Biology Unit 1 AQA Enzymes & The Digestive System

Biology Unit 1 AQA Enzymes & The Digestive System

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Digestion - process by which large molecules are hydrolysed by enzymes into small molecules, that can be absorbed and assimilated.

  • Oesophagus - carries food from mouth to stomach
  • Stomach - produces enzymes & hydochloric acid, stores & digests , mucus produced to stop enzymes digesting stomach.
  • Small Intestine - villi & microvilli on epithelial cells, large surface area, enzymes produced & products absorbed into the blood.
  • Large Intestine - absorbs water from digestive glands, forms faces due to lack of water.
  • Rectum - egestion , removal of faces.
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Two stages of digestion: Physical - broken down by teeth, ingested easily and provides large surface area for chemical digestion. Stomach muscles also churn to break down more. Chemical - large insoluble molecules broken down into soluble ones, carried out by hydrolysis by enzymes (hydrolases). Digestive enzymes -

  • Carbohydrases - hydrolyse carbohydrates into monosaccharides.
  • Lipases - hydrolyse carbohydrates into glycerol & fatty acids.
  • Proteases - hydrolyse proteins into amino acids.

These hydrolysed molecules can then be absorbed into blood from small intestine. Assimilation is the incorporation of these molecules into body tissues or in processes of the body.

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Carbohydrates -

Basic monomer unit is a sugar - monosaccharide, two - disaccharide, lots - polysaccharides.

Glucose is a monosaccharide (hexose - 6 carbons).


Reducing Sugar Test

All monosaccharides and some disaccharides are reducing sugars, they donate electrons to the benedicts reagent, from copper (II) sulphate to copper (I) oxide.

Dissolved in water, add Benedict's, boil for 5 minutes, precipitate is formed. Lots of reducing sugar = red/orange. Semi - quantitive test & cannot distinguish between very high amounts of reducing sugars due to copper (II) sulphate being used up/converted.

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NON Reducing Sugar Test

Disaccharide sucrose is not a reducing sugar. To detect a non reducing sugar it must first be broken down into its monosaccharides by hydrolysis.

1. Test with Benedict's reagent, boil for 5 minutes, if no change in colour then reducing sugar is not present.

2. Hydrolyse the disaccharide into its monosaccharides with dilute hydrochloric acid.

3. Neutralise with sodium hydrocarbonate.

4. Retest with Benedict's, boil for 5 minutes.

5. Orange/brown if non reducing sugar was present in the original sample. Due to the reducing sugars produced by the hydrolysis of the non reducing sugar.

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Disaccharides -

glucose + glucose = maltose

glucose + fructose = sucrose

glucose + galactose = lactose

Condensation reaction joins the monosaccharides. Removal of water, glycosidic bond is formed.

Hydrolysis breaks the glycosidic bond with the addition of water, occurring in digestion.

Polysaccharides - formed by combining many monosaccharides. Large insoluble molecules - suitable for storage.

Test for starch - potassium iodide solution from yellow - black/blue.

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Starch digestion -

  • Salivary amylase produced by salivary glands hydrolyses glycosidic bonds in starch >> maltose.
  • Acid in the stomach denatures the salivary amylase. Travels to the small intestine.
  • Pancreatic amylase produced by pancreas hydrolyses the rest of the starch >> maltose.
  • Epithelial cells in the small intestine produces maltase - hydrolyses maltose >> glucose.

Sucrose digestion -

  • Epithelial cells in the small intestine produces sucrase - hydrolyses sucrose >> glucose & fructose.

Lactose digestion -

  • Epithelial cells in the small intestine produces lactase - hydrolyses lactose >> glucose & galactose.
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Lactose Intolerance -

The production of lactase decreases as it becomes a smaller part of the diet, some produce very little/no lactase.

-some do not produce enough lactase to digest all the lactose they consume. Undigested lactose reaches the large intestine, microorganisms break it down, producing a lot of gas.

= bloating, nausea, diarrhoea & stomach cramps.

Therefore there will be a lack of calcium in diets - eat foods rich in calcium, or put enzyme lactase in milk before drinking.

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Proteins - Amino acids (monomers) make up polypeptides (polymers), polypeptides combine to form proteins.

Every amino acid has four groups attached to a central carbon:

  • Amino group, NH2
  • Carboxyl group, C=OO-H
  • Hydrogen group, H
  • R group, different for each amino acid

Dipeptides - joining of two amino acids Condensation reaction (removal of water) -OH on carboxyl group (C=OO-H) and -H on the amino group (NH2). PEPTIDE BOND IS FORMED BETWEEN THE C AND N. Peptide bond can be broken by hydrolysis (addition of water) Series of condensation reactions forms polypeptides, in a process called polymerisation. The sequence of amino acids in the polypeptides forms the primary structure of a protein.

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Lipids - carbon, hydrogen and oxygen.

Triglycerides - long term energy store, insulation, waterproofing and protection. 3 fatty acids and glycerol.

Glycerol C3H8O3 Fatty acid chain


H - C - OH O=C - C - C - C H


H - C -OH


Ester bond formed by condensation between the H on -OH and -HO on fatty acid.

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Primary Structure - amino acids in the polypeptide. The primary structure determines the ultimate shape and hence the function. If one single amino acid is changed in the primary sequence can lead to a change in the shape and function of the protein.

Secondary Structure - hydrogen bonds form between the -NH and -C=O on either side of the peptide bond. 3-D α helix.

Tertiary Structure - α helix twisted and folded.

  • Hydrogen bonds -NH & -C=O, weak & easily broken
  • Ionic bonds -C=OO-H and -NH2 not involved in peptide bonds
  • Disulfide bonds strongest bonds

Quaternary Structure - combination of a number of polypeptide chains and non protein chains. Test for proteins - Biuret test detects peptide bonds, purple colour is positive.

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Enzyme Action - globular proteins that act as catalysts, lower the activation energy and speed up reaction.

For a reaction to take place:

  • Substrate molecules need to collide with sufficient kinetic energy
  • The energy of products needs to be less then substrate energy
  • An initial boost of energy is needed to start the reaction, minimum is the activation energy.

Enzymes lower this energy level that is needed to be overcome meaning that reactions can take place a much mower temperatures. Enzyme structure - Active site & substrate - enzyme substrate complex. The substrate is held in the active site by temporary bonds between the amino acids and the substrate.

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Lock & Key - Each substrate will only fit in the active site of one particular enzyme, they fit together because they are complementary in shape. Limitations - the lock is a rigid structure.

Induced Fit - Enzymes active site is not fully complementary to the substrate, the enzyme changes shape around the substrate, as it changes shape the enzyme puts a strain on the substrate, pressure is put on a bond in the substrate - lowering the activation energy needed to break this bond. Better then the lock and key - explains how other molecules can effect enzyme activity, and how activation energy is lowered.

Factors effecting enzyme activity effect: the contact of the substrate and enzyme, the active site that fits the substrate.

Temperature -

Increasing temp to the optimum (maximum rate)

  • enzyme & substrate have more energy
  • number of enzyme-substrate complexes increase
  • more product produced
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Above optimum:

  • enzyme vibrates, hydrogen bonds break, tertiary structure and active site changed.
  • at first it is less easy to form enzyme-substrate complex - slower rate
  • the enzyme become so disrupted at the higher temp - denatured enzyme
  • enzyme-substrate complex cannot form.

Body temperature has evolved to be 37'c

  • if higher temp there would be a higher metabolic rate, but the body would need more food to maintain higher temp
  • proteins are denatured at higher temps
  • if the body is at higher temps when you become ill enzymes will become denatured
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Effect of pH - pH is the measure of the concentration of hydrogen ions, each enzyme has its optimum pH where it will work at its fastest.

  • change in pH alters the charges on the amino acids that make up the active site on the enzyme - substrate can no longer become attached to the active site therefore no enzyme-substrate complex can be formed.
  • change in pH can cause the bonds that maintain the enzymes tertiary structure (disulphide, ionic & hydrogen) to break. The enzymes active site changes shape of the active site and the substrate will no longer be able to fit. The enzyme is denatured.

Effect of substrate concentration - If the amount of enzyme is fixed & the substrate is slowly added, the rate of reaction will be proportional to the amount of substrate added. - low substrate concs, the enzymes only have a few substrate molecules to collide with therefore the enzymes are not working to their full capacity. - increase substrate concs, enzymes gradually become filled, until the point where they all are working as fast as they can. Maximum rate of reaction. - After the addition of more substrates, there will be no effect on the rate, when there is excess substrate the rate levels off, all the active sites are occupied at one time.

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Enzyme Inhibitation

Competitive Inhibitation -

  • similar in shape to the substrate molecule.
  • compete with the substrate for the active site.
  • inhibitor is not permanently bound to the active site.

If the substrate concentration is increased the effect of the inhibitor in decreased, as there is more chance of the substrate occupying the active site. Non - Competitive Inhibitation -

  • attach themselves to the enzyme away from the active site.
  • the inhibitor alters the shape of the enzyme and its active site.
  • the substrate molecule can no longer occupy the active site.

The inhibitor and the substrate are not competing for the active site, therefore an increase in substrate conc will not reduce the effect of the inhibitor. Some non-competitive inhibitors are reversible others irreversible.

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Metabolic Pathways -

End product inhibits the first enzyme, if there is an increase in the end product then there would be a greater inhibitation of the enzyme. Therefore less product will be produced from the enzyme. The amount of products is controlled by end-product inhibitation.

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Natalie Beard


You've missed out starch absorption, this is pretty vital, in every test i've done so far its been included, any chance you could stick it in? :) Thanks, aside from that, nice resource :)



Starch digestion is on page 6, other then that I think thats all I have to know! Unless there is something else about starch that I don't have notes about? Thanks

Natalie Beard


Yup, starch absorption is different from digestion and it was on the specimen paper we did in class in the last week of term, heres a short summary from my revision cards:

Glucose and sodium are co-transported into the epithelial cells of the villi via active transport and then moves into the capilary via facilitated diffusion, the capillaries of the villi drain into the hepatic portal vein and the glucose is taken to the liver.

AQA Specification:

:) sorry for being picky, aside from that its a really nice set of cards :)



Very nice and concise, thanks for the effort!



Thanks for this great resource :) it will really help with my ISA tomorrow.



thanks youu

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