4.2.4 Endocytosis & Exocytosis

• The processes of diffusion, osmosis and active transport are responsible for the transport of individual molecules or ions across cell membranes
• However, the bulk transport of larger quantities of materials into or out of cells is also possible
• Examples of these larger quantities of materials that might need to cross the membrane include:
  • Large molecules such as proteins or polysaccharides
  • Parts of cells
  • Whole cells eg. bacteria

• Bulk transport into cells = endocytosis
• Bulk transport out of cells = exocytosis
• These two processes require energy and are therefore forms of active transport

Endocytosis

• Endocytosis is the process by which the cell surface membrane engulfs material, forming a small sac (or ‘endocytic vacuole’) around it
• There are two forms of endocytosis:
  • Phagocytosis:
    • This is the bulk intake of solid material by a cell
    • Cells that specialise in this process are called phagocytes
    • The vacuoles formed are called phagocytic vacuoles
    • An example is the engulfing of bacteria by phagocytic white blood cells

  • Pinocytosis:
    • This is the bulk intake of liquids
    • If the vacuole (or vesicle) that is formed is extremely small then the process is called micropinocytosis

The process of phagocytosis of a bacterium by a phagocyte (white blood cell)

Exocytosis

• Exocytosis is the process by which materials are removed from, or transported out of, cells (the reverse of endocytosis)
• The substances to be released (such as enzymes, hormones or cell wall building materials) are packaged into secretory vesicles formed from the Golgi body
• These vesicles then travel to the cell surface membrane
• Here they fuse with the cell membrane and release their contents outside of the cell
• An example is the secretion of digestive enzymes from pancreatic cells

The process of exocytosis