unit 1, section 2 ; DIGESTION

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what is the chemical compisiton of food

most food molcules are polymers- large and complex

polymers are composed of monmers - small and basic molecules

in carbs. the monomers are monossachrides

in proteins the monomers are amino acids

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explain the 2 types of digestion

physical - food broken down in the mouth by the teeth,          also in the stomach where movement of the stomach muscles continues to break it down      smaller pieces = larger SA = faster chemical digestion

chemical - polymers are insoluble - cant be absorbed directly into bloodstream-  the polymers have to be hydolysed inro smaller soluble molecules by adding water, hydrolysis is catalysed by digestive enzymes

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what are the 6 parts of the digestive system

1 mouth

2 oesphagus

3 stomach

4 small intestine

5 large intestine

6 rectum

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explain the role of the mouth in digestion

teeth break down the food

the tongue is used to push food down into the oesphagus

saliva is secreted to make food easier to swallow , also contains enzymes so chemnical digestion can begin

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role of the oesphagus in digestion?

tube that takes food from the mouth to the stomach, using waves of muscle contraction called peristalsis

mucus is secreted from tissues in the walls to lubricate food

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role of the stomach ?

small sac

lots of folds allowing it to expand

extrance and exit to the stomach controlled by sphincter muscles

stomach walls produces gastric juice - (HCl , pepsin, mucus )

pepsib hydrolyses proteins - pepsin only works in acidic conditions

perastalsis of the stomach turns food into an acidic fluid called chyme

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role of the small intestine?

2 main parts - duodenum and ileum
chyme is moved along the small intestine by peristalsis

1. duodenum - bile (alkaline) + pancreatic juice neutralise the acidity of the chyme and break it down into smaller molecules

2. ileum - small soluble molecules are absorbed through villi that line the wall
molecules are absorbed by diffusion , f.diffusion, + active transport

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role of the large intestine?

absorbs water salts and minerals

has a folded wall = increase SA

bacteria that decompse undigested nutrients are found here 

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role of the rectum?

faeces stored in the rectum

then pass through the sphincter muscles of the anus

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glands involved with digestion

salivary glands -   secrete saliva which lubricates food ,  saliva contains mucus, mineral salts + salivary amylase . Salivary amylase breaks down starch into into maltose (dissachride)

pancreas - releases pancreatic juice into the duodenum . pancreatic juice contains enzymes + sodium hydrogen carbonate which neutralises acidity of the HCl from the stomach

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Types of digestive enzyme

Carbohydrates- catalysts hydrolysis of carbs.

Proteases- catalysts hydrolysis of proteins

Lipases - catalysts hydrolysis of lipids

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The digestion of carbohydrates

Salivary glands - amylase. Hydrolyses starch into maltose

Stomach - none

Pancreas - amylase Hydrolyses starch into maltose

Ileum - Maltase > maltose into glucose
Sucrase > sucrose into glucose + fructose
Lactase > lactose into glucose + galactose

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The digestion of proteins

Salivary glands - none

Stomach - pepsin > protein into peptides

Pancreas- trypsin > protein into peptides
Chymotrypsin > protein into peptides
Carboxypeptidase > peptides into amino acids

Ileum - peptidase> peptides into amino acids

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The digestion of lipids

Pancreas - lipase > lipids into fatty acids and glycerol

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What are proteins made from

Monomers of a protein - amino acids

A dipeptide - 2 amino acids joined together

A polypeptide - more than 2 amino acids joined together

Protein - one or more polypeptide

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Amino acid structure

A carboxyl group COOH

An amino group NH2

The difference between amino acids is the variable group

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Polypeptide formation

Amino acids link together via condensation reactions , forms polypeptides

A molecule,of H2O is released during this reaction

Bonds between amine acids are peptides bonds

The reverse reaction is hydrolysis

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The 4 levels of protein structure

1. Primary = sequence of amino acids in the polypeptide chain

2. Secondary = H-bonds form between the amino acids in the polypeptide chains, this makes it coil into an alpha helix or fold into a beta pleated sheet

3. Tertiary =. Coiled or folded chain is coiled or folded even further as more bonds form between different parts of the polypeptide chain , forms a 3D globular shape

4. Quaternary =. Proteins made of several different polypeptide chains. Haemoglobin, insulin, collagen Quaternary is the way the polypeptide chains are assembled together

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Protein shape and function

Shape of a protein determines its function
Enzymes - roughly spherical in shape, tertiary structure, soluble , some enzymes break down food others help to synthesise large molecules

Antibodies - immune response , 2 short polypeptide chains and 2 long polypeptide chains joined together . Antibodies have variable regions - amino acid sequences vary greatly

Transport proteins - cell membranes, hydrophobic and hydrophilic amino acids which cause the protein to fold up and form a channel , transport molecules and ions across membranes

Structural proteins - strong ; long polypeptide chains lying parallel to each other with cross links between them , keratin and collagen , collagen has 3 polypeptide chains all coiled tightly together

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How do you test for proteins

Biuret test
1. Test solution need to be alkaline = add a few drops of sodium hydroxide solution
2. Then add copper (Ii) sulfate solution

If a protein is present the solution turns purple
If no protein present, the solution will remain blue

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What are carbohydrates made from?

Most carbs are polymers

Monomers = monosaccharides eg glucose

Glucose is a hexose sugar, = a monosaccharides with 6 C atoms

There are two forms of glucose - alpha and beta


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Carbohydrate formation

Monosaccharides joined together by condensation reactions
A molecule of H2O is released and a glycosidic bond forms between two monosaccharides
A disaccharides = 2 monosaccharides joined together
A polysaccharide = more than 2 monosaccharides joined

Alpha glucose + alpha glucose = maltose

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Digestion of carbohydrates

Disaccharide. Hydrolysed by. Into
Maltose Maltase. Alpha glucose x2
Sucrose. Sucrase. Alpha glucose + fructose
Lactose. Lactase. Alpha glucose + galactose

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Lactose intolerance

Lactose = sugar found in milk
Digested by enzyme lactase
If you don't produce enough of the enzyme lactase , you won't be able to digest the milk properly = lactose intolerance
Undigested lactose is fermented by bacteria = stomach cramps, flatulance, diarrhoea
Common - affects 15% of Northern Europeans
Milk can be treated with purified lactase to make it suitable for intolerant people

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How to test for sugars

A sugar is = monosaccharides + disaccharides
All sugars are reducing or non-reducing

Reducing sugars -
All monosaccharides and some disaccharides
Heat sample with Benedictus reagent , if the sample stays blue no reducing sugar is present
If the sample turns brick red , there is a reducing sugar present

If no reducing sugar is present the you have to test for a non-reducing sugar -
First have to break down the monosaccharides,- new sample
Boil it with HCl , then neutralise it with sodium hydrogen carbonate , then heat sample with Benedictus reagent
If the sample stays blue , there is no non-reducing sugar present
If the sample turns brick red, there is a non-reducing sugar present

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Starch basic info

A mixture of 2 polyssacrhides , amylose and amylopectin

Both are long chains of alpha glucose linked by glycosidic bonds
When starch is digested , its first broken into maltose by amylase [salivary glands and pancreas]
Maltose is then broken down into alpha glucose by Maltase [ileum]

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How do you test for starch?

Iodine test

Add iodine dissolved in potassium iodide solution to the sample

If there is no starch present - the sample remains browny orange

If there is starch present - simple changes to a dark blue colour

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How do enzymes speed up reactions

Enzymes lower the amount of activation energy required for the chemical reaction to begin eg. Lower temperature starts the reaction = speeds up the rate

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how do enzymes lower the activation energy

When a substrate and enzyme join, a substrate-enzyme complex is made
This is what lowers the activation energy for 2 reasons

1. If 2 substrate molecules need to be joined, being attached to the enzyme holds them close together, reducing any repulsion between the 2 substrate molecules so that they can bond easily

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

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What are the two models of enzyme action

The lock and key model -
Substrate fits with the enzyme like a key fits a lock , complementary shapes and very specific , the enzyme remains unchanged after the reaction

The induced fit model-
Helps to explain enzyme specificity , the substrate has to have the complementary shape and has to make the enzymes active site change in the correct way , this locks the substrate even more tightly to the enzyme as the active site undergoes a conformational change

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Properties of an enzyme

Related to its tertiary structure
Very specific- they only catalyse one reaction
Only one substrate fits the active site - active site determined by tertiary shape
If the tertiary structure is altered in any way the shape of the active site will change = substrate will no longer fit into the active site so the enzyme can no longer carry out its function
Mutation can effect enzyme shape as it effects primary structure
If there is no enzyme-substrate complex the reaction won't be catalysed

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How can enzyme activity measured activity?

1. Measuring the amount of product produced
Different molecules at the end of a chemical reaction that there are at the beginning
Measuring the amount of end product present at different times = reaction rate can be calculated

2. Measuring the amount of substrate left
During a reaction substrate molecules are used up
Measuring the amount of substrate left at different time = reaction rate can be calculated

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How does temperature affect enzyme activity?

Temperature increase= increase in rate of reaction
More heat = more kinetic energy , so molecules move faster = substrate molecules are more likely to collide with enzymes active site

The energy of the collision also increases which means each collision is more likely to result in a reaction

BUT - if the temperature becomes too high = the reaction stops
The temperature rise makes the enzymes vibrate more
It the temp. Rises above a certain level, the vibration breaks some bonds in the enzymes tertiary structure = active site changes shape
The substrate no longer fits with the enzyme = no enzyme-substrate complex
Every enzyme has an optimum temperature

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How does pH affect enzyme activity?

All enzymes have an optimum pH eg pepsin pH2
Above and below the optimum pH the H+ and OH- ions found in acids and alkalis can disrupt the ionic bonds and the hydrogen bonds that hold the enzymes tertiary structure in place
the enzymes becomes DENATURED
The active site has changed shape, there is no longer an enzyme substrate complex

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How does substrate conc. affect enzyme activity?

Higher the substrate concentration = the faster the reaction
More substrate molecules means that a collision between substrate and enzyme is more likely = more enzyme substrate complexes

Only true until saturation point -
There are more substrate molecules than free active sites
Adding more substrate will no longer increase the rate of reaction.

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What are enzyme inhibitors?

Competitive inhibitors-
Competitive inhibitor molecules have a similar shape to that of substrate molecules
They compete with the substrate molecules to bind to the active site = no reaction will take place
Competitive inhibitors block the active site so no substrate molecules can fit in it
If there's a Hugh concentration of the CI molecules, it'll take up nearly all the active sites
If the is a high concentration Of substrate, less CI molecules , = less inhibition

Non- competitive inhibitors-
Non competitive inhibitors bind to the enzyme away from its active site = the active site changes shape, so the substrate molecules can no longer fit
Non competitive inhibitor don't compete for the active site because they are a different shape
Increasing substrate molecules concentration won't make a difference

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