AQA Biology Unit 1 Revision Notes

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Biology Unit 1 – Section 3.1.1

Microorganisms as Pathogens To be considered a pathogen it must:

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 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 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 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


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

the risk

Factors that increase the risk of CHD 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



Main parts of the digestive system

(image from teachpe.com)

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

into maltose

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

Two types of digestion, physical and chemical: 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.


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 Polymer

Glucose carbohydrates

Amino acids Proteins

Glycerol and fatty acids 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 1. Make sure the sample is liquid either naturally (already in liquid form) or by grinding it up with water

2. Add an equal volume of benedict’s solution to the sample

3. Heat in a water bath

4. If reducing sugar present the solution will turn orange-brown

Others colours may also happen:

Concentration of reducing sugar Colour of precipitate

None Blue

Very low Green

Low Yellow

Medium Brown

High 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


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 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 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 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 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


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 Enzymes have a rigid shape

The substrate is the exact complimentary shape to fit the active site

Limitations:

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

Induced fit model 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 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


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



Feature Optical Microscope TEM (transmission electron microscope)

SEM (scanning electron microscope)

Uses to get image Light Electrons Electrons

Wavelength Light has long wave length

Electrons have short wave length

Electrons have short wave length

Magnification low high High (but less than TEM)

Resolution ~200nm ~0.1nm ~10-20nm

Colour images Yes No No

2D or 3D 2D 2D 3D

Thin samples needed?

Depends – light needs to penetrate the sample

Yes yes

Vacuum needed No Yes yes

Complex staining process

No 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

ctsizeofobje

eSizeofimagionmagnificat

ionmagnificat

esizeofimagctsizeofobje

Resolution 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

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 cells are broken open to release organelles

done in a homogeniser

called homogenate


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

Ultracentrifugation 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 Organelle that is in the pellet

Low Nuclei

Medium Mitochondria

High Ribosomes

Structure of an epithelial cell

The nucleus

(Image from buzzle.com)

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


Mitochondria

(Image from microbewiki.kenyon.edu)

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:

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:

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:

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


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.

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 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)


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

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:

Hydrophilic head – which is attracted to water

Hydrophobic tail – which orients itself away from water

Test for lipids Known as the emulsion test

1. Take a test tube

2. To 2cm depth of the sample add 5cm depth of ethanol

3. Shake tube to dissolve lipid

4. Add 5cm depth of water

5. A cloudy white colour indicates the presence of a lipid


Cell Surface Membrane

(image from biologyguide.net)

Proteins – two types 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 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: 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


Facilitated diffusion

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 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

Water enters the cell

Cell swells and bursts

Hypotonic solution (with respect to cell)

Solution has equal water potential to the cell

No net movement of water particles

No change in cell

Solution is isotonic

Solution has lower water potential than cell

Water leaves cell

Cell shrinks

Solution hypertonic


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 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 1. Carrier proteins accept the molecule to be transported

2. Molecule enters one side of the transport protein

3. Molecule binds to carrier protein

4. ATP is used to swivel the protein in the membrane

5. Molecule is released on the other side of the membrane

6. 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.


Co-transport of glucose

(image from physioweb.uvm.edu)

First sodium ions are pumped out of the cell which requires ATP. This is due to the action of the sodium-

potassium 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) Eukaryotic Cells (animals and

plants)

Nucleus No Yes

Nucleolus No Yes

Membrane Bound organelles No Yes

Ribosomes Yes – 70S Yes – 80S

Flagella Yes No

Peptidoglycan cell wall Yes No (cellulose cell wall in plants)

Capsule Yes No

Cell Membrane Yes Yes

http://physioweb.uvm.edu/gi_physiology/digest_absorb/images/na_glucose_cotransport.gif


(image from bacterianamehere.pbworks.com)

Cholera

How cholera causes disease 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:

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)



Structure of the Lungs

(image from tcmcentral.com)

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.


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

nRateVentilatioeTidalVolumentilationPulmonaryV


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:

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:

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 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 Caused by scars forming on the epithelium of the lungs

Thickens the lung epithelium irreversibly


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 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 Localised allergic reaction

Caused by the release of histamine when an allergen such as pollen is detected

Histamine has the following effects:

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 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 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 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



Structure of the heart

(Image from alabelleddiagramofthehumanheart.net)

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)


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 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 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 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



Barriers to entry 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

Defence Mechanisms

Non-specific

(response is immediate and the same for all

pathogens)

Physical Barrier

(e.g. Skin)

Phyagocytosis

Specific

(response is slower and specific to each

pathogen)

Cell Mediated response

(T-lymphocytes)

Humoral Response

(B-lymphocytes)


Phagocytosis

(image © Nelson Thornes AQA AS biology text book)


Lymphocytes – The differences


T Lymphocytes



B Lymphocytes


Antibody structure

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:

Separation of a chemical from a mixture

Calculating the amount of substance in a mixture (pregnancy tests)


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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AQA Biology Unit 1 Revision Notes

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