Biology QCE Unit 2

What is the difference between endotoxins and exotoxins?

Exotoxins are proteins secreted by bacteria that directly damage host cells, whereas endotoxins are components of the outer membrane of gram-negative bacteria (lipopolysaccharides) that trigger strong immune and inflammatory responses when the cell dies.

What are endospores and why are they significant for bacterial survival?

Endospores are highly resistant, dormant bacterial structures that allow survival in extreme conditions such as heat, desiccation, and chemicals. They enable bacteria to persist until conditions become favourable again.

How do bacteria reproduce? How is this different from mitosis?

Bacteria reproduce asexually by binary fission, where the circular chromosome replicates and the cell splits in two. Unlike mitosis, binary fission does not involve a nucleus, spindle fibres, or distinct phases (prophase, metaphase, etc.)

Compare the roles of internal parasites and external parasites in causing disease.

Internal parasites (e.g., tapeworms) live inside the host and absorb nutrients, often damaging tissues and disrupting organ function. External parasites (e.g., ticks, lice) feed on blood or tissues from the outside, often acting as vectors that transmit pathogens between hosts.

Explain how fungal infections typically differ in severity between healthy and immunocompromised individuals.

In healthy individuals, fungal infections are usually superficial and controlled by the immune system (e.g., athlete’s foot). In immunocompromised individuals, fungi can spread systemically, causing life-threatening infections because the weakened immune system cannot contain them.

Contrast fungi and protists in terms of structure and mode of nutrition.

Fungi are eukaryotic, multicellular (except yeasts), with chitin cell walls, and they are heterotrophic decomposers that absorb nutrients externally. Protists are diverse, mostly unicellular eukaryotes, with variable nutrition — they may be autotrophic (algae), heterotrophic (amoeba), or both.

Explain why most bacteria in the human gut are beneficial rather than pathogenic.

Most gut bacteria are mutualistic: they aid in digestion, synthesise vitamins (e.g., vitamin K), and outcompete harmful microbes. Pathogenic bacteria are in the minority, but can cause disease when balance is disrupted.

Explain how rapid bacterial reproduction can contribute to virulence.

Because bacteria reproduce rapidly by binary fission, they can quickly increase population size, accumulate mutations that enhance survival, and overwhelm the host’s immune system before effective responses are mounted.

Tapeworms are internal parasites. Explain how their presence in the intestine disrupts normal body function and how this can spread to new hosts.

Tapeworms absorb nutrients directly from the host’s digested food, leading to malnutrition and weight loss. Eggs or segments are shed in faeces, contaminating food or water and infecting new hosts when ingested.

Examples of the inflammatory response include

the release of prostaglandins from damaged body cells.

In adaptive immunity, which cells directly destroy virally infected cells?

T cytotoxic cells

The blood of a newborn baby was tested and found to contain IgG antibodies against the influenza (flu) virus. The most likely explanation for the presence of these antibodies is that

the baby’s mother was immunised against the influenza virus whilst pregnant.

Antigen-presenting cells deliver antigens to lymphocytes found within lymphoid tissue. These lymphocytes recognise these antigens as being non-self, causing the lymphocytes to become activated and increase in number.

Which of the following lymphocytes are activated by antigen-presenting cells?

T Helper cells

In adaptive immunity, which part of this process allows long-term (sometimes lifetime) protection against pathogens?

Generation of memory cells

Which of the following best defines an innate immune defence?

An immune defence that requires no learning

What are the two branches of the adaptive immune response?

Humoral response (B cells + antibodies) and cell-mediated response (T cells).

What do B lymphocytes produce?

Antibodies that bind to antigens, neutralising pathogens or marking them for destruction.

What do cytotoxic T cells do?

Kill infected host cells by recognising antigen fragments on MHC I markers.

Role of helper T cells?

Activate B cells and cytotoxic T cells; release cytokines to regulate immune response.

Role of memory cells in adaptive immunity?

Provide faster and stronger response on second exposure to the same pathogen.

Main difference between humoral and cell-mediated immunity?

Humoral targets pathogens in body fluids (extracellular), cell-mediated targets infected cells (intracellular).

What is herd immunity?

Herd immunity occurs when a high proportion of a population is immune (through vaccination or previous infection), reducing the spread of disease and protecting individuals who are not immune.

Define vehicle transmission

The virus is spread by touching contaminated surfaces, or coming into contact with contaminated food or water.

Define inflammation

Your body's response to damage or infection. It's when fluid, proteins, and immune cells rush to the area, causing heat, pain, swelling, redness, and temporary loss of function.

Explain why a secondary immune response would be so much greater than the primary response

During a second infection with the virus a stronger response occurs because, during the first exposure, memory cells were created. There are so many of these that a strong and quick response can occur.

What is the adaptive immune response?

A highly specific, long-lasting defense system that learns to recognize and remember specific invaders (pathogens).

Two main branches of the adaptive immune response

• Cell-mediated immunity (T-cells directly kill infected cells).

• Humoral immunity (B-cells produce antibodies to tag pathogens).

Role of T-cells (cytotoxic)

Directly destroy body cells that have been infected by a pathogen (cell-mediated immunity).

Role of T-helper cells

T-helper cells are the managers of the adaptive immune response. They recognize pathogens and activate other immune cells, like B-cells and cytotoxic T-cells, to fight the infection.

Role of B-cells

Produce antibodies that attach to pathogens, marking them for destruction by other immune cells (humoral immunity).

How does immunological memory work?

After an infection, memory T-cells and B-cells remain in the body. If the same pathogen enters again, they launch a much faster and stronger response.

Adaptive vs. Innate Immunity

Innate immunity is the body's non-specific, first line of defense (like skin, fever). Adaptive immunity is the highly specific, learned response that creates memory.

How do vaccines relate to the adaptive immune response?

Vaccines introduce a harmless piece of a pathogen (an antigen) to the body, allowing the adaptive immune system to create memory cells without causing illness.