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Membrane Transport Lecture Notes
In today�s experiments we will explore membrane transport processes, focusing on passive transport, specifically diffusion of molecules through various types of matter and across semipermeable membranes
1. Lab Manual, Ch 5, Ex 5-1- Diffusion
Diffusion - the movement of molecules/atoms from an area of high density/concentration to an area of low density/concentration
all particles have kinetic energy; they vibrate and move randomly, in what is known as Brownian Motion - as particles move, they bump into each other and disperse, naturally moving into areas of lower concentration, until eventually there is no net diffusion occurring - this is known as equilibrium
the rate of diffusion can depend on several factors; one of these factors is the state of matter that the particle is in - instructor�s demonstration
Questions: diffusion is faster in what type of matter - gas, liquid or solid? Which state of matter has the highest kinetic energy? Which state has the most space between the particles?
During the instructor�s demonstration - keep in mind, what is the difference in time scale (seconds, minutes, hours?) for diffusion in a gas vs. liquid vs. solid?
Diffusion also depends on temperature - instructor�s demonstration - hypothesize - in which solution will the dye move faster? Why? Which solution has the greater kinetic energy?
Diffusion can also depend on the size or molecular weight of the particle - looking at the agarose plate, you can come up with a hypothesis about which particle has the highest molecular weight, and which the lowest, based on rates of diffusion - was your hypothesis correct? If not, what other characteristics of the molecules may have influenced their rates of diffusion?
Rates of diffusion also depend on the steepness of the concentration gradient - what we will observe during the rate of osmosis experiment
Diffusion is a passive process - does not require energy; therefore, particles can only move down their concentration gradients (from a region of high concentration to a region of low concentration); diffusion cannot occur against the concentration gradient of the diffusing particle
In contrast, active transport requires energy in the form of ATP - molecules can be actively transported against their concentration gradients (eg., the Na/K ATPase)
Cell membranes allow the diffusion of molecules into/out of the cell, but these membranes are selectively permeable - only some substances are allowed through the membrane. Whether a particle is allowed through depends on its size, charge, polarity and hydrophobicity:
Small, nonpolar molecules like oxygen, carbon dioxide can diffuse directly through the cell membrane
Molecules that are similar in structure to the cell membrane can also pass directly through, eg., steroid hormones
Molecules that are polar (water) or carry a charge (ions) must pass through channels that are selective for specific molecules
Other molecules diffuse through the cell membrane via carrier molecules (proteins and sugars)
In today�s experiments, the dialysis tubing you will use is made of a cellulose membrane that allows the diffusion of molecules according to size only (molecules less than/equal to 14000 daltons can diffuse through)
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