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Transport Across the Membrane

Transport Across the Membrane


By the end of this lesson, you will be able to:

  1. define diffusion and osmosis, explain the cause of these phenomena, and explain passive transport through membranes (including aquaporins).
  2. explain the role of osmotic pressure (hydrostatic pressure) in helping the cell to reach osmotic equilibrium, 
  3. define and use the terms hypotonic, hypertonic  and isotonic in explaining the movement of materials across a selectively permeable membrane and in osmoregulation, 
  4. describe and distinguish between A facilitated diffusion and active transport        B active and passive transport.            C endocytosis and exocytosis.          D phagocytosis, pinocytosis, receptor-mediated endocytosis.
  5. define and give the characteristics of specific active transport mechanisms.  Be able to give examples and explain the workings of the following specific examples of active transport:  A   . simple uniport pumps    B. the sodium-potassium pump (as an antiport system).   C   coupled channels/cotransport (as syport or antiport systems).

All materials that pass into the cell must pass through the cell membrane.  In some cases these materials pass through by passive transport (diffusion, osmosis), while in other cases something called active transport is involved.  Proteins are involved in one form of passive transport: facilitated diffusion. All active transport mechanisms involve the use the membrane proteins and metabolic energy in the form of ATP.  This tutorial examines the transport mechanics and how the structure of the membrane assists in this function

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Passive Transport

Passive transport uses the energy already present in the system. Diffusion and Osmosis are reviewed as well as Facilitated Diffusion which differs in that it involves a membrane protein. In all cases though, movement is down a concentration gradient.

Source: M. O'Mahony, open source figures

PhET Simulation of Diffusion and Osmosis

PhET always has amazing interactive simulations.  They have one for passive transport through membranes.  This url takes you to the PhET page for this simulation.  You can safely download and run it. It is fairly simplistic for you at this point- but IS an excellent review.

Source: PhET, University of Colorado Boulder

Molecular Workbench

Go to the site:

and select Diffusion

The Majority of this page is about passive transport but there is a nice simulation about active transport as well.

This simulator requires Java.

Source: Molecular Workbench, The Concord Consortium

Active Transport

This screencast explores the various types of transport across the membrane and into the cell that require and additional input of energy in the form of adenosine triphosphate (ATP). Movement can be Single Protein (Uniport, Symport or Antiport) or it could be Bulk Transport of greater amounts of materials either into (Endocytosis) or out of (Exocytosis) the cell. A number of excellent simulations are available on the web; a number of these are embedded into the slides that follow.

Source: M. O'Mahony, figures identified for non-commercial reuse, websites


Student notes template to accompany video


Source: M. O'Mahony, open source figures

Active Transport Animations

Source: American Society for Microbiology

McGraw-Hill Animations


Cotransport (Symport and Antiport)

Antiport:  Sodium-Potassium Pump

Endocytosis and Exocytosis     


Source: McGraw-Hill, Raven, et al., Chapter Animations