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.
An adult human carries around 5 litres of blood. It is made up of four main components:
| Component | What it looks like | Roughly what fraction of blood |
|---|---|---|
| Plasma | Pale yellow watery liquid | About 55% by volume |
| Red blood cells | Tiny biconcave discs, no nucleus, red colour | About 45% by volume |
| White blood cells | Larger cells with a nucleus, less than 1% by volume | A small fraction |
| Platelets | Tiny fragments of cells, no nucleus | A small fraction |
Plasma is the watery fluid the other components are suspended in. About 90% of it is water, with all sorts of useful substances dissolved in it:
Red blood cells are highly specialised for one job: carrying oxygen from the lungs to every respiring cell in the body. Three adaptations make them excellent at this:
A red blood cell only lives for about 4 months. Old ones are broken down in the liver, and new ones are made constantly in the bone marrow.
White blood cells belong to the immune system. Their job is to defend the body against pathogens. Two main types exist:
A typical white blood cell is much larger than a red blood cell and has a prominent nucleus when viewed under a microscope. Both types are covered in detail in section 6.
Platelets are not whole cells but tiny fragments broken off from much larger cells in the bone marrow. They have no nucleus. Their job is to make the blood clot at a wound, which seals the cut, prevents excessive blood loss, and stops bacteria getting in. Covered in section 8.