• 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