Biology (Holt)

The Cell Membrane

The cytoplasm of a cell is contained by its membrane. Cell mem-

branes are not rigid like an eggshell. Rather, they are fluid like a

soap bubble. The fluidity of cell membranes is caused by lipids,

which form the foundation of membranes. The lipids form a barrier

that separates the inside of the cell from the outside of the cell. This

barrier allows only certain substances in the cell’s environment to

pass through. This selective permeability of the cell membrane

determines which substances enter and leave the cell.

The Cell Membrane as a Barrier

The selective permeability of the cell membrane is caused mainly by

the way phospholipids interact with water. A is a lipid

made of a phosphate group and two fatty acids. As shown in

Figure 11, a phospholipid has both a polar “head” and two nonpolar

“tails.” You may recall that the polar ends of water molecules will

form weak bonds with other polar substances. The head of a phos-

pholipid, which contains a phosphate group, is polar and is

attracted to water. In contrast, the two fatty acids, or tails, are non-

polar and therefore are repelled by water.

In a cell membrane, the phospholipids are arranged in a double

layer called a ,as shown in Figure 11. The nonpolar tails

of the phospholipids make up the interior of the lipid bilayer.

Because water both inside and outside the cell repels the nonpolar

tails, they are forced to the inside of the lipid bilayer. Ions and most

polar molecules, including sugars and some proteins, are repelled

by the nonpolar interior of the lipid bilayer. The lipid bilayer allows

lipids and substances that dissolve in lipids to pass through.

lipid bilayer

phospholipid

60 CHAPTER 3Cell Structure

Real Life

Donated blood is frozen

in a special process

called cryopreservation.

Similar methods are used

to preserve human eggs,

embryos, and blood from

the umbilical cord, a rich

source of immune-system

cells.

Finding Information

Research how cryo-

preservation

methods enable

cells to with-

stand freezing.

Polar

Nonpolar

Polar

Lipid bilayer

Polar

head

Non-

polar

tails

The lipid bilayer is the foundation

of the cell membrane.

The arrangement of phospholipids

in the lipid bilayer makes the cell

membrane selectively permeable.

A phospholipid’s “head” is polar, and

its two fatty acid “tails” are nonpolar.

Cell membranes are made of a double layer of phospholipids, called a lipid bilayer.

Figure 11 Lipid bilayer