The Digestive System

AQA bio unit 1

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Break Down of molecules.

Digestion breaks down larger molecules into smaller molecules.

Many of the molecules in our food are polymers -  large complex molecules made up of long chains of monomers - small basic molecular units

Proteins and some carbohydrates are polymers. In carbohydrates, the monomers are called monosaccharides. They contain the elements, carbon, hydrogen and oxygen. In proteins the monomers are called amino acids - containing carbon. oxygen. nitrogen and hydrogen.

Polymers in our food are insoluble, therefore cannot be absorbed directly into our bloodstream and made into new products. They have to be hydrolised, broken into smaller molecules by the addition of water.This process happens during digestion and it catalysed by digestive enzymes.

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Specific functions within the digestive system

Oesophagus - The tube that takes food from the mouth to the stomach, using waves of muscle contractions called peristalsis. Mucus is secreted from the tissues in the walls to lubricate the food's passage downwards.

The stomach - A small sac. Its has lots of folds, allowing the stomach to expand, a 4ltre capacity. The entrance and exit to the stomach is controlled by spincter muscles. The stomach walls produce gastric juice contraining hydrochloric acid, pepsin(enzyme) and mucus - helping the breakdown of large molecules. Pepsin hydrolyses proteins into smaller polypeptide chains. Pepsin only works in acidic conditions. Peristalsis of the stomach turns food into an acidic fluid called chyme.

The small intestine - to main parts, 1) the duodenum and 2) the ileum. Chyme is moved along the small intestine by peristalsis. In the duodenum, bile which is produced by the pancreas to neutralise the acidity of the chyme and break it down into smaller molecules. The ileum, the small, soluble molecules e.g glucose and amino acids are absorbed through structures called villi that line the gut wall. Molecules are absorbed by diffusion, facilitated diffusion and active transport.

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Pancres and Salivary Glands

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

Salivary Glands: 

Three main parts of the saliary glands in the mouth - They secrete saliva that consists of mucus, mineral salts and salivary amylase. Salivary amylase breaks down starch into maltose (a disaccharide). Saliva also lubricates food making it easier to swallow.

The pancreas - releases pancreatic juice into the duodenum of the small intestine  through the pancreatic duct. Pancreatic juice contains amylase, trypsin, chymotrypsin and lipase - it also contains sodium hydrogencarbonate which neutralises the acidity of hydrochloric acid from the stomach.

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Enzymes in digestion

Carbohyrases - catalyes the hydrolysis of carbohydrates. Proteases catalyse the hyrolysis of proteins. Lipases catalyse the hydrolysis of lipids.

LOCATION        ENZYME            CLASS             HYDROLYSIS                INTO

salivaryglands       amylase                   carbohydrase                  starch                            maltose

stomach                pepsin                      protease                         protein                             peptides

pancreas             amylase                    carbohydrase                 starch                             maltose

                              trypsin                     protease                         protein                            peptides

                            chymotrypsin            protease                       protein                        peptides                    

                             lipase                       lipase                              lipids                                amino acids

ileum (small i)      maltase                     carbohydrase                 maltose                          glucose

                           sucrase                    carbohydrase               sucrose                      glucose&fructose

                         lactose                       carbohydrase              lactose                    glucose &galactose

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Proteins are made from long chains of amino acids - monomers of proteins are amino acids.

A dipeptide is formed when two amino acids join together, a polypeptide is formed when two or more amino acids join together - proteins are made up of one or more polypeptides.

Different amino acids have different variable groups(R groups). All amino acids have the same general structure, a carboxyl group (-cooh), an amino group (-NH2) attached to a carbon atom . The difference between amino acids is the R group they contain. e.g Glycine, the smallest amino acid's R group is a H atom.

Polypeptides are formed by condensation reactions - a molecule of water is released during the reaction, the bonds between the amino acids formed is called a peptide bond. (between the C of one acid and the N of another.)The reaction is reversed when water is added ( hydrolysis)

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Protein Structure

Proteins have four structural levels

Primary - is the sequence of the amino acids in the polypeptide chains

Secondary - the polypeptide chain doesn't remain flat and straight. Hydrogen bonds form between the amino acids in the chain, automatically coiling up the chain into an alpha helix or foliding up into a beta pleated sheet

Tertiary - the coiled or folded chain of amino acids has often coiled and folder further. More bonds form between different partd of the polypeptide chain. For proteins made from a single polypeptide chain, the tertiary structure forms their final 3D structure.

Quaternary - some proteins are made from several polypeptide chains held together by bonds. The quaternary structure is the way the chains are assembled together. For proteins made from more than one chain this is their final 3D structure.

Proteins fuction is defined by their shape - for example, haemoglobin is compact and soluble which makes it easy to transport, great for carrying around oxygen. Collagen has 3 chains held tightly together making it strong, great supportive tissue in animals.

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Protein Function

Proteins have different shapes and sizes which makes them specialised to carry out particular jobs.

Enzymes - roughly spherical, tight folding polypeptide chains, soluble, metabolism - breakdown large molecules. Some synthesise large molecules.

Antibodies - involved in the immune system, two short polypeptide chains and two heavy chains bonded together, antibodies have variable regions, meaning the amino acids sequences in these regions vary lots

Transport proteins - present in all cell membranes, hydrophobic and hydrophillic amino acids, causing protein to fold up and form a channel. These proteins transport molecules and ions across membranes

Structural proteins - strong, long polypeptide chains lying parallel to each other with cross links between them. Structural proteins include keratin(nails and hair) and collagen (connective tissue)

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Biuret Test for Proteins

In order to find out if a food sample contained protein you would use the biuret test.

2 stages to the test:

1) The solution needs to be alkaline, so first add sodium hydroxide solution

2) Then add copper(ii) sulphate solution.

If a protein is present a purple layer forms

If no protein is present the solution will remain blue.

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Carbohydrates contain the elements C, H and O. The monomers they are made from are monosaccaharides such as glucose, fructose and galactose.

Monosaccharides join together to form disaccharides and polysaccharides. The monosaccharides are joined together by condensation reaction - water is released and a glycocidic bond is formed.

Disaccharides and polysaccharides are broken down during digestion. Maltose -> glucose + glucose.Sucrose -> glucose and fructose. Lactose -> glucose and galactose.

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Quite useful :P 

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