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Topic 2: Enzymes and the Digestive system
By Tahmida Chowdhury
Digestive System
What is ingestion?
A: when your body takes in food
What is digestion?
A: breaking larger molecules into small, soluble ones
What is absorption?
A: the uptake of small molecules and water
What is assimilation?
A: when absorbed small molecules are built up to larger molecules to make cells or cell components or are broken down for energy
Digestive System
What is the role of the mouth?
A: (starts carbohydrate digestion) amylase in saliva breaks down starch into maltose
What is the role of the oesophagus?
A: to carry food to the stomach by peristalsis
What is the role of the stomach?
A: to produce enzymes and acid, and store and digest food
What does the mucus in the stomach lining prevent
A: the enzymes digesting the stomach
What is the role of the small intestine?
A: to absorb products of digestion into bloodstream
How is the small intestine adapted to this function?
A: contains villi and microvilli to increase surface area and maximise rate of absorption
What is the role of the pancreas and salivary glands?
A: to produce enzymes. The salivary glands produces amylase and the pancreas produces all 3
Digestive Enzymes
What is physical/mechanical digestion?
A: when large foods are broken down to smaller foods via the teeth-
What is an advantage of this?
A: this allows a larger surface area for chemical digestion
What is chemical digestion?
A: breakdown of large insoluble molecules into small soluble molecules by enzymes via hydrolysis
What is hydrolysis?
A: when water is added to break the bond between molecules
Name the 3 main digestive enzymes
A: Carbohydrase, lipases, proteases
What does Carbohydrase break down and to what?
A: starch to maltose
What does lipase break down and to what?
A: lipids to fatty acids and glycerol
What does protease break down and to what?
A: proteins to amino acids
Coeliac Disease
How is the microvilli of people with coeliac disease damaged?
A: large substances like proteins can get through the lining of the small intestine
As a result why can’t diffusion occur properly?
A: the surface area is less and the rate of diffusion is reduced
Monosaccharides
What are monomers?
A: monomers are the building blocks of a large polymer. They are small molecules ‘fused’ together in a series of condensation reactions to form a polymer.
What are polymers?
A: chains made of monomers
What four elements are polymers made of? (CHON)
A: carbon, hydrogen, oxygen and nitrogen
What is a monosaccharide?
A: individual monomer units from which all carbohydrates are built.
Monosaccharides
Give 3 properties of monosaccharides
A: They’re all reducing sugars, sweet and soluble
What do 2 monosaccharides form?
A: disaccharide
What is the general formula for a monosaccharide?
A: (CH2O)n (where n can be any number from 3-7)
What is the name of the bond between two monosaccharides?
A: glyosidic
How is the glyosidic bond formed?
A: in a condensation reaction- the removal of water
Reducing Sugars
What is a reducing sugar?
A: a sugar that reduces its electrons by donating them to benedict’s reagent or another chemical (all monosaccharides)
What is the name of the test for a reducing sugar?
A: benedict’s test
Describe the test for a reducing sugar
A: add equal volumes of the sample solution and benedict’s reagent in a test tube and heat for it five minutes in a water bath.
What is the indication for a positive result?
A: brick red/orange colour
What is the indication for a negative result?
A: blue
Non-Reducing Sugars
What are non-reducing sugars?A: sugars that don’t reduce the benedict’s solution, so you need to break them down first.After receiving a negative result from the benedict’s test, why would you add 2cm3 of hydrochloric acid to the solution? A: to hydrolase any spare disaccharides into monosaccharides.After the hydrochloric acid why would you add sodium hydrogen to the solution?A: to neutralise the acid What do you do after this step?A: do the reducing sugar test again (heat the solution again with benedict’s reagent)
Disaccharides
Name 3 common disaccharides
A: maltose, sucrose, lactose
What forms to make maltose?
A: glucose + glucose
What forms to make sucrose?
A: glucose + fructose
What forms to make lactose?
A: glucose + galactose
What is a condensation reaction?
A: when water is removed to make the glyosidic bond between two monosaccharides
Why can’t disaccharides be absorbed into the blood?
A: they’re too large
Polysaccharides
How are polysaccharides formed?
A: when 3 or more monosaccharides form together
Why are polysaccharides suitable for storage?
A: they are large and insoluble
What are the 3 main types of polysaccharides?
A: starch, cellulose and glycogen
Describe and name the test for starch?
A: iodine test; 2cm3 of sample solution and same volume of iodine
What indication shows a positive result?
A: blue/black colour
What indication shows a negative result?
A: yellow
Starch Give 3 properties starch molecules have
A: insoluble, compact, osmotically inactive
What 2 compounds is starch a mixture of?
A: amylose and amylopectin
Describe the structure of amylose
A: 1,4 glyosidic bonds, un-branched compact chains of a-glucose that form a helix
Describe the structure of amylopectin
A: 1,6 glyosidic bonds, branched chains of a-glucose molecules
Why can glucose from amylopectin be hydrolysed quicker?
A: glucose can only be released from the ends of the chains and amylopectin has branched chains where the glucose is easier to reach
What disaccharide unit is starch broken into and by which enzyme?
A: into maltose by amylase
What monosaccharide unit is maltose broken into and by which enzyme?
A: glucose by maltase
Starch Digestion
What happens to starch in the mouth?A: saliva containing amylase hydrolyses the starch into maltoseWhat happens to starch once it reaches the small intestine?A: further hydrolysis of the starch occurs by amylase and then the maltase on the epithelial lining breaks maltose into glucose because there is no use for maltoseWhat is the ileum?A: the epithelial cells lining the small intestine However, when food enters the stomach what does the acid do to amylase?A: it denatures the enzyme amylase so further hydrolysis of the starch can’t occur
Lactose Intolerant
Where is lactose found?
A: milk products
Why can’t lactose be digested?
A: it is a disaccharide (too big)
Why don’t some adults produce the enzyme lactase?
A: their diet doesn’t contain enough milk so their body outgrows the production of the enzyme or it’s due to a faulty gene
How does a person become lactose intolerance?
A: when their bodies consume more lactose than lactase can digest
What happens do the undigested milk (lactose)?
A: it reaches the large intestine where microorganisms break it down
Why do people who are lactose intolerant get diarrhoea?
A: the water potential is lowered by the small soluble substances, therefore water moves in naturally by osmosis from the epithelial cells
Proteins
What do amino acids make up?
A: a polypeptide chain of a protein
What is the bond called between 2 amino acids
A: peptide
What four elements are amino acids made from?
A: carbon, hydrogen, oxygen and nitrogen (sometimes sulphur)
What forms the primary structure?
A: the order/sequence of amino acids in a polypeptide chain
What does the primary structure determine?
A: the overall shape and function of the protein
Describe the relationship between the protein’s shape and its function
A: they are specific to each other
Protein Structure
How is the secondary structure formed?
A: when the primary structure has further coil and pleats
What 2 types of common secondary structures are there?
A: a-helix and beta pleated
What bonds are secondary structures held with?
A: weak hydrogen bonds formed between the –NH and –C=O groups in the amino acids
How is the 3D tertiary structure formed
A: from further coil and pleats of the secondary structure
What bonds hold the tertiary structure together?
A: disulphide bridges, ionic bonds and hydrogen bonds
How is the quaternary structure formed?
A: when 2 or more polypeptide chains link together
Protein Denaturation
When can a protein become denatured?
A: high temperature or changes in pH
Which bonds break due to high temperatures and why?
A: hydrogen because they’re weak
Why don’t disulphide and ionic bonds break to high temperatures?
A: they’re stronger and can withhold high temperatures
What can break the ionic bonds?
A: strong acids and alkalis and detergents and solvents
What happens when the tertiary structure is broken?
A: it loses its function
Why is this particularly important for denaturing in enzymes?
A: when the shape of the active site is altered the substrate can no longer fit in
Test for Proteins
What is the name for test for proteins and what does this detect?
A: biuret test- it detects for peptide bonds
Describe the biuret test in 2 steps
1) sample solution and sodium hydroxide in a tube
2) Mix this with 0.05% of dilute copper sulphate solution
What does a positive indication show?
A: a purple colour
What does a negative indication show?
A: blue
Fibrous Proteins
Name 2 types of basic protein shapes
A: fibrous and globular
What is the overall function of fibrous proteins?
A: structural functions
What kind of tertiary structure are in fibrous proteins?
A: alpha helix
Describe the basic structure of a fibrous protein
A: long chains that run parallel to one another
What kind of bonds are in the fibrous proteins?
A: polypeptides linked by cross bridges
What is the solubility of fibrous proteins
A: insoluble
Give 2 examples of a fibrous protein and where is it found
A: collagen, and is found in bone tendons. Keratin, found in hair and nails
Globular Proteins
What is the overall function of globular proteins?
A: metabolic functions
What is the basic structure of globular proteins?
A: round, and deeply folded
What kind of tertiary structure are globular proteins?
A: alpha helix and beta pleated
What is the solubility of globular proteins?
A: soluble
What kind of bonds hold the tertiary structure together?
A: hydrogen, ionic, disulphide bridges
Name examples of globular proteins
A: antibodies, enzymes, insulin and haemoglobin
Enzymes
What are enzymes?A: biological catalysts that speed up the rate of reactions inside living things without being used up themselvesHow is an enzyme-substrate complex formed?A: the enzyme has a specific shaped active site complementary to the shape of the substrate molecule so they bind What is the activation energy?A: the minimum amount of energy required to start a reaction Why happens at the beginning of the reaction?A: the bonds of the substrate are broken first in order for them to be changed into products- an E-S complex is formed
Enzyme Models
What does the lock and key method explain?
A: that enzymes have a rigid shape and the substrate is the exact complementary shape to the active site.
What is a limitation of this model?
A: it explains how the enzyme is a rigid structure but doesn’t explain how its active site can change when other substances bind to the enzyme
How is the induced-fit model a better explanation of scientific observations?
A: it explains how other molecules can change the shape of the enzyme and how the activation energy is lowered
Describe the induced-fit model
A: the enzyme isn’t’ rigid but flexible. It moulds itself around the substrate in order to bind
How does an enzyme lower the activation energy in a substrate?
A: when it changes shape to bind to the substrate it puts strain on the substrate which distorts a bond and lowers the energy to break that bond
Factors Affecting Enzymes
What does the effect of pH have on enzymes?
A: as this alters the charge on the amino acids in the active site and the substrate can no longer bind and make enzyme-substrate complexes
How can the effect of a small temperature increase have on enzymes?
A: as temperature increases the kinetic energy increases, which increases the amount of collisions and more E-S complex form
Describe what happens in the tertiary structure when the temperature increase is too high?
A: the atoms vibrate rapidly and the hydrogen bonds break. Reducing the number of E-S complex and causing the tertiary structure to change
What is the effect of enzyme concentration on the rate of reaction?
A: the rate of reaction is directly proportional to the enzyme concentration- there are no limiting factors
What is the affect of substrate concentration on the rate of reaction?
A: initially it will increase the rate of reaction until the active site are all in use as the concentration increases, so now the number of enzymes limit the rate of reaction
What is a buffer solution and how does it work?
A: maintains the pH by absorbing H+ ions when the solution is too acidic and releasing H+ ions when the solution is too alkaline
Inhibitors
How do competitive inhibitors affect enzyme activity?
A: they are a similar shape to the substrate so they slow down activity by competing for the active site
Where do these inhibitors bind?
A: the active site
How can these inhibitors be overcome?
A: by increasing substrate concentration
How do non-competitive inhibitors affect enzyme activity?
A: they stop activity by changing the shape of the active site so substrate molecules can no longer fit
Where does this inhibitor bind?
A: the allosteric site
