A-Level Biology OCR Notes

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Water

Water molecules consist of 2 hydrogen molecules covalently to an oxygen molecule.
The molecules are slightly polar because the oxygen nucleus pulls the shared electrons away from the hydrogen nuclei. Giving the oxygen nuclei a δ- charge, and the hydrogen nuclei a δ+ charge.
The polarity of water causes attraction between water molecules. This force of attraction is called a hydrogen bond.

Property of Water	Why is it useful
Liquid medium	Provides habitats for aquatic organisms, medium for chemical reactions & used for transport
Important metabolite	Used in hydrolysis & condensation reactions
High specific heat capacity	Keeps aquatic & cellular environments stable
High latent heat of vaporisation	Evaporation has a cooling effect on organisms
Cohesion of molecules	Water is drawn up the xylem
Surface tension	Allows pond-skaters to walk on the surface
Good solvent and transport medium	Dissolves ionic and polar molecules, allowing them to easily be transported
Good reaction medium	The cytoplasm in cells is an aqueous solution where many chemical reactions happen
Incompressible	Can prevent plants from wilting & act as a hydrostatic skeleton for invertebrates

Monomers & Polymers

Monomers are individual molecules that make up a polymer.
Polymers are long chains that are composed of many individual monomers that have been bonded together in a repeating pattern.
Condensation Reactions occurs when two molecules combine to form a more complex molecule with the removal of water.

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Hydrolysis Reactions occurs when larger molecules are broken down into smaller molecules with the addition of water.

Carbohydrates

Monosaccharides are the simplest carbohydrates, consisting of only one sugar molecule (e.g. Glucose, Fructose & Galactose)
Ribose sugars (pentose) are found in many important biological molecules such as ribonucleic acid (RNA), ATP, NAD

Disaccharides are sugars that are composed of two monosaccharides joined together in a condensation reaction, forming a glycosidic bond.

Disaccharide	Constituent monosaccharides
Maltose	2 × a-glucose
Sucrose	a-glucose and fructose
Lactose	b-glucose and galactose

Polysaccharides are formed by many monosaccharides joined together.
- Amylose, amylopectin (starch) is the main polysaccharide energy store in plants, is composed of α-glucose.
- In animals, the polysaccharide energy store is called glycogen, composed of α-glucose.
- Cellulose is a structural component of plant cell walls, composed of long unbranched chains of b-glucose.

Lipids

Fatty acids can be:
- Saturated – there are no C=C bonds and the molecule has as many hydrogen atoms as possible.
- Unsaturated – there is at least one C=C bond, therefore the molecule contains fewer hydrogen atoms than is maximally possible.
A triglyceride molecule is formed by joining one molecule of glycerol to three fatty acids through three condensation reactions, forming ester bonds.
Triglycerides have key roles in respiration and energy storage due to its insolubility and high carbon to hydrogen ratio.

Phospholipids replace one of the fatty acid chains in triglycerides with a phosphate molecule.
The hydrophobic tails and hydrophilic heads of phospholipids allow them to form phospholipid bilayers

Proteins

Dipeptides are formed when two amino acids are joined together by a condensation reaction, forming a peptide bond.
A polypeptide is a polymer made of many amino acids joined together by peptide bonds.
A protein may contain one or more polypeptide chains.
There are four structural levels:

Level	Definition	Bond Type
Primary	The specific sequence of amino acids in a polypeptide chain	Peptide Bonds
Secondary	The curling or folding of the polypeptide chain into α-helices and β-pleated sheets due to the formation of hydrogen bonds	Hydrogen Bonds
Tertiary	The overall specific 3-D shape of a protein, which is determined by interactions between R groups and the properties of R groups	Hydrogen bonds Ionic bonds Disulphide bridges
Quaternary	The specific 3-D shape of a protein that is determined by the multiple polypeptide chains and/or prosthetic groups bonded together	Hydrogen bonds Ionic bonds Disulphide bridges

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