Download Membrane Transport

Membrane Transport
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.
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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.
Carrier-Mediated Transport
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 T m.
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.
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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.
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2. The Na /K pump helps to maintain these concentration differences by transporting
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Na out of the cell and K into the cell.
(From: http://www.mhhe.com/biosci/ap/foxhumphys/student/olc/chap06summary.html)