Human Physiology, 14th edition (2016)

158 Chapter 6

Summary

c. Water moves by osmosis from the solution of lower

osmolality and osmotic pressure to the solution of

higher osmolality and osmotic pressure.

3. Solutions containing osmotically active solutes that

have the same osmotic pressure as plasma (such as 0.9%

NaCl and 5% glucose) are said to be isotonic to plasma.

a. Solutions with a lower osmotic pressure are hypotonic;

those with a higher osmotic pressure are hypertonic.

b. Cells in a hypotonic solution gain water and swell;

those in a hypertonic solution lose water and shrink

(crenate).

4. 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.

a. Increased osmolality of the blood stimulates the

osmoreceptors.

b. Stimulation of the osmoreceptors causes thirst and

triggers the release of antidiuretic hormone (ADH)

from the posterior pituitary.

c. ADH promotes water retention by the kidneys,

which serves to maintain a normal blood volume and

osmolality.

6.3 Carrier-Mediated Transport 142

A. The passage of glucose, amino acids, and other polar mol-

ecules through the plasma membrane is mediated by carrier

proteins in the cell membrane.

1. Carrier-mediated transport exhibits the properties of

specificity, competition, and saturation.

2. The transport rate of molecules such as glucose reaches

a maximum when the carriers are saturated. This

maximum rate is called the transport maximum ( T (^) m ).
B. The transport of molecules such as glucose from the side of
higher to the side of lower concentration by means of mem-
brane carriers is called facilitated diffusion.

1. Like simple diffusion, facilitated diffusion is passive
   transport—cellular energy is not required.


2. Unlike simple diffusion, facilitated diffusion displays
   the properties of specificity, competition, and saturation.
   C. The active transport of molecules and ions across a mem-
   brane requires the expenditure of cellular energy (ATP).


3. In active transport, carriers move molecules or ions from
   the side of lower to the side of higher concentration.


4. One example of active transport is the action of the Na^1 /
   K^1 pump.
   a. Sodium is more concentrated on the outside of the
   cell, whereas potassium is more concentrated on the
   inside of the cell.
   b. The Na^1 /K^1 pump helps to maintain these
   concentration differences by transporting Na^1 out of
   the cell and K^1 into the cell.
   6.1 Extracellular Environment 131
   A. Body fluids are divided into an intracellular compartment
   and an extracellular compartment.


5. The extracellular compartment consists of blood plasma
   and interstitial, or tissue fluid.


6. Interstitial fluid is derived from plasma and returns to
   plasma.
   B. The extracellular matrix consists of protein fibers of collagen
   and elastin and an amorphorus ground substance.


7. The collagen and elastin fibers provide structural
   support.


8. The ground substance contains glycoproteins and
   proteoglycans forming a hydrated gel, which contains
   most of the interstitial fluid.
   6.2 Diffusion and Osmosis 133
   A. Diffusion is the net movement of molecules or ions from
   regions of higher to regions of lower concentration.


9. This is a type of passive transport—energy is provided
   by the thermal energy of the molecules, not by cellular
   metabolism.


10. Net diffusion stops when the concentration is equal on
    both sides of the membrane.
    B. The rate of diffusion is dependent on a variety of factors.


11. The rate of diffusion depends on the concentration
    difference across the two sides of the membrane.


12. The rate depends on the permeability of the plasma
    membrane to the diffusing substance.


13. The rate depends on the temperature of the solution.


14. 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.
    C. Simple diffusion is the type of passive transport in which
    small molecules and inorganic ions move through the plasma
    membrane.


15. Inorganic ions such as Na^1 and K^1 pass through
    specific channels in the membrane.


16. Steroid hormones and other lipids can pass directly
    through the phospholipid layers of the membrane by
    simple diffusion.
    D. Osmosis is the simple diffusion of solvent (water) through a
    membrane that is more permeable to the solvent than it is to
    the solute.


17. Water moves from the solution that is more dilute to the
    solution that has a higher solute concentration.


18. Osmosis depends on a difference in total solute
    concentration, not on the chemical nature of the solute.
    a. The concentration of total solute, in moles per
    kilogram (liter) of water, is measured in osmolality
    units.
    b. The solution with the higher osmolality has the
    higher osmotic pressure.