Biology Now, 2e

64 ■ CHAPTER 04 Life Is Cellular

CELLS

Sugar

Transport

proteins

Simple diffusion Facilitated diffusion

Transport

proteins

Na+ Ca2+

Plasma

membrane

H 2 O K+

Energy

Active transport Passive transport

to move into and out of the cell, making it selec-

tively permeable (Figure 4.4). Selective perme-

ability means that some substances can cross

the membrane, others are excluded, and still

others can pass through the membrane when

aided by transport proteins.

All movement of substances through the

plasma membrane occurs by either active or

passive transport. Some transport proteins

facilitate active transport, the movement of

a substance that requires an input of energy

(Figure 4.4, left). Molecules move across the

plasma membrane by active transport when

they need to move from a region of lower concen-

tration to a region of higher concentration. In

contrast, passive transport is the movement

of a substance without the addition of energy

(Figure 4.4, middle and right). Movement via

passive transport is spontaneous.

to create a self-assembling membrane. They first

mixed together oil and a detergent. Then they

added copper, a metal ion, as a catalyst to spark

a chemical reaction. With the addition of copper,

sturdy membranes begin to bud off the oil; these

were self-assembling structures. “There’s no

equivalent whatsoever in nature,” says Devaraj.

“Our goal was simply to mimic biology.”

Through the Barrier

Devaraj admits that his artificial membrane is

far simpler than a real cell’s plasma membrane,

which is dotted with numerous proteins, includ-

ing transport proteins. Transport proteins are

gates, channels, and pumps that allow molecules

Figure 4.4

The plasma membrane is a barrier and a gatekeeper

The plasma membrane moves substances in a highly selective fashion, determined in large part by the types of membrane proteins

embedded in the phospholipid bilayer. M

Q1: In what ways is the plasma membrane a barrier, and in what ways is it a gatekeeper?

Q2: Why can’t ions (such as Na+) cross the plasma membrane without the help of a transport protein?

Q3: If no energy were available to the cell, what forms of transport would not be able to occur? What forms of transport

could occur? (Hint: Look ahead at Figures 4.5 and 4.6.)