Passive Transport is Diffusion of a Substance Across a Membrane with No Energy Investment

Related Posts

The diffusion of solutes across a synthetic membrane. Each of the large arrows under the diagrams shows the net diffusion of the dye molecules of that color.
Source: Urry, Lisa A.. Campbell Biology (p. 133). Pearson Education. Kindle Edition.

Passive Transport is Diffusion of a Substance Across a Membrane with No Energy Investment (Campbell Biology)

Molecules have a type of energy called thermal energy, due to their constant motion. One result of this motion is diffusion, the movement of particles of any substance so that they spread out into the available space. Each molecule moves randomly, yet diffusion of a population of molecules may be directional. To understand this process, let’s imagine a synthetic membrane separating pure water from a solution of a dye in water. Study the photos carefully to appreciate how diffusion would result in both solutions having equal concentrations of the dye molecules. Once that point is reached, there will be a dynamic equilibrium, with roughly as many dye molecules crossing the membrane each second in one direction as in the other.

We can now state a simple rule of diffusion: In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient, the region along which the density of a chemical substance increases or decreases (in this case, decreases). No work must be done to make this happen; diffusion is a spontaneous process, needing no input of energy. Note that each substance diffuses down its own concentration gradient, unaffected by the concentration gradients of other substances.

Much of the traffic across cell membranes occurs by diffusion. When a substance is more concentrated on one side of a membrane than on the other, there is a tendency for the substance to diffuse across the membrane down its concentration gradient (assuming that the membrane is permeable to that substance). One important example is the uptake of oxygen by a cell performing cellular respiration. Dissolved oxygen diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the oxygen as it enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction.

The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion. Remember, however, that membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. In the case of water, the presence of aquaporin proteins allows water to diffuse very rapidly across the membranes of certain cells compared with diffusion in the absence of aquaporins. As we’ll see next, the movement of water across the plasma membrane has important consequences for cells.


Urry, Lisa A.. Campbell Biology. Pearson Education. Kindle Edition.

Related Research

Research Article: Psychosocial and Environmental Correlates of Walking, Cycling, Public Transport and Passive Transport to Various Destinations in Flemish Older Adolescents

Date Published: January 19, 2016 Publisher: Public Library of Science Author(s): Hannah Verhoeven, Dorien Simons, Delfien Van Dyck, Jelle Van Cauwenberg, Peter Clarys, Ilse De Bourdeaudhuij, Bas de Geus, Corneel Vandelanotte, Benedicte Deforche, Harry Zhang. Abstract: Active transport is a convenient way to incorporate physical activity in adolescents’ daily life. The present study aimed … Continue reading

Research Article: Genetic Evidence of Expansion by Passive Transport of Aedes (Stegomyia) aegypti in Eastern Argentina

Date Published: September 1, 2016 Publisher: Public Library of Science Author(s): Leonardo M. Díaz-Nieto, Marina B. Chiappero, Clara Díaz de Astarloa, Arnaldo Maciá, Cristina N. Gardenal, Corina M. Berón, Christopher M. Barker. Abstract: None Partial Text: Aedes (Stegomyia) aegypti (Linnaeus) (Diptera: Culicidae) is the principal vector of the yellow fever virus, the five dengue virus … Continue reading

Research Article: Which Socio-Ecological Factors Associate with a Switch to or Maintenance of Active and Passive Transport during the Transition from Primary to Secondary School?

Date Published: May 27, 2016 Publisher: Public Library of Science Author(s): Griet Vanwolleghem, Delfien Van Dyck, Femke De Meester, Ilse De Bourdeaudhuij, Greet Cardon, Freja Gheysen, Jake Olivier. Abstract: The aim was to investigate which individual, psychosocial and physical neighborhood environmental factors associate with children’s switch to or maintenance of active/passive transport to school … Continue reading

Research Article: Psychosocial and environmental correlates of active and passive transport behaviors in college educated and non-college educated working young adults

Date Published: March 20, 2017 Publisher: Public Library of Science Author(s): Dorien Simons, Ilse De Bourdeaudhuij, Peter Clarys, Katrien De Cocker, Bas de Geus, Corneel Vandelanotte, Jelle Van Cauwenberg, Benedicte Deforche, Ruth Jepson. Abstract: This study aimed to examine potential differences in walking, cycling, public transport and passive transport (car/moped/motorcycle) to work and to … Continue reading

Research Article: Personal and Environmental Characteristics Associated with Choice of Active Transport Modes versus Car Use for Different Trip Purposes of Trips up to 7.5 Kilometers in The Netherlands

Date Published: September 5, 2013 Publisher: Public Library of Science Author(s): Eline Scheepers, Wanda Wendel-Vos, Elise van Kempen, Luc Int Panis, Jolanda Maas, Henk Stipdonk, Menno Moerman, Frank den Hertog, Brigit Staatsen, Pieter van Wesemael, Jantine Schuit, Noel Christopher Barengo. Abstract: This explorative study examines personal and neighbourhood characteristics associated with short-distance trips made … Continue reading

Research Article: FGT-1 Is a Mammalian GLUT2-Like Facilitative Glucose Transporter in Caenorhabditis elegans Whose Malfunction Induces Fat Accumulation in Intestinal Cells

Date Published: June 24, 2013 Publisher: Public Library of Science Author(s): Shun Kitaoka, Anthony D. Morielli, Feng-Qi Zhao, Todd Lamitina. Abstract Caenorhabditis elegans (C. elegans) is an attractive animal model for biological and biomedical research because it permits relatively easy genetic dissection of cellular pathways, including insulin/IGF-like signaling (IIS), that are conserved in mammalian … Continue reading

Research Article: The Balance of Fluid and Osmotic Pressures across Active Biological Membranes with Application to the Corneal Endothelium

Date Published: December 31, 2015 Publisher: Public Library of Science Author(s): Xi Cheng, Peter M. Pinsky, Paul J Atzberger. Abstract: The movement of fluid and solutes across biological membranes facilitates the transport of nutrients for living organisms and maintains the fluid and osmotic pressures in biological systems. Understanding the pressure balances across membranes is … Continue reading

Research Article: Elucidating the Signal Responses of Multi-Parametric Surface Plasmon Resonance Living Cell Sensing: A Comparison between Optical Modeling and Drug–MDCKII Cell Interaction Measurements

Date Published: August 27, 2013 Publisher: Public Library of Science Author(s): Tapani Viitala, Niko Granqvist, Susanna Hallila, Manuela Raviña, Marjo Yliperttula, Mark J. van Raaij. Abstract: In vitro cell-based assays are widely used during the drug discovery and development process to test the biological activity of new drugs. Most of the commonly used cell-based … Continue reading

Research Article: A Synthesis of Tagging Studies Examining the Behaviour and Survival of Anadromous Salmonids in Marine Environments

Date Published: March 14, 2012 Publisher: Public Library of Science Author(s): S. Matthew Drenner, Timothy D. Clark, Charlotte K. Whitney, Eduardo G. Martins, Steven J. Cooke, Scott G. Hinch, A. Peter Klimley. Abstract: This paper synthesizes tagging studies to highlight the current state of knowledge concerning the behaviour and survival of anadromous salmonids in the … Continue reading