AQA Biology AS Unit 1: Cause of Disease/ Enzymes 1 & 2

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1 Causes Of Disease

Pathogens

  • Microorganisms include bacteria, fungi and virus.
  • Some microorganisms can be beneficial to our health.
  • Certain microorganisms cause disease, these are known as pathogens.
  • Disease is a description of certain symptoms which can be physical or mental.

Microorganisms as Pathogens

  • To be a pathogen a microorganism must:
    • Gain entry to the host.
    • Colonise the tissues of the host. 
    • Resist defences of the host. 
    • Cause damage to the host tissues. 
  •  When a pathogen gets into a host and colonises this is an infection.
  • Disease occurs when infection leads to symptoms.
  • Transmission is when a pathogen is transferred to someone.
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Pathogens continued

How do microorganisms get into the body?

  • Pathogens enter by penetration an organism's interfaces
  • The skin acts as a barrier to infection since it is thick and continuous
  • If the skin is broken e.g. cut, pathogens can get in
  • The body linings are thin, moist, have large surface area and have good blood supply, this allows entry for pathogenic microorganisms.
  • Entry points include:
  • Gas Exchange System e.g. pathogens like influenza and tuberculosis
  • Digestive System e.g. food and water carries pathogens into stomach like cholera and typhoid.
  • The body has natural defences:
  • Mucous layer covers exchange surfaces to form barrier
  • Enzymes break down pathogens
  • Stomach acid kills microorganisms
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Pathogens Continued

How Pathogens cause disease

  • Pathogens affect the body in two main ways:
  • Damaging host tissues
  • Producing toxins
  • Diseases can have many causes such as pathogens, lifestyle and genetics
  • The Faster a pathogen divides the quicker symptoms appear
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1.2 Data and Disease

Correlations and Causal Relationships

  • Correlation occurs when a change in one of two variables is reflected by a change in the other
  • When just one factor has caused a change in a variable it is said to be causal
  • If there are other factors to consider then there is a correlation NOT causation

Is the Data reliable?

  • Was the right factor measured which the right questions asked?
  • How was data gathered?
  • Who is collecting the data do they have a motive?
  • Has the study been repeated with similar results?
  • Still unanswered questions?
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1.3 Lifestyle and Health

What is Risk?

  • A measure of the probability that damage to health will occur as a result of a given hazard
  • Risk has two elements: probability of a hazardous event and consequence of the event

Cancer Risk Factors

  • Causal factors can be age and genetic
  • Some factors like lifestyle can be changed
  • Things that contribute to cancer:
  • Smoking
  • Diet
  • Obesity
  • Little Physical activity
  • Sunlight
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Lifestyle and Health Continued

Coronary Heart Disease (CHD)

Lifestyle Factors

  • Smoking
  • High Blood Pressure: due to prolonged stress, certain diets and lack of exercise
  • Blood Cholesterol Levels: reduced by lowering saturated fatty acids
  • Obesity
  • Diet: high salt raises blood pressure, high saturated fatty acids increase blood cholesterol
  • Physical Activity: Aerobic lowers blood pressure and cholesterol, reduces obesity

All these factors can be changed reducing the risk of CHD and Cancer

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2 Enzymes and the Digestive System

Enzymes and Digestion

  • Glands of the digestive system produce enzymes
  • The enzymes break large molecules into smaller ones for absorption
  • The digestive system provides an interface due to having food substances entering it

Major parts of the Digestive System

  • Oesophagus: carries food from mouth to stomach, made up of thick muscular wall
  • Stomach: muscular sac with inner layer that produces enzymes, stores and digests food, glands in the stomach produce mucus stops the stomach digesting its enzymes
  • Small Intestine: Long muscular tube, food is further digested, inner walls folded into villi giving large surface area, villi have microvilli allowing for absorption of digestion products into blood
  • Large Intestine: absorbs water forming the faeces
  • Rectum: Faeces are stored before removed via egestion
  • Salivary Glands: near mouth, contain amylase which breaks down starch
  • Pancreas: large gland below stomach, produces secretion containing proteases, lipases and amylase
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Enzymes and Digestion Continued

What is Digestion

  • It takes place in two ways:
    • Physical Breakdown: food broken down into smaller pieces e.g. by teeth, creates a large surface area, food is also churned by stomach muscles
    • Chemical Digestion: Large insoluble molecules broken down using enzymes by hydrolysis
  • Enzymes are specific so more than one is needed to break large molecules
  • Carboydrases: break down carbohydrates to monosaccharides
  • Lipases: Break down lipids to glycerol and fatty acids
  • Proteases: Break down proteins to amino acids
  • The products are absorbed into the bloodstream and then used to form large molecules again.
  • The molecules are incorporated into body issues which is assimilation.
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2.2 Carbohydrates- Monosaccharides

  • Carbohydrates contain carbon, oxygen and hydrogen.
  • Carbon readily bonds with other carbons allowing various sequences of carbon.
  • Every living thing is based on carbon

Making Large Molecules

  • Individual molecules that make up a chain is a monomer.
  • The carbon atoms join to form longer chains known as polymers
  • Polymers can be made up of carbon, hydrogen, oxygen and nitrogen
  • In carbohydrates the basic monomer unit is sugar (saccharide)
  • Single monomer is a monosaccharide, a pair is a disaccharide and larger are known as polysaccharides
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Carbohydrates- Monosaccharides Continued

Monosaccharides

  • Sweet tasting, soluble substances
  • General formula (CH2O)n
  • Glucose is a monosaccharide , it is a hexose sugar (C6H12O6)

Test for Reducing Sugars - Benedict's Test

  • Monosaccharides and some disaccharides are reducing sugars
  • Reduction involves the gain of electrons
  • A reducing sugar is a sugar that can donate electrons, in this case Benedict's reagent 
  • Benedict's is an alkaline solution of Copper (II) Sulfate
    • Add the food sample to a test tube making sure it is liquid
    • Add equal Benedict's reagent
    • Heat the mixture gently for 5 minutes it should turn orange/brown
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2.3 Carbohydrates-- Disaccharides and Polysacchari

Disacchaides

  • Glucose-Glucose = Maltose
  • Glucose-Fructose = Sucrose
  • Glucose-Galactose = Lactose
  • The reaction to form a disaccharide is called a condensation reaction since a molecule of water is removed, a glycosidic bond is formed
  • When water is added to a disaccharide the glycosidic bonds are broken this is hydrolysis

Test for Non Reducing Sugars

  • Disaccharides e.g. sucrose are non reducing sugars
  • To detect them you must first break them down into the monosaccharides
  • Add Benedict's solution to the liquid food sample and gently boil
  • If it remains blue a non reducing sugar is present, add hydrochloric acid and boil
  • The acid hydrolysis the disaccharide, add sodium hydrogencarbonate to neutralise it
  • Retest by adding Benedict's reagent and boiling, it should bow turn orange/brown
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Carbohydrates-- Disaccharides and Polysaccharides

Polysaccharides

  • Monosaccharides are joined by glycosidic bonds to form polysaccharides
  • The bonds are formed in a condensation reaction
  • Polysaccharides are insoluble so are suitable for storage
  • Cellulose is a polysaccharide used for structural support in plant cells
  • Starch is found in many parts of plants as small granules of grains.
  • Starch is formed by linking 200 - 100000 alpha glucose molecules by glycosidic bonds

Test for Starch

  • Starch changes the colour of iodine in potassium iodide solution
  • Add the sample to a test tube then add to drops of iodine solution and shake
  • The solution should turn from yellow to blue/black at room temperature
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2.4 Carbohydrate Digestion

Starch Digestion

  • Food is chewed up by the teeth giving a larger surface area
  • Salivary glands produce amylase which hydrolyses the starch into maltose by breaking alternate glycosidic bonds of the starch
  • The food passes into the small intestine where more amylase hydrolyses the starch
  • Epithelial lining of the small intestine produces maltase which hydrolyses the maltose into alpha glucose

Disaccharide Digestion

Sucrose

  • Sucrose is usually contained in cells so the teeth physically break them down
  • Epithelial lining releases sucrase which hydrolyses the single glycosidic bond producing the monosaccharides glucose and fructose
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Carbohydrate Digestion

Lactose

  • It is found in milk and is digested in the small intestine by lactase
  • Lactase hydrolyses the glycosidic bond that links glucose and galactose

Lactose Intolerance

  • Since milk forms a lower part of adult diets lactase production diminishes.
  • This reduction can be so drastic that they produce little to no lactase
  • If undigested lactase reaches the large intestine microorganisms break it down.
  • The microorganisms produce gas which causes bloating, nausea and cramps
  • Main difficulty is getting enough calcium and vitamin D in absence of milk
  • Special none milk food is provided
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2.5 Proteins

  • Proteins are very large molecules
  • Enzymes are proteins which are involved in almost every living process

Structure of an Amino Acid

  • Amino acids are basic monomer units that combine to form the polymer, polypeptide
  • Polypeptides combine to form proteins
  • Every amino acid has a central carbon atom that has; an amino group (NH2), carboxyl group (COOH), hydrogen atom and R group

The Formation of a peptide bond

  • Amino acids combine to form dipeptides due to condensation reactions
  • The OH from the carboxyl group and the H from the other amino acid join.
  • A peptide bond between the carbon and nitrogen forms
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Proteins Continued

The Primary Structure - Polypeptides

  • Many amino acids can join in condensation in a process called polymerisation
  • Hundreds of amino acids joined form a polypeptide
  • The sequence of amino acids in a polypeptide chain forms the primary structure
  • There are almost limitless combinations and primary protein structures
  • Primary Structure determines  ultimate shape and function
  • Changing a single amino acid in the sequence leads to a shape change thus a stop in function

The Secondary Structure

  • The linked amino acids posses both NH and C=O groups on either side of every peptide bond
  • The Hydrogen of the NH has an overall positive charge and the O of the C=O has a negative charge, these groups form hydrogen bonds
  • This causes the polypeptide chain to twist into a 3D shape known as an alpha helix
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Proteins Continued

Tertiary Structure

  • The Alpha helix can be twisted even more this is the tertiary structure
  • Bonds that are included:
  • Disulfide bonds: fairly strong
  • Ionic bonds: formed between carboxyl and amino groups (not involved in peptide bonds)
  • Hydrogen bonds: easily broken
  • The 3D shape is important as it makes the protein distinctive and allows it to recognise, and be recognised by other molecules

Quaternary Structure

  • Large molecules often form complex molecules containing individual polypeptide chains
  • Also non-protein (prosthetic) groups associated with the molecules

Test for Proteins

  • Add sodium hydroxide solution to the sample add few drops of dilute copper (II) sulphate and mix, if a protein is present goes from blue to purple
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2.6 Enzyme Action

  • Enzymes are globular proteins that act of catalysts
  • They speed up the reaction that occur

Enzymes as Catalysts lowering activation energy

  • For reactions to take place:
  • The substrates must collide with sufficient energy to change arrangement of their atoms
  • Energy of products must be less than substrates
  • Activation Energy: minimum energy required to activate the reaction

Enzyme Structure

  • Enzymes specific shape is due to sequence of amino acids
  • The active site of the enzyme is a small region, it forms a small hollow depression within the larger enzyme molecule
  • When the substrate and enzyme bind it is called the enzyme-substrate complex
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Enzyme Action Continued

Lock and Key model of enzyme action

  • Enzymes work like a lock and key
  • The enzyme has a fixed shape that the substrate fits into
  • This idea is limited as it suggests that enzymes are rigid however it is actually flexible

Induced fit model of enzyme action

  • Enzymes change shape to fit the substrate, it moulds itself around it
  • As it changes shape it puts pressure on the substrate which distorts a particular bond lowering the activation energy
  • This model is better than the lock and key since it explains better how other molecules affect enzyme activity and how activation energy is lowered
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2.7 Factors affecting enzyme action

Temperature

  • Temperature rise increases the kinetic energy of molecules
  • Molecules move more rapidly and collide more often
  • After a certain temperature the enzymes become denatured
  • Denaturation is a permanent change that stops the enzyme functioning

pH

  • pH measures the hydrogen ion concentration
  • Each enzyme has an optimum pH
  • pH change alters charges on the amino acids that make up the active site stopping it from allowing the substrate to bind
  • Cause bonds to break that maintain tertiary structure causing it to change shape
  • Arrangement of the active site depends partly on the hydrogen and ionic bonds between NH and COOH groups of the polypeptides that form the enzymes
  • Change in H+ ions affects the bonding changing the active site
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Factors affecting enzyme action continued

Effects of Substrate Concentration on the Rate of Enzyme Action

  • If enzyme amount is fixed at a constant level and substrate slowly increases the rate of reaction increases.
  • Since with low substrate concentration enzymes have limited substrate to collide with so active sites won't work to full capacity
  • More substrate added the active sites become filled until they work as fast as they can
  • Rate of reaction reaches its maximum
  • When there is excess substrate the rate of reaction levels off.
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2.8 Enzyme Inhibition

  • Enzyme Inhibitors directly/indirectly interfere with the active site functioning
  • There are two reversible inhibitors:
  • Competitive: bind to active site
  • Non-Competitive: bind to other position

Competitive Inhibitors

  • Molecular shape similar to substrate so can occupy the enzyme
  • They compete with the substrate for the active site
  • If substrate concentration is increased the effect of the inhibitor is reduced
  • The inhibitor isn't permanently bound so other molecules can replace it
  • Example: The respiratory enzyme that acts on succinic acid can be bound to malonic acid instead blocking the succinic from combining with the active site

Non-Competitive Inhibitors

  • This inhibitor alters the shape of the enzyme active site so substrate cannot attach
  • An increase is substrate doesnt decrease the inhibitor's effect
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Comments

Phil Murphy

Covers key points

Changavy

this is really helpful-thank u!

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