A student prepares a stained blood smear and examines it under a microscope, identifying two different types of white blood cell. - Cell P has a large, round nucleus that almost fills the whole cell. - Cell Q has a nucleus divided into several lobes, and appears to be surroundin…

Answer: ((a)(i)) Cell P is a lymphocyte; its function is to produce antibodies ((a)(ii)) Cell Q is a phagocyte; its function is to engulf pathogens by phagocytosis ((b)) When a blood vessel is damaged, the soluble plasma protein fibrinogen is converted into the insoluble protein fibrin, which forms a mesh of fibres across the wound; this mesh traps blood cells and forms a clot that seals the wound ((c)) Any two from: it prevents further loss of blood from the body; it prevents pathogens from entering the body through the open wound. Part (a)(i): Identifying cell P Cell P has a large, round nucleus that almost fills the whole cell. This is the typical appearance of a lymphocyte. Its function is to produce antibodies, which help to defend the body against pathogens. Par…

IGCSE Biology 0610: Lymphocytes, Phagocytes and Blood Clotting

A student prepares a stained blood smear and examines it under a microscope, identifying two different types of white blood cell.

Cell P has a large, round nucleus that almost fills the whole cell.
Cell Q has a nucleus divided into several lobes, and appears to be surrounding and taking in a small rod-shaped bacterium.

Later, the same student pricks their own finger with a sterile lancet to obtain the blood sample, and notices that the small wound stops bleeding and seals over within a few minutes.

(a) (i) Identify cell P and state its function. [2]

(a) (ii) Identify cell Q and state its function. [2]

(b) Describe the process that seals the finger wound, in terms of the conversion of one protein into another to form a mesh. [3]

Part (a)(i): Identifying cell P

Cell P has a large, round nucleus that almost fills the whole cell. This is the typical appearance of a lymphocyte. Its function is to produce antibodies, which help to defend the body against pathogens.

Part (a)(ii): Identifying cell Q

Cell Q has a lobed nucleus and is shown surrounding and taking in a bacterium. This behaviour and appearance identify it as a phagocyte. Its function is to engulf pathogens by phagocytosis, destroying them after taking them into the cell.

Part (b): Sealing the wound. Blood clotting

When the finger is pricked, the wall of a small blood vessel is damaged. This triggers a series of reactions in which the soluble plasma protein fibrinogen is converted into the insoluble protein fibrin. Threads of fibrin form a mesh across the wound. This mesh traps red blood cells and platelets, forming a clot that seals the damaged vessel and stops the bleeding within a few minutes, exactly as the student observes.

Part (c): Why this sealing process is important

Clotting is important for two main reasons. First, it prevents further loss of blood from the body through the wound. Second, by sealing the wound, it prevents pathogens from entering the body through the break in the skin, reducing the risk of infection.

Final answers

(a)(i) Cell P: lymphocyte; produces antibodies.
(a)(ii) Cell Q: phagocyte; engulfs pathogens by phagocytosis.
(b) Fibrinogen (soluble) is converted into fibrin (insoluble), which forms a mesh across the wound, trapping cells to form a clot.
(c) Any two of: prevents further blood loss; prevents pathogens entering the body through the wound.