The Digestive System

Digestive system, digestive enzymes, polymers, monomers, proteins, carbohydrates

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The Digestive System Overview

  • (1) Mouth- is has salivary glands which secrete saliva (mucus, mineral salts + salivary amylase) which helps to lubricate the food being broken up by teeth.
  • (2) Oesophagus is a tube that takes food from the mouth to the stomach using muscle contractions called peristalisis. Food is lubricated by mucus secreted by the wall tissue
  • (3) Stomach- It has lots of folds allowing it to expand, the entrance and exits are controlled by sphincter muscles. The stomach wall produces gastric juice (HCl, pepsin + mucus) which helps to break down food. The pepsin hydrolyses proteins into smaller polypeptide chains. The HCl provides acidic conditions for this to work. Peristalsis of the stomach churns food into chyme (acidic fluid)
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  • (4) Small Intestine is in two parts
    • Duodenum- Bile and pancreatic juice neutralise the acidity of the chyme and break it down into smaller molecules. The pancreas secretes the pancreatic juice (amylase, trypsin, chromotypsin, lipase and sodium hydrocarbonate- which neutralises the HCl)
    • Ileum- The smaller soluble molecules are absorbed through the villi in the gut wall via diffusion, facilitated diffusion and active transport.
  • (5) Large Intestine- Absorbs water, salts and minerals. It has a folded wall to increase the surface area for absorption. Bacteria decompose the undigested nutrients.
  • (6) Rectum- Faeces are stored in the rectum and pass through the sphincter muscles at the anus during defecation.
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Digestion breaks down the large molecules into smaller molecules, as most of the molecules in our food are polymers (which are large molecules composed of long chains of monomers, smaller molecular units) Polymers are insoluble, which means they cannot be directed absorbed into the blood stream and cannot be assimulated into new products.

Polymers have to be hydrolysed. Hydrolysis is when a water molecule is added in aid to break the bond in a polymer chain. This occurs during digestion and is catalysed by digestive enzymes.


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Digestive Enzymes

Carbohydrases catalyse the hydrolysis of carbohydrates. The main ones are:

Starch ---- + amylase -----> Maltose 

Maltose ---- + maltase -----> 2x Glucose

Sucrose ---- + sucrase -----> Glucose + Fructose

Lactose ---- + lactase -----> Glucose + Galactose

Proteases catalyse the hydrolysis of proteins.

Protein ---- + tripsin/pepsin -----> Peptides

Lipases catalyse the hydrolysis of lipids.

Lipids---- lipase -----> Fatty acids + Glycerol

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Proteins are made up of long chains of amino acids. Amino acids are the monomers of proteins.


Amino acid structure:


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Polypeptides and Biuret Test

Polypeptides are formed by condensation reactions. This is when a molecule of water is released in aid for the amino acids to form a petide bond. This reaction is the opposite of a hydrolysis reaction.


Biuret Test is used to find out if a substance contains protein.(

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Protein Structure


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Proteins shape determines its function


The main structural proteins to understand are haemoglobin and collagen. Haemoglobin is a compact protein used for transporting oxygen, it is very compact. Collagen has 3 highly coiled peptide chains, which are very strong making it useful for supportive tissue in animals.

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Most carbohydrates are polymers made up of monosaccharides containing the elements C, H, O. E.g glucose, fructose, galactose.

Monosaccharides are joined together by condensation reactions to form disaccharides or monosaccharides. A glycosidic bond forms between monosaccharides are broken down in digestion where they are hydrolysed.

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Lactose Intolerance and Benedict's Test

Lactose is a sugar found in milk and it is digested by the enzyme lactase, found in the intestines. If there isn't enough lactase to break down the lactose then it is called lactose intolerance. Undigested lactose is fermented by bacteria, which can cause problems such as flatulence, stomach cramps and diarrhoea.

It's uncommon to be lactose tolerant- but milk can be treated with purified lactase.

Benedicts test is used to see if a sugar is present (sugar is a term for mono/disaccharides)


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Starch is made up of the polysaccharides amylose and amylopectin. These are both long chains of glucose linked by glycosidic bonds. It is digested into maltose by amylase (which is released by the pancreas and the salivary glands) The maltose is then broken down by maltase into alpha glucose molecules.

The Iodine test is used to test whether starch is present.

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