Chapter 4: Cell Membrane Transport Flashcards
When epithelia transport materials into the internal environment from the outside, the process is called __________. When they transport from the internal environment to the outside, it is called _________.
absorption secretion (p. 118)
How does simple diffusion explain our ability to smell an open perfume bottle across a room? The molecules travel away from the bottle because they are more concentrated near the bottle and less concentrated farther away, and because these molecules, like any others, move spontaneously down their _____________ ________ because they are in a state of constant _______ motion.
concentration gradient thermal [p. 102 - 103 in Principles of Human Physiology]
Diffusion is a general term that applies to molecules moving through any medium. Diffusion is defined as…
…the movement of molecules from one location to another as a result of their own thermal motion. [p. 103, Principles of Human Physiology]
Thermal motion is often called ______ thermal motion because individual molecules move helter-skelter in various directions because of collisions with other molecules. As a consequence, molecules do not go in any particular direction for long before changing course, and when this change occurs, the direction is unpredictable.
random [p. 103, Principles of Human Physiology]
What are the 3 factors affecting the rate at which a substance is transported passively across a membrane (simple diffusion)?
1.) the magnitude of the driving force 2.) the membrane surface area 3.) the permeability of the membrane [p. 103, Principles of Human Physiology]
When a driving force acts on molecules crossing a membrane, it influences not only the direction in which they move but also the rate at which they are transported. In most cases, the net flux _________ as the magnitude of the driving force increases, but not _______.
increases always [p. 103, Principles of Human Physiology]
Consider two spaces divided by a membrane. The concentrations change less rapidly as they become more nearly equal; that is, the net flux decreases as the size of the concentration gradient _________. Once the concentrations become equal, they no longer change, because the net flux is zero. Under these conditions, molecules still move back and forth across the membrane, but at _____ _____ in both directions.
decreases equal rates [p. 103, Principles of Human Physiology]
In simple diffusion, the net flux of a substance is directly proportional to the size of the concentration gradient, or to the size of the __________ ________, if the substance is charged. As the concentration gradient increases or decreases by a given factor, the flux increases or decreases by that ____ ______.
electrochemical gradient same factor [p. 103, Principles of Human Physiology]
As a general rule, the rate at which molecules are transported across a membrane varies in ______ proportion to the membrane’s surface area. If the surface area were to double, the number of molecules crossing the membrane per unit time would also double. Note that this would be true regardless of the _________ of transport.
direct mechanism [p. 104, Principles of Human Physiology]
For a given concentration gradient, when permeability is higher, the net flux is ______.
larger [p. 104, Principles of Human Physiology]
Permeability has a precise mathematical definition. We have seen that when a substance crosses a membrane by simple diffusion, the net flux is proportional to the size of the concentration gradient. This relationship is expressed mathematically by Fick’s law, which can be written as follows:
net flux = PA (ΔC)
In the expression, A is the ________ _______ ____, ΔC is the __________ ________, and the coefficient P represents __________.
membrane surface area
concentration gradient
permeability
[Keep in mind that this equation can be represented differently. Following is a paragraph from a Science Direct article:
Diffusion is described by Fick’s law:
j = −DA (Δ concentration/Δ distance)
where J is the net flux (movement) of the compound; — indicates that the movement is from an area of higher concentration to an area of lower concentration; D is the diffusion (permeability) coefficient, specific for the compound and for the barrier; A is the surface area involved; Δ concentration is the concentration gradient; and Δ distance is the distance over which the compound must travel. The diffusional movement of a compound is increased by increasing the surface area for exchange, by increasing the concentration gradient, or by decreasing the distance across which the compound has to move.
From https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/net-flux ]
Note that the term permeability applies to membranes not to __________ _________ - even though a membrane’s permeability is not the same for all substances and therefore depends on the nature of the transported molecules. A substances that crosses a membrane is ________, whereas the membrane itself is permeable (able to be permeated).
transported molecules
permeant
[p. 104, Principles of Human Physiology]
Name 4 factors which influence the permeability of a cell membrane:
- lipid solubility of the diffusing substance
- size and shape of diffusing molecules
- temperature
- membrane thickness
[p. 104 - 105]
__________ (________) substances are the most lipid soluble, whereas __________ (polar or ________) substances are the least lipid soluble. Given that molecules must move through a lipid bilayer in simple diffusion, we would expect lipid-soluble molecules to be able to enter the bilayer more readily.
Hydrophobic (non-polar)
hydrophilic (polar or ionized)
[p. 105 of Principles of Human Physiology]
Of all the factors, which one has the strongest influence on permeability?
lipid solubility
[p. 105 of Principles of Human Physiology]
