Biology Unit 2 Revision

Biology Unit 2 re-take revision. This exam is worth 100 marks. 


2.1.1 - Water

Functions of water:

  • A reactant in many important chemical reactions- like photosynthesis and hydrolysis
  • A solvent which means that substances can dissolve in it
  • Transports substances, because it is a solvent. Substances such as water and glucose
  • Helps with temperature control. Carries heat energy away when it evaporates, which helps to cool

Structure of water molecules 

  • Shared negative hydrogen electrons are pulled towards oxygen, the other side of each hydrogen atom is left with a slight positive charge
  • Unshared negative electrons on the oxygen atom give it a slight negative charge. 
  • Makes water a polar molecule 
  • Negatively charged atoms of water attract the positively charged hydrogen atoms or other water molecules. This is called hydrogen bonding

Structure related to its properties & functions

  • Specific heat capacity- Is the energy required to raise the temperature of 1g of a substance by 1 degree. Hydrogen bonds between water molecules can absorb a lot of energy, so water has a high specific heat capacity. Takes a lot of energy to heat it up. Useful for living organisms because it stops rapid temperature changes, allowing them to keep temperature fairly stable
  • High latent heat of evaporation- Takes a lot of energy to break hydrogen bonds between molecules, so a lot of energy is used up when water evaporates. Useful because it allows organisms to cool down quickly
  • Water's polarity makes it very cohesive- Water molecules are cohesive which allows it to flow and also to transport substances
  • Water's polarity makes it a good solvent- Because its polar, the positive end of a water molecule will be attracted to the negative ion, and the negative end of a water molecule will be attracted to the positive ion. So they will dissolve
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2.1.2 - Proteins

  • Made from long chains of amino acids. Dipeptide is formed when two amino acids join together. A polypeptide is formed when two or more amino acids join together. Proteins are made up of many polypeptides. 
  • Amino acids are made up of a carbon surrounded by a hydrogen, an NH2 group, a carboxyl group and an R-group. The r group is a variable group which is different on different amino acids. 
  • Proteins are joined together by peptide bonds to form dipeptides and polypeptides. A molecule of water is released when this reaction takes place. This is a condensation reaction. The reverse of this reaction (i.e. breaking the peptide bonds) adds a molecule of water. This is called a hydrolysis reaction. 

Proteins have four strcutural levels:

  • Primary- Sequence/order of amino acids in the polypeptide chain. Held together by peptide bonds
  • Secondary- The way the hydrogen bonds between the amino acids allow the protein to coil into an alpha helix or fold into a beta pleated sheet. Held together by hydrogen bonds
  • Tertiary- Coiled and folded chain of amino acids is often coiled and folded further. More bonds form between the different parts of the polypeptide chain. Tertiary structure forms their final 3D structure. Affected by different types of bonds such as: ionic interactions, disulphide bonds, hydrophobic and hydrophilic interactions and hydrogen bonds
  • Quaternary- The way that some proteins that are made up of several different polypeptide chains held together by bonds. The quaternary structure is the way that these polypeptide chains are assembled together. Bonds involved tend to depend on the tertiary structure.

Protein shape and its function:

  • Collagen - a fibrous protein that forms tissues in animals so it needs to be strong. Made of three polypeptide chains that are tightly coiled into a strong triple helix. The chains are interlinked by strong covalent bonds. They are tough and rope shaped. 
  • Haemoglobin- Globular protein with an iron-containing haem group that binds to oxygen.Its structure is curled up so that hydrophilic side chains are on the outside of the molecule and hydrophobic side chains face inwards. Makes it soluble in water, which also makes it good for transport in the blood

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