CIE A Level Biology (9700) 2019-2021

Revision Notes

4.2.4 Endocytosis & Exocytosis

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

Phagocytosis of a bacterium, downloadable AS & A Level Biology revision notes

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

Exocytosis in a secretory cell, downloadable AS & A Level Biology revision notes

The process of exocytosis

Exam Tip

Remember – active transport, endocytosis and exocytosis all require energy. This energy is provided by ATP produced during respiration.

Use the following terms in questions on the bulk transport of materials – endocytosis (phagocytosis, pinocytosis, micropinocytosis, macropinocytosis) and exocytosis NOT bulk transport.

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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