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

Covers chapter 1 and 2

HideShow resource information

1 Causes Of Disease


  • 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.
1 of 22

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
2 of 22

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
3 of 22

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?
4 of 22

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
5 of 22

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

6 of 22

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
7 of 22

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.
8 of 22

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
9 of 22

Carbohydrates- Monosaccharides Continued


  • 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
10 of 22

2.3 Carbohydrates-- Disaccharides and Polysacchari


  • 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
11 of 22

Carbohydrates-- Disaccharides and 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
12 of 22

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 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
13 of 22

Carbohydrate Digestion


  • 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
14 of 22

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
15 of 22

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
16 of 22

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
17 of 22

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
18 of 22

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
19 of 22

2.7 Factors affecting enzyme action


  • 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 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
20 of 22

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.
21 of 22

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
22 of 22


Phil Murphy


Covers key points



this is really helpful-thank u!

Similar Biology resources:

See all Biology resources »See all Health, illness and disease resources »