The specific immune response


A pathogen has an antigen on its surface that tells white blood cells that it is foreign.

sending distress signals

When one of your cells is infected, it will send out distress signals to generate an immune response

Antigen presentation aids clonal selection. It is performed by macrophages in the lymph nodes

Cytokines are hormone-like proteins produced by cells which are used for communication between cells, allowing some cells to regulate the activity of others

There are 3 types of cytokines:


  • macrophages release monokines
  • these attract other neutrophils by the process of chemotaxis
  • the monokines also stimulate the B cells to differentiate into B plasma cells and release antibodies


  • T cells, B cells and macrophages release interleukins
  • interleukins stimulate the differentiation of B and T cells


  • stop viruses replicating by inhibiting their replication and stimulating the activity of T killer cells

starting the immune response

  • an invading pathogen has foreign antigens
  • these are detected by the T and B lymphocytes
  • they carry correct receptor molecules on their membranes (complementary to antigen
  • however it may take time for the lymphocytes to find the antigens

Cells that are attacked by  pathogens will display antigens on their surface that attracts B and T lymphocytes. Macrophages can also become antigen-presenting cells.  This increases the chance that the correct B and T will locate the antigens.

Clonal selection

The selection for the correct B and T for the immune response is known as Clonal selection, these lymphocytes must increase in number to become effective. They divide by mitosis in a process called clonal expansion

Differentiation of T lymphocytes

they develop into:

  • T helper cells: Which release cytokines that stimulate the B cells to differentiate
  • T killer cells: which search the body for pathogens and attach to foreign antigens and secrete and produce toxi substances
  • T memory cells: cells that are involved in immunological memory. remain in body for years and become very active in secondary response

differentiation of B lymphocytes

  • plasma cells: manufacture and release antibodies
  • memory: remain in body for years and provide immunological memory



A deliberate exposure to antgenic material which activates the immune system to make an immune response and provides immunity

  • You can have vaccinations against specific diseases
  • once vaccinated you can have artificial immunity
  • the immune system will treat the antigen material as a real disease
  • this results in lymphocytes producing antibodies and memory cells that provide long term immunity

What type of antigenic material is used?

  1. Whole live, microorganisms
  • small amount of antigens, so no symptoms
  • its kept alive so it reproduces
  • body is constantly exposed to antigens
  • very effective as memory cells develop good response

2. live and harmless microorganisms

  • micro-organism is changed in a  minor way so it does not initiate a response
  • is living so it can reproduce
  • good, but not as good as just live micro-organisms

3. dead and whole microorganisms

  • micro-organism is killed with heat or chemicals, so it is still intact
  • cannot reproduce so will not be large numbers of pathogens
  • not as effective as many antigen sources

4. extracted antigens

  • when the pahtogens antigens are isolated
  • not very effective and no good if the pathogens and the antigens change
  • requires new vaccine for new strain

5. toxoids

  • some pathogens produce toxins
  • toxins are weak but still contain antigens
  • not harmful
  • only gives immunity to one particular toxin
  • boosters are required every 10 years

Vaccination is used in 2 ways:

Herd vaccination

  • provides imunity to all the population at risk and once enough people immune the disease can no longer spread
  • to erradicate small pox, 85% of the population had to be vaccinated

Ring vaccination

  • it is a response vaccination
  • if one person is reported as having the diease-
  • all people in the immediate vicinity are vaccinated

Pathogens do tend to mutate however….

  • when this happens it means that existing vaccinations will not be effective
  • when new slightly changed viruses are created, this can cause an epidemic
  • large scale outbreaks are called pandemics

Active immunity is where your immune system is activated by your lymphocyte cells. This leads to long term immunoligical memory. Passive immunity is provided by the antibodies and not made in your system. Could be provided by mum across placenta or breast milk.

Natural immunity is gained normally and with no help. Can be gained because of an infection and generates a secondary immune response.  Artificial immunity is gained by deliberate exposure to antigens or antibodies.




Non specific response to disease

The primary defences of humans

  • pathogens must enter the body
  • evolution has selected hosts to defend themselves against such entries
  • this is called our primary defence

Physical barriers to entry include:

The skin

  • contains keratinocytes produced by mitosis in the epidermis
  • once made they migrate to the surface of the skin
  • They prevent pathogens and bacteria from entering the body
  • lasts 30 days

Mucous membranes

  • found in areas of your body which are exposed to the external environment, either directly or indirectly
  • these areas are protected by mucous membranes
  • they cover and protect and provide secretory and absorptive functions
  • goblet cells stimulate mucus which traps bacteria and pathogens
  • cilia waft the mucus so it can digested and the pathogens can be destroyed

The eyes and ears

  • contains antibodies and lysosomes
  • contains wax which traps pathogens


  • kept in acidic conditions so pathogens do not enter

The secondary defence of humans

There are 2 types of phagocytes that kill pathogens that are once inside when they have reproduced and caused symptoms


  • multiobed nucleas, made in the bone marrow
  • travel in blood and squeeze through tissue fluid
  • short lived and released in large numbers when an infection occurs


  • larger than neutrophils and also made in the bone marrow
  • travel in blood as monocytes
  • located in lymph nodes and around organs
  • provides a non specific immune response
  • also initiates an immune response by activating lymphocytes

cells that have become infected with a pathogen release a chemical messenger called histamine. this attracts other neutrophils and makes your capillaries more leaky so that fluid enters your lymphatic system. So as the fluid leaves the capillaries it takes pathogens with it, into the lymhatic system where lymphocytes are waiting to engulf the pathogens.


Phagocytes engulf and destroy pathogenic cells. When a pathogen invades a body, it is recognized as foregin as it ihas foreign antigens. Antigens are specific ot the organism. Protiens called antibodies have membrane bound receptors that are complementary shape to the antigen. It attaches to the Antigen preventing it from attacking the host cell. The antibodies attaches to the phagocyte as the phagocyte has a membrane bound receptor that is complementary to the antibody. Then Phagocytosis takes place…

  1. pathogen attatches to phagocyte by antibody and surface receptors
  2. pathogen engulfed by infolding of phagocyte membrane
  3. pathogen trapped inside a vacoule called a phagosome
  4. lysosomes fuze with the phagosomes and release the enzyme lysin into it
  5. harmless end products of  digestion are absorbed



An antigen is a molecule that stimulates an immune response,

  • it has a specific shape
  • foreign antigens are detected by your immune system
  • this results in antibdodies being produced
  • Antigens are usually a protein or a glycoprotein

B lymphocytes produce antibodies in response to an infection, the B lymphocyte does not recognise the foreign antigen so will release antibodies. The antibodeies that attach to the antigens are complemtary in shape and makes them harmless.

The structure of the antibody

  • 4 polypeptide chains held together by disulphide bridges
  • a constant region which is the same in all antibodies. This enables the antibody to attach to the phagocyte and helps in the process of phagocytosis
  • A variable region which is a specific shape and differs from one antibody to another. This is because of the amino acid sequence. The shape of the variable region is complementary to that of an antigen
  • hinge regions which allow some flexibility, so the antibodye can bind to more than one antigen as the branches of the Y shaped molecule can move further apart

An antibody is made up of two heavy chains and...

Antibodies work by attaching to the antigens on the pathogen. A pathogen may use the antigen as a binding site for the host cell it may target. If the antibody can block the binding site by attaching the antigen then the pathogen cannot bind to its host cell. This is called neutralisation. some antibodies are larger than average. They may look like several antibodies attached together. They may have specific variable regions. If the antibody has a number of binding sites, it maybe able to attach to more than one antigen on a pathogen at the same time, this is called agglutination. when the pathogens are stuck together, they cannot enter the host cell.

Initially when foreign antigens are first detected it takes a few days for the number of antibodies to rise highe enough to combat the infection. This is the primary immune response. and once the pathogens have been dealt with, the number of antibodies in the blood will fall.

If the same pathogen infects the body again then the concentration of the antibodies will increase but much quicker. Therefore the concentration rises quicker and to a greater amount, so this is known as the secondary response.


Smoking and what effects it has on you.

So what does a regular cigarette

  • tar
  • butane
  • ammonia
  • methanol
  • formoladehyde
  • cadmium
  • radon
  • hydrogen Cyanide
  • arsenic
  • acetone
  • nicotine
  • carbon monoxide

Carbon monoxide

  • carbon monoxide enters the blood and combines with the red blood cells haemoglobin to form carboxyhemoglobin
  • reduces the oxygen capacity of RBC
  • smokers feel this when they exercise
  •  also damages the endothelium of the arteries

Damaged arteries can lead to artherosclerosis

  • damage is repaired by the phagocytes
  • this encourages the growth of smooth muscle
  • this leads to deposition of fatty substances (plaque)
  • these deposits develop into atheromas

The build up of Artheroma’s results in tears and restriction of the lumen and the blood flow

This can lead to many conditions such as Thrombosis, coronary heart disease and strokes. These are all types of cardiovascular disease.


  • blood flowing past a built up plaque cannot flow smoothly
  • this can lead to a blood clot forming, which increases the risk of sticky platelets
  • blood clot is a thrombus
  • it can block arteries


when the lumen of the coronary arteries is narrowed by plaque, it can reduce the blood flow to the heart, which can result in:

  • Heart attack, where part of the heart muscle dies
  • Angina: Severe pain in chest as heart does not receive enough O2
  • heart failure: where the heart cannot pump due to blockage of CA

So what factors increase the risk of CHD?

•Sex (men under 50 much more likely to die than women).
•High blood cholesterol.
•Lack of exercise.
•General diet, inc. high salt intake.
  • Death of brain tissue
  • caused by either thrombus blocks an artery leading to the brain
  • artery leading to brain burst (haemorrhage)


cardiovascular diseases are those diseases that affects the heart and circulatory system.

  • Atherosclerosis
  • Coronary heart disease (CHD)
  • stroke
  • arteriosclerosis

This disease is the most common that affects the economically developed countries.

Symptoms of Cardiovascular disease:

High blood pressure and hypertension are usually the first signs, they result from the narrowing of the artery lumen caused by the deposition of atheroma. The narrowing lumen increases the friction between the blood and the artery wall. The heart pumps against this increase friction between the blood and the artery wall. The heart pumps against this increased friction which raises the blood pressure. The atheroma makes the artery wall less elastic. WHich means that they cannot recoil and dilate as easily.

Coronary heart disease (CHD)

A person with CHD may find it more harder to exercise. They may feel out of breath after only a small amount of exertion. This is because of the atherosclerosis in the coronary arteries. The atheroma narrows the lumen of the artery, reducing blood flow to the cardiac muscle. When exercising the heart increases its output. The heart muscle does not receive enough oxygen for aerobic respiration and is put under strain in an effort to pump more blood. As it does not have enough oxygen it cannot pump sufficient blood to the rest of the body.


The symptoms of stroke are always sudden, it results from part of the brain receiving not enough oxygen

  • sudden numbness or weakness of the face, especially on one side of the body
  • sudden confusion and difficulty in speaking or understanding
  • sudden difficulty in seeing
  • sudden trouble in walking
  • sudden severe headache, unknown causes

Nicotine mimics the action of transmitter substances between nerves, making the nervous system more sensitive, Nicotine causes adrenaline to be release, this increased Adrenaline can cause:

  • increased heart rate
  • breathing
  • arterioles constrict
  • raise blood pressure

Nicotine causes the arterioles to constrict. It can make platelets sticky which can contribute to the formation of clots and thrombus. and also causes the build up of plaque. A clot or thrombus can stop blood flow. Or move in blood until it reaches a narrow artery.

Short term affects that result from TAR

Tar is a combination of chemicals which end up settling on the lining of the airways and the alveoli and NOT in the blood, it is said to be carcinogenic.
  • Because it settles on the lining of the airways and the alveoli, it increases the diffusion distance for oxygen entering the blood and for carbon dioxide leaving the blood.
  • Results in smooth muscles in the walls of the airways to contract
  • the lumen of the airway gets smaller and this restricts the flow of air to the alveoli
  • cilia are unable to move the mucus
  • tar also stimulates goblet cells to enlarge and secrete more mucus
  • mucus collects in airways, pathogens are not removed, so multiply
  • bronchioles become blocked and more susceptible to infections

Long term effects of tar

Smokers cough is an attempt to move the bacteria laden mucus that collects on the lungs. A constant cough can lead to the delicate lining of the airways and the alveoli becoming damaged. This lining will eventually be replaced by scar tissue, which is thicker and less flexible. More smooth muscle grows on the bronchioles which reduces the lumen of the airway and the flow of air is permanently restricted.

Due to the build up of the pathogens in the bronchioles, means that the linings of the airways may become inflamed, this will damage the lining of the epithelium in particular. Also this then attracts white bloods cells which attempt to deal with the pathogenic micro-organisms. They have to make their way into the airway from the blood, so therefore they release enzymes which digest part of the linings of the lungs in order to get into the airways. The enzyme used is elastase. This damages the elastic tissue in the alveoli, so they alveoli can no longer contract and expel air as there is a loss of elasticity in the alveolus walls. As we exhale, the alveolus walls do not recoil to push air out. Which increases pressure in the alveoli as air gets trapped. The bronchioles collapse which cause the alveoli to burst and therefore reduce the surface area of the lungs.

Diseases associated with smoking


  • loss of elasticity in the alveoli, which causes it to burst
  • reduction in surface area, so less surface for gaseous exchange
  • shortness of breath which leads to more shallow and rapid breathing
  • blood is less well oxygenated which leads to fatigue

Lung cancer

  • carcinogenic compounds line the lungs
  • these chemicals (mutagens) enter nucleus and cause mutation of DNA which can effect cell division
  • Cancer often starts at the entrance of the bronchi and takes 20-30 years to develop and grows for years before it develops

Chronic bronchitis

inflammation of the lining of the airways and damage to the cilia and over production of mucus by goblet cells

  • irritation of the lungs
  • coughing
  • coughing up bacteria-laden mucus that will also contain white blood cells
  • can lead to infections

Chronic obstructive pulmonary disease

this is a combination of all the diseases mentioned above

Nucleic acids

nucleotides are the monomers of nucleic acids. If you put lots of nucleotides together and you build either RNA, or DNA, they are known as polynucleotides.

Nucleotides are nitrogen-containing organic substances that form the basis of the nucleic acids DNA and RNA. All nucleotides contain the following 3 groups.

In DNA the sugar is Deoxyribose, whereas in RNA it is Ribose. There are five bases split into 2 types:

  • Purine base: Adenine and Guanine
  • Pyramidine bases: Thymine, Cytosine and Uracil.

DNA contains A,G,T and C

RNA contains A,G,U and C

A DNA  molecule forms when two polynucleotide strands come together. They form something much like a ladder and it is also antiparrellel as the strands run in opposite directions to each other. The backbone consists of the Sugar and phosphate groups of the nucleotides covalently bonded to one another.

3 Hydrogen bonds form Between BASE G and BASE C

2 Hydrogen bonds form between BASE A and BASE T

lots of hydrogen bonds help to make the DNA molecule very stable. The base pairing that occurs between a Pyramadine and a Purine is described as being complementary. In a complete DNA molecule, the anti parrellel chains twist, which forms the final structure known as the double helix.


Semi conservative replication

  1. double helix untwists
  2. hydrogen bonds between the bases are broken apart to unzip the DNA with the enzyme Helicase, which exposes the bases
  3. free nucleotides floating around the nucleas are hydrogen bonded onto the exposed bases according to the base pairing rule
  4. bases that are complementary to each other form hydrogen bonds between each other
  5. covalent bonds are formed between the sugar group of one nucleotide and the phosphate group of another nucleotide with the enzyme DNA polymerase which seals and forms the backbone of the polynucleotide
  6. it is semic conservative replication as each new DNA molecule consists of one conserved strand and one newly built strand

the structure of DNA relates to its function by…

  • the sequence of bases is an example of information storage, the information is in the forms of codes to build proteins
  • the molecules are long, so large amount of information can be stored
  • the base pairing rule means that complementary strands of DNA can be replicated
  • the double helix structure gives the molecule, stability
  • Hydrogen bonds allow fo easy unzipping for copying and reading information



RNA is structurally different from DNA in a number of different ways

  • the sugar molecule that makes the nucleotides is ribose
  • the nitrogenous base URACIL is found instead of the organic base THYMINE
  • the polynucleotide chain is usually single stranded
  • three forms of RNA exist
  • RNA contains the  purines A,G and the pyramidines C,U not T!
  • Base pairing takes place between A and U, and G and C

The base pairing rule means that molecules of RNA can be made that are complementary to molecules of DNA. This is because exposed DNA nucleotides can have  free RNA nucleotides, hydrogen bonded to them and the sugar phosphate backbone is sealed up to form a chain of RNA nucleotides. This is is called transcription and is the basis of copying the genetic code of the DNA base sequence.

There are 3 forms of RNA

  • MRNA: is made as a strand that is complementary to one strand of a DNA molecule. It is therefore a copy of the other DNA strand of the double helix
  • RRNA is found in the ribosomes
  • TRNA carries the amino acids to the ribosomes where they are bonded together to form polypeptides. The primary structure sequence is made according to the MRNA

What are the instructions for?

  • the sequence of bases on the DNA make up codes for particular protein molecules
  • they code for the sequence of amino acids in a protein
  • the sequence coding for a particular protein (a gene) can be exposed by splitting the hydrogen bonds that hold the double helix together in that region
  • RNA nucleotides form a complementary strand (MRNA) this MRNA molecule is a copy of the DNA coding strand or GENE
  • the MRNA peels away from the DNA and leaves the nucleus through the nuclear pore
  • the MRNA attaches to the ribosomes
  • then TRNA brings amino acids to the ribosomes in the correct order, according to the base sequence on the MRNA
  • the amino acids are joined by peptide bonds to give a protein with a specific primary structure
  • this primary structure gives rise to the secondary and the tertiary structure

Enzymes and Enzyme action

What is an enzyme?

  • Globular protein
  • specific 3D tertiary structure
  • generally soluble in water
  • biological catalysts
  • specific to one type of substrate
  • contains an area known as an active site
  • affected by temperature and PH

A Biological Catalyst is something that speeds up a chemical reaction, they give a chemical reaction some activation energy so they can start more effectively, they are not used up in the reaction

Enzymes turn the substrate into the product, for example lactiose is broken down into Glucose and Galactose by the enzyme Lactase

Activation enery is the amount of energy that needs to be applied for a reaction to happen, different reactions require different levels of activation energy, enzymes reduce the amount of activation energy needed for a reaction to happen

Enzymes work by reducing the amount of activation energy required for the reaction to take place

Random collisons take place

When an enzyme active site randomly collides with the substrate, to form an enzyme-substrate complex, the enzyme catalyses the reaction, when heating the enzymes and the substrate molecules, more collisions result in an increase in the rate of reaction

Diagram illustrating the induced fit model of ...




Induced fit Hypothesis

  • as a substrate molecule collides with an active site, the active site changes shape slightly
  • this makes the active site fit more closely around the substrate. The substrate is held into place because opposite charges on the active site and the substrate are found near each other, therefore an enzyme-substrate complex forms
  • this change in the shape of the active site puts strain on the substrate molecule, this establishments the substrate molecule so the reaction can happen more easily
  • This produces a product, so now the complex is an enzyme-product complex
  • the product that is formed is no longer a similar shape to the substrate
  • the products no longer fits into the active site so moves away
  • the enzyme is able to catalyse the same reaction with another substrate molecule

Enzymes and temperature

The molecules in a liquid or gas are continually moving in random movements, sometimes these molecules collide. the more kinetic energy you give, the enzyme and the substrate, the quicker the movement, so the enzyme and the substrate collide more often.

  • more kinetic energy means more vibration, which puts more strain on the bonds
  • this can break the weaker bonds on the tertiary structure and change the active site
  • as you heat the enzyme, more and more bonds are broken
  • rate of reaction decreases, until tertiary structure unravels which basically means it denatured


The rate at which the enzyme works best, is known as the optimum temperature, however for each enzyme the optimum temperature may vary. The optimum temperature is related to the organisms environment, or to the internal body temperature maintained. There will be some leeway in the temperature before an enzyme becomes denatured.

Enzymes and PH

PH is

  • measure of H+ ions concentration
  • the higher the H ions the lower the PH value, so acids contain a high concentration of H ions
  • an acid is a proton donor, as H ions are known as protons

How does it affect the active sites?

  • H ions are charged and attract, negatively (-) charged molecules
  • tertiary structure is held together by alot of H bonds and Ionic bonds
  • H ions interfere with the H bonds and the ionic bonds, so if the amount of H ions increases you can change the shape of the active site.

All enzymes have an optimum PH

  • at the optimium Ph, the concentration of the H ions gives the tertiary structure of the enzyme the best shape
  • the shape holds the active site together that is complementary to the substrate
  • Enzymes work in fairly low ranges of PH
  • we use Buffers to control Ph in practicals



Concentration effects on Enzymes

If there is no substrate, then the enzyme has nothing to catalyse, which means that the reaction can not occur as enzyme-substrate complexes cannot form.

as the concentration of substrate increases, collisions between enzyme and substrate increase, more enzyme-substrate complexes form, so more product is formed which means that the reaction increases.

However a point will be reached that if the concentration of the substrate increases any further, then the rate of reaction will not increase and it reaches it maximum value. This is because all the enzymes are forming Enzyme-substrate complexes as fast as possible, which means that all the active sites are being occupied. So any further increase in the substrate concentration will have no effect on the reaction rate.


Increasing the enzyme concentration?

  • as the concentration of enzyme increases, more active sites become available
  • more enzyme substrate complexes form, so the reaction rate increases
  • If the enzyme concentration increases any further, a point will be reached where all the substrates are occupying the active sites
  • Therefore the substrate concentration is the limiting factor


Inhibitors of action

Inhibitors are defined as substances that reduce the rate of reaction as they have some effect on the enzyme molecule. They can either be competitive, in which they compete for the active site on the enzyme or non-competitive depending on how they block enzyme action as they may effect another area of the enzyme (the allostiric site)

competitive inhibitors have a shape that is similar to that of the substrate. Therefore they can form an enzyme-inhibitor complex with the active site. They do not lead to the formation of products as it is not identical to the substrate. The enzyme therefore does not catalyse the reaction. When there is an inhibitor complex, the substrate cannot enter, so the amount of ESC (enzyme substrate complexes) is reduced, which reduces the rate of reaction and slows it down.

If the substrate concentration increases, then the rate of reaction can increase as there would be more of a chance of a substrate colliding with the active site compared to an inhibitor. Competitive inhibitors tend to be reversible.

non-competitive do not compete with the substrate molecules for a place on the active site. instead they attach elsewhere on the enzyme. this distorts the tertiary structures 3D shape of the enzyme molecule. This leads to a change in shape of the active site which means that the substrate can no longer fit. So ESC can longer form and the rate of reaction decreases.

If there are enough inhibitors to bind to all the enzymes, then the enzyme controlled reaction would stop, changing the substrate concentration will have no effect. Non competitive tend to be permanent or irreversible because the inhibitor denatures the enzyme. Some metabolic pathways use this type of inhibition.


Prosthetic groups and enzymes

some enzymes require a non-protein substance to be present before they can catalyze a reaction at an appropriate rate, they are known as….

Co enzymes

  • small organic and non protein
  • temporarily bind to active site
  • takes part in the reaction
  • used again and again
  • can carry chemical groups between enzymes for metabolic pathways

Prosthetic groups

  • permanent part of enzymes
  • contribute to overall shape and charge

inorganic ion cofactors

  • sometimes the presence of ions increases the rate of an enzyme controlled reaction
  • ions can combine with the enzyme or substrate so the enzyme-substrate complex can form more easily by changing charge distribution and sometimes shape


Evidence for evolution

What can fossils tell us about Evolution?

  • past species are different from today
  • old species have died out
  • new species have arisen
  • new species are similar to those found in the same place

But there are issues with relying on fossils!

  • the fossil evidence is incomplete
  • only the hard bit becomes a fossil
  • if the living thing does not have hard parts, fossils may not form
  • very easily destroyed or damaged

Other evidence for evidence for evolution would include


How evolution works

  1. variation must occur before evolution can take place
  2. once variety exists, the environment can select, it will select those variations that give advantages
  3. individuals with an advantage will survive and reproduce
  4. therefore they pass on their advantageous characteristics (inheritance)
  5. The next generation will be better adapted to the environment. Over time the group of organisms will be better adapted its environment (adaptation)

Evolution and diversity:

  • Sometimes a group of individuals within a species will evolve into a new species.
  • this increases diversity
  • some members of a species will be better adapted than others
  • they are still the same species so they can interbreed
  • having a lot of local populations that are genetically different increases diversity

Evolution is still occuring, it does depend on the selection pressure, favorable characteristics will be selected.


pesticides are chemicals designed to kill insects, insects are pests and damage our crops and can even act as vectors of diseases (malaria) therefore pesticide was developed to kill these insects and prevent them from causing damage and harm. Insectide however applies a very strong selection pressure which means means that if the insect does not show some genetic variation and is not able to fight off the insectide, it will die.Insects that show genetic variation and show some form of resistance, it will survive and produce some offspring. This characteristic will pass to the offspring. So the resistance spreads quickly through the whole population.

Resistance towards the insectides has been developed in different ways.

  • The insects maybe able to break down the insectide (using enzyme) and metabolise them
  • the target receptor protein on the cell membrane maybe modified

Diagram showing development of pesticide resis...

 Another problem with pesticide, is that it effects the food chain. If these insects  show some resistance to the insectide, then they will survive application of these chemicals. The insects may then be eaten by their predators. The predators would then recieve a larger dose of the insecticide. this can move up all the way in the food chain, with the risk that humans can recieve large doses of insecticide The use of antibiotics makes it a very powerful selection pressure for bacteria. when you take antibiotics, most of the bacteria are killed. However there maybe a few that show some resistance to the antibiotics. So when people begin to feel better, they stop taking the antibiotics before they have finished the prescribed course. This allows the resistant bacteria to survive and reproduce and create a new strain of resistant bacteria.Some Bacteria have gained some form of wide resistance to antibiotics. MRSA is an example. It has developed resistance to stronger and stronger drugs, this is an example of the “evolutionary arms race” as doctors are finding it harder to combat the bacteria, and find it hard to develop and find new, effective drugs.   

A schematic representation of how antibiotic r...

A schematic representation of how antibiotic resistance is enhanced by natural selection (Photo credit: Wikipedia)

Evolution: Natural selection

Charles Darwin was born on the 12th of February 1809 and died on the 19th April 1882, He   did

His observations

  1. offspring were similar to parents
  2. no two organisms are identical
  3. organisms may produce many offspring
  4. populations in nature tend to remain stable in size

Therefore Variation is key to a species ability to survive.



Darwin's finches or Galapagos finches. Darwin,...

Darwin’s finches

His conclusions

  • there is a struggle to survive
  • better adapted individuals survive and pass on their characteristics
  • over time, a number of changes will give rise to a new species that would not be able to interbreed successfully

Natural selection is the selection by the environment of particular individuals that show certain variations, these individuals will survive to reproduce and pass on their variation to the next generation



English: Charles Darwin (1809-1882) in his lat...

English: Charles Darwin (1809-1882) in his later years. This image is flipped. Darwin’s mole was on the right side of his nose. (Photo credit: Wikipedia)
























Selection Pressure drives natural selection

  • availability of food
  • predators
  • disease
  • physical and chemical factors

The sequence of events leading to natural selection include

  1. there is competition within a population for resources that are limited
  2. there is genetic variation within the population
  3. some individuals possess a favorable characteristic which means that they are better adapted to the environment
  4. individuals with the favorable characteristics will have more offspring than the individuals with the less favorable characteristics
  5. The possessors of the favorable characteristic will pass the favorable characteristics on to some of their offspring
  6. this is repeated over many generations and natural selection occurs

Speciation is the formation of a new species from an existing one


  • long, slow accumulation of changes over many generations
  • eventually species can no longer interbreed

Speciation requires a reproductive or geographical barrier to stop some organisms within a group are unable to interbreed,

  • beneficial changes spread down generations
  • these changes can end up affecting part of a group
  • this isolated group can becomes different from the others
  • eventually so different that they become a new species

Geographical separation (allopatric speciation)

  • prevents interbreeding between populations on different islands
  • speciation likely to occur
  • This permitted new species

Reproductive Barriers Cause Sympatric Speciation

A reproductive barrier is any factor that prevents reproduction between the species, there are 3 main types of reproductive barrier

  1. Biochemical changes prevents fertilization
  2. behavioral change prevents mating
  3. physical change prevents intercourse