Topic 1 - Biological Molecules

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Carbohydrates

  • Functions: substrate for respiration, energy storage, structural purposes
  • Monosaccharides: 'simple sugars', used as energy storage, taste sweet and dissolve in water, eg: alpha-glucose.
  • Disaccharides: two monomers joined together with a glycosidic bond, condensation reaction, 3 types:
    Lactose - galactose + glucose (-H2O)
    Maltose - 2x alpha glucose (-H2O)
    Sucrose - glucose + fructose (-H2O)
  • Polysaccharides: lots of monomers joined together with glycosidic bonds, starch, cellulose and glycogen
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Starch

  • Found in plant cells, never in animal cells
  • Chains of alpha-glucose molecules joined by glycosidic bonds, condensation reaction
  • Chains may be branched or unbranched
  • Ubranched chains are wound tightly in a coil which makes it very compact
  • The main role of starch is energy storage, it is good for this because:
    1. Insoluble - doesn't affect water potential
    2. Large - doesn't diffuse out of cells
    3. Compact
    4. Forms a-glucose when hydrolysed - easily transported and ready to use
    5. Branched form has many ends - each of which can be acted on by an enzyme
      simultaneously for rapid release of monomers
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Glycogen

  • Found in animals and bacteria, never in plants
  • Similar structure to starch but shorter chains and more highly branched
  • Major carbohydrate storage, it suits this becasuse:
    1. Insolube
    2. Compact
    3. More highly branched than starch - can be acted on by many enzymes
      simultaneously, so it can rapidly form glucose monomers for animals' higher
      respiratory rate and metabolic rate as they are more active.
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Cellulose

  • Made up of monomers of beta-glucose rather than of alpha-glucose
  • Straight unbranched chains that run parallel to eachother, allowing H bonds to form between them
  • The sheer number of H bonds makes a considerable contribution to strengthening cellulose, making it a valuable structural material
  • Cellulose are grouped together to form microfibrils, which are arranged in parallel groups called fibres
  • Provides rigidity in plant cell walls
  • Provents plant cell from bursting during osmosis
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Lipids

  • Carbon, hydrogen, oxygen
  • Insoluble in water
  • Soluble in organic solvents, eg: alcohol
  • Organic compounds that can either be fatty, oily or waxy
  • Functions: storage, thermal insulation, water-proofing, hormone production, digestion, etc.
  • Test for lipids: 2ml of sample + 5ml of ethanol, shake tube thoroughly, add 5ml of water, shake gently, cloudy-white emulsion = presence of lipids
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Saturation

  • Saturated: no double bonds between C atoms, insoluble, can be stored
  • Mono-unsaturated: one carbon to carbon double bond
  • Poly-unsaturated: more than one carbon to carbon double bond, molecule can bend easily so is a liquid at room temp
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Proteins

  • Functions: enzymes (amylase), structural (collagen), signalling hormones (insulin), transport (haemoglobin)
  • Structure:
  • Primary: a unique sequence of amino acids in a polypeptide chain
  • Secondary: the unique sequence of amino acids decides how it folds and where the H bonds go, beta pleated sheets or alpha helix
  • Tertiary: folds again with ionic and H bonds, if the R region contains sulphur a disulphide bond is formed. A unique active site is formed complementary to a specific substrate
  • Quarternary: proteins that have 2 or more polypeptide chains
  • Test for proteins- sample + equal volume of sodium hydroxide solution, add a few drops of copper (II) sulphate solution and gently mix, purple colouration = peptide bonds and hence a protein. Will remain blue if not.
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Enzymes

  • Globular proteins with a specific 3D shape
  • Active site + substrate = enzyme-substrate complex
  • Lock and key model: enzyme represents lock, substrate the key, they are complementary
  • Enzymes catalyse reactions
  • Induced fit model: the active site can mould around the substrate and change to fit the specific shape of the substrate exactly
  • Environmental factors that can change the 3D structure of the enzyme molecules:
  • Temp
  • pH
  • Substrate concentration
  • Enzyme concentration
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Inhibitors

  • Competitive inhibitors: have a shape that closely resembles that of the enzyme's normal substrate. The competitive inhibitor binds to the active site, blocking the substrate, enzyme-inhibitor complex. Reversible inhibitors have weak bonds which can be easily broken.
  • Non-competitive inhibitors: do not compete for the active site, instead they bind to the allosteric site. It then forms bonds with the enzyme and changes its tertiary structure so the active site no longer fits with the substrate. If it is reversible it can be removed and the enzyme will go back to its original shape.
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DNA

  • Function: responsible for passing genetic information from cell to cell
    The vast number of sequences provides genetic diversity
    Only rarely does it mutate
    Can carry immense amounts of genetic information
    Can transfer and replicate information as mRNA
  • Structure: 2 strands joined by only hydrogen bonds which allow them to seperate easily during DNA replication and protein synthesis
    The phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
    3 hydrogen bonds between C and G, the higher the proportion of C to G pairings the more stable the DNA molecule
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DNA Replication

  • DNA helicase breaks down the hydrogen bonds between the complementary base pairs, separating the 2 strands
  • Free nucleotides are activated and bind to their complementary bases
  • They are joined by DNA polymerase which creates phosphodiester bonds up the backbone of the strand
  • 2 identical molecules of DNA are formed
  • This is semi-conservative replication because each molecule has half of the original DNA material.
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Adenosine Triphosphate (ATP)

  • When energy is needed by a cell, ATP is broken down into Adenosine Disphosphate and an inorganic phosphate:
    ATP + H2O = ADP + inorganic phosphate + energy
  • Hydrolysis reaction catalysed by ATP hydrolase
  • The opposite occurs during synthesising
  • Bonds between phosphates unstable, low activation energy, easily broken, release a lot of energy
  • Advantages of ATP:
  • Universal energy carrier
  • Instant small amounts of energy as needed
  • Transports energy to cell
  • Soluble
  • Disadvantages of ATP:
  • Can't be stored
  • Large amounts needed to synthesise
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Phosphorylation

  • The inorganic phosphate released can be used to lower the activation energy of compounds and make them more reactive:
  • Photophosphorylation: during photosynthesis
  • Oxidative phosphorylation: during respiration
  • Substrate level phosphorylati: when phosphate groups are transferred from donor molecules to ADP
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Test for reducing sugars

  • 2ml of sample in liquid form
    • equal volume of Benedicts reagant
  • Heat mixture in water bath for 5 minutes
  • Reducing sugar will form an insoluble red precipitate of copper oxide
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