Endocytosis & Exocytosis (Cambridge (CIE) AS Biology): Revision Note

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 volumes 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 e.g., bacteria

• Bulk transport into cells = endocytosis

• Bulk transport out of cells = exocytosis

  • These two processes require energy input and are therefore forms of active transport

Endocytosis

• Endocytosis is the process by which the cell surface membrane folds inwards and engulfs material, forming a small sac (or ‘endocytic vesicle’) 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

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 or hormones) 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