-soluble

  substances can’t easily pass through it — the membrane acts as a barrier to those substances.