USMLE Step 1 Physiology Review 52 05 Solutions (1 of 2)

USMLE Step 1 Physiology Review 52 05 Solutions (1 of 2)

 

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Play USMLE Audio MP3 52 05 Solutions (1 of 2) Below

Begin 52 05 Solutions (1 of 2) Transcription

Now let’s review some information on solutions.

What is the true or molecular solution?

  • It is a homogenous molecular mixture of two or more substances.

What is the solvent in biological systems?

  • Water.

For a solute to interact with water, it must have certain characteristics.  Solutes must show some degree of what or what to interact with water?

  • Solutes must show some degree of polarity or charge to interact with water.

What process makes small, inorganic compounds soluble in water?

  • Ionization.

How are organic compounds made soluble in water?

  • Organic compounds are made soluble in water by binding with alcoholic hydroxyl groups, ionization and salt formation.

There are large organic molecules that have both water and lipid soluble portions.  How are they valuable in biological systems?  Here’s a clue: they form special carriers for fat in plasma.

  • They serve as an interface between organic compounds that are soluble in water and organic compounds that are not soluble in water.

Student Doctor, please pause the tape and summarize the information discussed so far on solutions.

  • A true or molecular solution is a homogenous molecular mixture of two or more substances.  Water is the solvent in biological systems.  Solutes must show some degree of polarity or charge to interact with water.  Ionization makes small inorganic compounds soluble in water.  Organic compounds are made soluble in water by binding with alcoholic hydroxyl groups, ionization, and salt formation.  Large organic molecules that have both water soluble and lipid soluble portions serve as an interface between organic compounds that are soluble in water and organic compounds that are not soluble in water.

What kind of solution is this a description of?  Particles of about one nanometer in diameter that are uniformly mixed in a solvent, but exist in distinct phases?

  • Colloidal solution.

How do the particles in colloidal solutions remain in solution rather than precipitating or settling out?  Pause the tape.

  • The particles in colloidal solutions remain in solutions rather than precipitating or settling out due to interaction with the dispersion medium, which is water, molecular motion and mutual repulsion due to electrical charges.

In general, do proteins form colloidal solutions or molecular solutions?

  • In general, proteins form colloidal solutions.

Which parts of protein molecules interact with water?

  • Protein molecules interact with water through the carboxyl and amino groups. 

Proteins are least soluble at the pH when the carboxyl groups and the amino groups interact more with one another, rather than with water.  What is this point called?

  • The iso-electric point.

Can plasma proteins pass through capillary walls?

  • No.

Since plasma proteins cannot pass through capillary walls, these proteins create what kind of force?

  • They create osmotic force.

What effect does this osmotic force have on capillaries?

  • It helps to keep fluid in capillaries.

Student Doctor, please pause the tape and summarize the information discussed so far on solutions.

Particles of one nanometer in diameter that are uniformly mixed in a solvent but exist in distinct phases are said to be in colloidal solution.  The particles in colloidal solutions remain in solutions rather than precipitating or settling out due to interaction with the dispersion medium, which is water, molecular motion and mutual repulsion due to electrical charges.  In general, proteins form colloidal solutions.  Proteins interact with water through the carboxyl and amino groups.  Proteins are least soluble at the pH when the carboxyl and amino groups interact more with one another rather than with water.  This is called the iso-electric point.  Plasma proteins cannot pass through capillary walls, and so they create osmotic force.  This osmotic force helps to keep fluid in capillaries.

 

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