Daily Newsletter January 25, 2012

 

 Daily Newsletter                                        January 24, 2012

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Today's Topic: Passive Transport


The cell membrane is selectively permeable, so some things can make it through the phospholipid bilayer.  For other chemicals, we need to provide a protein to serve as a pore, channel or transporter.  In general, there are two ways that a chemical can be moved across the membrane:  Down the chemical's concentration gradient, or Against the chemical's concentration gradient.  When a chemical moves down it's concentration gradient, we do not need to add energy to the process.  The concentration gradient basically powers it's self.


There are three basic forms of passive transport through the membrane:

1. Diffusion - Using the inherent Brownian motion of molecules, chemicals move from points of high concentration to points of low concentration.
2. Osmosis - The movement of water across a selectively permeable membrane. 
• Water goes to where the party is, meaning it will move to a compartment that has a higher solute concentration.
• Since we are talking about two fluid compartments on either side of a membrane, we are ultimately talking about relative solute concentrations.
• In biology we always use the inside of the cell as our reference, so:
• An isotonic fluid has the same solute concentration as the inside of the cell.
• A hypertonic solution has a solute concentration that is higher than the solute concentration inside the cell.
• A hypotonic solution has a solute concentration that is lower than the solute concentration inside the cell.
• Remember that we are looking at solute concentrations, not the concentration of a single chemical.
• Diffusion gradients are specific for each chemical.
• Osmosis is determined by total solute concentration.
3. Facilitated Diffusion - Ultimately, this is diffusion, but the cell has had to provide a passage for the chemical.  So, this only applies to chemicals that can not normally pass through the phospholipid bilayer.
• The cell provides a protein channel or pore for the chemical to pass through.
• Each channel or pore is specific to a single chemical or set of chemicals.
• Since this is an protein (enzyme) mediated action, the ability to transport follows standard enzyme kinetics.
• The core idea about facilitated diffusion following enzyme kinetics is that diffusion becomes limited by the number of channels or pore available.  Standard diffusion is not limited.

In each case, the cell does not need to expend energy to move the chemical across the membrane.  The motive force is built in to the chemical gradients.  Remember, the cell membrane is going to provide an internal vs. an external space (compartment).  Each side of the membrane will have a unique chemical profile.  As such, there will be concentration gradients across the membrane.

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Daily Challenge:  Passive transport
In your own words, describe passive transport.  Concentrate on osmosis and facilitated diffusion by providing an example that illustrates each action.  Make sure you use an example that uses a cell (not beakers, flasks, etc...).