The Digestive System

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Digestive System Organs 1


Uses waves of contractions called peristalsis which make the food travel down the tube. Mucus is secreted from tissues in the tissues to lubricate the food's passage downwards.


Has lots of folds for it to expand. Stomach walls produce gastric juice which contains hydrochloric acid, pepsin and mucus. Pepsin hydrolyses proteins into smaller polypeptide chains. This only works in acidic conditions (by HCL). Peristalsis of the stomach turns food into chyme (acidic fluid).

Small Intestine
Chyme is moved along by peristalsis. In the duodenum, bile and pancreatic juice neutralise the acidity of the chyme and break it down into smaller molecules. In the ilium, the small soluble molecules are absorbed through structures such as villi that line the gut wall. Molecules are absorbed by diffusion, faciliated diffusion and active transport.

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Digestive System Organs 2

Large Intestine
Absorbs water, salts and minerals. It has a folded wall which provides a large surface area for absorption. Bacteria which decompose some of the undigested nutrients are found in the large intestine.

Faeces are stored in the rectum and then pass through sphincter muscles at the anum during defacation.

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The Salivary Glands and the Pancreas

Glands along the digestive system release enzymes to help break down food.

Salivary Glands

In the mouth, they secrete saliva that consist of mucus, mineral salts and salivary amylase. Salivary amylase breaks down starch into maltose. Saliva also helps to lubricate food, making it easier to swallow.


Releases pancreatic juice into duodenum through the pancreatic duct. Pancreatic juice contains amylase, trypsin, chymotrypsin and lipase. Also contains sodium hydrogencarbonate which neutralises the acidity of hydrochloric acid from the stomach.

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Enzyme Reactions

Salivary Glands

-Amylase hydrolyses starch into maltose


-Pepsin hydrolyses proteins into peptides


-Amylase hydrolyses starch into maltose

-Lipase hydrolyses lipids into fatty acids and glycerol


-Maltase hydrolyses maltose into glucose

-Sucrase hydrolyses sucrose into glucose and fructose

-Lactase hydrolyses lactose into glucose and galactose

-Peptidase hydrolyses peptides into amino acids

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Monomers of proteins are amino acids. A dipeptide is two amino acids joined together by a condensation reaction by a peptide bond. Polypeptide, more than two.

Amino acids have a carboxyl group (-COOH), an amino acid group (-NH2) attached to a carbon atom. The variable which makes a difference in an amino acid is the R group. The reverse happens during digestion.

The primary structure is a sequence of amino acids in a polypeptide chain.

The secondary structure is still a chain of amino acids but with hydrogen bonds forming between the amino acids in the chain. This makes it automatically coil into an alpha helix or fold into a beta pleated sheet. 

The tertiary structure is coiled and folded further. More bonds form between different parts of the polypeptide chain. For proteins made from a single polypeptide chain, the tertiary structure forms their final 3D structure.

The quaternary structure is when it is made from several different amino acid chains held together by bonds. The quaternary structure is the way these polypeptide chains are assembled together. The quaternary structure is the proteins last structure when made from lots of polypeptide chains.

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Biuret Test

Biuret Test

The test solution needs to be alkaline, so first you add a few drops of sodium hydroxide solution.

Then you add copper sulfate solution.

If protein is present, a purple layer forms.

If there is no protein then the solution will stay blue. The colours are pale so you need to look carefully.

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Carbohydrates contain, hydrogen, oxygen and carbon. They are made from monosaccharides such as glucose, fructose and galactose. Alpha glucose has the H on top and for beta it is the other way round.

They join together by conensation reactions and have a glycosidic bond.

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Lactose Intolerance

Lactose is a sugar found in milk, it is digested by lactase, found in the intestine.

If you dont have enough of the enzyme lactase, you will not be able to break down the lactose in milk properly so will have a lacose intolerance.

Undigested lactose is fermented by bacteria and can cause lots of intestinal problems such as cramps.

Milk can be artificially treated with purified lactase to make it suitable for lactose-intolerant people.

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Reducing and Non-Reducing Sugars

Reducing Sugars

For all monosaccharides and some disaccharides.

You add benedicts reagent to a sample and heat it.

If the sample contains reducing sugars then it gradually turns brick red due to the formation of a red precipitate.

Non-reducing Sugars

You first have to break them down into monosaccharides. This is done by boiling the test solution with dilute hydrochloric acid and then neutralising it with sodium hydrogencarbonate.

Then carry out the benedicts test like for reducing sugars.

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Starch is made up of a mixture of two polysaccharides. Both are composed of long chains of alpha glucose linked together by glycosidic bonds, formed in condensation reactions.

When starch is digested, its broken down into maltose by amylase (produced in the salivary glands and pancreas. Maltose is then broken down into alpha glucose molecules by maltase which is released by the intestinal epithelium.

Iodine Test

Add iodine dissolved in potassium iodide soluton to the test sample.

If there is starch present, the sample changes from browny-orange to a dark, blue-black colour.

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Enzyme Action

Enzymes have an active site which has a specific shape. The active site is the part of the enzyme where the substrate molecules (the substance that the enzyme interacts with) bind to. Enzymes are highly specific due to their tertiary structure.

In a reaction a certain amount of energy is needed to start the reaction. This is activation energy - often provided as heat. Enzymes lower the amount of activation energy needed, lowering the temperature usually needed. This speeds up the reaction.

When a substrate fits into the enzymes active site it forms an enzyme-substrate complex. It is this which lowers the activation energy, as:

-If two substrate molecules need to be joined, being attached to the enzyme holds them close together, reducing the force needed to bring them together so they can bond more easily.

-If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate , so the substrate molecules break up more easily.

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Models of Enzyme Action

Lock and Key Model

Where the substrate fits into the enzyme in the same way a key fits into a lock.

However it doesnt give a full story, as scientists found that enzyme-substrate complexes slighly change shape to make it fit.

Induced Fit

The substrate doesnt have to be only the same shape, but make the enzyme's active site change shape in the right way as well.

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Enzyme Properties

As enzymes are specific, they can only catalyse one reaction. This is because only one substrate will fit in the active site.

The active sites shape is determined by its tertiary strucuture.

Each different enzyme has a different tertiary structure and so a different shaped active site. If the substrate doesnt match the shape, the reaction will not be catalysed.

If the tertiary structure of a protein is altered in any way, the shape of the active site will also change. This means the substrate won't fit into the active site and the enzyme will not longer be able to carry out its function.

The tertiary structure may be altered by changes in the pH or temperature.

The primary structure of a protein is determined by a gene. If a mutation occurs in that gene, it could change the tertiary structure of the enzyme produced.

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Factors Affecting Enzyme Activity - Temperature

Rate of any chemical reaction increases when the temperature increases, as there is more kinetic energy, so the molecules move faster, meaning more collisions. However if the temperature gets too high, the reaction stops.

A rise in temperature makes the enzyme molecules vibrate more. If the temperature goes above a certain point, this vibration breaks some of the bonds that hold the enzyme in shape. The active site changes shape and the enzyme and substrate no longer fit together. At this point the enzyme is denatured and so no longer catalyes the reaction.

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Factors Affecting Enzyme Activity - pH and Concent

Above and below the optimum pH, the H and OH ions found in the acids and alkalis can mess up the ionic bonds and hydrogen bonds. These hold the tertiary structure together. This means the active site changes shape, so the enzyme is denatured.

The higher the substrate concentration, the faster the reaction. As, more molecules means more collisions can happen and so a faster reaction.
This is only true up to a 'saturation' point. After that there are so many molecules that the enzymes have as much as they can cope (the active sites are full), so adding more makes no difference.

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Competitive and Non-Competitive Inhibitors

Competitive Inhibition
They have a similar shape to that of the substrate molecules. They compete with the substrate molecules to bind to the active site, but no reaction takes place. Instead they block the active site so no substrate molecules can fit in it.
How much the enzyme is inhibited depends on the concentration of the inhibitor and substrate. If there is a high concentration of the inhibitor, it will take up nearly all the active sites and hardly any of the substrate will be able to bind.
Non-Competitive Inhibition
Bind to the enzyme away from the active site. This causes the active site to change shape so the substrate molecules can no longer bind to it. They don't compete with the active site because they are a different shape. Increasing the enzyme of substrate won't make any difference as the enzyme activity will still be inhibited.

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