Explain How Microorganisms Enter The Body

Biology Unit 1 – Section 3.1.1
Microorganisms as Pathogens
To be considered a pathogen it must:
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Gain entry
Colonise the tissues
Resist the defences
Cause damage to the tissues

Pathogens include bacteria, viruses and fungi

How do microorganisms enter the body
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Many pathogens enter through the gas exchange system (including ones that cause flu and
TB)
Food and water can carry pathogens into the stomach and intestines via the mouth and into the digestive system (including ones that cause cholera)

Preventing pathogens entering
 Mucous layer that covers the exchange surfaces and forms a thick sticky barrier that is difficult to penetrate
 Enzymes that break down pathogens
 Stomach acid which kills pathogens
How do pathogens cause disease
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By damaging host tissues – the sheer number of pathogens causes damage and stops tissues functioning properly – e.g. viruses stop DNA and RNA synthesis
By producing toxins – most bacteria produce toxins which cause damage to the body – e.g. the cholera bacterium produces a toxin which leads to diarrhoea

Correlations and causal relationships
REMEMBER – CORRELATION DOESN ’T MEAN CAUSATION
Data that shows there is a correlation between two variables e.g. cancer and smoking can never prove that smoking is the cause of cancer so therefore always look at it critically as it could be that stress is the cause and people smoke to help with stress nothing can be explicitly proved! Factors that increase the risk of cancer
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Smoking – if they smoke the risk is higher
Diet – low fat and high fibre rich in fruit and vegetables lowers risk
Obesity – if someone is overweight the risk is higher
Physical activity – more of this lowers risk

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Sunlight – more someone is exposed to sunbeds or sunlight (without sun cream) the greater the risk

Factors that increase the risk of CHD
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If you smoke
If you have high blood pressure
If your blood cholesterol is high
If you’re obese
Diet – if you have a high amount of salt this increases blood pressure which increases the risk and if you have a high amount of saturated fatty acids this increases blood cholesterol which increases risk
If you don’t do a lot of physical activity

Therefore you can reduce the risk of CHD and cancer by:
 Giving up or not taking up smoking
 Avoiding becoming overweight
 Reducing salt intake in the diet
 Reducing saturated fats in the diet
 Doing regular exercise
 Keeping alcohol consumption within the recommended limits
 Increasing intake of fibre and antioxidants in the diet

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Biology Unit 1 – Section 3.1.2

Main parts of the digestive system
(image from teachpe.com)

Roles of the major parts of the digestive system
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Mouth – starts carbohydrate digestion by adding amylase in saliva to it braking large carbohydrates down into maltose

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Stomach – contains enzymes (proteases) which digest proteins breaking them down into amino acids. It also produces a lot of mucus to prevent the stomach itself from getting digested by its own enzymes
Small intestine – where most digestion happens. Lots of enzymes secreted either into it or by its walls.
However it is adapted for the absorption of digestive products also occurs here.
Pancreas and salivary glands – produce enzymes, salivary glands produce saliva in the mouth which contains amylase and the pancreas produces pancreatic juice which contains proteases, lipase and amylase. To digest proteins, lipids and starch

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Two types of digestion, physical and chemical:
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Physical digestion is food being broken down into smaller pieces by both the teeth and the churning of the stomach. This not only enables us to swallow food but increases the surface area of the food so enzymes can act on it better
Chemical digestion is breaking down large insoluble molecules into smaller soluble ones. It is done by a process called hydrolysis, this is carried out by enzymes.

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Monomers and polymers
Monomers are the building blocks of the large polymer. They are small molecules which are ‘fused’ together in a series of condensation reactions to form a polymer
Monomer
Glucose
Amino acids
Glycerol and fatty acids

Polymer carbohydrates Proteins lipids Monosaccharides
Best known of these is glucose (C6H12O6)
These are all reducing sugars. A reducing sugar is one that can donate electron to (or reduce) another chemical.

Benedict’s test for reducing sugars
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Make sure the sample is liquid either naturally (already in liquid form) or by grinding it up with water
Add an equal volume of benedict’s solution to the sample
Heat in a water bath
If reducing sugar present the solution will turn orange-brown

Others colours may also happen:
Concentration of reducing sugar
None
Very low
Low
Medium
High

Colour of precipitate
Blue
Green
Yellow
Brown
Red

Glycosidic Bond
(Thanks to biology-innovation.co.uk for the diagram)

This shows two alpha glucose molecules linking together in a condensation reaction and removing the water to form a glycosidic bond

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Test for non-reducing sugars
These sugars don’t reduce the benedict’s solution so you need to break them down first. This is done by adding hydrochloric acid to your sample and heating it in a water bath. Then add sodium hydrogencarbonate to the sample to neutralise the acid. Test with pH paper to check it’s alkaline then carry out the test for reducing sugars (above)

Test for Starch
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Place some of the sample in a test tube
Add iodine solution (a few drops)
Presence of starch is indicated by the sample going blue-black

Starch Digestion
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Starts in the mouth by being chopped up by the teeth and mixed with saliva
Saliva contains amylase which breaks starch into maltose
Starch digestion then continues in the small intestine
Pancreatic juice contains amylase and other enzymes
Amylase continues to break starch down into maltose
However maltose isn’t a useful product for the body so maltase breaks this down into glucose which can then be absorbed

Sucrose and lactose digestion
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These both pass into the small intestine
In the small intestine the enzyme sucrase breaks down sucrose into glucose and fructose
The enzyme lactase breaks down lactose into glucose and galactose
These products can then be absorbed

Lactose intolerance
Lactose is found in milk. Some people can’t produce then enzyme lactase due to a faulty gene. This means as they don’t have the enzyme Lactose can’t be broken down and therefore it can’t be digested so these people have to avoid lactose containing foods such as milk and milk products.

Proteins
Formation of a peptide bond
Water is removed and the two amino acids join together, this happen lots of times and a polymer is formed.

Protein structure
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Primary structure – amino acid sequence – the order of the different amino acids in the peptide chain
Secondary structure – the coiling or folding of the protein to make either an alpha-helix or a beta pleated sheet – this is achieved by the formation of hydrogen bonds between the –NH and –C=O groups in the individual amino acids
Tertiary structure – the alpha helices and beta pleated sheets can fold and twist more to give a complex and often unique 3-d structure – this is achieved by disulphide bonds, ionic bonds and hydrogen bonds
Quaternary structure – the final 3-d structure of the different polypeptide chains and sometimes non – protein groups as well (e.g. haemoglobin – the haem group isn’t a protein chain but the rest is 4 protein chains) Bonds
Disulphide bonds – really strong and not easily broken
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Ionic bonds – formed between different polypeptides, weaker than disulphide bonds and are broken easily by changes in pH
Hydrogen bonds – numerous but easily broken

Enzymes
Enzymes are catalysts. A catalyst works by lowering the amount of energy you have to put into a reaction in order for it to happen (the activation energy). They do this by having an active site that the molecule or molecules can bind to for the reaction to happen. The molecule that enters the enzymes active site is the substrate.

Lock and key model
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Enzymes have a rigid shape
The substrate is the exact complimentary shape to fit the active site

Limitations:
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Enzymes not rigid structures as other molecules can bind to other sites on the enzyme and change it’s shape

Induced fit model
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Now accepted model
Enzyme isn’t rigid but flexible
Shape changes slightly of the active site to accommodate the substrate
Moulds around the substrate like a glove
Enzyme has a certain general shape but alters slightly when the substrate is present
This puts a strain on the substrate molecule
This strain distorts bonds in the substrate which helps lower the activation energy required to break these bonds Factors that affect enzymes
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pH – as this alters the charge on the amino acids in the active site and the substrate can no longer bind and make enzyme-substrate complexes
Temperature – this increases the kinetic energy of molecules and initially speeds up the rate of an enzyme controlled reaction, however when the temperature is too high the bonds (especially hydrogen bonds) begin to break and this causes the shape of the enzyme to change and the active site changes shape and can no longer accept substrate molecules.
Substrate concentration – increasing this initially will increase the rate of reaction but a pointy will come when all the active sites are full and no matter how much you increase the substrate concentration the rate of reaction will stay the same

DENATURATION
When an enzyme is said to be denatured then its active site has changed shape permanently and it can no longer function. Inhibition
Two types competitive and non-competitive
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Competitive
 Similar molecular shape to substrate
 Compete with substrate for active sites
 Inhibitor not permanently bound to active site so just slows down the reaction
 The greater the concentration of the inhibitor the greater the effect
 Similarly the lower the concentration of the substrate the greater the effect
Non-competitive
 Fit in a site other than the active site
 Alters the enzymes shape so substrate molecules fit into the active site but not in a way that allows the reaction to proceed
 Enzyme can’t function
 As non-competitive altering the substrate concentration will not decrease the effect of the inhibitor

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Biology Unit 1 – Section 3.1.3
Feature

Optical Microscope

Uses to get image
Wavelength

Light
Light has long wave length low
~200nm
Yes
2D
Depends – light needs to penetrate the sample
No
Yes
No
Yes

Magnification
Resolution
Colour images
2D or 3D
Thin samples needed? Vacuum needed
Complex staining process TEM (transmission electron microscope)
Electrons
Electrons have short wave length high ~0.1nm
No
2D
Yes

SEM (scanning electron microscope) Electrons
Electrons have short wave length
High (but less than TEM)
~10-20nm
No
3D
yes yes yes

Because of the complex staining process TEM and SEM images may contain artefacts which are things that result from the way the specimen is prepared and appear on the finished micrograph (the image obtained with the microscope) but are not part of the natural specimen.

Magnification magnificat ion 

sizeofobje ct 

Sizeofimag e sizeofobje ct

sizeofimage magnificat ion

Resolution
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The minimum distance apart two objects can be for them to appear as separate items
Depends on the wavelength used
Increasing magnification will increase the size of the image but won’t always increase the resolution- every microscope has a limit.

Cell Fractionation
Preparation
Tissue is placed in ice cold isotonic buffer solution before cell fractionation can begin
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Ice cold – to slow down enzyme activity that may break down the organelles
Isotonic – to prevent a water potential gradient forming which would cause organelles to burst or shrink
Buffer solution – to create a constant pH

Homogenation
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cells are broken open to release organelles done in a homogeniser called homogenate

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then filtered to remove any cells and large pieces of cell membrane

Ultracentrifugation
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tube containing filtered homogenate placed in ultracentrifuge and spun at whatever speed desired the organelles fall to the bottom at different speeds depending on their mass each time a pellet and supernatant will be formed
Pellet contains the organelles ( that have been separated by mass)
Supernatant can be re-spun to separate more organelles

Speed
Low
Medium
High

Organelle that is in the pellet
Nuclei
Mitochondria
Ribosomes

Structure of an epithelial cell
The nucleus

(Image from buzzle.com)
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Nuclear envelope – double membrane that surrounds the nucleus it controls the entry and exit of materials in and out of the nucleus and contains the reactions happening within
Nuclear pores – allows the passage of large molecules such as messenger RNA out of the nucleus
Nucleoplasm – like cell cytoplasm – a granular jelly like substance that makes up the bulk of the nucleus (in the middle on the diagram but not labelled)
Chromatin – is the DNA found in the nucleoplasm it only turns to chromosomes when the cell is replicating
The Nucleolus – small spherical structure within the nucleus that manufactures RNA and assembles ribosomes Functions of the nucleus:
 Controls the cell
 Retain the genetic material of the cell as DNA or chromosomes
 Manufacture RNA and ribosomes

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Mitochondria

(Image from microbewiki.kenyon.edu)
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Double membrane – surrounds the organelle the outer one controlling entry and exit of materials and the inner one folded to give a large surface area (the folds are called cristae)
Cristae – provide large surface area for the attachment of enzymes involved in respiration
The matrix – fluid that is semi-rigid and contains protein lipids and some DNA that allows it to produce the enzymes involved in respiration

Function
They are the site of certain stages of respiration so are responsible for the production of ATP.

Endoplasmic Reticulum (ER)
Two types:
Rough ER (RER)
Has ribosomes present on its outer surface. Its functions are to:
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Provide a large surface area for the synthesis of proteins
Provide a pathway for the transport of materials throughout the cell

Smooth ER (SER)
Lacks ribosomes on its surface. Its functions are to:
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Synthesise, store and transport lipids
Synthesise, store and transport carbohydrates

Golgi apparatus (also called Golgi Body)
Transport centre of the cell – transports materials across the cell surface membrane using vesicles. It’s functions are to: 
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Add carbohydrates to proteins to make glycoproteins
Produce secretory enzymes (ones that work outside the cell) such as those in pancreatic juice
Secrete carbohydrates such as those used in making cell walls in plants

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Transport modify and store lipids
Form lysosomes

Lysosomes
Lysosomes are vesicles made by the Golgi body that contain enzymes. They are used to isolate these potentially harmful enzymes from the rest of the cell and make sure they just act on what they are supposed to either outside the cell or inside.
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Release enzymes to break down pathogens (phagocytic cells)
Release enzymes outside of the cell (e.g. proteases in the stomach)
Digest worn out organelles
Break down cells after they have died

Ribosomes
Two types 80s and 70s. 80s found in eukaryotic cells, 70s found in prokaryotic cells.
They are important in protein synthesis

Lipids
Roles
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Plasma membrane – provide the flexibility in the plasma membrane and enable lipid soluble materials to pass through it
An energy source – when oxidised lipids provide more than twice the energy of the same mass of carbohydrate Waterproofing – lipids aren’t soluble in water so are effective waterproofing for example a plant’s waxy cuticle on the leaf
Insulation – they are slow conductors of heat so help the body stay warm
Protection – they are often stored around delicate organs such as the kidneys

Triglycerides

(image from raw-milk-facts.com)
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The formula for glycerol needs to be remembered and how it links to the fatty acids but the structure of the fatty acids doesn’t need to be remembered
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Saturated – no carbon to carbon double bonds
Mono-unsaturated – 1 carbon to carbon double bond
Poly-unsaturated – more than 1 carbon to carbon double bond

Phospholipids
Similar to triglycerides except only 2 fatty acids attached and a phosphate is also attached

(image from uber-life.net)
They have:
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Hydrophilic head – which is attracted to water
Hydrophobic tail – which orients itself away from water

Test for lipids
Known as the emulsion test
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Take a test tube
To 2cm depth of the sample add 5cm depth of ethanol
Shake tube to dissolve lipid
Add 5cm depth of water
A cloudy white colour indicates the presence of a lipid

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Cell Surface Membrane

(image from biologyguide.net)

Proteins – two types
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Extrinsic – occur on either side of the phospholipid bilayer but don’t go all the way through (labelled as peripherial proteins on diagram)
Intrinsic – go through the bilayer from one side to the other (labelled as integral proteins on diagram)

Function of the proteins is to provide structural support and act as carrier to transport things through the membrane. This is called the fluid mosaic model
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Fluid – because the individual phospholipids can move relative to each other
Mosaic – because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern like a mosaic

Diffusion
Diffusion is the net movement of molecules or ions from an area of high concentration to an area of low concentration. Rate of diffusion depends on:
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Concentration gradient – the greater the difference in concentration between the two sides of the exchange surface the faster the molecules or ions will move
Area over which diffusion takes place – the larger the area of an exchange surface the faster the rate of diffusion Thickness of an exchange surface – the thinner the exchange surface the faster the rate of diffusion

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Facilitated diffusion

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Passive process so requires no energy from respiration
Channels are selective – only open for one particular molecule
Water filled channels so only transport things that are water-soluble
The rate of this is effected by the same factors as diffusion but also effected by the number of pores available to a particular substance

Osmosis
The passage of water from a region of high water potential to a region of low water potential through a partially permeable membrane

Water potential
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Pure water has a water potential of 0
If you add anything to this the water potential decreases
Water potential always a negative value

Water potential and cells
Solution has higher (less negative) water potential than the cell
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Water enters the cell
Cell swells and bursts
Hypotonic solution (with respect to cell)

Solution has equal water potential to the cell
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No net movement of water particles
No change in cell
Solution is isotonic

Solution has lower water potential than cell
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Water leaves cell
Cell shrinks
Solution hypertonic

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Active Transport
The movement of molecules or ions into or out of a cell from a region of low concentration to a region of high concentration using ATP and carrier molecules

Features
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Requires energy in the form ATP
Molecules or ions moved against a concentration gradient
Carrier proteins needed
Carrier proteins are specific so only certain molecules can be transported this way

Active transport of a single molecule
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Carrier proteins accept the molecule to be transported
Molecule enters one side of the transport protein
Molecule binds to carrier protein
ATP is used to swivel the protein in the membrane
Molecule is released on the other side of the membrane
This caused the carrier protein to go back to normal

Villi and microvilli
Villi
Villi are the folds inside the lining of the small intestine these increase the surface area available for the absorbtion of the products of digestion. Finger like projections

Microvilli

(image from poohbah.ndo.co.uk)
Microvilli are finger-like projections from the surface of the cells lining the intestine. They increase the surface area even more for absorption of digestive products.

Glucose Absorption
Diffusion occurs and glucose into the cells along a concentration gradient. However this can’t always occur as eventually the concentration of glucose inside the cell and in the small instestine will be the same and no more moves into the cell as there isn’t a concentration gradient. This would lead to a massive loss of glucose which wouldn’t be able to be absorbed. This is where co-transport comes in.
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Co-transport of glucose

(image from physioweb.uvm.edu)
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First sodium ions are pumped out of the cell which requires ATP. This is due to the action of the sodiumpotassium pump.
This means a concentration gradient is established which means more sodium ions can be drawn into the cell A co-transport protein in the cell membrane is used which can transport both glucose and sodium ions so more glucose is also drawn in with the sodium ions

Prokaryotic vs. Eukaryotic
Feature

Prokaryotic cells (bacteria)

Nucleus
Nucleolus
Membrane Bound organelles
Ribosomes
Flagella
Peptidoglycan cell wall
Capsule
Cell Membrane

No
No
No
Yes – 70S
Yes
Yes
Yes
Yes

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Eukaryotic Cells (animals and plants) Yes
Yes
Yes
Yes – 80S
No
No (cellulose cell wall in plants)
No
Yes

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(image from bacterianamehere.pbworks.com)

Cholera
How cholera causes disease
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Almost all the cholera bacteria are killed by the stomach acid but due to sheer numbers some may survive and continue onto the small intestine
They use their flagella to propel themselves through the mucus lining
They then start to produce a toxic protein which causes the ion channels in the cell to open and chloride ions that are usually stored within the cell flood into the lumen of the small intestine
This causes the water potential in the small intestine to drop which draws water in from the surrounding cells and this causes diarrhoea

ORS
Oral rehydration solutions (ORS) which is also called oral rehydration therapy (ORT) is the treatment for cholera. It contains: 
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Water (for rehydration)
Sodium ions (to replace those lost from the epithelium)
Glucose (to provide energy and stimulate the uptake of sodium ions by co transport)
Potassium ions (to replace those lost and to stimulate appetite)
Other electrolytes (prevents electrolyte imbalance)

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Biology Unit 1 – Section 3.1.4
Structure of the Lungs

(image from tcmcentral.com)
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Air flows into the lungs through the trachea which is a flexible airway supported by rings of cartilage
Then into the bronchi which are similar in structure to the trachea and contain mucus to trap dirt and dust particles and pathogens to stop them entering the lungs – also have cilli which are tiny hairs that get rid o0f this mucus which has dust trapped in it.
Air then travels into the bronchioles which are smaller divisions of the bronchi, they are made of muscle which allows them to constrict and therefore control the flow in and out of the alveoli
The air then travels into the alveoli which are tiny air sacs. These allow the oxygen to diffuse into the blood and for the carbon dioxide in the blood to diffuse out and into the air.

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Mechanism of breathing
Inspiration (or inhalation)

 The external intercostal muscles contract (muscles on the outside of the ribcage) and the internal ones relax.
 This forces the ribs upwards and outwards increasing the volume of the chest cavity
 The diaphragm also contracts and flattens also increasing the volume of the chest cavity
 This increased volume leads to a reduction in the pressure in the lungs
 This then draws air from outside into the lungs
Expiration

 The internal intercostal muscles contract while the while the eternal ones relax
 This moves the ribs down and in decreasing the volume of the chest cavity
 The diaphragm muscles relax too and this returns it to its upwardly domed position which again decreases the volume of the chest cavity
 This increases the pressure on the lungs and air is forced out into the atmosphere
(Images from tutorvista.com)

Pulmonary ventilation
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A measure of how much air is taken in and out of the lungs in a given time

PulmonaryVentilation  TidalVolum e  Ventilatio nRate
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Features of exchange surfaces
The alveoli are exchange surfaces as they allow oxygen and carbon dioxide to be exchanged to and from the blood
Exchange surfaces must be:
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Large surface area to volume ratio – speeds up rate of exchange
Very thin – short diffusion pathway
Partially permeable – to be selective about what can be transported
There must be movement – (e.g. of blood and air) to maintain a steep concentration gradient (speeds up diffusion) Alveoli have this in order to efficiently allow oxygen to diffuse into the blood and allow carbon dioxide to diffuse out

Pulmonary TB
It is caused by bacteria (m. bovis and m. tuberculosis ) and is spread via infected people coughing or sneezing droplets into the air and another person breathing this in.

Transmission
People are more likely to suffer from TB if:
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They are in close contact with infected individuals over long periods, e.g. living and sleeping in overcrowded conditions If they work or live in long-term care facilities where relatively large numbers of people live close together
(e.g. care homes, hospitals)
They are from countries where TB is common
They have reduced immunity (e.g. the very young or very old, people with AIDS or other medical conditions that lower their immunity)

Course of infection
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Bacteria grow and divide in the upper regions of the lungs were there is more oxygen
The body’s immune system responds and white blood cells accumulate at the site of infection to destroy the bacteria This leads to inflammation and the enlargement of the lymph nodes at that region of the lungs. This is the primary infection and in a healthy individual there are few symptoms if any and the infection is controlled in a few weeks, however some bacteria usually remain.
These can re-emerge and cause a second infection which typically occurs in adults
This also happens in the upper part of the lungs but this time isn’t son easily controlled
The bacteria destroy the tissue of the lungs this results in cavities and when the lungs then repair these scar tissue arises
The sufferer then coughs up the damaged lung tissue containing bacteria and if left untreated the infection spreads to the rest of the body and can be fatal
Pulmonary Fibrosis
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Caused by scars forming on the epithelium of the lungs
Thickens the lung epithelium irreversibly

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Diffusion of oxygen into the blood is less efficient as diffusion pathway is longer
The volume of air the lungs can contain is also reduced
Reduced elasticity so difficulty ventilating the lungs (getting air in and out)

Effects of fibrosis
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Shortness of breath especially when exercising – due to less oxygen being able to diffuse into the blood
Chronic dry cough – because the fibrous tissue obstructs the airway
Pain and discomfort in the chest – due to pressure and damage from the mass of fibrous tissue
Weakness and fatigue – due to the reduced intake of oxygen into the blood so less available for respiration

Asthma
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Localised allergic reaction
Caused by the release of histamine when an allergen such as pollen is detected

Histamine has the following effects:
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The lining of the airways becomes inflamed
The lining cells (epithelial cells) secrete more mucus than usual
Fluid leaves the capillaries and enters the airways
The muscle surrounding the bronchioles contracts and constricts the airways

Effects of Asthma
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Difficulty breathing - due to all the effects above having the overall effect of a much greater resistance to air flowing into the lungs
A wheezing sound when breathing – due to air passing through the very constricted airways
A tight feeling in the chest – consequence of not being able to ventilate the lungs properly due to constricted airway
Coughing – reflex response to the obstructed bronchi and bronchioles in an effort to clear them

Emphysema
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Normal lungs contain lots of elastic tissue which helps them inflate and deflate
Emphysema normally develops in smokers
With emphysema the lung tissue’s elastin is permanently stretched and this means the lungs are no longer able to force air out of the alveoli
Surface area of the alveoli is reduced and they sometimes burst
As a result little of any gas exchange can take place across the stretched and damaged air sacs

Effects of Emphysema
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Shortness of breath – concentration gradients are reduced as air is struggled to be forced from the lungs and replaced with fresh oxygen rich air
Chronic cough – body’s effort to remove the damaged lung tissue and mucus that cannot be removed naturally due to the cilla being destroyed
Bluish skin coloration – due to the low levels of oxygen in the blood as a result of poor gas exchange

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Biology Unit 1 – Section 3.1.5
Structure of the heart

(Image from alabelleddiagramofthehumanheart.net)
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Aorta – carries oxygenated blood to the body
Vena cava – carries deoxygenated blood to the heart from the body
Pulmonary artery – carries deoxygenated blood to the lungs
Pulmonary Vein – carries oxygenated blood from the lungs to the heart

Cardiac Cycle

(image © nelson thornes AS AQA Biology textbook)
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Control during the cardiac cycle
The cardiac cycle is controlled by two nodes that pass electrical activity to each other and the muscle in order to make the heart contract – these are the AVN (atrioventricular node) and the SAN (sinoatrial node)  A wave of electrical activity spreads from the SAN to both atria causing them to contract
 A layer of non-conductive tissue stops this passing to the ventricles
 This wave of electrical activity then passes to the AVN which delays sending the response
 This is to allow the atria to fully empty and the atrioventricular valves to close preventing the blood from flowing back into the atria
 The AVN after this short delay then sends a wave of electrical activity to the bundles of his. These are exposed muscle fibres in the ventricle walls.
 This then makes the ventricles contract quickly and at the same time forcing the blood out of the heart

Heart Disease
Atheroma
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Fatty deposit that forms on the wall of an artery
Accumulations of white blood cells that have taken on LDL’s (low density lipoproteins)
These enlarge and form an atheromatous plaque on the wall of the artery
Made up of cholesterol, fibres and dead muscle cells
These cause the lumen to narrow restricting the flow of blood
This increases the chances of thrombosis and aneurysms

Thrombosis
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Formed when an atheroma breaks through the lining of the artery
This forms a rough surface
As the body tries to repair this a blood clot may be formed
This a thrombus
This my block the blood vessel reducing or preventing the supply of blood to tissues beyond it
This tissue normally dies as a result of lack of oxygen and nutrients such as glucose
The clot may also detach and move with the blood and could block another artery such as the coronary artery starving the heart of oxygen

Aneurysm
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This is caused by a thrombosis as it weakens the artery walls
These weakened points swell to form a balloon like blood filled structure called an aneurysm
These frequently burst causing to haemorrhaging and massive blood loss
If this happens in the brain it is called a stoke

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Biology Unit 1 – Section 3.1.6
Defence Mechanisms

Non-specific

Specific

(response is immediate and the same for all pathogens) (response is slower and specific to each pathogen) Physical Barrier

Cell Mediated response

(e.g. Skin)

(T-lymphocytes)

Phyagocytosis

Humoral Response
(B-lymphocytes)

Barriers to entry
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Protective covering – skin – physical barrier pathogens find hard to penetrate
Epithelia covered in mucus – pathogens get stick in the mucus and removed from the body by the cilia
Hydrochloric acid in the stomach – provides such a low pH enzymes in most pathogens are denatured and then the pathogens die

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Phagocytosis

(image © Nelson Thornes AQA AS biology text book)

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Lymphocytes – The differences

(image © Nelson Thornes AQA AS biology text book)

T Lymphocytes

(image © Nelson Thornes AQA AS biology text book)

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B Lymphocytes

(image © Nelson Thornes AQA AS biology text book)

Antibody structure

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Polypeptide chains (protein)
Antigen binding sites are specific to one antigen
Like enzymes but antigen-antibody complexes are formed

Monoclonal antibodies
Normally there are lots of different antibodies in your system so they can respond to lots of different pathogens.
However for some uses antibodies can be used but we need them to target one specific antigen so this antibody is grown outside the body and they are used for:
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Separation of a chemical from a mixture
Calculating the amount of substance in a mixture (pregnancy tests)

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Cancer treatment
Transplant surgery (stop the organ being rejected)

Vaccination
Vaccination is the introduction of a substance into the body with the intention of the body producing antigens against it and destroying it in order to produce memory cells so that when the pathogen enters the body again you don’t have any of the symptoms of the disease

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