Biology Now, 2e

Engineering Life ■ 67

bulge inward or outward to form packages called

vesicles. Vesicles move molecules from place to

place inside a cell but also transport substances

into and out of the cell (Figure 4.7).

Cells expel materials in vesicles via exocy-

tosis (Figure 4.7, top left). The substance to be

exported from the cell is packaged into a vesi-

cle, and as the vesicle approaches the plasma

membrane, a portion of the vesicle’s membrane

fuses with the plasma membrane. The inside of

the vesicle then opens to the exterior of the cell,

discharging its contents.

Endocytosis (Figure 4.7, right and lower

left) is the opposite of exocytosis. In this process,

a section of plasma membrane bulges inward

to form a pocket around extracellular fluid,

events inside the cell that causes the cell to do

something. For example, the receptors in nerve

cells receive molecular signals from other nerve

cells that cause the cells to fire. Receptor proteins

are key components of a cell’s communication

system, enabling it to respond appropriately to

changes in the organism.

Another Way Through

In addition to transport proteins, there is

another way that molecules move into and out

of a cell. Sections of the plasma membrane can

Figure 4.7

Substances move into cells by endocytosis and out of cells by exocytosis

Q1: If endocytosis itself is nonspecific, how does receptor-mediated endocytosis bring only certain molecules into a cell?

Q2: What sorts of molecules could be moved by endocytosis or exocytosis, but not by diffusion?

Q3: How does the fluid that enters a cell via pinocytosis differ from the fluid that enters by osmosis?

Endocytosis is the reverse of

exocytosis, bringing material

from the outside of the cell to

the inside, enclosed in

vesicles. Endocytosis may be

nonselective, as shown here,

drawing in any and all

molecules near the opening of

the vesicle.

Exocytosis Endocytosis

Receptor-mediated endocytosis

Exocytosis is used to eject

substances from the cell.

Here, a cell ejects waste

material into the outside

environment.

Here, a white blood cell

engulfs a yeast cell

through phagocytosis,

a form of nonspecific

endocytosis.

Cells lining blood vessels

have created fluid-filled

vesicles through pinocytosis,

nonspecific endocytosis of

fluid.

Receptor-mediated

endocytosis is a

selective process in

which only certain

molecules bind to

receptor proteins.

Here, low-density

lipoprotein (LDL)

particles bind to

LDL receptors and

are transported to

the cell interior.