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

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.

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

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

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?

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

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

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

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.

• 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

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

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

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

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

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

• 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

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

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

• 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

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

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

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.

• 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