Biological Molecules - Unit 2 OCR

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  • Biological Molecules
    • Water
      • Functions
        • Reactant in loads of important chemical reactions like hydrolysis reactions
        • Transports substances - liquid solvent and cohesive so easily transports all substances
        • Helps with temperature control - high specific heat capacity and high latent heat of evaporation
        • Habitat - helps with temperature control, is a solvent and becomes less dense when freezes means organisms can survive and reproduce
      • Structure
        • Polarity - one atom of oxygen (O) joined to two hydrogen (H2) by shared electrons
          • As shared negative H electrons pulled towards oxygen the other side of H atom is left with slightly positive charge
            • Unshared negative electron on O gives slightly negative charge
              • This makes water polar - slightly negative on one side and slightly positive on the other
            • This makes water polar - slightly negative on one side and slightly positive on the other
        • Hydrogen Bonding
          • Weak bonds between slightly positive H atom and slightly negative H in another molecule
          • Form between water molecules as slightly negative O attracts slightly positive H of other water
          • Bonds give water some of useful properties
      • Properties
        • High Specific Heat Capacity
          • H bonds give water energy needed to raise temperature of 1 gram of substance by 1 degree
            • When heated lots of energy is used to break H bonds so less heat energy is available to increase water temperature
              • So water takes a lot of energy to heat up which is useful for living organisms as it stops rapid changes allowing fairly stable temperature
        • High Latent Heat of Evaporation
          • Water evaporates when H bonds holding molecules together break allowing water on surface to escape as gas
            • Lots of energy needed to break H bonds so lots is used up when water evaporates
              • Lots of heat used to change water from liquid to gas
                • Useful for living organisms as water's great for cooling things as carries away heat energy when evaporates from surface, cooling surface and helps lower temperature
        • Very Cohesive
          • Attraction of two same type molecules
            • Water molecules very cohesive as they're polar, helps water to flow making it great for transport
        • Lower Density When Solid
          • At  low temperature water freezes turning from liquid to solid so molecules held further apart so ice floats
            • Useful for living organisms as ice forms insulating layer on top so water below doesn't freeze, organisms do not freeze in cold and can move around
        • Good Solvent
          • Water is polar so slightly positive attracts negative ion and vice versa
            • Ions become surrounded by water so dissolve so polarity makes it useful as a solvent
              • Means living organisms can take up useful substances dissolved in water that can be transoprted around the organisms body
    • Proteins
      • Made From...
        • Polymers - large complex molecules composed of long chains of monomers (small basic molecular units), monomers or proteins are amino acids
        • Dipeptide formed when two amino acids join, Polypeptide formed when more that two amino acids join - Proteins made of one or more polypeptides
      • Amino Acid Structure
        • All have the same general structure - carboxyl group            (-COOH) and amino group (-NH2) attached to carbon atom
        • Difference between amino acids is the variable (residual) group they contain - R group
      • Polypeptide Formation
        • Condensation reactions link amino acids to form dipeptides and polypeptides
        • Water molecule released and peptide bonds are formed between the amino group of one and the carboxyl group of another
        • The reverse reaction (hydrolysis) adds the water molecule back to break the peptide bond
      • Protein Structure
        • Big complicated molecules easily described in 4 levels that are primary, secondary, tertiary and quaternary structures held together with different bonds
        • Primary - sequence of amino acids in polypeptide chain, held together by peptide bonds
        • Secondary - polypeptide chain does not remain flat and straight, H bonds form between -NH and -CO groups in chain making it coil into alpha helix of folded into beta pleated sheet
        • Tertiary - coiled of folded chain is often coiled or folded further, more bonds form between parts of chain
          • Ionic Interactions - weak between negative and positive R groups on different parts
          • Disulphide Bonds - whenever two cystine molecules come close the sulphur in one bonds with the other
          • Hydrophobic and Hydrophilic - when hydrophobic R groups get close they tend to clump together meaning hydrophilic R groups pushed  to outside affecting how protein folds
          • Hydrogen Bonds - weak bonds between slightly positive H atoms in R groups and slightly negative atoms in other R groups
        • Quaternary - some proteins made from two or more polypeptide chains, this is the way they assemble together, tends to be determined by tertiary of individual chains, can be influenced by all bonds, for protein made from more than one this structure is 3D
      • Function of Proteins
        • Fibrous - tough rope shaped and tend to be found in connective tissue (collagen : supportive tissue so needs to be strong and flexible, three polypeptide chains in triple helix, interlinked by covalent bonds, minerals can bind to increase rigidity
        • Globular - round and compact, soluble so easily transported in fluid (haemoglobin : carries O around body, curled up making it good for transport in blood
    • Carbohydrates
      • Made From
        • Most are polymers
        • Monomers of carbohydrate are monosaccharides
          • Single monosaccharides are called carbohydrates though
            • Glucose - six carbon atoms : two forms - alpha and beta
              • Structure relates to function - main energy source, soluble so easily transported, chemical bonds contain energy
      • Polysaccharide Formation
        • Monosaccharides joined by glycosidic bonds, during synthesis hydrogen on one bonds to hydroxyl (OH) on another releasing water
          • Like a polysaccharide the reverse is hydrolysis where water is added to break the glycosidic bond
        • Disaccharide formed when two mono join together, Polysaccharide formed when more than two mono join together
          • Two alpha joined by glycosidic bond to form maltose (disaccharide)
            • Lots of alpha joined form amylose (polysaccharide)
      • Functions
        • Starch - main energy store in plants, cells get energy from glucose, plant store excess at starch, it is insoluble in water so doesn't cause water to enter cells by osmosis, good for storage and is a mixture of two polysaccharides
          • Amylose - long unbranched chain of alpha, angles of bonds are different between alpha than beta giving amylose a coiled structure, makes it compact so good for storage as fits more in a small space
          • Amylopectin - long branched chain of alpha, side branches allow break down enzymes to get at glycosidic bonds easier meaning quick glucose release
        • Glycogen - main energy store in animals, get energy from glucose too, similar structure to amylopectin except lots more side branches so stored glucose released quicker important for energy release, also very compact so good for storage
        • Cellulose - major component of plant cell wall, long unbranched beta chains, straight bonds between sugars so chains are straight, linked by hydrogen bonds to form strong fibres meaning it provides structural support
    • Lipids
      • Made From
        • Variety of different components, al containing hydrocarbons (molecules containing only hydrogen and carbon atoms), components made from related to lipid function
      • Triglycerides - one molecule of glycerol and three fatty acids each joined to glycerol by ester bond, fatty acids have long tails made of hydrocarbons and tails are hydrophobic making lipids insoluble
        • Fatty Acids - same basic structure but hydrocarbon tails vary, two kinds : saturated and unsaturated
          • Saturated - don't have any double bonds between carbon atoms, fatty acid saturated with hydrogen
          • Unsaturated - do have double bonds between carbon atoms causing chain to kink
        • Mainly used as storage molecules, good as long hydrocarbon tails of fatty acids contain lots of chemical energy so lots released when broken down, because of tails lipids contain twice as much energy per gram as carbohydrate
          • Insoluble so don't cause water to enter cell by osmosis, they bundle together as insoluble droplets in cells as fatty acid tails are hydrophobic - tails face inwards shielding themselves from water with glycerol heads
      • Phospholipids- lipids found in cells, similar to triglycerides but one fatty acid is replaced with phosphate group, group is ionised making it attract water molecules, so phosphate pare is hydrophilic while the rest is hydrophobic
        • Make up bilayer of membranes which control what enters and leaves a cell, heads are hydrophilic and tails are hydrophobic so form a double layer with heads facing outwards towards water on either side
          • Centre of bilayer is hydrophobic so water-soluble substances can't pass through it easily so membrane acts as a barrier for those substances
      • Cholesterol - type of lipid often in cell membranes, used to make steroids, hydrocarbon ring structure attached to hydrocarbon tail, ring structure has polar hydroxyl group attached making cholesterol soluble, insoluble in blood so carried around the body by lipoproteins
        • Help strengthen cell membrane by  interacting with phospholipid bilayer, small size and flattened shape allow it to fit in between phospholipids in membrane
      • Functions

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