Cell Transport


In order to survive, cells must maintain a biological balance (homeostasis) with their environment; they do this by regulating what comes into and goes out of the cell


-Passive vs. Active Transport

  Passive  Active
Cell Energy (ATP) Required No Yes
Concentration Gradient & Molecular Movement molecules move
 along the gradient
("high to low")
molecules are forced
 against the gradient
("low to high")
Transport Methods

facilitated diffusion

transport pumps


-Crossing the Membrane

-to enter or leave a cell you can:

     1. move directly through the lipid bilayer
          -works for small molecules that are chemically similar 
            to the phospholipids
                -gases (O2 and CO2) and water
          -typically associated with diffusion and osmosis

     2. cross the bilayer through a channel / transport protein
           -works for small, polar molecules 
                 -sugars and ions
           -typically associated with facilitated diffusion

     3. manipulate the bilayer with the use of vacuoles / vesicles
           -works for large substances
                  -enzymes, hormones, food bits and waste particles
           -typically associated with endo- and exocytosis


-The Movement of Water

water molecules do move via diffusion but water is so important to a cell that its natural movement gets its own term (osmosis) 

     -the diffusion of water
          -movement of water from high to low concentration
                -passive transport - no energy required

-Sometimes, cells will force water against the concentration gradient
     -ex: paramecium in a pond must expel water to avoid cytolysis
     -this requires energy & is not osmosis

active transport is involved anytime water is forced against its natural flow 



to great cell transport animations

-Short Animations of
diffusion: Ohio University 
-osmosis: Westminster Secondary School
-transport proteins: The College of St. Scholastica  

Animated Movies
Pearson Prentice Hall  (includes audio narration)   
-Northland Community and Technical College 


to more detailed notes

-Florida State University
-Clinton Community College
-University of Connecticut