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2.1.2 Biological Molecules (Lipids and Proteins) (Structure of Lipids…
2.1.2 Biological Molecules (Lipids and Proteins)
Structure of Lipids (Macromolecules)
Triglyceride
glycerol: 3-carbon molecule with 3 alcohol groups
three fatty acid chains; 2-20 carbon chain with carboxyl group on one end; can be saturated (no double C=C bonds), or unsaturated (contain C=C bonds)
having one (monounsaturated) or more than one (polyunsaturated) gives the carbon change a kink; decreases melting point and makes lipid more fluid
Phospholipid
same structure as triglyceride but one of the fatty acid chains are replaced by a phosphate group
fatty acid chains normally have even number of carbons (usually 16 or 18); commonly one saturated, and one unstaurated
Lipid: a group of substances that are soluble in alcohol rather than water; non-polar; not polymers
macromolecule: a very large organic molecule
Synthesis and Breakdown of Triglycerides
Esterification
formation of ester (-COO-) bonds between fatty acids and glycerol
-COOH group of fatty acid chain reacts with -OH group of glycerol to form a covalent ester bond
water molecule is lost, per ester bond formed (when triglycerides are formed, three water molecules are lost in total)
Hydrolysis
breakage of ester bonds between fatty acids and glycerol
covalent ester bond is broken, and a water molecule is lost
Properties of Lipids, and Function
Phospholipids
polar, hydrophilic phosphate head
non-polar hydrophobic fatty acid tails
ampipathic
forma plasma membrane; phospholipid bilayer; individual phospholipids are free to move around but tails will not be exposed to water; stable membrane e.g. most plant and animal cells
membrane is selectively permeable; only small, non-polar molecules can pass through the bilayer; plasma membrane controls what goes in and out of the cell
Triglycerides
energy source; broken down in respiration to release energy
energy store: are insoluble in water so can be stored without affecting water potential of the cell; releases more energy than glucose
insulation e.g. blubber in whales
buoyancy: less dense than water, so help animals float e.g. aquatic animals
protection of delicate organs: shock absorber
Cholesterol
steroid alcohol made of four carbon based rings (isoprene units)
small and hydrophobic molecule; can sit in middle of hydrophobic part of bilayer; regulates fluidity of membrane
made in liver in animals
steroid hormones e.g. testosterone, oestrogen are made from cholesterol so can pass though plasma membrane as they are small and non-polar
Synthesis and Breakdown of Polypeptides
Condensation
formation of peptide bonds (-CN-)
two amino acids join together to form a dipeptide molecule
water molecule is lost
Hydrolysis
breakdown of peptide bonds (-CN-)
dipeptide molecule is broken down into two amino acids, and a water molecule is lost
Levels of Protein Structure
Primary
sequence of amino acids in a chain; determines function of end protein, and secondary, tertiary, quaternary structure
amino acid general structure: amine group (-NH2) on one end, and carboxyl group (-COOH) on other end of central C atom, and an R group
linked together by covalent peptide bonds
Secondary
folding of primary structure
α-helix
β-pleated sheet
bonds:
Hydrogen bonds: between -NH of one amino acid, and -CO of another
Tertiary
3D folding of the secondary structure
bonds:
hydrogen bonding
disulphide bridges: bonds between S in R group of amino acid (cystine); strong covalent bonds
ionic bonds: bonds between carboxyl groups and amino groups; ionise into NH3+ and COO-
hydrophobic and hydrophilic interactions: hydrophobic R groups in the centre of the peptide chains to avoid water, and hydrophilic R groups found at the edge of the structure, close to water; case twisting of amino acid chain; stable structure
Quaternary
two or more polypeptide chains joined together
bonds:
hydrogen bonds
ionic bonds
disulfide bridges
hydrophobic and hydrophilic interactions
Globular Proteins
Haemoglobin (conjugated protein)
quaternary structure; four polypeptide chains (2 α and 2 β)
prosthetic haem group: a non-protein component that forms a permanent part of the functioning protein; contains and iron ion
function: carry oxygen form lungs to tissue; oxygen binds to iron
Insulin
made of two polypeptide chains; first chain starts of α helix and second chain ends β pleated sheet; fold into tertiary structure
soluble, due to hydrophilic R groups on outside of molecule
function: increase rate of uptake of glucose from blood into muscle and fat cells
Pepsin (enzyme)
function: digest protein in the stomach
made up of single polypeptide chain folded into symmetrical tertiary structure; contains many more acidic R groups than basic R groups, so is stable in acidic stomach, as there are less OH- to accept H+ ions and change structure of enzyme
held together by hydrogen bonds and disulphide bridges
molecules of relatively spherical shape; soluble in water; often have metabolic roles in organism
Fibrous Proteins
Collagen
provide mechanical strenght
e.g. in artery wall; withstand high blood pressure and prevent the artery all from bursting
e.g. tendons; connect muscle and bone
e.g. bones, cartilage, and connective tissue; are strong
Keratin
rich in cystine; many disulphide bridges are formed
e.g. hair, nails, hooves, horns, feathers
mechanical protection
impermeable barrier to pathogens
waterproof
Elastin
cross-linking and coiling
e.g. skin; stretch around bones and muscles
e.g. in lungs; lungs can expand and recoil without bursting
e.g. in blood vessels; prevent bursting
relatively long, thin structure; insoluble in water; metabolically inactive; usually have a structural role in organism
Organic Ions
Cations:
Ca2+
strong bones; clotting blood; muscle contraction; cell wall development in plants; regulate permeability of plasma membrane
Na+
control water levels; ; maintain pH; muscle contraction; vacuoles maintain turgidity
K+
control of water level in body fluid; maintain pH; assist active transport across plasma membrane; healthy leaves and flowers; muscle contraction; vacuoles retain turgidity
H+
photosynthesis; respiration; transport of oxygen and carbon dioxide in the blood; regulate blood pH
NH4+
component of amino acids; nucleic acids; regulates pH; in nitrogen cycle
Anions
NO3-
component of amino acids; nucleic acids; nitrogen cycle
HCO3-
regulate blood pH
transport CO2 into and out of the blood
Cl-
PO43-
OH-
Sammer Sheikh