Structures and Functions in Living Organisms · 6 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 15% of your exam marks.
The heart, blood vessels, and blood components are regularly tested, particularly structure-function links.
Phagocytes are the rapid-response part of the immune system. They patrol the blood and tissues looking for anything that does not belong (bacteria, viruses, fungi). When they find a pathogen they engulf it whole (a process called ) and digest it inside themselves using digestive enzymes. This is a non-specific response: a phagocyte will engulf any pathogen it meets, not just one type.
Lymphocytes are the targeted-response part of the immune system. They produce antibodies: proteins that exactly fit a specific feature on the surface of one type of pathogen. This is a specific response, since each lymphocyte makes antibodies against only one type of pathogen.
When antibodies bind to pathogens, they:
Describing how a phagocyte destroys a pathogen
What comes up: a "describe how a phagocyte destroys a bacterium" question (often 4 marks, awarded for distinct stages).
Write (four marks): (1) The phagocyte engulfs the bacterium / surrounds it with pseudopodia (1). (2) The bacterium ends up inside a vesicle within the phagocyte (1). (3) Vesicles containing digestive enzymes fuse with the vesicle holding the bacterium (1). (4) The enzymes digest / break down the bacterium (1).
Watch out: simply writing "the phagocyte engulfs and digests the bacterium" scores only 1–2 marks. You need to include the vesicle and the enzyme-fusion step to reach full marks.
The first time you meet a new pathogen, the immune response is slow (it takes the lymphocytes time to find the right one and multiply), and you feel ill. By the time the pathogen is cleared, the body has a stock of memory cells specific to that pathogen.
If the same pathogen invades a second time, the memory cells recognise it immediately and produce more antibodies, faster, and in greater quantities than the first time around. The pathogen is destroyed before it has a chance to cause symptoms. You are now immune to that pathogen.
Vaccination is a way of getting the memory-cell benefit without ever needing to suffer the disease. A vaccine contains a harmless version of a pathogen, prepared in one of three ways:
The vaccine carries the pathogen's without being able to cause the disease. The body responds as if it has been infected:
If the real pathogen ever attacks later, the memory cells trigger a fast, strong antibody response and the person remains symptom-free. Vaccination has dramatically reduced or eradicated diseases including smallpox (eradicated worldwide), polio (almost eradicated), measles, rubella, tetanus and many others.