 The CM determines what gets in and out of cells.  Some molecules pass through easily including lipids that dissolve in the lipid layer of the CM.  Small molecules such as water, glucose and amino acids also pass through easily.

 Some molecules do not pass through easily including large molecules (starch, protein) and those that carry charges (attract to opposite charge on CM).  How cells move substances across the cell membrane involves a number of different processes.

 Molecules of liquids and gases are always in motion. These molecules collide with each other,and thus undergo diffusion.  Diffusion: Movement of molecules from an area of high concentration, to one of low concentration(Ex. Perfume,coffee)

 The difference in concentration between a region of greater concentration and a region of lesser concentration is called a concentration gradient (shown below).

Low O 2 level High CO 2 level High O 2 level Low CO 2 level Movement of gases in cellular respiration involves concentration gradients.Note the direction of movement from high to low.

 Click here for animation. Click here for animation.

 This is a “fast” form of diffusion.  Molecules get some help crossing the membrane.  Evidence shows that there are proteins in the cell membrane with hollow channels.  Molecules pass through these channels along a concentration gradient.

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 This is another way to move materials into and out of cells.  In this case the cell must use energy to move substances against a concentration gradient.  This allows the cell to maintain internal conditions that are chemically different from the external environment.

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 Osmosis is a type of diffusion.  In this case, water moves across a semipermeable membrane from an area of high water concentration to one of low water concentration.  Animation Animation

Name Type of Transport Direction of Movement ConditionsExamples Diffusion Passive towards lower concentrati on Concentrati on gradient Water, gases (0 2 and CO 2 ), and steroid hormones. Facilitated Diffusion Passive towards lower concentrati on Concentrati on gradient, plus channel or carrier proteins Water, glucose, and amino acids. Osmosispassive towards lower concentrati on Concentrati on gradient, channel proteins optional Water 0nly. Active Transport active towards higher concentrati on Carrier protein and ATP energy Ions, sugars, and amino acids.