|Human Physiology 6/e Fox|
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Chapter 6: Membrane Transport and the Membrane Potential
Diffusion and Osmosis
I. Diffusion is the net movement of molecules or ions from regions of higher to regions of lower concentration.
A. This is a type of passive transport, energy is provided by the thermal energy of the molecules, not by cellular metabolism.
B. Net diffusion stops when the concentration is equal on both sides of the membrane.
II. The rate of diffusion is dependent on a variety of factors.
A. The rate of diffusion depends on the concentration difference across the two sides of the membrane.
B. The rate depends on the permeability of the cell membrane to the diffusing substance.
C. The rate depends on the temperature of the solution.
D. The rate of diffusion through a membrane is also directly proportional to the surface area of the membrane, which can be increased by such adaptations as microvilli.
III. Simple diffusion is the type of passive transport in which small molecules and inorganic ions move through the cell membrane.
A. Inorganic ions, such as Na+ and K+ pass through specific channels in the membrane.
B. Steroid hormones and other lipids can pass by simple diffusion directly through the phospholipid layers of the membrane.
IV. Osmosis is the simple diffusion of solvent (water) through a membrane that is more permeable to the solvent than it is to the solute.
A. Water moves from the solution that is more dilute to the solution that has a higher solute concentration.
B. Osmosis depends on a difference in total solute concentration, not on the chemical nature of the solute.
1. The concentration of total solute, in moles per kilogram (liter) of water, is measured in osmolality units.
2. The solution with the higher osmolality has the higher osmotic pressure.
3. Water moves by osmosis from the solution of lower osmolality and osmotic pressure to the solution of higher osmolality and osmotic pressure.
C. Solutions that have the same osmotic pressure as plasma (such as 0.9% NaCl and 5% glucose) are said to be isotonic to plasma.
1. Solutions with a lower osmotic pressure are hypotonic; those with a higher osmotic pressure are hypertonic.
2. Cells in a hypotonic solution gain water and swell; those in a hypertonic solution lose water and shrink (crenate).
D. The osmolality and osmotic pressure of the plasma is detected by osmoreceptors in the hypothalamus of the brain and maintained within a normal range by the action of antidiuretic hormone (ADH) released from the posterior pituitary.
1. Increased osmolality of the blood stimulates the osmoreceptors.
2. Stimulation of the osmoreceptors causes thirst and triggers the secretion of antidiuretic hormone (ADH) from the posterior pituitary.
3. ADH promotes water retention by the kidneys, which serves to maintain a normal blood volume and osmolality.
I. The passage of glucose, amino acids, and other polar molecules through the cell membrane is mediated by carrier proteins in the cell membrane.
A. Carrier-mediated transport exhibits the properties of specificity, competition, and saturation.
B. The transport rate of molecules such as glucose reaches a maximum when the carriers are saturated. This maximum rate is called the transport maximum, or Tm.
II. The transport of molecules such as glucose from the side of higher to the side of lower concentration by means of membrane carriers is called facilitated diffusion.
A. Like simple diffusion, this is passive transport, cellular energy is not required.
B. Unlike simple diffusion, facilitated diffusion displays the properties of specificity, competition, and saturation.
III. The active transport of molecules and ions across a membrane requires the expenditure of cellular energy (ATP).
A. In active transport, carriers move molecules or ions from the side of lower to the side of higher concentration.
B. One example of active transport is the action of the Na+/K+ pump.
1. Sodium is more concentrated on the outside of the cell, whereas potassium is more concentrated on the inside of the cell.
2. The Na+/K+ pump helps to maintain these concentration differences by transporting Na+ out of the cell and K+ into the cell.
The Membrane Potential
I. The cytoplasm of the cell contains negatively charged organic ions (anions) that cannot leave the cell; they are "fixed" anions.
A. These fixed anions attract K+, which is the inorganic ion that can pass through the cell membrane most easily.
B. As a result of this electrical attraction, the concentration of K+ within the cell is greater than the concentration of K+ in the extracellular fluid.
C. If K+ were the only diffusible ion, the concentration of K+ on the inside and outside of the cell would reach an equilibrium.
1. At this point, the rate of K+ entry (due to electrical attraction) would equal the rate of K+ exit (due to diffusion).
2. At this equilibrium, there would still be a higher concentration of negative charges within the cell (because of the fixed anions) than outside the cell.
3. At this equilibrium, the inside of the cell would be ninety millivolts negative (-90 mV) compared to the outside of the cell. This is called the K+ equilibrium potential (EK).
D. The resting membrane potential is less than EK; it is usually -65 mV to -85 mV. This is because some Na+ can also enter the cell.
1. Na+ is more highly concentrated outside than inside the cell, and the inside of the cell is negative. These forces attract Na+ into the cell.
2. The rate of Na+ entry is generally slow because the membrane is usually not very permeable to Na+.
II. The slow rate of Na+ entry is accompanied by a slow rate of K+ pump, which maintains constant concentrations and a constant resting membrane potential.
A. The Na+/K+ pump counters this leakage, thus maintaining constant concentrations and a constant resting membrane potential.
B. Most cells in the body contain numerous Na+/K+ pumps that require a constant expenditure of energy.
C. The Na+/K+ pump itself contributes to the membrane potential because it pumps more Na+ out than it pumps K+ in (by a ratio of three to two).
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