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The digestive system
The digestive system
Organs/process
ð Mouth:
ð Mechanical digestion
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ð Movement by peristalsis
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ð Stomach:
ð Muscular sac, entrance controlled by cardiac
sphincter
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ð Churning of food (peristalsis) into chyme – mastication
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ð Moved into duodenum via pyloric sphincter – chyme neutralised and broken down by bile and
pancreatic juices
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ð Pancreas:
ð Pancreatic juices – amylase, trypsin, chymotrypsin, carboxypeptidase, lipase and sodium
hydrogencarbonate
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ð Large intestines (colon):
ð Absorbs water, minerals and salts
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Process
1
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Digestion
3
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Assimilation
5
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ð Condensation – reaction of joining molecules and the release of water
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1

Proteins
Amino acids
ð Amino group, carboxyl group, hydrogen atom and R side group attached to a
central chiral carbon atom
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ð Joining of more than two amino acids forms a polypeptide chain
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Structure
ð Prim ary structure – sequence of amino acids in a polypeptide chains joined by peptide bonds
through condensation reactions
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ð Tertiary structure – formation of more bonds (ID-ID, disulphide bridges, ionic interactions and
hydrogen bonding) between the R groups on the polypeptide chain
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Gives the final 3D shape of the protein
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Have chemical functions
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g
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Generally long and thin
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Structure maintained by strong
bonds e
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disulphide bridges
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Keratin, actin and myosin
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ð Glycine amino acid allows 3
polypeptide chains to pack together
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Osteogenesis imperfecta EXTRA:
Mutation causes glycine to be replaced
by another amino acid, meaning the
triple helix of collagen can no longer be
maintained
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Biuret test
Test detects the peptide bonds in proteins
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Sample needs to be dissolved in water
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NaOH solution is added to make the sample alkaline
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Add copper (II) sulphate solution added, presence of protein purple layer forms but remains blue if
not
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ð Lower activation energy needed to achieve the transition state (state where existing bonds are
strained) by providing an alternative pathway by forming an enzyme-substrate complex
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ð Used in extracellular and intracellular processes
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Lock and key

Induced fit

M echanism
Active site and substrate are
complementary to each other
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Properties
ð Active site of enzymes is determined by the tertiary structure of the protein (which is determined by
the primary structure - determined by genes)
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Factors affecting enzyme activity
Tem perature
ð Rate of reaction increases with an increase in temperature
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ð Too high temperatures cause molecules to vibrate more and weaker bonds e
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hydrogen bonds will
break
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pH
ð Above or below the optimum temperature alters the ionic and hydrogen bonds:
ð Charges on active site are no longer complementary to those on the substrate
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Substrate concentration
ð Increase in substrate increases enzyme activity
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Inhibitors
ð Competitive inhibitors – have a similar shape to the substrate; can bind with active site and block
substrates from attaching
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ð Non-competitive inhibitors – binds to a different active site on the enzyme, altering the shape of the
other active site preventing substrates from attaching
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[EXTRA]
ð ACE inhibitors:
ð ACE – angiotensin converting enzyme
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p
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ð Irreversible inhibition:
ð Enzyme- inhibitor binding is covalent and the distortion of active permanent
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g
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ð E
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Cl- needed for activity of salivary amylase (chemical digestion)
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ð Monosaccharides - single sugar (carbohydrate)
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ð Glucose (α) – hexose sugar, main energy source in humans, used for ATP
production
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(Isomer of glucose)
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ð Maltose - the condensation of two α-glucose molecules joined together
by a α, 1-4 glycosidic bond (hydrolysed by maltase)
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ð Lactose (milk sugar) – condensation of α-glucose and galactose (hydrolysed by lactase)
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Add reagent (blue) to the sample and heat
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If reducing sugar present solution should turn brick red
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Boil sample with HCl – break the bonds
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Add sodium hydrogencarbonate – neutralise solution
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Add reagent (blue) to the sample and heat
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If reducing sugar present solution should turn brick red
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ð Starch (about 300 glucose subunits joined by α, 1-4 glycosidic bonds):
ð Main energy storage in plants
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ð Good for storage because spiral shape of amylose makes it compact, large in size makes it
insoluble and branching of amylopectin means glucose can be released easily
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ð Stored mainly in liver and muscles
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Starch test
1
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2
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Principle: Iodine bonds to centre of starch helix, forming a iodine-starch complex which is blue-black
in colour
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ð Lactose cannot be hydrolysed and digested
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ð Results in watery diarrhoea
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ð Results in bloated feeling

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