AQA A Level Biology

what are killer t-cells?

- cytotoxic cells
- bind to non-self antigens

how to killer t-cells work?

1. virus attaches to and enters host cell
2. viral DNA causes host cell to express viral antigens on its surface
3. killer t-cells attaches to viral antigens on infected cells
4. killer t-cells induces death of infected cell (apoptosis)

what is apoptosis?

programmed cell death

how do killer t-cells destroy cancer?

- cancerous = mutations
- mutations = abnormal antigens
- killer t-cell binds to abnormal antigens
- killer t-cells induces death of cancerous cell

What does perforin do?

creates holes in the cell membranes in order to destroy the infected cell

what do helper t-cells do?

- produce cytokines
- cytokines stimulate b-cells to divide into plasma and memory cells
- activate killer t-cells

define helper t-cells

help humoral response by involvement in the maturation of b-cells and by triggering the production of antibodies by b-cells

what do suppressor t-cells do?

- halt immune response
- prevent wastage of energy and resources
- prevent overactive immune response
- causes reduction of white blood cells after being ill

define memory t-cells

work in the same way as memory b-cells, ensuring rapid response on subsequent exposure to the same non-self antigens

define suppressor t-cells

regulate the immune response by suppressing activity of killer t-cells and b-cells

define 'microorganism'

organism that is too small to be seen without a microscope

define 'pathogen'

microorganism that causes disease

outline phagocytosis

1. phagocyte follows conc gradient of chemoattractants
2. phagocyte attaches to pathogen
3. pathogen engulfed and placed in a vesicle (phagosome)
4. lysosomes migrate to phagosome and bind with it to form a phagolysosome
--> lytic enzymes released from lysosome
5. pathogen digested
6. useful parts absorbed
7. all waste products released by exocytosis
8. non-self antigens of the pathogen displayed on the phagocytes surface (antigen-presenting)

where are t-cells made?

thymus gland

where do b-cells mature?

bone marrow

what do b-cells do?

- humoral response
- release antibodies
- complementary to a specific antigen

what response are plasma cells?

primary

what response are memory b-cells?

secondary

outline the humoral response

1. antigen attaches to complementary antibody on b-cell
2. antigen taken into cell (endocytosis)
3. presented on cell surface membrane
4. t-helper cells bind to antigen and stimulate b-cell to divide

what does mitosis do in humoral response?

- forms clones of b-cells (plasma and memory)
- clonal response

what do clones do?

each clone can produce one type of antibody (monoclonal) which can be used in medicine

outline plasma cells

- primary response
- secrete antibodies
- only survive for a few day
- slow response = person gets ill before pathogen is killed

outline memory cells

- secondary response
- circulated in blood and tissue fluid
- divide rapidly
- rapid response so person doesn't get ill

what is an antibody?

y-shaped globular protein produced by b-cells

where are antibodies produced?

plasma b-cells

what are antibodies made of?

4 polypeptide chains -- 2 heavy, 2 light

what are the two regions an antigen is made of?

- variable
- constant

what is special about the variable region?

complementary and specific to an antigen

what are the chains bonded by?

disulphide bridge

why do antibodies have a hinge region?

to allow the antibody to move to bind to more than one antigen

what is 'monoclonal antibody production'

the process by which large quantities of antibodies (targeted against a particular antigen) can be produced

how are monoclonal antibodies produced?

- mouse immunised by injection of antigen to stimulate production of antibodies
- plasma cells isolated from spleen
- plasma cells fuse with tumor cells to form a hybridoma
- hybridoma produces large amounts of identical antibody molecules

what is the scientific term for a tumor cell?

myeloma

what can be monoclonal antibodies be used as?

specific probe to track down and purify specific protein that induced its transformation

what needs to happen to monoclonal antibodies before human use?

humanisation

why do monoclonal antibodies need to be 'humanised' before used by humans?

to stop the trigger of an immune response

what are monoclonal antibodies used for?

- pregnancy tests
- diagnostic tools for AIDS
- industrial production of interferon

what is a vaccine?

a dead or weakened version of a disease with relevant antigens

what is a vaccination?

introduction of a vaccine containing appropriate disease antigens into the body to induce artificial immunity

how do vaccines work?

- pathogen stimulaties wbc into making antigens
- dead or inactive pathogen enter body via injection
- antibodies stick to antigens and clump them together
- memory cells = immunity

what are the pros of immunisation?

- protected from certain diseases
- economic benefits
- eradicate diseases
- prevent epidemics

what are the cons of immunisation?

- chance of contracting diseases from vaccine
- religions might be against it
- side effects
- only work for a certain amount of time
- antigenic variability
- certain pathogens hide from immune system

what is HIV

a retrovirus that attacks the body's immune system

what are the modes of transmission of HIV

- blood (transfusions)
- sharing needles
- breast feeding/ giving birth
- sex

what is herd immunity'?

vaccinating enough people so there isn't enough hosts for the virus to spread

define 'epidemic'

infection of a country

define 'endemic'

infection of a specific area

define 'pandemic'

infection of more than one country

give 2 types of natural immunity

- infection
- maternal antibodies

give 2 types of artificial immunity

- vaccine
- monoclonal antibodies

how does HIV result in the death of a t-cell?

- HIV recognises helper t-cell
- apoptosis

how might the global distribution of HIV/AIDS change in coming years?

- could decrease due to potential cures and treatments being discovered
- could increase due to new strains

how does HIV replicate?

- virus fuses with cells plasma membrane
- reverse transcriptase synthesises dna to rna
- second strand is synthesised by reverse transcriptase to be complementary
- double strand is incorporated as provirus is cell's dna
- proviral gene transcribed into rna
- rna transcriped as mrna
- capsids assemble around viral genomes
- new viruses bud off from host cell

HIV structure

- core containing RNA and proteins such as reverse transcriptase
- capsid
- envelope made from membrane stolen from cell membrane if previous host cell
- attachment protein

what is an antigen

molecules found on the surface of cells, usually proteins or polysaccharides, which can stimulate the production of antibodies

what are antigens used to identify?

- pathogens
- toxins
- abnormal body cells
- cells from other organisms of the same species

define 'antigen-presenting'

a 'self' cell that has been infected so displays the pathogen's antigens on its cell surface

how does antigenic variability cause disease?

- pathogens can change their surface antigens
= antigenic variation
- when infected again memory cells produced from first infection DO NOT recognise different antigens

what is the problem with antigenic variability?

makes it difficult to develop vaccines

how does antigenic variability affect the production of flu vaccines?

- antigens of flu virus change regularly = new strain
- memory cells from vaccination do not recognise new antigens
- different strains = different vaccinations

what is the cellular response?

the t-cells and other immune system cells that they interact with e.g phagocytes form the cellular response

what is 'clonal selection'

- memory b-cells activated and divide into plasma cells
- plasma cells produce antibody
- memory t-cells activated and divide into correct t-cell to kill cell carrying the antigen

what is an antigen-antibody complex

where antibodies bind to the antigens on the surface of a pathogen

what is 'neutralisation'

where antibodies covering the pathogen binding sites prevent the pathogen from binding to a host cell and entering the cell

what is 'agglutination'

where pathogens become clumped due to the antibody being able to bind to more than one pathogen at once

how does neutralisation and agglutination lead to the destruction of pathogens

once clumped together the phagocytes bind to antibodies and phagocytosis destroys many of the pathogens at once

outline active immunity

- immune system makes its own antibodies after being stimulated by an antigen
- natural = after catching a disease
- artificial = vaccination

outline passive immunity

- immunity you get from being given antibodies made by a different organism
- natural = maternal antibodies
- atirifical = monoclonal antibodies

active immunity characteristics

- requires exposure to antigen
- takes a while for production to develop
- memory cells produced
- long-term protection

passive immunity characteristics

- doesn't require exposure to antigen
- protection is immediate
- memory cells NOT produced
- protection is short-ternn as antibodies are broken down

what does HIV lead to?

AIDS

what is AIDS?

- condition where the immune system deteriorates and fails

when people are infected with HIV when does it develop into AIDS?

when the helper t-cell numbers reach a critically low level

ethical issues around vaccines

- tested on animals before humans
- animal based substances may be used to produce a vaccine which some disagree with
- human testing can be tricky and cause unnecessary risk
- some people don't want to risk side effects
- if there was an epidemic it would be difficult to decide who recieves the vaccine first

ethical issues around monoclonal antibodies

animals used to produce the cells which some people might disagree with

why don't antibiotics work against viruses?

- antibiotics kill bacteria by interfering with metabolic reactions
- target enzymes and ribosomes
- viruses don't have their own enzymes and ribosomes so use the host cells
- antibiotics can't inhibit human virus as they don't target human processes
- most antiviral drugs are designed to target the few virus-specific enzymes which human cells don't use = drugs can't be designed to inhibit them without affecting the host cell

progression of AIDS symptoms

1. minor infection of mucous membranes and respiratory
2. immune system cells decrease further and are more susceptible to more serious infections e.g chronic diarrhoea
3. last stages = low no of immune system cells and a range of serious infections such as toxoplasmosis of the brain
INFECTIONS KILL AIDS PATIENTS NOT HIV ITSELF

how are monoclonal antibodies used as targeting drugs to cancer cells?

- different cells = different surface antigens
- cancer cells have tumour markers
- mono. antibodies can be made that will bind to markers
- able to attach anti-cancer drugs to antibodies
- contact = bind with markers
- drug will only accumulate in the body where cancer cells are

ELISA test

1. apply sample to slide - antigens will attach
2. wash surface to remove unattached antigens
3. add specific antibody to antigen you're trying to idenitfy
4. wash to remove unwanted antibodies
5 add 2nd antibody with enzyme attached that binds with the first
6. add colourless substrate
= if attached to enzyme a colour will form
= amount of antigen present relates to colour intensity

what is a carbohydrate?

a group of organic molecules that includes sugars and starch

what are the 3 types of carbohydrate?

• monosaccharides
• disaccharides
• polysaccharides

give some examples of monosaccharides

• glucose
• fructose
• galactose

give some examples of disaccharides

• maltose
• lactose
• sucrose

give some examples of polysaccharides

• cellulose
• starch
• glycogen

describe monosaccharides

• commonly referred to as sugars
• structural isomers
• single carbohydrate molecules
• general formula - (CH2O)n
• ose = sugar e.g pentose(5), triose(3), hexose(6)

describe disaccharides

• when two monosaccharides join together
—> bond formed = glycosidic bond
• reaction is known as a condensation reaction

what is a glycosidic bond?

• a bond which it's formation involves the loss of a water molecule

give some disaccharide formation examples

• glucose + glucose = maltose + water
• glucose + fructose = sucrose + water
• glucose + galactose = lactose + water

explain how disaccharides are broken down

• during digestion
• reaction involves the breaking of a bond by reacting it with water
• HYDROLYSIS REACTION

give some examples of reducing sugars

• glucose
• fructose
• maltose

how are sugars reducing?

• the reducing group donates an electron (normally carbon number one)
• glucose + maltose - aldehyde group
• fructose - ketone group

what is always a part of the reducing group?

the double c-c bond

why is maltose half as reducing as 2 glucose

one reducing group is used in the glycosidic bond but one is left and able to reduce other chemicals

describe the benedicts test

• place the sugars into test tubes (&control)
• add benedicts to all the test tubes
• heat a water bath and add test tubes for 5 mins

what are the results of the benedicts test

• reducing sugar = brick red
• not reducing = blue/ no colour change

what is the test for reducing sugars?

benedicts

what are the two forms of glucose?

• alpha
• beta

what are the differences between alpha and beta glucose?

• alpha glucose hydroxyl group sticks below the ring
• beta glucose hydroxyl group sticks above the ring (BUP)

what is a polysaccharide?

monosaccharides joined together by many glycosidic bonds

what is starch?

a storage polysaccharide stored in starch grains