Enzymes and Digestion

Biology AQA new AS level, enzymes and digestion.

HideShow resource information


3 pain processes: Digestion, Absorption (taking molecules into body) and Assimilation (absorbed molecules incorporated into body tissues). 2 stages of digestion:

1. Physical breakdown:

  • Large food into smaller by structures such as teeth or stomach muscles.
  • Makes ingestion possible.
  • Provides large surface area for chemicals.

2. Chemical digestion:

  • Conversion of large, complex, insoluble molecules into smaller, soluble ones.
  • Carried out by enzymes functioning by hydrolysis.
    • Carbohydrases - carbohydrates into monosaccharides in the s. intestine.
    • Lipases - lipids into glycerol and fatty acids in the s. intestine.
    • Proteases - proteins into amino acids in the stomach/s. intestine.
    • Amylase - starch into glucose and maltose in the mouth/s.intestine.
  • They're absorbed into the blood and re-built into larger molecules for tissues.
1 of 10

Major Parts of the Digestive System.

  • Oesophagus - transports food from mouth to stomach.
  • Stomach - is a muscular sac producing enzymes. It stores and digests.
  • S. intestine - is a long muscular tube to further digest by enzymes. Uses villi.
  • L. intestine - absorbs water from secretion of digestive glands. Food thickens to form faeces.
  • Rectum - the final part of the intestine, faeces is stored and removed by egestion.
  • Salivary glands - pass secretions to mouth. Includes amylase.
  • Pancreas - is a gland to secretes pancreatic juices (protease, lipase and amylase).
  • Mouth - uses tongue to move food, saliva lubricates and teeth for breakdown.
  • Liver - bile breaks down fats. It processes food from s. intestine, breaks down toxins and converts ammonia to urea.
  • Gall bladder - bile breaks down fats and neutralises acids.
  • Bile duct - carriers bile secreted by liver.

Emulsification - making 2 substances that would not normally mix, mix.

Emmisable - 2 substances that won't mix.

2 of 10

Carbohydrates - Monosaccharides.

  • Monomer - individual organic molecule that makes up part of a larger 1.
  • Polymer - is a long chain of monomers.
  • Hydrolysis - adding water to break a bond.
  • Condensation - removing water to make a bond.
  • In carbohydrates, the basic monomer is a sugar (saccharide).
  • MONOSACCHARIDES are sweet tasting and soluble.
  • General formula = (CH2O)n.
  • Glucose + glucose = maltose.
  • Glucose + fructose = sucrose.
  • Glucose + galactose = lactose.
  • When monosaccharides join water is removed: condensation reaction making glycosidic bonds.
  • Polymers.
  • Many combined monosaccharide molecules.
  • Insoluble so good for storage and some structual support in plants.
  • E.g. starch.
3 of 10

Food Tests.

  • Reducing Sugars - BENEDICT'S TEST.
  • All monosaccharides and some disaccharides are reducing sugars. This means they donate electrons to/reduce another chemical.
  • Add Benedict's reagent to food and heat gently.
  • Blue, green, yellow, orange, red (semi-quantitative).
  • Non-Reducing Sugars - BENEDICT'S TEST.
  • No change with Benedict's.
  • Add HCl to food and heat to hydrolyse disaccharide.
  • Add sodium hydrogencarbonate to neutralise HCl, check it's alkaline.
  • Re-test with Benedict's and heat.
  • Test for Starch - IODINE.
  • Add iodine and shake.
  • Yellow to blue/black.
  • Test for Proteins - BIURET TEST.
  • Detects peptide bonds.
  • Add sodium hydroxide and few drops of dilute copper sulfate and mix.
  • Blue to purple.
4 of 10

Carbohydrate Digestion.

Starch Digestion:

  • Uses many enzymes as they're specific and converts molecules to monomers.
  • Enzymes produced in different places due to different optimum pH's.
  • Amylase hydrolyses alternate glycosidic bonds to produce disaccharide maltose, the hydrolysed into monosaccharide alpha-glucose by maltase.
  • Food chewed so larger surface area: Salivary amylase + food = maltose.
  • Food moves to stomach, acid denatures amylase. Then to s. intestine and mixed with pancreatic juices: pancreatic amylase + starch = maltose.
  • Epithelial lining produces maltase: maltase + maltose = alpha-glucose.
  • Mineral/alkaline salts maintain neutral pH in mouth and s. intestine.

Disaccharide Digestion:

  • Sucrose- found inside cells so teeth required to get it, epithelium of s. intetsine produce sucrase: sucrase + single glycosidic bond = glucose + fructose.
  • Lactose - sugar found in milk. Lactase in s. intestine hydrolyses glycosidic bond between glucose and galactose (some people intolerant as have no lactase).
5 of 10

Amino Acids, Peptide Bonds and Protein Function.

Amino acids - basic monomer units. They combine to form a polymer - polypeptide which combine to form proteins. They have an amino group (NH2), a carboxyl group (C=OOH) and a R group which is unique.

Peptide bonds- amino acid monomers form dipeptides. Bonds form between C of 1 and N of another by condensation.

Proteins have 2 basic shapes:

1. Fibrous proteins - have structural functions e.g. collagen.

2. Globular proteins - have metabolic functions e.g. enzymes.

Fibrous proteins form long chains that run parallel and are linked by cross-bridges so are very stable.

6 of 10

Structure of Proteins/Polypeptides.

  • Polymerisation - joining of amino acid monomers through condensation to form polypeptide.
  • 20 amino acids in any sequence.
  • Determines shape which is very specific to the function
  • Linked amino acids have -NH and -C=O groups on either side of bond. H is positive and O is negative so form hydrogen bonds.
  • Polypeptide twists into 3D coil called alpha-helix
  • A-helices further twist and fold to form unique structure.
  • Maintained by disulphide, ionic(between carboxyl and amino groups and broken by pH change) and hydrogen (numerous and weak) bonds
  • Large proteins have complex molecules with individual linked polypeptide chains
7 of 10


These are globular (dissolve in water) proteins that are catalysts. They have a specific shape with an active site (functional part).

For reactions to take place naturally: molecules must collide with enough energy to alter atom arrangement; energy particles must be less than substrate ones; an initial boost of energy is needed to start the reaction - activation energy.

The molecules that act on enzymes are substrates, these form an enzyme-substrate complex. The substrate is held in the active site by temporary bonds between some amino acids of enzyme and groups of substrate.

Lock and Key Model - enzyme is rigid with active site complementary to substrate. They fit exactly as have a specific shape.

Induced Fit Model - when a substrate binds with an enzyme it induces changes in enzyme's shape i.e. enzyme flexible. As it changes shape the strain on the substrate distorts a bond and so lowers activation energy needed to break it.

8 of 10

Factors Affecting Enzymes.

  • Reactions measured by: formation of products and disappearance of substrate.
  • Increased kinetic energy so collide more often, therefore enzyme and substrate come together more often.
  • If too high hydrogen and other bonds break, altering active site and slowing reaction (45 degrees).
  • Denaturation - enzyme so disrupted it stops working (60 degrees).
  • Body at 37 degrees otherwise additional energy would be needed to
  • maintain higher temp, other proteins may denature and illnesses that further increase temp would denature enzymes.
  • pH:
  • Change from optimum reduces effectiveness until denatured.
  • Alters charges on amino acids of active site so no complex can form.
  • Causes bonds in tertiary structure to break, altering shapes and denaturing.
  • Rate of reaction proportional to amount of substrate added.
  • If concentration low enzymes have limited number to collide with.
  • When all active sites used, more addition will have no affect.
9 of 10

Enzyme Inhibition.

Inhibitors - interfere with functioning of active site so reduce activity. Reversible inhibitors make temporary attachments to the active site, there are 2 types:

1. Competitive Inhibitors:

  • Similar shape to substrate so occupy active site: enzyme-inhibitor complex.
  • Inhibitor and substrate compete for active site.
  • At high sub. concns inhibitor has little effect. At low ones the rate is lower.
  • Eventually, all substrate molecules occupy an active site, the greater the inhibitor concentration the longer it takes.

2. Non-competitive Inhibitors:

  • Attach to site other than active one as have no structural similarity to substrate.
  • Alters shape of active site so substrate can't bind and enzyme cannot function.
  • An increase in substrate concentration does not decrease effect of inhibitor.

End-product inhibition - a product acts as a non-competitive inhibitor on earlier enzyme.

10 of 10


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Biological molecules, organic chemistry and biochemistry resources »