OCR A Level Biology 4.1: Diseases and Pathogens

OCR A Level Biology 4.1

• Communicable diseases are caused by pathogens that can be transmitted between individuals.
• Non-infectious diseases are not caused by pathogens. Examples include lung cancer, cardiovascular disease, and depression.
• Pathogens include bacteria, viruses, fungi, and protoctists.
• Bacteria are prokaryotic organisms. Some are non-pathogenic, while others are pathogenic. Pathogenic bacteria can remain in body cavities (eg. M. tuberculosis).
• Viruses have no cellular structure and rely on host cells to replicate. TMV (tobacco mosaic virus) infects plants, causing yellowing leaves. Influenza viruses cause the flu (influenza A, B, and C). HIV (human immunodeficiency virus) is an enveloped retrovirus.
• Protoctists are unicellular eukaryotes. Plasmodium falciparum is a protist that causes malaria. P. infestans causes potato blight.
• Fungi have cell walls and reproduce via hyphae. They cause diseases like athletes foot and ring rot in plants (potatoes, tomatoes). Black Sigatoka affects bananas.
• Transmission of pathogens can occur through physical contact, vectors (e.g., mosquitoes for malaria), airborne droplets (e.g., influenza), or contaminated surfaces (e.g., contaminated food).
• Disease transmission is the transfer of pathogens from an infected host to an uninfected host.
• Transmission by a vector involves an organism that transfers a pathogen to an uninfected individual. Vectors, such as mosquitoes, can carry pathogens without being harmed themselves, increasing the likelihood of transmission.
• Factors affecting transmission include population density, living conditions (crowded housing, sanitation facilities, water treatment facilities), and human behaviours.
• Resistance to diseases and susceptibility levels are related to genetic factors and environmental factors.
• Vaccination relies on intentionally introducing an antigen to stimulate the immune response.

Plant Defences

• Plants use both passive and active mechanisms to defend against pathogens.
• Passive defences are always present. Examples include a waxy cuticle, cellulose cell walls, and the Casparian strip.
• Active defences are triggered by pathogen invasion. This includes responses like the production of callose and lignin, and the narrowing of plasmodesmata (channels connecting plant cells).
• Chemical defences are also employed, such as the production of toxic compounds (catechol) and sticky resins in bark to trap pathogens. Microorganisms that are naturally present can compete against harmful pathogens.
• Ethylene is a signalling compound allowing communication between plants to stimulate other parts of the plant to prepare defenses against infections.

Non-specific Immune Response

• Skin and mucous membranes provide the first line of defense, acting as physical barriers.
• Chemical secretions like lysozymes and hydrochloric acid create an inhospitable environment for pathogens.
• Commensal microorganisms compete with pathogens for resources.
• Expulsive reflexes like coughing and sneezing remove pathogens.

Specific Immune Response: T Lymphocytes

• T-lymphocytes (T cells) mature in the thymus gland.
• They have specific T cell receptors (TCRs) that recognize antigens, triggering clonal selection and expansion.
• T cells differentiate into helper T cells, killer T cells and memory T cells.

Specific Immune Response: B Lymphocytes

• B-lymphocytes (B cells) mature in the bone marrow.
• They produce antibody receptors (BCRs) that recognize specific antigens, triggering clonal selection and expansion.
• Activated B cells differentiate into plasma cells and memory B cells.

Antibody Structure and Function

• Antibodies (immunoglobulins) are Y-shaped proteins with heavy and light chains.
• The variable region of the antibody is unique to each antigen, allowing for specific binding.
• Antibodies act as opsonins, agglutinins, and anti-toxins, facilitating pathogen destruction.

Primary & Secondary Immune Responses

• Primary response is slower, requiring time for clonal selection and antibody production after first exposure to an antigen.
• Secondary response is much faster and stronger due to memory cells, which have a faster response time and antibody production.

Types of Immunity

• Active Immunity is acquired either naturally (exposure to a pathogen) or artificially (vaccination). It results in the production of memory cells, that enables the body to respond faster during subsequent exposures.
• Passive Immunity is acquired without an immune response. Antibodies from another organism (e.g. mother's breast milk) are received. Passive immunity doesn't trigger memory cells and provides only short-term protection.

Autoimmune Diseases

• Autoimmune diseases occur when the body's immune system attacks its own tissues.
• These diseases can affect a single organ or the entire body. Rheumatoid arthritis affects joints.

Vaccination

• Vaccination is an artificial way to gain active immunity.
• It involves introducing an antigen to stimulate an immune response without causing disease.
• Vaccines can be live attenuated (weakened) or inactivated (killed) pathogens, or contain specific parts of pathogens.
• Herd immunity occurs when a large percentage of the population is vaccinated, providing protection for those who cannot be vaccinated.

Antibiotics

• Antibiotics are chemical substances used to treat bacterial infections.
• They are either bactericidal (killing bacteria) or bacteriostatic (inhibiting bacterial growth).
• Antibiotics can have broad-spectrum effects or be more targeted towards specific bacteria.
• Antibiotic resistance arises when bacteria evolve resistance genes, often through mutation.

Sources of Medicine

• New drugs are often discovered by studying the natural world, e.g., by examining the genomes of microbes and analyzing compounds from plants and animals.
• Identifying molecules that target pathogens or enzymes involved in disease processes plays a role in drug discovery.
• Modifying existing drugs to improve their effectiveness is another approach.
• Personalized medicine aims to tailor treatments to individual genetic characteristics.

Test your knowledge on communicable and non-infectious diseases in OCR A Level Biology. This quiz covers various pathogens, including bacteria, viruses, fungi, and protoctists, as well as their effects on health. Discover the intricacies of how these diseases manifest and their impact on organisms.