Principles of Anatomy and Physiology 14e with Atlas of the Skeleton Set (14th Edition)

Published by Wiley
ISBN 10: 1-11877-456-6
ISBN 13: 978-1-11877-456-4

Chapter 3 - The Cellular Level of Organization - Checkpoint - Page 69: 9

Answer

Diffusion is simply the movement of a solute in a solvent from an area of higher concentration to one of lower concentration under the force of kinetic energy. Both solute and solvent particles move down their concentration gradients. If and when equilibrium is reached, the particles continue to move but the concentrations do not change. If a selectively permeable membrane is interposed between solutions of different concentrations, diffusion will also occur. The selectively permeable membrane will allow some molecules, ions, and particles to pass through and diffuse down their concentration gradients in a simple process. But the membrane will retard the passage of some molecular species and probably prevent the passage of others. This will all depend on the characteristics, both chemical and structural, of the membrane. The plasma membrane of a human cell has a lipid bilayer that allows simple diffusion of non-polar hydrophobic molecules but retards the passage of ions and large charged polar particles. This is where facilitated diffusion comes in. Integral membrane proteins can act as channels and form pores to allow retarded molecular species in. Facilitated diffusion can also be affected by carrier or transporter membrane proteins, which can bind with solute molecules on one side of the membrane, change shape, pass through the membrane and deliver the bound solute membrane to the other side of the membrane.

Work Step by Step

Neither simple nor facilitated diffusion needs an energy source like ATP. Whereas simple diffusion does not require the agency of integral membrane protein channels or carriers, facilitated diffusion utilizes both. Thus a glucose transporter protein (inserted into membranes by the hormone insulin) greatly increases the speed of the movement of glucose into cells, a case of facilitated diffusion by carrier.
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