USMLE Step 1 Physiology Review 52 09 Membrane Transport (1 of 2)

USMLE Step 1 Physiology Review 52 09 Membrane Transport (1 of 2)

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Play USMLE Audio MP3 52 09 Membrane Transport (1 of 2) Below

Begin 52 09 Membrane Transport (1 of 2) Transcription

Now let’s learn about the electrophysiology of membrane transport.  The function of the human body depends on the transport of substances across cell membranes.

What is this a description of: the migration of molecules from a region of high concentration to one of lower concentration as a result of random motion?

  • Diffusion.

Certain substances can move across cell membranes by simple diffusion.  All of these substances possess a certain quality related to solubility.  What is that quality?  Pause the tape.

  • Substances that can move across cell membranes by simple diffusion are lipid soluble substances, such as steroids, oxygen and anaesthetic acids.

The net diffusion of water across selectively permeable membranes is called what?

  • Osmosis.

What determines the osmolarity of a solution?

  • The total number of particles in the solution.

Is a solution with low osmolarity one that has proportionally more or less water than a solution with high osmolarity?

  • A solution with low osmolarity is one that has proportionally more water.  Remember, osmolarity refers to the number of particles in the solution.

Student Doctor, please pause the tape and summarize the information discussed this far about diffusion and osmosis.

  • The function of the human body depends on the transport of substances across cell membranes.  Diffusion is the migration of molecules from a region of high concentration to one of lower concentration as a result of random motion.  Lipid soluble substances such as steroids, oxygen and anesthetic acids can move across cell membranes by simple diffusion.  The net diffusion of water across selectively permeable membranes is called osmosis.  The total number of particles in a solution determines its osmolarity.  A solution with low osmolarity is one that has proportionally more water than a solution with high osmolarity.

What is the term for hydrostatic pressure that prevents the flow of water across a semi-permeable membrane?

  •  Osmotic pressure.

If you have two solutions, one high osmolarity and one low osmolarity, separated by a semi-permeable membrane, will water diffuse from high to low osmolarity or from low to high osmolarity?  Pause the tape.

  •  Water will diffuse from a low osmolarity solution to a high osmolarity solution until water concentration on both sides of the membrane is equal.

What quality of the solution is determined by the concentration of non-penetrating solutes?

  • The tonicity of a solution.

If you put a cell in a hypertonic solution, will water diffuse out of the cell or into the cell?

  • Water will diffuse out of the cell, and the cell will shrink.

What will happen if you put a cell in a hypotonic solution?

  • Water will diffuse into the cell and the cell will swell.

Student Doctor, please pause the tape and summarize the information about osmosis discussed since the last summary.

  • We started out with a simple definition of osmotic pressure.

Osmotic pressure is the term for the hydrostatic pressure that prevents the flow of water across a semi-permeable membrane.  If you have two solutions separated by a semi-permeable membrane, water diffuse from a low osmolarity solution to a high osmolarity solution until water concentration on both sides of the membrane is equal.  The tonicity of a solution is determined by the concentration of non-penetrating solutes.  If you put a cell in a hypertonic solution, water will diffuse out of the cell, and the cell will shrink.  If you put a cell in a hypotonic solution, water will diffuse into the cell, and the cell will swell.

Now for some questions about mediated transport systems.

Many substances cannot cross the cell membrane by simple diffusion because they are hydrophilic and/or too large.  What are the molecules that facilitate the transfer of hydrophilic or lipid insoluble substances across the cell membrane?  Pause the tape.

  • They are carrier proteins that are embedded in the membrane and permit the transport of lipid insoluble substances across the membrane.

Binding sites of the carrier proteins are specific for certain kinds of substances.  How can structurally related compounds inhibit the cross membrane transfer of the intended solutes?

  •  Structurally related compounds compete for the binding sites on the carrier proteins and inhibit the binding of the intended solute.

Student Doctor, please pause the tape and summarize the information discussed this far about mediated transport systems.

Many substances cannot cross the cell membrane by simple diffusion because they are hydrophilic and/or too large.  Carrier proteins that are embedded in the membrane permit the transport of lipid insoluble substances across the membrane. The binding sites of the carrier proteins are specific for certain kinds of substances.  Structurally related compounds can compete for the binding sites on the carrier proteins and inhibit the binding of the intended solute.

 

 

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