Work for 16+ AS Biology Mrs White The following powerpoint is intended to explain why hydrogen bonds form, and where they are found in the biological molecules we have studied so far. Read through the slides carefully, making a note of anything you do not understand well. There are some questions contained in the slides. Try to answer these too. The final slide consists of an activity for you to do.

The Hydrogen Bond 1.Sometimes in a covalent bond the electrons are not shared equally. 2.For example, in water there are two O-H covalent bonds, and the electron belonging to hydrogen is drawn towards the oxygen atom. 3.This gives the hydrogen atom a slight positive charge (  + ) and the oxygen atom has a slight negative charge (  - ). 4.The slightly positive hydrogen atom can form a weak electrostatic bond with a slightly negative atom. This is called a hydrogen bond and it is written as a dotted line between the atoms involved.

Hydrogen Bonding in Water The hydrogen bond shown by the dotted line is between an oxygen atom (shown in red), and a hydrogen atom (shown in white). How many hydrogen bonds can one molecule of water form?

Covalent Bonds showing a Charge Separation 1.  - O-H  + found in the glucose molecule, and causes hydrogen bonding between chains of beta glucose in cellulose, also on some amino acid side chains, and of course, in water. 2.  + C=O  - found in the acid part of an amino acid, and part of the peptide bond after condensation 3.  - N-H  + found in the amino part of an amino acid, and part of the peptide bond after condensation.

Where are Hydrogen Bonds Found? 1.Between molecules of water 2.Between cellulose molecules 3.Between C=O and N-H in the polypeptide chain to stabilise  -helix and  -pleated sheet (secondary structure of protein) 4.Between some R groups of amino acids (tertiary structure of a protein) 5.Between polypeptide chains in collagen 6.Between the bases of the two strands of DNA

Polar Molecules 1.The term we use for a molecule with a charge separation is a polar molecule. 2.There is not a significant charge separation between C and H in a C-H bond, so molecules that are completely hydrocarbon in nature are not polar. 3.Which of the following molecules are polar? a hydrogen molecule,water, glucose, methane, amino acid, an oxygen molecule, ethanol, a nitrogen molecule.

The Significance of Water as a Polar Molecule 1.Water acts as a solvent to ions and other polar molecules. This makes it ideal for dissolving the chemicals found in cells. 2.If a positive ion dissolves, it becomes surrounded by water molecules with the  - oxygen atoms of the water molecules next to the ion. 3.If a negative ion dissolves it becomes surrounded by water molecules with the  + hydrogen atoms of the water molecules next to the ion (See next slide). 4.Similarly, water will attract to oppositely charged areas of polar molecules.

Water Dissolving Sodium Chloride

Hydrophilic and Hydrophobic Molecules. 1.Polar molecules are hydrophilic because they ‘like’ water and can dissolve in it. 2.Non-polar molecules are hydrophobic because they ‘hate’ water and cannot dissolve in it. 3.Most of the chemicals in the cell are hydrophilic and can dissolve in water. However, the lipids are hydrophobic and do not dissolve in water. (Just think about cooking oil and water not mixing).

Hydrophilic and Hydrophobic Side Chains. 1.Some of the R groups (amino acid side chains) are hydrophobic. 2.What type are these? 3.The fourth factor contributing to the tertiary structure of proteins is the hydrophobic nature of these side chains. They go into the centre of the molecule. When they get close to each other they form very weak bonds (called van der Waals forces) with each other. 4.The hydrophilic side chains go to the outside of the protein molecule and interact with water to dissolve the molecule.

Fibrous Proteins – Collagen 1.The polypeptide chain forms a helix structure. 2.Three polypeptide chains wind around each other to form a three-stranded rope. 3.These strands are held together by hydrogen bonds and covalent bonds. 4.Covalent cross-links form between collagen molecules to make a fibre. 5.The hydrophobic nature of most of the R groups makes collagen insoluble.

Exercise – not Optional!!! Using the knowledge you have gained from reading through the slides, make rough (but readable!) notes about ‘The importance of water in living organisms’. Look up in any suitable text book, AS revision guide, or the internet. Things you should think about are: water as a solvent, water as a chemical reactant, water as a habitat, the physical properties of water – cohesion, latent heat of vapourisation, specific heat capacity, etc. You will need to have some of these things explained to you when we meet so please don’t panic!!