# Physiology 2130 Lecture Notes - Lecture 4: Pressure Gradient, Osmotic Concentration, Osmosis

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5 Feb 2013
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Physiology Lecture 4 Osmosis
Osmosis
o Movement of water across a membrane in response to a solute concentration gradient
o Water moves to dilute the more concentrated solution
o Osmosis is affected by 3 things
Permeability of the membrane
Osmotic Pressure
o Pressure applied to oppose the osmotic movement of water
o In the previous example, the osmotic pressure would be the pressure needed to prevent
the movement of water from A to B
Predict Osmosis
o By knowing the osmolarity of solutions we can predict osmosis
o Osmolarity: the number of particles per litre of solution (osmol/L)
o Looking at the ability of a solution to cause osmosis
o Only depends on the number of particles not their size or composition
Osmolarity
o To calculate osmolarity we need to know
The number of moles of the substance in solution
Whether the substance dissociates in solution
Tonicity
o The ability of a solution to cause osmosis across biological cell membranes
o Osmolarity of body fluids: 300mOSM
o Isotonic: same osmolarity as body fluids
o Hypotonic: lower osmolarity than body fluids
o Hypertonic: higher osmolarity than body fluids
Movement of Ions
o The plasma membrane can also control the movement of ions (such as Na+) across its
membrane
o The movement of ions is affected by both the chemical and electrical gradients
o Molecules move from areas where they are in high concentration to areas wher they are
in low concentration: i.e. they move down their chemical gradient
o Many of the ions within our body carry a charge
o Electrically charged molecules move towards areas of opposite charge( i.e. positive
charged ions move towards negatively charged areas or down their electrical gradient)
Resting Membrane Potential
o The uneven distribution of ions across the cell membrane
Caused by selective permeability of the membrane to certain ions
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