Biological Molecules

Unit 1: Biological molecules - water, lipids, proteins, carbohydrate and enzymes.
Detailing important parts.

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Water Structure:

  • Water contains - 1 oxygen atom, 2 hydrogen atoms (H2O).
  •  Covalently bonded (shared electrons).
  •  Due to shared negative hydrogen atoms are pulled towards oxygen atom. Hydrogen atoms left with slight positive charge.
  • Unshared negative electrons (oxygen's), there is a slight negavtive charge.
  • Therefore water molecule is dipolar.
  • Negatively charged oxygen atoms attracted to positively carged hydrogen atoms, called hydrogen bonding.


  • Dipole nature: Cohesion (attraction between molecules), good solvent (ionic substances dissolve, charged ions surrounded by water molecules).
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  • Made from monosaccharides (single or more than one).
  • Long, large chains.
  • Glucose (monosaccharide) 6 carbon atoms, alpha and beta. Soluble, easily transported.

Disaccharides and polysaccharides:

  • Monosaccharides join together through glycosidic bonds in condensation reaction (H2O molecule released).
  • Hydrogen bonds with Hydroxyl.
  • 2 monosaccharides form disaccharide.
  • Need to know: Maltose (glucose & glucose).
                           Lactose (glucose & glacatose).
                           Sucrose (glucose & fuctose).
  • More than 2 monosaccharides form a polysaccharide.


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Starch & Glycogen

Starch - main energy store in plants. Insoluble, dosen't change water potential in cells, good for storage.

  • Energy from glucose stored as starch, when more glucose needed starch is broken down.
  • Mixture of 2 polysaccharides:
    1. Amylose: unbranched chain, 1-4 glycosidic bonds, coiled structure, good for storage.
    2. Amylopectin: branched chain, 1-4 and 1-6 glycosidic bonds, side branches allow enzymes to break down easily, energy released quickly.

 Glycogen - main energy store in animals.

  • Energy from glucose, stored as glycogen.
  • 1-4 and 1-6 glycosidic bonds (more side branches). Energy released quickly, important for energy release.
  • Compact and insoluble, good for storage.
  • Large molecule, lots of energy.
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Triglycerides (lipid):

  • One glycerol, 3 fatty acids.
  • Fatty acid tails are hydrocarbon chains.
  • Tails are hydrophobic, therefore insoluble, tail structure varies.

Formed in condensation reactions:

  • Joined by Ester bonds, hydrolysis and condensation reactions.
  • Hydrogen bonded to hydroxyl.

Saturated and usaturated fats:

  • Saturated (animal fats), unsaturated (plant fats).
  • Unsaturated melts at lower temperatures.
  • Differences between the two is the hydrocarbon tails.
  • Saturated fats have no double bonds.
  • Unsaturated fats have double bonds, more than one and it becomes a polyunsaturated fat.
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What are proteins made of:

  • Long chains of amino acids.
  • Lots of smaller molecules (monomers)
  • Dipeptide - two amino acids.
  • Polypeptide - more than two amino acids.
  • Proteins - one or more polypeptides.

Different amino acids:

  • Have different variable groups.
  • Same main structure - carboxyl group (COOH) & amino acid groups (NH2).
  • Joined by peptide bonds.
  • Polypeptides formed from condensation reactions.
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Protein Structure:

  • Primary Structure - sequence of amino acids, in a polypeptide chain.
  • Secondary Structure - hydrogen bonds cause coiling into alpha helix or beta pleated sheet.
  • Tertiary Structure - coiled and folded further. More bonds formed. For single polypeptide chain final structure.
  • Quaternary Structure - several different polypeptide chains held by bonds. Quaternary made from more than one, final structure.

Different bonds between structual levels:

  • Primary structure - peptide bonds.
  • Secondary structure - hydrogen bonds, create alpha helix and beta folded sheet.
  • Tertiary Structure - ionic interactions (weak attractions between negative and positive charges), disulfide bonds (when two amino acids cysteine come close, sulfur atom bonds), hydrophobic and hydrophillic interactions (hydrophobic groups clump together, hydrophillic pushed outside, structure folds) and hydrogen bonds.
  • Quaternary Structure - determined by teriary structure, influenced by all the bonds.
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Primary Structure (3D structure and propeties):

  • Amino acid sequence determines bonds and how protein folds. (eg. disulfide bonds form if there are many cysteines).
  • 3D structure determines properties.

Types of protein Structure:

Globular - Round, compact, multiple polypeptide chains. Coiled up, hydrophillic on outside of chain & hydrophbic on inside. (eg. haemoglobin, four polypeptide chain. Carries oxygen, soluble, contains haem qroups bind to oxygen).


Fibrous - Long insolublepolypeptide chains, tightly coiled, rope shape. Held by lots of bonds which makes them strong. Found in supportive tissue. (eg. collagen is strong, fibrous proteins, supportive tissue in animals).

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Enzymes are Biological Catalysts:

  • Speeds up chemical reactions as biological catalysts.
  • Catalyse metabolic reactions (eg. digestion, respiration).
  • Can be : Intracellular - within cells
                   Extracellular - outside cells
  • Globular proteins.
  • Enzymes have a active site, it as a specific shape.

Lower activation energy of a reaction:

  • Activation energy (energy supplied to chemicals before reaction) often provided as heat.
  • Enzymes lower activation energy needed, therefore lowers the temperature.
  • When substrate fits enzymes active site this forms a enzyme substrate, and lowers reaction temperature:
    1. Enzyme holds substrates together reducing repulsion.
    2. If enzyme is catalysing breakdown reaction, puts strain on bonds, therefore molecule breals up more easily.
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Lock and key model - Substrate fits like a key into enzyme the lock.
Induced fit (better theory) - Active site changes shape to fit substrate.

3D structure causes specific enzymes:

  • Only catalyse in one type of reaction.
  • Only one substrate will fit.
  • 3D structure determines active site shape (determined by primary structure).
  • Different 3D structure, different active site, different enzyme.

Enzyme concentraition effects rate of reaction:

  • More enzyme molecule present, more likely for substrate to colide with them and form enzyme-substrate complex, increasing rate of reaction.
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