Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 133

b. Simple diffusion of ions through membrane channel

proteins in the plasma membrane

c. Simple diffusion of water molecules (osmosis) through

aquaporin (water) channels in the plasma membrane

Membrane transport processes may also be categorized by

their energy requirements. Passive transport is the net move-

ment of molecules and ions across a membrane from higher to

lower concentration (down a concentration gradient); it does

not require metabolic energy. Passive transport includes all of

the non-carrier-mediated diffusion processes (the simple dif-

fusion of lipid-soluble molecules, ions, and water), plus the

carrier-mediated facilitated diffusion ( fig. 6.2 ). Active transport

is the net movement of molecules and ions across a membrane

from the region of lower to the region of higher concentra-

tions. Because active transport occurs against the concentra-

tion gradient, it requires the expenditure of metabolic energy

(ATP) that powers specific carrier proteins, which are often

called pumps.

2. Non-carrier-mediated transport
   a. Simple diffusion (diffusion that is not carrier-mediated)
   of lipid-soluble molecules through the phospholipid
   layers of the plasma membrane

Figure 6.2 Three types of passive transport.
( a ) Nonpolar molecules can move by simple diffusion through the
double phospholipid layers of the plasma membrane. ( b ) Inorganic
ions and water molecules can move by simple diffusion through
protein channels in the plasma membrane. ( c ) Small organic
molecules, such as glucose, can move by facilitated diffusion
through the plasma membrane using carrier proteins.

Channel proteins

Nonpolar molecules

(a)

(b)

(c)

Carrier protein

Plasma membrane

| CHECKPOINT

1a. Describe the distribution of fluid in the body.

1b. Describe the composition of the extracellular

matrix and explain the importance of the matrix

metalloproteinases.

2. List the subcategories of passive transport and
   distinguish between passive transport and active
   transport.

6.2 Diffusion and Osmosis

Net diffusion of a molecule or ion through a membrane

always occurs in the direction of its lower concentration.

Nonpolar molecules can penetrate the phospholipid barrier

of the plasma membrane, and small inorganic ions can pass

through protein channels in the plasma membrane. The net

diffusion of water through a membrane is known as osmosis.

LEARNING OUTCOMES

After studying this section, you should be able to:

3. Define diffusion and describe the factors that
   influence the rate of diffusion.


4. Define osmosis, describe the conditions required
   for it to occur, and explain how osmosis relates to
   osmolality and osmotic pressure.


5. Explain the nature and significance of hypotonic,
   isotonic, and hypertonic solutions.


6. Explain how homeostasis of plasma osmolality is
   maintained.