A-Level Biology OCR Notes

Disease

Communicable disease are caused by pathogens (viruses, bacteria, fungi & protoctista) and can be transmitted from one person to another. They are infectious.
Pathogens can be transmitted directly (e.g. physical contact, ingestion, droplet infection) or indirectly (e.g. via vectors)
Bacteria are prokaryotes that can usually damage cells directly or release toxins
Protoctists are unicellular eukaryotic organisms, which can produce sexually and asexually
Fungi are eukaryotic organism which cannot photosynthesis therefore are parasties
Viruses are acellular, non-living particles, they can only replicate inside living host cells where they hijack the host machinery to replicate and then burst the cell to be released

Disease	Pathogen	Description
HIV/AIDs	Virus	Attacks immune cells
Influenza	Virus	Attack mucous membrane in the respiratory system
Tobacco mosaic virus	Virus	Moult and discolour leaves on tobacco and tomato plants
Ringworm	Fungi	Causes a skin rash in cattle
Athletes foot	Fungi	Causes a rash on the foot of humans
Black Sigatoka	Fungi	Causes leaf spots in banana plants
Blight	Protoctista	Affects potato tubers and tomato and potatoes leaves
Malaria	Protoctista	Blood parasite Plasmodium spread by mosquitos
Tuberculosis	Bacteria	Kills cells and tissues, mainly in the lungs
Bacterial meningitis	Bacteria	Causes swelling of the meninges damaging the brain and nerves
Ring rot	Bacteria	Decays vascular tissue in tomato and potato plants

Autoimmune diseases occur when the immune system mistakenly attacks its own antigens
- In Arthritis, antibodies attack membranes around the joints
- In Lupus antibodies attack proteins in the nucleus of cells.

Components of the Immune System

Antigens are any part of an organism/substance which is recognised as foreign by the immune system and goes on to trigger an immune response.
Antibodies are a protein produced by lymphocytes in response to the presence of the corresponding antigen

Antibodies agglutinate pathogens by forming antigen-antibody complexes, leading to phagocytosis & neutralise toxins.

Cell-Mediated Immunity

Antigen from the pathogen is displayed on the cell surface of body cells or phagocytes after phagocytosis
T cells with the correct specific receptor bind with the antigen and are activated
They divide by mitosis (clonal expansion) and differentiate into T helper, cytotoxic and memory cells.

Humoral Immunity

The humoral response is best at fighting pathogens which are free in the bodily fluids

Free antigen binds to a complementary B cell receptor, activating the B cell (clonal selection)
The pathogen is endocytosed, and the antigen presented on the plasma membrane
T helper cell binds to the presented antigen and stimulates the B cell to divide by mitosis (clonal expansion)
The B cell differentiates to plasma and memory cells

Primary & Secondary Immune Response

The primary immune response is when a pathogen infects the body for the first time the initial immune response is slow.
The secondary immune response is a more rapid and vigorous response caused by a second or subsequent infection by the same pathogens. This is due to the presence of memory cells

Vaccination & Medicines

Vaccination is the introduction into the body of a vaccine containing disease antigens, by injection or mouth, in order to induce artificial immunity
Vaccines work by injecting weakened/dead pathogens into the body to stimulate an immune response, to form memory cells against the specific antigen, which destroy the pathogen quickly upon infection.
Herd immunity is when the vaccination of a significant proportion of the population provides protection for individuals who have not developed immunity
Pathogen may mutate so that its antigens change suddenly (antigenic variability) So the vaccine is now ineffective to the new antigens.
Ethical considerations: side effects, financial cost, right to choose, animal testing of vaccines, human trials
Active immunity occurs when specific antibodies are produced by the individual’s own immune system
Passive immunity occurs when specific antibodies are introduced to the individual from an outside source.

Immunity	Example
Natural Active	Direct contact with pathogen
Natural Passive	Antibodies through breastmilk
Artificial Active	Vaccination
Artificial Passive	Injection of antibodies

Antibiotics prevent the growth of bacteria. They are effective because they show specificity in killing bacteria without harming human cells. However, overuse has led to the spread of resistance in bacteria e.g. MRSA. To reduce spread prescription of antibiotics is controlled, patients must finish their course and prevent spread by control measures
New medicines can be discovered from plant compounds using DNA sequencing to screen plants and organisms for potential medical compounds. DNA sequencing can also be used to develop a specific drug suited to persons genome.

Defences

Chemical	Action
Phenols	Antibiotic and antifungal proteins. One example is the tannins present in tree bark
Alkloids	Compounds containing nitrogen (e.g. caffeine, nicotine, cocaine and morphine) are bitter to stop herbivores feeding on them and affect enzyme action.
Defensins	These cysteine-rich, defensive proteins have anti-microbial activity. They appear to affect the functioning of ion transport channels in the plasma membrane.
Hydrolytic enzymes	Present in the spaces between cells, they can have a variety of effects. Chitinases break down the chitin in fungal cell walls, glucanases hydrolyse the glycosidic bonds in glucans, and lysozymes destroy bacterial cell walls.