Global challenges Revision Notes

Physical Barriers

The human body's first line of defence against pathogens consists of physical barriers that prevent their entry. The skin acts as a tough, waterproof, and largely impermeable layer, forming a protective shield against all types of microorganisms. If the skin is broken, for example by a cut, pathogens can enter, which is why wound care is important. Mucous membranes line the respiratory, digestive, and urogenital tracts. These membranes secrete sticky mucus, which traps pathogens and dust particles, preventing them from reaching deeper tissues. In the respiratory tract, tiny hair-like structures called cilia work in conjunction with mucus to sweep trapped pathogens and debris upwards and outwards towards the throat, where they can be swallowed and destroyed by stomach acid or expelled.

Chemical Defences

Beyond physical barriers, the body employs several chemical defences that nonspecifically target and destroy pathogens. The stomach produces hydrochloric acid, creating a highly acidic environment (pH 1.5-3.5) that is lethal to most bacteria and other microorganisms ingested with food and drink. Tears and saliva contain an enzyme called lysozyme, which has antibacterial properties, breaking down the cell walls of certain bacteria. Ear wax also plays a role, trapping pathogens and containing acidic compounds that inhibit bacterial growth. The acidic pH of urine and vaginal secretions also contributes to preventing infection in those areas, by creating an unsuitable environment for many types of pathogenic bacteria.

Cellular Defences: Phagocytosis

If pathogens manage to breach the physical and chemical barriers, the body's internal non-specific defence involves specialised white blood cells. Phagocytes, a type of white blood cell, are crucial for this. They circulate in the blood and tissues, constantly seeking out and engulfing foreign particles, including bacteria, viruses, and cellular debris through a process called phagocytosis. Once engulfed, the pathogens are contained within a vesicle where they are broken down and destroyed by digestive enzymes. This process is a rapid and essential response to eliminate invading microorganisms and prevent the spread of infection. Macrophages and neutrophils are examples of phagocytes.

Positive Human Impacts on Biodiversity

Human actions can significantly benefit biodiversity. Conservation efforts like creating national parks and marine protected areas safeguard habitats and species from destruction. Reforestation programmes aim to restore deforested land, increasing tree cover and forming new ecosystems for various organisms. Sustainable agricultural practices, such as organic farming or using natural pest control, reduce the use of harmful chemicals, protecting soil organisms and pollinators. Breeding programs for endangered species in zoos or botanical gardens, followed by reintroduction into the wild, help to boost populations and genetic diversity.

Negative Human Impacts on Biodiversity

Unfortunately, many human activities threaten biodiversity. Deforestation, driven by human expansion and resource needs, destroys critical habitats, leading to species loss. Pollution, including air, water, and soil contamination from industries, agriculture, and domestic waste, can poison organisms and disrupt their reproductive cycles. Over-exploitation, such as unsustainable fishing or hunting, depletes species populations, sometimes to extinction. The introduction of non-native (invasive) species can outcompete native species for food and habitat, leading to declines in indigenous populations and even extinctions. Climate change, largely due to human-induced greenhouse gas emissions, causes habitat shifts, extreme weather events, and changes in species distribution, all impacting biodiversity negatively.

Measuring Biodiversity - A Practical Approach

Biodiversity can be quantitively measured and monitored. A common method involves sampling an area, for example, using quadrats for plants or small, slow-moving animals, or pitfall traps for ground-dwelling insects. For larger or more mobile animals, direct observation or camera traps might be used. The number of different species found in a defined area is known as species richness. The evenness of species abundance (how many individuals of each species are present) contributes to overall biodiversity. Repeat sampling over time and in different areas allows scientists to identify changes and assess the impact of human activities or conservation efforts. This data is vital for making informed decisions about ecosystem management.

Layers of Defence

The immune system has multiple layers of defence. The first line includes physical barriers such as the skin, which acts as a tough, impenetrable shield, and mucous membranes found in the respiratory and digestive tracts, which trap pathogens. Chemical barriers, like stomach acid, which kills most ingested microbes, and tears, which contain antimicrobial enzymes, also provide protection. These initial defences aim to prevent pathogens from entering the body altogether.

Nonspecific (Innate) Immunity

If pathogens bypass the first line of defence, the innate immune system provides an immediate, general response. Phagocytes, a type of white blood cell, are key players here. They detect foreign invaders and engulf them through a process called phagocytosis, effectively digesting and destroying the pathogens. Inflammation, another part of the innate response, localises infection, preventing its spread and attracting more immune cells to the affected area. This response is not specific to any particular pathogen.

Specific (Adaptive) Immunity

The adaptive immune system provides a highly targeted and long-lasting defence. It involves lymphocytes, specifically B-cells and T-cells. B-cells produce antibodies that are unique to specific pathogens. These antibodies flag pathogens for destruction or neutralise them directly. T-cells directly kill infected body cells or coordinate the immune response. A crucial aspect of adaptive immunity is immunological memory, where the immune system 'remembers' previous encounters with pathogens, allowing for a faster and stronger response upon re-exposure, leading to immunity.

The Role of White Blood Cells

White blood cells (leucocytes) are central to the immune response. Phagocytes, such as neutrophils and macrophages, are involved in innate immunity, engulfing and destroying pathogens. Lymphocytes, including B-cells and T-cells, are responsible for adaptive immunity. B-cells produce specific antibodies to target pathogens. T-cells identify and destroy infected cells and coordinate other immune cells. The coordinated action of these cells forms the backbone of the body's defence against disease.

What is Cancer?

Cancer is a disease caused by fundamental changes within cells that lead to uncontrolled growth and division. Normally, cells in the body grow, divide, and die in a carefully regulated cycle to maintain tissue health and function. This regulation ensures old or damaged cells are replaced appropriately and new cells are produced only when needed. In cancer, this normal control over cell division is lost, leading to an abnormal increase in cell numbers.

Changes in Cells Leading to Cancer

The 'changes' in cells that cause cancer typically involve alterations to their genetic material (DNA). These alterations, often called mutations, can affect genes that control cell growth, division, and death. When these control mechanisms are disrupted, cells no longer respond to the signals that would normally stop their division or trigger programmed cell death. This allows them to replicate without restraint, forming a mass of cells.

Uncontrolled Cell Growth and Division

Uncontrolled cell growth and division are the defining characteristics of cancer. Instead of dividing only when necessary, cancerous cells divide continuously and rapidly. This excessive proliferation leads to the formation of tumours, which are abnormal masses of tissue. These cells do not mature and specialise correctly, often remaining in an immature state, and they can compete with healthy cells for nutrients and space, disrupting normal tissue function.

Types of Tumours

There are two main types of tumours: benign and malignant. Benign tumours are typically non-cancerous. They grow slowly, do not invade surrounding tissues, and do not spread to other parts of the body. They are usually encapsulated and can often be removed surgically. Malignant tumours, on the other hand, are cancerous. They grow rapidly, invade nearby tissues, and can spread to distant parts of the body through the bloodstream or lymphatic system, a process known as metastasis, forming secondary tumours.