AQA Biology Key Topics

• Antibodies are produced by B-lymphocytes (a type of white blood cell)
• Monoclonal antibodies are produced from clones of a single white blood cell - this means that all of the antibodies are identical and will only target one specific protein antigen
• To produce the monoclonal antibodies, a mouse B-lymphocyte must be fused with a tumour cell (which divide rapidly) to form a hybridoma - the mouse may have previously been injected with the chosen antigen, so that the B-lymphocyte with the intended antibody can be cloned
• Hybridoma cells can be cloned to produce many identical hybridoma cells - these cells all produce the same antibodies (monoclonal antibodies) which can then be collected and purified
• Monoclonal antibodies can be made to bind to any single antigen. This makes them useful as they only bind to one specific molecule (or type of molecule), so they can be used to target a specific cell or chemical in the body

A hormone called HCG is found in the urine of pregnant women. Pregnancy tests detect this hormone using monoclonal antibodies.

• The section of the stick that the women urinates on has some antibodies to the HCG hormone, with blue beads attached to them
• The indicator (test section - which indicates pregnancy) also has HCG-specifc antibodies stuck onto it

If the woman is pregnant:

• The HCG hormone binds to the antibodies attached to the blue beads
• The urine moves up the stick, carrying the hormone and the beads with it
• The beads and hormone bind to the antibodies on the indicator
• The blue beads get stuck on the indicator, turning it blue

If the woman is not pregnant, the urine still flows up the stick carrying the blue beads, however there is nothing to stick the blue beads onto the indicator section, so it doesn't turn blue

• Different body cells have different antigens, so monoclonal antibodies can be produced that will bind to specific cells in the body
• Cancer cells have antigens on their cell membranes which differ to normal body cells - these are called tumour markers
• Monoclonal antibodies can be produced which bind to tumour markers
• Anti-cancer drugs can be attached to monoclonal antibodies - this could be a radioactive substance, a toxic drug or a chemical which stops the cancer cells growing and dividing
• The antibodies are given to the patient through a drip. They only target the cancer cells, which means that they do not kill any normal body cells near the tumour

• They can bind to hormones and other chemicals in blood to measure their level
• Test blood samples in laboratories for certain pathogens

Monoclonal antibodies can also be used to locate specific molecules on a cell or in a tissue:

• Monoclonal antibodies are produced to bind to the desirable molecule
• The antibodies are then bound to a fluorescent dye
• If the molecules are present in the sample, the monoclonal antibodies will attach to them and they can then be detected using the dye

• Monoclonal antibodies have some obvious advantages, such as in cancer treatment - they have less side effects than standard chemotherapy or radiotherapy, as they only target and kill specific cancer cells
• However, monoclonal antibodies do cause more side effects than were originally expected - they can cause fever, vomiting and low blood pressure (when they were first developed, scientists thought that they would not cause many side effects, as they only target specifc cells or molecules)
• These unexpected side effects mean that monoclonal antibodies are not as widely used as treatments as scientists had originally thought they would be

Pathogens are microorganisms that cause communicable diseases for plants and animals.

• Bacteria: Very small cells that can reproduce rapidly inside the body
• They produce toxins that damage cells and tissues

• Viruses: Much smaller than cells
• Live inside cells and replicate themselves using the cells' machinery
• Eventually cause the cells to burst, releasing all the new viruses
• Cause illness through the damage to cells

• Protists: Mainly single-celled eukaryotes
• Some protists are parasites, they are often transferred to an organism by a vector

• Fungi: Can grow and penetrate human skin and the surface of plants
• Some are single-celled, others have a body made up of hyphae (thread-like structures)

• Measles:
• spread by droplets (from an infected person's sneeze or cough)
• causes a red skin rash and signs of a fever (high temperature)
• can be fatal if there are complications - measles can lead to pneumonia (a lung infection) or a encephalitis (a brain infection)
• most people are vaccinated against measles at a young age
• HIV:
• spread by sexual contact, or the exchange of bodily fluids (e.g. when people share needles)
• initially causes flu-like symptoms for a few weeks
• can be controlled with antiretroviral drugs which stops the virus replicating in the body
• virus attacks the immune cells and, if the immune system is badly damaged, it cannot cope with other infections or cancers - at this stage the virus is known as AIDS
• Tobacco Mosaic Virus (TMV):
• affects many species of plants (e.g. tomatoes)
• causes a mosaic pattern on the leaves of the plants - as they become discoloured
• the discolouration means that the plant can't carry out photosynthesis as well, so the virus also affects growth

Rose Black Spot:

• a fungus that causes purple or black spots to develop on the leaves of rose plants - the leaves can then turn yellow and drop off
• this means less photosynthesis can happen, so the plant's growth is also stunted
• the fungus spreads through the environment in water or by wind
• gardeners treat the disease using fungicides and by ridding the plant of its affected leaves - these leaves then need to be destroyed so that the fungus cannot spread to any other rose plants

Malaria:

• mosquitoes act as vectors for the protist - they pick up the malarial protist when they feed on an infected animal
• every time the mosquito feeds on another animal, it infects it by inserting the protist into the animal's blood vessels
• cause repeating episodes of fever which can be fatal
• the spread of malaria can be controlled by stopping mosquitoes from breeding, using insecticides and using mosquito nets to prevent being bitten

Salmonella:

• bacteria causes food poisoning by producing toxins
• symptoms include fevers, stomach cramps, vomiting and diarrhoea
• is passed on when someone eats food that is contaminated with Salmonella bacteria (e.g. eating chicken that has caught the disease whilst alive or eating food that has been contaminated whilst being prepared in unhygienic conditions)
• in the UK, most poultry is given a vaccination against Salmonella - this controls its spread

Gonorrhoea:

• is an STD (sexually transmitted disease), so is passed on by sexual contact (unprotected sex)
• symptoms include pain when urinating and thick yellow/green discharge from the penis/vagina
• was originally treated with penicillin (an antibiotic), but now some strains of the bacteria have become resistant to it
• can be treated with antibiotics and people should wear barrier methods of contraception (e.g. condoms) to prevent spreading the disease

Water:

• some pathogens can be picked up by drinking dirty water
• cholera is a bacterial infection that's spread by drinking water that is contaminated with the diarrhoea of other sufferers

Air:

• pathogens can be carried in the air and then inhaled
• influenza virus is an airborne virus that is carried in the air in droplets produced when a sufferer coughs or sneezes

Direct Contact:

• some pathogens can be picked up by touching a contaminated surface, including the skin
• athlete's foot is a fungus which makes skin itch and flake off - it is spread by touching surfaces that have already been touched by an infected person (e.g. shower floors or towels)

• Being Hygienic - using simple hygiene methods can prevent the spread of disease (e.g. washing hands before preparing food, or after sneezing can stop the carrier infecting other people)
• Destroying Vectors - getting rid of the organisms that spread the disease can prevent the disease from being passed on - vectors that are insects can be killed using insecticides or by destroying their habitat so that they can no longer breed
• Isolating Infected Individuals - isolating someone with a communicable disease prevents them from passing it on to anyone else
• Vaccination - vaccinating people and animals against communicable diseases means that they cannot develop the infection and then pass it on to someone else

• the skin acts as a barrier to pathogens and secretes antimicrobial substances to kill them
• hairs and mucus in the nose trap particles that could contain pathogens
• the trachea and bronchi secrete mucus to trap pathogens
• the trachea and bronchi are lined with cilia - hair-like structures which waft the mucus up to the back of the throat where it can be swallowed
• the stomach produces hydrochloric acid (HCl) which kills pathogens

If pathogens get inside the body, the immune system responds to destroy them. The most important part of the immune system is the white blood cells - they are a component of the blood which respond to the threat of microbes. They can target microbes in 3 ways:

• WBCs can engulf foreign cells and digest them - this is called phagocytosis
• They can produce specific antibodies to lock onto invading cells, making it easier for other WBCs to find and destroy them
• They can also produce antitoxins - these counteract the toxins produced by invading bacteria

Some types of white blood cell (e.g. B-lymphocytes) have the ability to produce antibodies which make it easier for other WBCs to find and destroy (through phagocytosis) invading microbes/pathogens. This is the process for the production of antibodies:

• every invading pathogen has unique molecules - called antigens - on their surface
• when the WBCs which are capable of producing antibodies come across a foreign antigen, they start to produce proteins called antibodies - antibodies are specific to one antigen, they won't lock on to any others (this process is the primary response to the pathogen - the specific antobody is stored in memory cells)
• antibodies are then produced rapidly and are carried around the body to find all similar bacteria or viruses
• if the person if infected again (secondary response), the WBCs rapidly produce the specific antibodies needed to kill it - the person is naturally immune to that pathogen and will not get ill

Vaccinations are made to protect humans/animals from future infections.

• if an organism is infected with a large dose of a new pathogen, it can be very serious and potentially fatal, as it takes white blood cells time to learn and adapt to the pathogen and its antigen (this is the primary response and it is usually fairly slow)
• vaccinations involve injecting small amounts of dead/inactive pathogens - these pathogens carry antigens, which cause WBCs to produce antibodies to attack them (however as the pathogens are dead/inactive, the vaccination is harmless)
• if live pathogens of the same type enter the body again, the WBCs are much quicker at mass-producing the antibodies specific to the pathogen's antigens, so the pathogen can be killed off much faster, without causing too much harm (this is the secondary response, which is a lot faster and more efficient than the primary response)

Pros:

• vaccines help to control a lot of communicable diseases that were once common (e.g. smallpox, polio, measles, mumps and rubella)
• big outbreaks of disease (epidemics) can be prevented if a large percentage of the population is vaccinated - this is called herd immunity - it means that even the people who are not vaccinated are unlikely to catch the disease as fewer people will be able to pass it on (conversely, if a significant number of people are not vaccinated, the disease can spread quickly through them and many people will get ill at the same time)

Cons:

• vaccines do not always work - sometimes they do not give you immunity
• they can cause side effects or bad reactions (such as swelling, fevers, or seizures) - however, bad reactions are very rare

• Painkillers (such as aspirin) are drugs that relieve pain, however they don't tackle the cause of the disease or kill pathogens - they just help to relieve the symptoms
• Other drugs (such as cold remedies) also reduce symptoms without tackling the underlying cause

• Antibiotics (such as penicillin) actually kill (or prevent the growth of) the bacteria that cause the problem without killing body cells - different antibiotics kill different types of bacteria (so it is important to be treated with the right antibiotic)
• Antibiotics have greatly reduced the number of deaths from communicable diseases caused by bacteria
• Antibiotics do not destroy viruses (such as flu or cold viruses)

• Viruses reproduce inside cells, so it is difficult to develop drugs which kill just the virus and not the body's cells
• Antivirals must be taken to inhibit the growth, reproductiona and development of viruses

• Bacteria are able to mutate - sometimes they mutate resistance to an antibiotic
• This means that only non-resistant strains of bacteria are killed when infections are treated with antibiotics
• The resistant strains of bacteria survive and reproduce, so the population of the resistant strain increases (this is an example of natural selection - the favoured allele for resistance is passed on)
• Resistant strains can cause serious infections that cannot be treated ny antibiotics - MRSA can cause serious wound infections and is resistant to the antibiotic methicillin

To slow down the rate of development of resistant strains:

• doctors should avoid over-prescribing antibiotics (so there is less opportunity for the bacteria to develop resistance)
• patients should finish the whole course of antibiotics that they are on (so that all of the bacteria are killed, and they cannot mutate to develop resistance)

• Plants (and micro-organisms) produce a variety of chemicals to protect themselves against pathogens - some of these chemicals can be used as drugs to treat human diseases or relieve symptoms

• Aspirin is used as a painkiller and to lower fever - it was developed from a chemical in willow

• Digitalis is a drug used to treat heart conditions - it comes from a chemical in foxgloves

• Alexander Fleming was testing Petri dishes containing bacteria - he saw that one of the dishes had mould on it and the area surrounding the mould was free of bacteria
• He found that the mould (called Penicillium notatum) on the Petri dish was producing a substance that killed bacteria - this was penicillin (the first antibiotic)

• Most new drugs are synthesised by chemists in the pharmaceutical industry - however, this process may still be started with a chemical extracted from a plant

• In the first stage of preclinical testing, drugs are tested on human cells and tissues in the lab
• However, human cells and tissues cannot be used to test drugs that affect whole or multiple body systems (e.g. a drug for blood pressure must be tested be tested on a whole animal, as it must have an intact circulatory system)

• The next stage of preclinical testing is to test the drug on live animals. This tests three things:
• efficacy - whether the drugs actually works and has the desired effect
• toxicity - how harmful the drug is
• dosage - the best concentration that should be given and how often
• British law states that all new drugs must tested on two different live mammals
• some people believe that it is cruel and unethical to test drugs on animals
• others believe that it is the safest way to make sure a drug isn't dangerous before giving it to humans

If the drug passes preclinical trials (the tests on animals), then it is tested on human volunteers in clinical trials:

• 1) The drug is tested on healthy human volunteers to make sure that the drug does not have any harmful side effects (tests toxicity) - a low dosage is given to begin with, but this is gradually increased
• 2) If the drug passes the previous test, it is then given to human volunteers who are suffering from the illness - this is to find the optimum dosage (the dosage that is most effective with the fewest side effects
• 3) To test the efficacy of the drug, patients are randomly put into 2 groups - one group is given the new drug, the other group is given a placebo (often a sugar pill, which has no effect)
• Clinical trials are blind, which allows the doctor to see the actual difference that the drug makes, in comparison to the placebo effect (when a person assumes the drug will work, so they feel better even though the treatment isn't doing anything)
• 4) A lot of clinical trials are double-blind - this means that neither the doctor nor the patients know whether they are getting the real drug or a placebo - this is so the doctors aren't subconciously influenced by their knowledge
• 5) The results of drug testing need to go through peer review before they are published to prevent false claims

Mineral Ion Deficiencies:

• Nitrates - needed for protein synthesis and therefore growth - deficiency causes stunted growth
• Magnesium - needed for making chlorophyll, which is needed for photosynthesis - deficiency causes chlorosis (yellowing of the leaves)

Plants can be also be infected by:

• Viruses (Tobacco Mosaic Virus)
• Bacteria (Fire Blight)
• Fungi (Rose Black Spot)
• Insects (Aphids)

Plant diseases can be detected by:

• stunted growth
• spots on leaves
• areas of decay (rot)
• abnormal grwoths
• malformed stems or leaves
• discolouration
• infestation of pests

Plant diseases can be identified by:

• looking up symptoms in a gardening manual or on a gardening website
• taking the infected plant to a laboratory, where it can be identified by scientists
• using testing kits that use monoclonal antibodies

Physical Defences:

• waxy cuticle - provides barrier to stop pathogens entering
• strong cell walls made from cellulose - physical barrier to protect plant cells
• layers of dead cells around their stems (e.g. bark on trees) - act as a barrier

Chemical Defences:

• production of antibacterial chemicals - kill bacteria (e.g. witch hazel and the mint plant)
• production of poisons - deter herbivores (e.g. tobacco plant, foxgloves and deadly nightshade)

Mechanical Defences:

• thorns and hairs - stop animals from touching and eating them
• leaves that droop or curl when touched - can knock off insects and move away from dangers
• mimicry - can mimic other organisms to deter insects from laying their eggs on them (e.g. passion flower) and can trick herbivores into not eating them (e.g. ice plant)