OCR A Level Biology:复习笔记2.5.6 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

       

     

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

Exam Tip

Remember – active transport, endocytosis and exocytosis all require energy. This energy is provided by ATP produced during respiration.To get the mark in the exam you have to specifically state 'exocytosis' for bulk transport out of the cell and 'endocytosis' (or even better: phagocytosis, pinocytosis, micropinocytosis, macropinocytosis) for bulk transport into the cell. Simply stating 'bulk transport' is not specific enough, the examiner will want to know what type of bulk transport and for this you need to state the scientific name!

 

 

转载自savemyexams

更多Alevel课程
翰林国际教育资讯二维码