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Cambridge IGCSE Biology · 0610

Chapter 9: Transport in animals (Part 3)

Components of blood

Blood is a tissue consisting of four main components:

  1. Red blood cells: Biconcave cells that contain haemoglobin (a protein) which binds to oxygen for transport. They lack a nucleus to provide more space for haemoglobin.
  2. White blood cells: Involved in fighting off infection and pathogens.
  3. Platelets: Fragments of cells involved in blood clotting.
  4. Plasma: The liquid part of the blood that transports blood cells, ions, soluble nutrients, urea, hormones, and carbon dioxide.

Types of white blood cells

Phagocytes
These cells engulf pathogens through the process of phagocytosis.
  • Process: The phagocyte recognises a pathogen, engulfs it into the cell membrane, and releases enzymes to break it down and kill it.
Lymphocytes
These cells produce antibodies.
  • Function: Antibodies are specific to the antigens on the membrane of a pathogen. They bind to antigens, causing foreign cells to clump together, preventing them from entering body cells.
Four stages of phagocytosis showing a phagocyte approaching, engulfing, trapping, and digesting a pathogen
Diagram 1: Phagocytosis. Four stages show a phagocyte approaching a pathogen, extending its membrane to surround it, trapping the pathogen in a vacuole, and using enzymes to digest and destroy it.

Exam Traps

  • Avoid saying antibodies kill pathogens inside cells — they bind antigens on the pathogen surface.

Blood clotting

Clotting is vital to prevent blood loss and the entry of pathogens into the body through injuries.

Mechanism
When a vessel is broken, platelets stick to the wall and clump together to block the cut.
Fibrin
A soluble protein called fibrinogen is converted into an insoluble protein called fibrin. Fibrin forms a mesh of fibres across the wound that traps more platelets and red blood cells to form a scab.
Blood clotting process showing platelets at a damaged vessel wall and fibrin mesh trapping red blood cells
Diagram 2: Blood clotting process. A damaged blood vessel wall with platelets accumulating at the site. The mesh of fibrin fibres traps red blood cells to form a solid plug or scab.

Exam Traps

  • Do not say platelets produce fibrin — fibrin forms from fibrinogen during the clotting cascade.
  • Avoid describing fibrin as soluble; the mesh that forms the scab is insoluble fibrin.

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